Antibodies blocking programmed death 1 (anti-PD-1) or its ligand (anti-PD-L1) are associated with modest response rates as monotherapy in metastatic breast cancer, but are generally well tolerated and capable of generating dramatic and durable benefit in a minority of patients. Anti-PD-1/L1 antibodies are also safe when administered in combination with a variety of systemic therapies , as well as with radiotherapy. We summarize preclinical, translational, and preliminary clinical data in support of combination approaches with anti-PD-1/L1 in metastatic breast cancer, focusing on potential mechanisms of synergy, and considerations for clinical practice and future investigation. In addition, the robust negative predictive value of the integral PD-L1 biomarker (SP142 antibody) was promising, allowing for future selection of individuals most likely to derive benefit. Numerous randomized phase III studies combining anti-PD-1/L1 with standard-of-care therapies are ongoing and will be reported over the next several years, potentially increasing the breadth of options for combination immunotherapy in breast cancer.21Cytotoxic chemotherapy has pleiotropic immunomodulatory effects that may synergize with anti-PD-1/L1. Recently, the first randomized anti-PD-1/L1 combination trial in metastatic breast cancer, IMpassion130, provided proof-of-concept that anti-PD-1/L1 plus chemotherapy can be safe and more effective than chemotherapy alone. In the trial, atezolizumab (anti-PD-L1) prolonged progression-free survival (PFS) in combination with first-line nab-paclitaxel  in the entire population, with a preliminary analysis suggesting prolonged OS in the 41% of subjects with tumors containing at least 1% PD-L1-positive immune cells .However, given the perils of cross-trial comparison, one foreseeable clinical challenge is to ascertain the relative efficacy of dozens of feasible anti-PD-1/L1 combination approaches in metastatic breast cancer. The goals of this review are to describe immunologic mechanisms of synergy of various standard therapeutic approaches with anti-PD-1/L1, summarize available preclinical data, and discuss clinical use and future investigations of anti-PD-1/L1 combination approaches in metastatic breast cancer.22 and include expansion or activation of effector cell populations , depletion or inhibition of suppressor cell populations , and induction of immunogenic cell death (ICD), a stress response associated with release of danger-associated molecular patterns (DAMPs) signals and enhanced antigen presentation.27 Chemotherapy is also associated with interferon gamma secretion and adaptive PD-L1 upregulation.29 For all these reasons, there has been significant interest in evaluating the efficacy of combining chemotherapy with anti-PD-1/L1. On the other hand, patients who have been extensively pretreated with cytotoxic therapy seem less likely to respond to immunotherapy, suggesting immunosuppressive mechanisms may dominate in the context of more extensive therapy.Cytotoxic chemotherapy remains a standard-of-care for metastatic breast cancer, with commonly employed agents including microtubule-targeting agents , anthracyclines , anti-metabolites , alkylating agents (cyclophosphamide), and platinums . The immunomodulatory effects of chemotherapy have been the subject of extensive review,30 there are fewer data comparing the relative efficacy of the various chemotherapy agents plus anti-PD-1/L1, and results across animal models are inconsistent. For example, cyclophosphamide-containing regimens were among the most effective potentiators of anti-PD-1/L1 response in one study,30 whereas cyclophosphamide plus anthracycline failed to enhance anti-PD-1/L1 response in another.31 Because immune effects of chemotherapy are varied, it becomes difficult to compare the effects on the basis of pharmacodynamic activity alone. For example, anti-metabolites (5-FU and gemcitabine) may be superior to anthracycline or cyclophosphamide in depleting MDSCs, doxorubicin may be superior in inducing ICD, whereas cyclophosphamide may be superior in depleting Tregs.While there are preclinical models demonstrating the efficacy of anti-PD-1/L1 plus various chemotherapy agents,3 These results led to regulatory approval in the first-line setting for PD-L1-positive disease. However, an earlier phase I study demonstrated a response rate of 24% and durable responses in the first-line setting with single-agent atezolizumab,32 raising the possibility that the benefits observed with the addition of atezolizumab are additive rather than synergistic. Ongoing studies will address whether benefits of the taxane/atezolizumab combination can be seen in patients with a shorter disease-free interval than 12 months as studied in IMpassion130 and whether alternative chemotherapy backbones could offer similar or greater clinical benefit. The IMpassion131 study is similar to IMpassion130, however evaluating atezolizumab plus paclitaxel rather than nab-paclitaxel (NCT03125902). Importantly, the results of this study may provide clarity on whether prophylactic steroids impair clinical benefit to anti-PD-L1. The multi-arm, non-comparative phase II “TONIC” trial evaluated various induction chemotherapy regimens or radiation followed by anti-PD-1 (nivolumab) in metastatic TNBC. In this small study, highest objective responses were observed following induction cisplatin (23% ORR) and induction doxorubicin (35% ORR), however these findings must be confirmed in a larger study.33 The optimal sequencing of anti-PD-1/L1 with other therapies remains a topic of considerable debate. The KEYNOTE-355 phase III trial will provide additional randomized data of pembrolizumab versus various chemotherapy backbones (NCT02819518). Of note, this trial uses a different PD-L1 IHC assay (DAKO 22c3 antibody) for patient selection, which recently was found to classify more TNBCs as PD-L1-positive, compared to the SP142 assay.34 The impact of PD-L1-discordance may require additional investigation.In the phase III IMpassion130 trial, atezolizumab improved PFS when added to first-line nab-paclitaxel in metastatic triple-negative breast cancer (TNBC) in the entire study population, but OS was only prolonged in patients with tumors bearing PD-L1-positive immune cells.36 However, it has been shown that the vaccine-effect of radiotherapy is modest, and that synergy with checkpoint blockade may depend on pre-existing immunity.37 Similar to chemotherapy, radiotherapy is associated with release of DAMPs such as uric acid, high mobility group box 1 (HMGB1), calreticulin, and double stranded DNA, which act as immunologic adjuvants to activate myeloid cells and facilitate subsequent chemokine release and T-cell recruitment. Radiotherapy may also upregulate MHC class I and FAS adhesion molecules, which may counteract adaptive loss of MHC or beta 2 microglobulin.38 Conversely, radiotherapy can cause immunosuppressive effects, including upregulation of the PD1/PDL1 axis, upregulation of suppressive macrophage receptors including Mertk,39 expansion of Tregs, and possibly apoptosis of tumor infiltrating lymphocytes (TILs).In the metastatic setting, ionizing radiotherapy is frequently employed to palliate symptoms  or to delay progression of central nervous system metastases using either stereotactic radiosurgery or whole brain radiotherapy. The principal mechanism of radiotherapy is to induce lethal DNA damage to tumor cells or tumor-associated stroma. However, radiotherapy can enhance anti-tumor immunity by engaging both innate and adaptive responses. In some cases, radiotherapy may be associated with regression of non-irradiated tumors, coined the “abscopal effect.” Radiation-induced DNA damage may lead to cell death and serve as a source of antigen and danger signals that facilitate DC maturation and cross-presentation of tumor antigens to prime tumor-specific T cell responses.40 It is difficult to ascertain the optimal dose and schedule of radiation plus immune checkpoint inhibitor. Increased dose is associated with more profound release of DAMPs including ATP and HMGB1, but may also promote immunosuppressive effects such as induction of exonucleases that eliminate cytosolic DNA, a key messenger of DC activation and downstream T-cell priming.42 In a comparison of various fractionation schedules plus anti-PD-1 using MOC1 and MC38 murine models, higher-dose hypofractionated radiotherapy (8 Gy x 2) was superior to low-dose fractionated radiotherapy (2 Gy× 10) in controlling tumor, enhancing interferon production, and upregulating PD-L1.43 In a breast cancer model, hypofractionated (8 Gy × 3) was superior to high single dose therapy (20–30 Gy).41 Radiation may also cause systemic lymphopenia (with fractionated radiotherapy causing more profound lymphopenia compared to hypofractionated),44 and conversely, systemic immunosuppression may influence efficacy.45 The timing of radiation may also influence response, with one study showing concurrent therapy superior to sequential.46 Radiation combined with anti-PD-1/L1 has been well tolerated in patients with metastatic breast cancer with preliminary reports of tumor response in lesions outside the radiation field.47 Optimizing radiotherapy dose and timing will likely be the subject of future clinical trials. Furthermore, other immune stimulatory agents such as toll-like receptor 3 agonists and fms related tyrosine kinase 3 ligand (Flt3L), may synergize with radiotherapy and may hold unique promise in conjunction with anti-PD-1/L1.48In preclinical models, suppressive effects of radiotherapy can be mitigated in combination with anti-PD-1/L1. In a melanoma model, anti-CTLA4 plus radiotherapy was associated with PD-L1 upregulation, and the addition of anti-PD-L1 reversed T-cell exhaustion, promoted clonal T-cell expansion within the tumor, and enhanced response.49 Androgen signaling is known to play a negative regulatory role in central (thymic) T-cell production, and androgen ablation/blockade has been shown to facilitate increases in thymus size, lymphocyte count, thymic recombination of the T-cell receptor, and T-cell cytolytic activity.50 In murine breast cancer models, androgen blockade was associated with enhanced T-cell killing via upregulation of the apoptosis ligand, TRAIL.51 In prostate cancer models, AR blockade increased immune responses to vaccination.52 Finally, in a prostate cancer trial, pembrolizumab plus enzalutamide was associated with increased tumor and DC PD-L1 expression, increased circulating PD-1-positive T-cells, and clinical response following enzalutamide progression.54 Anti-PD-1/L1 agents combined with androgen blockade are currently being evaluated across a number of clinical trials in the metastatic breast cancer setting . Combinations with anti-estrogens are also ongoing, including the multi-arm MORPHEUS trial that combines fulvestrant with atezolizumab +/− other targeted approaches (NCT03280563).Estrogen/progesterone modulation remains a cornerstone of palliative therapy of hormone receptor (HR)-positive metastatic breast cancer. FDA-approved estrogen-directed therapies include a selective estrogen receptor modulator (tamoxifen), aromatase inhibitors , and a selective estrogen receptor degrader (fulvestrant). These agents may be used as monotherapy (with or without ovarian suppression), or in combination with targeted agents such as mammalian target of rapamycin (mTOR) inhibitors (everolimus) or cyclin-dependent kinase 4/6 (CDK4/6) inhibitors. Most HR-positive breast cancers and about half of TNBCs express the androgen receptor (AR) to some degree, prompting emerging interest in evaluating AR inhibition as a therapeutic strategy.55 Preclinical evidence suggests that CDK4/6i promote anti-tumor immunity by increasing antigen processing and presentation. CDK4/6i also activate tumor cell expression of endogenous retroviral elements and stimulate interferon signaling, resulting in enhanced tumor antigen presentation.57 In human epidermal growth factor receptor 2 (HER2)-positive breast cancers, CDK4/6i also increase expression of multiple antigen processing and presentation genes, including MHC Class I and Class II.56 They may also modulate NK cell activity.58 Teo and colleagues observed increased expression of cell-surface calreticulin in TNBC cell lines (HCC1806 and MDA-MB-231) after treatment with ribociclib, suggesting that CDK4/6i can induce ICD.59 In addition, CDK4/6i augment T cell effector function while markedly suppressing proliferation of regulatory T cells. As cell cycle inhibitors, CDK4/6i decrease T cell proliferation; however, CDK4/6i increase the activation of effector T cells and modulate gene expression.60 Preclinical and clinical studies have confirmed increased tumor infiltrating T cells61 and decreased Tregs within treated tumors.61Cyclin dependent kinase 4 and 6 inhibitors (CDK4/6i) have dramatically changed the treatment of metastatic HR-positive breast cancer. There are three FDA-approved agents: palbociclib, ribociclib, and abemaciclib. CDK4/6i are thought to work primarily by inducing cytostasis via G1 cell-cycle arrest, but have also been shown to induce apoptosis in vitro.62 For example, in a CT26 model, the clinical activity of abemaciclib was dependent on immunity, and combination anti-PD-L1 plus abemaciclib resulted in superior disease control with complete responses. Of note, concurrent therapy was superior to sequential therapy in this model. A phase Ib study of pembrolizumab plus abemaciclib in heavily pretreated patients with PD-L1-positive estrogen receptor-positive/HER2-negative advanced cancer showed an acceptable safety profile and clinical activity 63 compared to historical controls for single agent pembrolizumab (ORR 12%)65 or single agent abemaciclib (ORR 20% with a 42% disease control rate).66Given their place in standard treatment, a favorable side effect profile, and the documented beneficial immune effects, CDK4/6i may be a promising agent to combine with anti-PD-1/L1. CDK4/6i increase PD-L1 expression in vivo, with mounting preclinical data suggesting synergy with PD-1/PD-L1 blockade.67 Both trastuzumab and pertuzumab are capable of eliciting antibody-dependent cellular cytotoxicity (ADCC) via interactions of the antibody fragment crystallizable region (Fc) with Fc receptors found on NK cells and macrophages.68 Trastuzumab emtansine (T-DM1) is an antibody-drug conjugate, approved in the second-line trastuzumab-resistant setting, that augments the cytotoxic effect of trastuzumab via conjugation with the DM1 chemotherapy moiety. DM1 induces DC maturation and stimulates anti-tumor immunity.69 In murine models, T-DM1 therapy is associated with robust increases in T-cell infiltration, Th1 polarization, PD-1/PD-L1 expression, and intratumoral Tregs infiltration. Combination anti-PD-1 plus anti-CTLA-4 plus T-DM-1 was superior to T-DM-1 or anti-PD-1/CTLA-4 in a preclinical model.69 Lapatinib is an oral targeted inhibitor of EGFR and HER2, approved in combination with capecitabine or trastuzumab for metastatic HER2-positive breast cancer. Because lapatinib stabilizes HER2 protein at the cell membrane, it may also enhance the ADCC-effect of trastuzumab.70 Chemotherapeutic agents including taxanes may also enhance trastuzumab-mediated ADCC.71 Additional agents, including margetuximab, are being developed to maximize the ADCC-mediated immunotherapeutic effect of HER2-targeted therapy.72Overexpression of HER2 is observed in ~20% of breast carcinomas and is associated with an aggressive phenotype. The standard-of-care first-line therapy for metastatic HER2-positive breast cancer is systemic therapy with taxane plus dual anti-HER2 antibody therapy (trastuzumab and pertuzumab), which is associated with impressive gains in OS, and survival correlates with the degree of TILs.73 In a murine model, the activity of anti-HER-2 was dependent on cytotoxic T-cells and interferon secretion, and was improved in combination with anti-PD-1.74 One additional consideration for HER2-positive breast cancer is the antigenic potential of the HER2 protein. The E75 peptide vaccine, derived from an immunodominant epitope of the HER2 extracellular domain, has been shown to induce E75-specific cytotoxic T-cell responses in humans, and is being evaluated for clinical efficacy in the adjuvant setting in a phase III clinical trial.76 Trastuzumab was shown to facilitate DC uptake and antigen presentation of HER2, and increase E75-specific T-cell responses.75 HER2 signaling is also associated with downstream activation of the PI3K/mTOR/AKT pathway; therefore blockade may have secondary immune effects including PD-L1 upregulation. However, analysis of the TCGA database found no significant correlation between the mRNA expression levels of HER2 and PD-L1 in 790 available cases of breast cancer.77In addition to modulating ADCC, anti-HER2 antibodies may also interact with adaptive immune responses.13 In a similar trial, durvalumab plus trastuzumab was safe at standard full doses, but no responses were observed in a heavily pre-treated population.78 Ongoing clinical trials will evaluate whether combination therapy with anti-PD-1/L1 is effective in earlier lines of therapy, including a first-line trial evaluating standard-of-care first-line taxane/trastuzumab/pertuzumab +/− atezolizumab (NCT03199885). Of note, in the randomized phase II KATE2 study, the addition of atezolizumab to second-line T-DM1 only improved PFS, but only in the PD-L1-positive cohort.14In a phase Ib/II trial, a 20% ORR was observed with pembrolizumab plus trastuzumab in trastuzumab-resistant PD-L1-positive tumors.79 The mTOR inhibitor, everolimus, is approved in metastatic HR-positive/HER2-negative breast cancer in combination with exemestane,80 and also improves PFS when added to fulvestrant.81 Recently, the PI3K inhibitor, alpelisib, was approved in combination with fulvestrant for tumors bearing an activating PIK3CA mutation.82 Inhibitors of AKT are being developed and show promise in clinical trials. Recent studies have implicated this pathway in promoting an immunosuppressive tumor microenvironment83 via two mechanisms: (1) increased expression of immunosuppressive cytokines and chemokines which promote recruitment of MDSCs and Tregs84 and (2) constitutive expression of PD-L1.85 However, the association with PTEN loss/PIK3CA activation and PD-L1 expression was not observed in a small set of human TNBC specimens.86 Several pre-clinical studies have suggested that inhibition of the PI3K/AKT/mTOR pathway may decrease Tregs and promote CD8+ memory T-cell differentiation.89 Preclinical models evaluating the utility of combination therapy are limited, but the addition of anti-PD-1 was found to enhance the benefit of dual blockade of PI3K and CDK4/6 in an AT3OVA in vivo model.59 The PI3K-γ isoform has been specifically implicated in the function of TAM, and inhibitors of PI3K-γ are being clinically evaluated in combination with atezolizumab in TNBC (NCT03961698).A recent analysis of 13,349 genomic profiles identified an association of tumor mutational burden with common breast cancer oncogenic driver mutations, including mutations of both the PI3K/AKT/mTOR and RAS/MAPK pathways.P < 0.001), however, OS was not prolonged.91 Similarly the EMBRACA trial evaluated talazoparib in a similar cohort of patients with a median improvement in PFS of 8.6 months vs. 5.6 months .92 On the basis of these results, olaparib and talazoparib are now FDA-approved for gBRCA metastatic breast cancer.Phase III clinical trials have demonstrated efficacy of PARP inhibitors (PARPi) in metastatic breast cancer patients with a germline BRCA1/2 pathogenic variants (gBRCA). In the Olympiad study, olaparib was associated with an improvement in PFS relative to physician’s choice of non-platinum chemotherapy, showing a median PFS of 7.0 months vs. 4.2 months  pathway.95 Conversely, PARPi may also interfere with healthy immune function. PARP is known to interact with and activate NF-κB, a master regulator of innate immune function.96 PARP is necessary for optimal DC differentiation, activation, and stimulation of T cells.97 PARP deficiency has been attributed to increases in frequency and function of Tregs, decreased Th1 cytokine/chemokine function, deficiencies in Th2 differentiation, deficiencies in B-cell antibody class switching and somatic immunoglobulin hypermutation.99 Given the anticipated pleotropic effects of PARPi on anti-tumor immunity, more mechanistic studies in the context of ongoing clinical trials as well as randomized trials (such as NCT02849496) will be required to evaluate the synergistic potential of PARP inhibition in combination with anti-PD-1/L1. Recently the phase II single arm MEDIOLA trial evaluated olaparib in combination with durvalumab in patients with gBRCA, and demonstrated a disease control rate at 12 weeks of 80%.100In addition to direct antitumor effects, PARP inhibition may have immunomodulatory properties that improve or impair therapeutic efficacy in breast cancer. PARP inhibition has been associated with an increased number and effector function of cytotoxic T cells and NK cells, showing synergy with CTLA-4 inhibition in an immunocompetent BRCA1−/− model of ovarian cancer, with efficacy driven by improved peritoneal T cell effector function and IFNγ production with combination therapy.101 This effect may be due at least in part to inhibition of PARP-mediated CD8+ T cell apoptosis driven by reactive oxygen species produced by tumor cells.102 The Topacio/Keynote-162 trial evaluated niraparib and pembrolizumab in a phase II single arm trial with an ORR of 28% and disease control rate of 50%, with the best responses being demonstrated in patients with a tumor BRCA mutation.15In breast cancer, PARPi have been shown to increase tumor cell expression of PD-L1, thereby suppressing the antitumor T cell response, but also to have a synergistic effect when given with PD-1 inhibition.104 HDACi target epigenetic pathways inducing transcription modifications associated with growth inhibition, apoptosis, cell differentiation and ultimately anti-tumor effects.105 MDSCs which can suppress T-cell responses, pose an important limitation to immune therapy for breast cancer, but may also serve as a potential target for amplifying host immunity. This has been shown in animal models and in patients with breast cancer.106 Preclinical work demonstrates that HDACi may reduce the activity of MDSCs and Tregs,107 upregulate MHCI/II, increase sensitivity of breast cancer cells to cytotoxic T-cell mediated lysis, direct NK cell-mediated lysis, and facilitate ADCC.108 Exploratory analyses from the phase II clinical trial ENCORE 301 (exemestane +/− entinostat) demonstrated an increase in HLA-DR-positive monocytes and a decrease in granulocytic and monocytic MDSCs in patients treated with HDACi.109 Immunomodulatory activity was correlated with histone acetylation of peripheral mononuclear cells (suggested biomarker of response) and clinical benefit. Given the immunomodulatory effects of HDACi, it is not surprising that multiple preclinical studies have found synergy with the combination of HDACi and checkpoint blockade in breast cancer and other solid tumors.111 DNA methyltransferase inhibitors  and various systemic chemotherapies  also increase MHCI and tumor antigen and reduce systemic and intratumoral MDSCs, potentially augmenting anti-PD-1/L1.104Epigenetic modifying agents, including histone deacetylase inhibitors (HDACi), are undergoing phase III evaluation in metastatic breast cancer and may be immunomodulatory.112 RAS-MAPK pathway activity has been shown to suppress antigen presentation by decreasing expression of MHC-I and MHC-II. Furthermore, MEK inhibition has been demonstrated to upregulate MHC and PD-L1 expression, suggesting that combining MEK inhibitor plus anti-PD-1/L1 may be a promising therapeutic strategy. Indeed, this combination has yielded preclinical anti-tumor activity and is now being explored in phase I/II clinical trials. However, additional pre-clinical studies suggest that while MEK inhibition may augment TIL infiltration in TNBC, it may also have the unintended consequence of encumbering T cell proliferation, but may extend the survival and fitness of antigen-specific T-cells in the microenvironment.113 MEK signaling occurs downstream of T cell receptor activation. Therefore, inhibition of MEK may also decrease T cell proliferation and cytokine production, which can be overcome by co-administration of T-cell agonists such as anti-OX40.113Targeted inhibition of the oncogenic RAS-MAPK pathway, a driver of some breast cancers, may also have immunostimulatory effects. Genomic or transcriptomic activation of the RAS-MAPK pathway has been associated with decreased TIL infiltration in residual disease specimens of patients with TNBC treated with neoadjuvant chemotherapy.114 Activation of adenosine receptors (A2A-R or A2B-R) suppresses T-cell proliferation, cytokine production, and cytotoxicity.116 In 4T1 TNBC mouse models, A2A/B inhibition plus anti-PD-l was superior to monotherapy, with the observed benefit dependent on interferon secretion, NK-cells, and CD8+ T-cells.117 The adenosine receptor inhibitor, CPI-44, has been evaluated in conjunction with atezolizumab in early clinical trials, but has not been specifically evaluated in breast cancer patients. IDO1 is induced in DCs and macrophages at sites of inflammation, and degrades tryptophan into immune-suppressive metabolites that are associated with T-cell apoptosis, reduced activation, and Treg phenotype differentiation.118 In 4T1 TNBC orthotopic mouse models, IDO1 knockout results in reduced lung metastasis and improved survival.119 The IDO1 inhibitor, epacadostat, was well tolerated when combined with pembrolizumab, and was associated with a 10% ORR in a small TNBC patient cohort.120 Recently, however, epacadostat failed to improve PFS when combined with pembrolizumab in stage IV melanoma.121 Finally, OX40 is a T-cell agonist molecule which, when stimulated, may reduce the threshold required for initial T-cell activation. In both MMTV-PyMT and 4T1 mammary carcinoma models, combination anti-OX40 plus anti-PD-1/L1 was associated with improved tumor control, but the synergistic effect was demonstrated only when anti-OX40 was administered in sequence with anti-PD-1/L1. Concurrent therapy was more toxic to mice and associated with surges in both Th1 and Th2 cytokines, highlighting the possibility that compensatory feedback mechanisms could modulate efficacy of combination immunotherapy.122Additional immunotherapeutic agents, including agents targeting immune-metabolic pathways  or T-cell agonists (OX40) are being evaluated in conjunction with anti-PD-1/L1 in breast cancer. Adenosine mediates the pro-tumor effects of the ectoenzyme CD73, which is expressed in TNBC and associated with chemotherapy resistance.Pre-clinical, translational, and early clinical data support ongoing efforts to combine anti-PD-1/L1 with standard-of-care and emerging therapies including chemotherapy, radiotherapy, endocrine therapy, and targeted therapy. A number of putative mechanisms of synergy have been demonstrated, some of which are shared across therapeutic modalities  in IMpassion130 provides level I evidence supporting atezolizumab (anti-PD-L1) plus nab-paclitaxel as a standard approach for first-line therapy for patients with a >12 month distant recurrence free interval and PD-L1-positivity. There are insufficient data to guide whether anti-PD-1/L1 can be effectively combined with alternative chemotherapy regimens. Ongoing randomized trials  are addressing this question. In phase I/II studies of anti-PD-1/L1 monotherapy, efficacy diminishes substantially in later lines of therapy, suggesting that earlier treatment may be more effective. Biomarker assessments from ongoing trials may guide future personalization of chemotherapy plus anti-PD-1/L1 according to patient and/or tumor characteristics. Subjects with PD-L1-negative tumors did not benefit from the addition of atezolizumab, and therefore should be considered for clinical trials evaluating anti-PD-1/L1 in combination with novel agents. A number of existing therapies can induce PD-L1 upregulation, and may be promising for study in the PD-L1-negative TNBC population.64 Several combination strategies have mechanistic basis, including anti-PD-1/L1 plus CDK4/6i (with or without aromatase inhibitor), chemotherapy, mTOR inhibition, HDACi, DNMTi, AR blockade, or radiotherapy. With further research, novel biomarkers including high-throughput genomic/genetic profiling and advanced histologic approaches may be developed to personalize therapy.For subjects with HR-positive metastatic breast cancer, tumors are less likely to be PD-L1-positive.Subjects with HER2-positive breast cancer benefit from a multitude of approaches. An ongoing phase III randomized trial will evaluate first-line pertuzumab/trastuzumab/paclitaxel +/− atezolizumab (NCT03199885). An additional combination to be considered is T-DM1 plus anti-PD-1/L1, which in a phase II trial was associated with improved PFS but only in PD-L1-positive tumors (NCT02924883). Combinations with novel agents, such as HER2-directed vaccines, are promising and warrant clinical evaluation.123 The toxicity information of previously treated patients is adaptively combined with new patient data using a Bayesian approach, allowing for continuous reassessment of toxicity estimates. Owing to the allowance of staggered enrollment without the need for accrual suspension during DLT follow-up, this design has the potential to substantially shorten the trial duration compared to traditional phase I designs. Furthermore, it has been shown that this design assigns a greater proportion of patients to the target dose.Despite the relative safety of anti-PD-1/L1 combination therapies, the potential for long-term toxicity exists. A prominent example is immune-related endocrinopathy , which has been observed with anti-PD-1/L1 combination therapy and may require lifelong hormone replacement therapy. Resources should be devoted to evaluate patient reported outcomes and extend the time period for such measures to be assessed. Furthermore, novel phase I statistical designs should be employed to capture late toxicities in dose decision-making. For example, the time-to-event continual reassessment method starts with a target dose limiting toxicity (DLT) rate that the investigators deem acceptable, and the first patient is followed for DLT.The IMpassion130 clinical trial serves as proof-of-principle that anti-PD-1/L1 combination approaches can be safe and effective in metastatic breast cancer. A vast body of preclinical, translational, and clinical data supports ongoing efforts to evaluate a variety of combination approaches. As additional clinical trials are completed and various combination approaches are found to be beneficial, careful evaluation must be made to select the optimal combination strategy given unique patient and tumor characteristics. Moreover, robust, systematic, and streamlined biomarker studies are critical if immunotherapy combination strategies are to become applicable for the majority of breast cancer patients.


Furthermore, the nanoscale vacuum channel could enable to scale down the size of vacuum devices with high integration, making NVCT a promising candidate for high speed applications.We report the fabrication and electrical performance of nanoscale vacuum channel transistor (NVCT) based on graphene. Ninety-nanometer-width vacuum nano-channel could be precisely fabricated with standard electron beam lithography process. The optimization and treatment of surface damage and adhesive residue on graphene are carried out by ultrasonic cleaning and thermal annealing. Additionally, in situ electric characteristics are directly performed inside a vacuum chamber of scanning electron microscope (SEM) with the nanomanipulator. By modulating the gate voltage, the NVCT could be switched from off-state to on-state, exhibiting an on/off current ratio up to 10The online version of this article (10.1186/s11671-018-2736-6) contains supplementary material, which is available to authorized users. As the traditional Si-based technology gradually reaches the minimize limitation, many efforts have been made in the novel nanostructures or low-dimensional materials –7. AmongAs a result, many attempts have been made to downscale the vacuum channel into nanogap and construct three terminal junctions. For instance, the vertical structure was widely utilized in the traditional vacuum electronic devices , 15. ResHere, we report on the fabrication of a graphene-based NVCT using optimized wet transfer method and standard EBL processing. Vacuum nano-channel of 90 nm has been achieved with a back-gate structure, which could modulate the electric field of emitting area and the electron transmission through emitter to collector. In situ electric characteristics are performed in the vacuum chamber of scanning electron microscope (SEM) with a nanomanipulator, showing the basic functionality with high on/off current ratio, low work voltage, and leakage current. Importantly, we believe that further downscaling of the channel size could fulfill high speed, high reliability, and low-cost applications for modern electronics.4 (20 sccm) and H2 (40 sccm) [3:HCl:H2O solution (molar mass ratio of 1:1:1) for 90 min, the remaining PMMA/graphene film was transferred and soaked in the deionized water for 5 min. This cleaning operation was repeated four or five times to fully remove the etching solution residue. Then, the PMMA/graphene film was transferred to the SiO2/Si substrates and dried at 100 °C for 5 min, removing the residual water between the membrane and substrate. Lastly, the sample was soaked in the acetone solution for an hour to remove the PMMA support layer.In this report, large-scale graphene was directly grown on the Cu foil by thermal chemical vapor deposition (CVD) at 1020 °C with CH40 sccm) . Among v2/Si substrates with a post-annealing process based on the traditional wet transfer method, as shown in Fig. 2 (100 sccm) and H2 (40 sccm) at 300 °C for 3 h. The details and discussion of optimization process are shown in Additional file However, we observed that the traditional wet transfer process could lead to cracks or wrinkles on the graphene surface with massive PMMA residue, which may greatly influence the electrical performance afterwards. As a result, we further utilized the ultrasound  to cleann Fig. 2 00 sccm a2/Si substrate, indicating the excellent transparence of graphene. The graphene/SiO2 was characterized by field emission scanning electron microscope (Quanta 200 FEI), as is shown in Fig. 2/Si substrate. With unconspicuous D peak located at 1349 cm−1, the G and 2D peaks could be clearly observed at 1587 and 2685 cm−1 with a 2D/G ratio of 2.19. The low intensity of the D peak demonstrates that few additional defects were generated during the transfer process. The 2D peak is narrow with ratio IG/I2D below 0.5, which indicates the basic features of single-layer graphene. The Raman spectrum results show high quality and continuity of the graphene with our optimized chemical transfer method.Figure 2 insulator was deposited by the PECVD  method, with graphene chemical transferred onto the substrate subsequently. Gold contacts were deposited on graphene by electron beam evaporation (5 nm Cr and 80 nm Au) with a subsequent lift-off process. After PMMA spin-coated on the surface of graphene, the nano-vacuum channel was formed by standard EBL  with a followed O2-plasma etching. The nanogaps were positioned to cut the graphene membrane into two halves. The samples were cleaned with acetone, isopropyl alcohol, and deionized water, respectively. Lastly, the samples were processed via 1 h of annealing at 300 °C with the flow of hydrogen (40 sccm) and argon (100 sccm). Figure Figure hydrogen 0 sccm anhydrogen 0 sccm an−4 Pa), as shown in Fig. To study the mechanism of electron transport through the vacuum nano-channel, the in-situ field emission measurements are performed with a nanomanipulator in the vacuum chamber of SEM  and transfer (Vg vs. Ic) characteristics are investigated, as is shown in Fig. Vg increasing from 0 to 15 V. It is noticed that no obvious electron emission Ic were measured when Vg is less than the threshold voltage, indicating that the NVCT is in the off-state. As the Vg increases and exceed the threshold voltage, the NVCT switches to the on-state that Ic exhibit an exponentially growth with collector voltage Vc. The transfer characteristic with Vc = 7.5 V is shown in Fig. Ic grows rapidly when Vg is larger than the threshold voltage. Also, the curves plotted in exponential scale (black line) exhibit an on/off ratio exceeding 102, which is superior to the intrinsic graphene FETs due to the lack of bandgap. Wei et al. supposes that the electron emission properties are related to the surface topography of graphene or the distance from emitter to collector [To further explore the electric properties and extend the applications of graphene-based NVCT, the output . The inset clearly exhibits the fracture and cracks on the graphene surface after stability test. It is supposed that the Joule heat aggregates at the graphene emitter and damages the surface morphology, leading to the emission current degradation in low vacuum [To rule out the possibility of electron emission through the insulator, we also detect the leakage current during the measurement. Low and negligible leakage current (less than 0.5 nA) is observed, owing to the 100-nm thick SiOnsulator –31, coulw vacuum , 33. We To compare the performances of nanoscale vacuum channel transistors based on different types or materials, the channel width, operating voltage, working current, on/off ratio, gate current, and stability test are listed in Table 2/Si substrates with a post-annealing process based on the traditional wet transfer method that a 2 cm × 2 cm graphene membrane could be continuously transferred to the substrate. The electrical properties of NVCT were investigated. By modulating the gate voltage, the NVCT could be switched from off-state to on-state, exhibiting an on/off current ratio up to 102 with low working voltages (< 20 V) and leakage current (< 0.5 nA). Further improvement of the graphene-based NVCT by structure optimization may pave the way for high speed, high reliability, and low cost applications for modern vacuum nanoelectronics.In conclusion, a graphene-based NVCT was successfully fabricated with standard CMOS process. We utilized the ultrasound to clean the SiOAdditional file 1:Figure S1. The schematic diagram and optical picture of the CVD system. Figure S2. The whole structure of the devices. Figure S3. Schematic diagrams of the carriers in solid-state device and the electrons in the vacuum nanogap. (DOCX 719 kb)


Photonic crystals are commonly implemented in media with periodically varying optical properties. Photonic crystals enable exquisite control of light propagation in integrated optical circuits, and also emulate advanced physical concepts. However, common photonic crystals are unfit for in-operando on/off controls. We overcome this limitation and demonstrate a broadly tunable two-dimensional photonic crystal for surface plasmon polaritons. Our platform consists of a continuous graphene monolayer integrated in a back-gated platform with nano-structured gate insulators. Infrared nano-imaging reveals the formation of a photonic bandgap and strong modulation of the local plasmonic density of states that can be turned on/off or gradually tuned by the applied gate voltage. We also implement an artificial domain wall which supports highly confined one-dimensional plasmonic modes. Our electrostatically-tunable photonic crystals are derived from standard metal oxide semiconductor field effect transistor technology and pave a way for practical on-chip light manipulation. Traditional photonic crystals consist of periodic media with a pre-defined optical response. Here, the authors combine nanostructured back-gate insulators with a continuous layer of graphene to demonstrate an electrically tunable two-dimensional photonic crystal suitable for controlling the propagation of surface plasmon polaritons. Photonic crystals support intriguing states of matter, including topological photonic states5, unidirectional edge modes7 and pseudo-magnetic fields for photons8. To date, photonic crystals have been fabricated exclusively in discrete media with built-in local electrodynamics responses11. Because the photonic crystal effect cannot be readily turned off, conventional systems typically have very limited tunability9.When light propagates in media with periodically varying optical properties, a collection of optical phenomena emerges which can be described under the notion of photonic crystal12, and mechanical pressure13. Specifically, in monolayer graphene heterostructures, electrostatic gating can be used to greatly modify the optical properties15. Under the illumination of infrared (IR) radiation, graphene heterostructures support low-loss surface plasmon polaritons (SPPs) modes that are hybrid excitations of IR photons and Dirac electrons19. At a laser frequency ω, the wavelength λp of graphene SPPs scales with the carrier density ns as ε is the dielectric constant for the environment surrounding the graphene layer. By varying ns through field effect, the propagation of SPP waves in graphene can be readily controlled25, making monolayer graphene heterostructures a suitable platform for implementing tunable photonic crystals.A promising alternative platform for tunable photonic crystals can be constructed using two-dimensional (2D) van der Waals heterostructures. Unlike conventional materials, van der Waals heterostructures are widely tunable by a variety of readily attainable stimuli, including field-effect, optical pumping2 gate insulator layer26. Near-field imaging techniques reveal the formation of a complete plasmonic bandgap, in which SPPs propagation is prohibited. In the vicinity of the bandgap, plasmonic density of states shows strong spatial modulations and can be turned on/off and tuned by the applied gate voltage. An artificially designed domain wall in the photonic crystal supports highly confined one-dimensional (1D) plasmonic mode. Our graphene photonic crystal platform provides highly desired tunability for SPPs propagation, and is designed for on-chip in-operando light manipulation.Here, we propose, design and fabricate a tunable photonic crystal for SPPs out of a continuous graphene monolayer. The nanoscale spatial modulation of the graphene optical response is controlled by patterning a periodic structure in the SiO2 gate insulator layer. Due to the presence of the patterned substrate underneath, the graphene carrier density is enhanced at the location of the pillars compared to regions above the voids in SiO2 22, which prompts the formation of a 2D Brillouin zone (BZ) and of plasmonic band structure28. In the specific case of graphene, the SPP dispersion critically depends on the gate voltage Vg and the attendant average carrier density ω − k) into a third dimension given by average carrier density Vg  and phase ϕ of the corresponding near-field signal are extracted by a proper demodulation scheme (Methods). We analyze the near-field amplitude images s  is illuminated by focused IR radiation from a laser source. The tip acts as an optical antenna which resonantly enhances the electric field at the apexω) Figs.  which caVg = −90 V, we observed SPPs launched by the gold antenna on the left and propagating along the photonic crystal , in the sense that the state from one valley of the plasmonic band propagates only in one direction. Experimental verification of the valley polarization requires a chiral launcher that selectively couples to a single plasmonic valley and is beyond the scope of this study. Provided the photonic crystal breaks inversion symmetry, these domain wall states can become topologically-nontrivial, protected by valley Chern number39, topological edge states43 and adds to the toolkit of photonic-crystal-controlled light-matter interactions45. The top–down fabrication technique for the photonic crystal presented here is fully compatible with the state-of-the-art MOSFET devices, making our system a viable candidate for future electrostatically-reconfigurable plasmonic integrated circuits46. Our patterned dielectric arrays also modify the electronic properties of graphene26 and the interplay between electronic and plasmonic properties would be of interest to future exploration.Our gate-imprinted platform paves the way to the design of more elaborate infrared circuits that host valley plasmons29. The system is coupled to a continuous-wave CO2 laser (Access Laser). The s-SNOM setup is based on a taping mode AFM operating in ultra-high vacuum (UHV) and cryogenic temperatures. The incoming laser beam is focused on the AFM tip using a high-NA off-axis parabolic mirror inside the UHV chamber. The metallic AFM tip is tapped at a frequency of ~250 kHz. A pseudo-heterodyne interferometric detection module is implemented to extract both near-field amplitude s and phase ϕ from the tip-scattered light. The images showed are the near-field amplitude s normalized to the gold contact next to graphene. Thermally evaporated gold serves as a good reference for the detected signal. To properly suppress background contributions to the near-field signal, we demodulate at the third harmonic of the tip tapping frequency.Cryogenic near-field imaging is accomplished using a home-built scattering-type scanning near-field optical microscope (s-SNOM)26 were fabricated by plasma etching the SiO2 using a thin PMMA mask. Si substrates with thermally grown SiO2 of thickness 285 nm were spin coated with a layer of 495 A2 PMMA of thickness 50 nm. The hexagonal pattern was written by e-beam lithography in a Nanobeam nB4 system at a current of 300–400 pA. SiO2 pillars were etched in an Oxford Plasmalab 80 Plus system using a mixture of CHF3 gas (40 sccm) and Ar gas (5 sccm) to a height of ~50 nm. The PMMA mask was removed through an O2 plasma etch and the PDSL cleaned by piranha chemical etching. The resulting superlattice is an array of pillars with 46 nm diameter and 58 nm height that form a hexagonal lattice with a period of 80 nm. A graphene and hexagonal boron nitride heterostructure was placed onto the PDSL by mechanical transfer. The heterostructures consisted of a top h11BN of 7 nm, a layer of graphene, and a bottom h11BN layer of 4 nm. Metal contacts and launchers were deposited using standard e-beam lithography processes. All the reported back-gate voltages are measured from charge neutrality point of the devices.The hexagonal pillar patterned dielectric superlattices (PDSLs)11BN single crystals were synthesized by metal flux method. High purity elemental 11B (99.41 at%) was first mixed with Ni and Cr powders in a weight ratio of 1:12:12. These materials were then loaded into an alumina crucible and heated to a molten state with a flowing mixture of N2 (95%) and hydrogen (5%) at 1550 °C. The h11BN crystals were precipitated on the flux surface by slowly cooling (1 °C/h) to 1500 °C. After the growth process, it was quickly quenched to room temperature. The crystal flakes were obtained from the solidified flux with tape47. Optical properties of the h11BN single crystals were measured using broadband Fourier-transform infrared spectroscopy method (Supplementary Fig. The monoisotopic hSupplementary Information


A substantial amount of attention has been paid to the development of transparent electrodes based on graphene and carbon nanotubes (CNTs), owing to their exceptional characteristics, such as mechanical and chemical stability, high carrier mobility, high optical transmittance, and high conductivity. This review highlights the latest works on semitransparent solar cells (SSCs) that exploit graphene- and CNT-based electrodes. Their prominent optoelectronic properties and various fabrication methods, which rely on laminated graphene/CNT, doped graphene/CNT, a hybrid graphene/metal grid, and a solution-processed graphene mesh, with applications in SSCs are described in detail. The current difficulties and prospects for future research are also discussed. Semitransparent solar cells (SSCs), which exhibit eye-catching appearances by creating neutral- and multi-colors, have recently gained significant interest for their potential use in a variety of applications, such as power-generating windows for buildings and vehicles, high-efficiency tandem solar cells, power-generating color filters for displays, and wearable electronics ,18,19,20An enormous amount of effort is being devoted to the development of transparent conductive films employing thin metallic films, conductive polymers, metal oxides, metal meshes, nanowire networks, carbon nanotubes (CNTs), and graphene ,52,53,54This article reviews recent advances in the fabrication of transparent electrodes based on graphene and CNTs, as well as their characteristic performances and application, particularly in SSCs. Diverse approaches, which involve laminated stacked multilayer graphene, highly doped single-layer graphene, a hybrid monolayer graphene/metal grid, a solution-processed graphene mesh network, a laminated freestanding multi-walled CNT network, and p-doped single-walled CNTs, are examined. This paper concludes with challenges and future perspectives.Electrode materials that are visibly transparent and yet electrically conductive are a key component of a wide variety of electronic and optoelectronic devices, including solar cells. Doped metal oxides are the most common materials, such as indium–tin oxide (ITO), which offers a high transparency and low sheet resistance. However, owing to its chemical instability, its ceramic nature resulting in brittleness, and the scarcity of indium leading to high cost, C-nanomaterial-based transparent electrodes have emerged as an attractive alternative to these conventional transparent electrodes. Numerous studies have been conducted on the development of graphene-based transparent electrodes owing to their high electrical conductivity, high transparency, chemical stability, flexibility, and mechanical robustness. In this section, we discuss a laminated multilayer graphene-based transparent electrode for SSCs.61-butyric acid methyl ester (P3HT:PCBM)/graphene oxide (GO)/graphene top electrode, were demonstrated by employing a top-laminated chemical vapor deposition (CVD)-grown multilayer graphene electrode as shown in Because it is difficult to achieve a sufficiently low sheet resistance with monolayer graphene, multilayer graphene films are typically needed to attain a low sheet resistance. Although the ITO’s bottom electrode is easily replaced by a graphene-based transparent electrode in organic and perovskite solar cells ,61,62,632 layer, a perovskite semiconductor, a hole-transporting 2,2′,7,7′-tetrakis-9,9-spirobifluorene (Spiro-OMeTAD) layer, poly:poly(styrenesulfonic acid) (PEDOT:PSS), and a graphene top electrode. A thin layer of PEDOT:PSS film whose Fermi level (5.0 eV) is higher than the Dirac point of graphene (4.6 eV) is introduced, where more holes are injected into the graphene film by electrostatic doping, leading to an improved conductivity and hence a low sheet resistance of the graphene film dithiophene-alt5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3–c] pyrrole-1,4-dione} (PBDTT-DPP)(electron donor)/-phenyl-C71-butyric acid methyl ester (PC71BM) (electron acceptor)/TiO2/Ag nanowire electrode [J–V characteristics of a single-junction front subcell exhibiting a PCE of 4.29% with an 80-nm-thick PSEHTT:IC60BA layer, a rear subcell exhibiting a PCE of 5.60% with a 100-nm-thick PBDTT-DPP:PC71BM layer, and tandem solar cell exhibiting a PCE of 8.02%. The conductivity of the single-layer graphene film can be significantly enhanced by incorporating the metal grid as discussed above. However, Au is not a good candidate for the transparent electrode, because the price of Au is high and vacuum deposition techniques must be used, limiting its potential for low-cost and large-area applications. Although other metals can be used to reduce the cost for practical applications, tuning the work function of the hybrid transparent monolayer graphene/metal grid electrode can be an issue. In addition, Au shows strong absorption of light at 470 nm owing to the presence of the interband transition as well as a low transmission efficiency in the near-infrared (NIR) wavelength region . Furtherlectrode ,93,94. I71BM [J–V characteristics of three SSC devices with different buffer layers on both sides of the graphene cathode. The best solar-cell performance, which is due to the highest electrical conductivity, is achieved with the graphene cathode that is modified with the ZnO NPs/PEDOT:PSS buffer layer, where a PCE of 3.35% (3.06%) is attained under light illumination from the top (bottom) graphene electrode.Recent studies have also demonstrated the fabrication of SSCs with graphene transparent electrodes as both the anode and cathode, which can potentially improve the optical transparency, mechanical flexibility, and stability of SSCs ,99. Addi71BM . A photo71-butyric acid methyl ester (PC70BM)/molybdenum trioxide (MoO3)/two-layer graphene anode. Because PDTP-DFBT mainly absorbs light in the wavelength range of 600 to 900 nm whereas PC70BM primarily absorbs the shorter wavelengths of visible light, high transparency in the visible wavelength region can be accomplished by a bulk heterojunction blend, as shown in 70BM. An improved optical transmittance but a reduced PCE of the SSC device can be obtained by using PDTP-DFBT:PC60BM. J–V curves of the two SSC devices with all-graphene electrodes and an ITO cathode and a graphene anode (the inset schematic shows the SSC device configuration). The highest short-circuit current density (scJ) of 12.5 mA cm-2, leading to a PCE of 4.1%, is attained from the SSC with all-graphene electrodes under illumination from the top plastic stamp side.Although the optical transparency of the SSC device can be improved by reducing the thickness of the organic photoactive layer, the PCEs are reduced, which is not desired. For enhancing the optical transmittance while preserving or even enhancing the PCEs, an SSC wherein the organic active layer mainly absorbs light in the ultraviolet (UV) and NIR wavelength ranges with a single (two)-layer graphene cathode (anode) was demonstrated . In this2(hpp)4) doped C60 layer/C60/zinc phthalocyanine (ZnPc):fullerene C60 bulk heterojunction photoactive layer/hole transporting 2,2′-dimalononitrile (F6TCNNQ) doped N,N′-9,9,-dimethyl-fluoren-2-yl)-benzidine (BF-DPB) layer/f-CNT. Optical, atomic force microscopy (AFM), and SEM images of the f-CNT film, which has a predominant orientation, laminated on top of the SSC after the densification process are presented in J–V characteristics of the semitransparent small-molecule organic solar cells with different thicknesses of the hole-transporting layer under light illumination from the bottom (ITO cathode) and top (f-CNT anode) sides, respectively. PCEs of 1.3% and 0.9%, along with an average visible transmittance of 27% with negligible scattering effects, are achieved by the SSCs with the 50-nm-thick hole-transporting layer under bottom and top illumination, respectively, both of which present comparable PCEs of the reference cell , where a thin metal layer is used for the top electrode instead of f-CNT. One of the challenges of the CNT electrodes is that their surface is typically fairly rough, causing electrical shortage of the solar cells, which can be resolved via a planarization method with PEDOT:PSS [In addition to graphene-based transparent electrodes, there have been increasing efforts to develop CNT-based transparent electrodes, as CNT is chemically stable, mechanically flexible, electrically conductive, solution-processable, and transparent compared with the other transparent conductive oxides (TCOs). The lamination of a CNT network film as a top electrode for semitransparent small-molecule organic solar cells has been reported, where the fabrication of the transparent electrodes without damaging the underlying organic photoactive layer is crucial . In thisEDOT:PSS ,110. In 2 is used as a scaffold. A sequential deposition method is employed to deposit the perovskite photoactive layer, and a floating-catalyst CVD method is used to synthesize the thin f-CNT network film that is transferred on top of the light-absorbing perovskite semiconductor in an ambient atmosphere at room temperature [J–V characteristics and the IPCE profiles of the semitransparent perovskite solar cells with the CNT top electrode under illumination from both sides are presented in scJ. This is confirmed by the IPCE spectra presented in A laminated CNT network film as a top electrode was also demonstrated in semitransparent perovskite solar cells . Figure perature ,112. A s3 via the sandwich transfer method and MoO3 via the bridge transfer process [71BM/MoOx/CNT, where the CNT network films with high purity and long nanotube bundle lengths were prepared via the aerosol CVD method, and the doping was performed on the CNT films through the direct lamination and dry lamination methods [3 acid is an effective p-type dopant, direct application is not possible owing to the high acidity of HNO3 [3 doping sandwich transfer procedure is illustrated in 3 is placed on the CNT film followed by post-annealing treatment at 80 °C for 5 min, which causes the CNT film to appear reddish, indicating that the CNT network film is p-doped. This can be validated by checking the reduction in the Fermi level value from −5.0 to −6.0 eV through photoelectron yield spectroscopy (PYS). Then, the HNO3-doped CNT network film is placed onto a MoO3 film, where UV resin is used to promote the adhesion between the HNO3–CNT film and MoO3. When the device is illuminated from the ITO side, a PCE of 3.7% is achieved. x doping bridge transfer process, which is found to be more stable than the HNO3 sandwich transfer method [3 is thermally evaporated with a shadow mask to pattern the contact area of the electrode followed by post-annealing treatment at 400 °C. The p-doping of the CNT network film is confirmed by measuring the Fermi-level value, which decreases from −5.0 to −5.6 eV, using PYS. The MoOx-doped CNT film is transferred directly onto the photoactive layer of the partially fabricated organic solar-cell device, achieving a PCE of 3.1% under light illumination from the ITO side. The transmission spectra of glass/ITO, glass/pristine CNT film exhibiting an average visible transmittance of ~85%, glass/ITO/ZnO/PTB7:PC71BM/MoO3, and glass/CNT/MoO3/PTB7:PC71BM/ZnO exhibiting an average visible transmittance of ~40% are presented in J–V characteristics that are attributed to the mechanical variability and the sensitivity of the transfer methods for the CNT film doping. Such instability of the electrical properties with the thin CNT film can be easily resolved by increasing the thickness of the original CNT network film, which can also improve the electrical conductivity despite the reduced average visible transmittance. By using thicker CNT films that show a transmission efficiency of ~60% at 550 nm, stable J–V curves are obtained, with PCEs of 4.1% and 3.4% for SSC devices with HNO3-doped and MoOx-doped CNT electrodes, respectively. The thicker CNT network film is less sensitive to the microwrinkles formed during the transfer process, leading to a higher ocV. In addition to the improved ocV, the enhanced electrical conductivity of the thicker CNT film yields a higher FF. Improved PCEs can be attained by sacrificing the transparency of the device.Although the semitransparent performance can be accomplished by using the laminated CNT film as the top electrode, the lamination process is not easy, and the sheet resistance of the laminated CNT film is high for practical applications. Recent work revealed neutral-colored semitransparent organic solar cells, where the CNT films were p-doped with HNO process . Figure  methods ,115. Alt of HNO3 . The HNOr method . The CNTThe PCEs of SSC devices are significantly lower than those of the conventional solar cells with an opaque electrode. For the further efficiency enhancement of the SSC, various schemes can be exploited: (1) increased light absorption due to light-trapping effects in plasmonic nanostructures and nanostructure-textured surfaces; (2) broadband anti-reflection coatings yielding increased absorption in the photoactive layer and thus increased photocurrent generation; (3) integration of perfect light-absorbing mechanisms in the infrared (IR) regime for thermophotovoltaic energy-conversion applications; and (4) the design of tandem solar cells wherein more semiconductors, which can efficiently harvest the UV and IR portions of the solar spectrum rather than visible light, are employed. In addition to an SSC with a window-like appearance, multi-colored SSC devices can be developed for aesthetic solar panels and power-generating color filters for display technologies. Further enhancing the PCEs of SSCs and incorporating more functionalities into SSCs can introduce new avenues for the commercialization of SSCs.Regarding the transparent electrodes that have been widely used in a variety of the optoelectronic devices, the TCOs—including ITO—are still dominant in the market, but their brittleness and expensive fabrication remain a challenge, restricting their potential in many future flexible and wearable applications. Although considerable effort is directed towards the development of transparent electrodes based on conducting polymers, their poor stability and durability are significant issues. Owing to their high transparency with optically neutral colors, high electrical conductivity, and excellent mechanical flexibility, both CNT and graphene—allotropes of C—have been extensively explored for transparent electrode materials. Various fabrication methods, modification approaches, and patterning strategies for CNT- and graphene-based transparent electrodes for diverse electronic and optoelectronic devices have been demonstrated. However, there are many challenges to be overcome for the industrialization of both CNT- and graphene-based transparent electrodes for numerous optoelectronic applications. First, both the optical transparency and electrical conductivity of the CNT- and graphene-based transparent electrodes must be improved, both of which are still lower than those of the ITO electrode. This is very important for SSC applications. New fabrication methods for preparing CNT and graphene with high quality, different morphologies, and a rapid growth rate should be developed. The second challenge is creating CNT- and graphene-based transparent electrodes over a large area with reduced costs, which is essential for commercialization. Third, although significant efforts have been devoted to investigating various dopants for enhancing the conductivity of CNT and graphene, these dopants have poor stability, which should be resolved, especially for SSC applications that require long-term stability. Furthermore, creating a smooth surface of the CNT- and graphene-based transparent electrodes remains a bottleneck; it is difficult to achieve clear background images through the SSC devices, limiting their widespread use in practical applications. To industrialize the CNT- and graphene-based transparent electrodes for SSC applications, the aforementioned challenges should be addressed. Despite these issues, we envision a bright future for CNT- and graphene-based transparent electrodes for SSC applications, owing to their unique characteristics that are unattainable with other transparent electrode approaches.


Background: The outlook for patients with esophageal and gastric (EG) cancer remains poor. Hence, there is a compelling need to identify novel treatment strategies and complementary biomarkers. Programmed death ligand 1 (PD-L1) and mismatch repair deficiency (dMMR) are putative biomarkers of response to immune-checkpoint blockade, but their prognostic value and interrelationship in EG cancer have been sparsely investigated.Methods: Immunohistochemical expression of PD-L1 on tumour cells (TC) and tumour-infiltrating immune cells (TIC), and of PD-1 (programmed death receptor 1) on TIC was assessed using tissue microarrays with primary tumours and a subset of paired lymph node metastases from a consecutive, retrospective cohort of 174 patients with chemoradiotherapy-naïve EG adenocarcinoma. MMR proteins MLH1, PMS2, MSH2, and MSH6 were assessed by immunohistochemistry. The total number  of CD8+ T cells in each core was calculated by automated analysis.Results: High PD-L1 expression on both TC and TIC, but not PD-1 expression, was significantly associated with dMMR. PD-L1 expression on TIC was significantly higher in lymph node metastases than in primary tumours. High expression of PD-L1 or PD-1 on TIC was significantly associated with a prolonged survival, the former independently of established prognostic factors. A significant stepwise positive association was found between CD8+ T cells and categories of PD-L1 expression on TIC.Conclusion: PD-L1 expression on TIC is higher in lymph node metastases compared to primary tumours, correlates with dMMR, and is an independent factor of prolonged survival in patients with chemoradiotherapy-naïve EG adenocarcinoma. These findings suggest that PD-L1 expression on TIC may be a useful biomarker for identifying patients who may not need additional chemo- or chemoradiotherapy, and who may benefit from PD-1/PD-L1 immune-checkpoint blockade. PD-1 status was not investigated in that study  cancers constitute a considerable proportion of cancer cases and deaths worldwide . The out placebo . Express placebo , but the placebo . Some st placebo , 15. In n = 259), investigating the response rate of pembrolizumab, a humanized IgG4-κ monoclonal antibody inhibitor of PD-1, in previously treated gastric and esophago-gastric junction cancer, patients with MSI-High (MSI-H) tumours had a higher objective response rate (ORR) than non-MSI-H tumours, but, notably, the majority of responders were non-MSI-H patients and only 4% of the tumours were MSI-H (n = 472), an improved prognosis was demonstrated for patients with MSI-H gastric tumours, even tumours with more advanced nodal status, but the benefit was only confirmed in the non-cardia subgroup, with intestinal-type or tubular/poorly differentiated histology according to the WHO classification (n = 1990), there was no difference in disease-free survival according to MSI status  trial, that patients with dMMR tumours had a prolonged OS when treated with surgery alone, compared to surgery and perioperative chemotherapy together, proposing that perioperative chemotherapy may not be beneficial for patients with dMMR tumours (Mismatch repair deficiency (dMMR), or microsatellite instability (MSI), is another putative predictive biomarker of response to immune-checkpoint blockade. In the KEYNOTE-059 trial by Fuchs et al. . The association between CD8The study cohort encompasses a previously described consecutive series of 174 patients with chemoradiotherapy-naïve esophageal and gastric adenocarcinoma –22. All Tissue microarrays (TMAs) were constructed as previously described using a semiautomated arraying device  . Duplica+ T cells has been described previously , PMS2 , MSH2  and MSH6 using a monoclonal antibody  diluted 1:50. Immunohistochemical stainings were denoted as negative when all TCs showed loss of nuclear staining. Surrounding stromal cells and TICs served as internal controls for each TMA core. Tumour samples lacking nuclear staining of MLH1, PMS2, MSH2, or MSH6 were considered to have dMMR status. For immunohistochemical analyses of PD-1 and PD-L1, 4 μm tissue sections were pre-treated using the DAKO PT link system  and stained in an Autostainer Plus  with the anti PD-1 antibody NAT105, ab52587; Abcam, Cambridge, MA, USA, diluted 1:50 and the anti PD-L1 E1L3N, Cell Signaling Technology, Inc. (CST), Danvers, MA, USA) diluted 1:200. For scoring of PD-L1 and PD-1 expression the percentage of stained TIC was categorized as <10, 10–49, and 50–100%. For scoring of PD-L1 expression on TC the percentage of stained cells was categorized as <1, 1–4, 5–9, 10–49% and 50–100%, in accordance with a previous study by Berntsson et al.  following the manufacturer's protocol. The EBER 1 DNP probe  was used for detection, ISH iVIEW Blue Plus Detection kit  was used to produce the chromogenic reaction, and slides were counterstained with Red Counterstain II .For detection of Epstein-Barr virus (EBV)-infected tumours, the presence of EBV-encoded RNA (EBER) transcripts was analysed by chromogenic Gastric and esophageal tumour samples were collected from TCGA project from the Genomic Data Commons (GDC). The fragments per kilobase of exon per million mapped reads (FPKM) values were retrieved and the average FPKM value was used for all individual samples for each tissue to estimate PD-L1 and PD-1 gene expression levels. A cut-off value of 1 FPKM was used as a detection limit across all tissues. The patients were classified into two groups and their prognoses examined based on the FPKM values. Genes with low expression were excluded i.e., those with a median expression among samples <1 during the analysis. The prognosis of each group of patients was examined by Kaplan-Meier survival estimates. To choose the best FPKM cut-offs for the most significant grouping of the patients, FPKM values from the 20 to 80th percentiles were used and significant differences in the survival outcomes of the groups were examined.+ T cells with PD-L1 expression on TIC in primary tumours, Mann-Whitney U-test was used.To evaluate associations between categories of PD-L1 expression on TC and TIC, and PD-1 expression on TIC, and clinicopathological factors, Kruskal–Wallis test was used for continuous variables and Chi-Square (linear-by-linear) test for categorical variables. Wilcoxon signed-rank test was applied to compare the distribution of PD-L1 and PD-1 expression in primary tumours and paired lymph node metastases. Kaplan-Meier analysis and the log rank test were applied to detect differences in TTR and OS. Cox proportional hazards models were used to calculate univariable and multivariable hazard ratios (HR) for TTR and OS. To assess the association between CD8The multivariable model included age, primary tumour location, adjuvant treatment, T stage, N stage, M stage, differentiation grade, residual tumour status, and MMR status. The proportional hazards assumption was tested using Cox regression with a time-dependent covariate analysis, whereby the proportional hazard assumption was considered to be satisfied when the factor x time interaction was non-significant.p ≤ 0.05 were considered significant. All calculations were performed using IBM SPSS Statistics for Mac version 24.0 .All tests were two-tailed and In primary tumours, PD-L1 expression on TC and TIC could be evaluated in 165/174 (94.8%) cases. Positive (≥ 1%) PD-L1 TC expression was denoted in 38 (23.0%) cases and positive (>10%) PD-L1 TIC expression in 68 (41.2%) cases. In paired lymph node metastases, PD-L1 expression on TC and TIC could be evaluated in 68/81 (84.0%) cases. Positive PD-L1 TC expression was denoted in 12 (17.6%) cases and positive PD-L1 TIC expression in 37 (54.4%) cases. In primary tumours, PD-1 expression on TIC could be evaluated in 170/174 (97.7%) cases. Positive (>10%) PD-1 TIC expression was denoted in 86 (50.6%) cases. In paired lymph node metastases, PD-1 expression on TIC could be evaluated in 75 (92.6%) cases. Positive PD-1 TIC expression was denoted in 40 (53.3%) cases. Sample immunohistochemical images are shown in p = 0.009). There was no significant difference in PD-L1 expression on TC or PD-1 expression on TIC between primary tumours and lymph node metastases.Bar charts visualising the distribution of PD-L1 and PD-1 expression in primary tumours and paired lymph node metastases, respectively, are shown in p = 0.013) and dMMR (p < 0.001). Increased PD-L1 expression on TIC was significantly associated with lower T stage (p = 0.018), lower N stage (p = 0.001), lower M stage (p = 0.044) and dMMR (p < 0.001). Increased PD-1 expression on TIC was significantly associated with lower T stage (p = 0.049) and lower N stage (p = 0.027). Furthermore, there were moderate significant intercorrelations between PD-L1 and PD-1 expression on TC and TIC, and PD-1 expression on TIC, respectively . The associations of PD-L1 and PD-1 expression with tumour and patient characteristics, as well as the intercorrelations of PD-L1 and PD-1 expression in esophageal and gastric cancer, respectively, are shown in The associations of PD-L1 and PD-1 expression with tumour and patient characteristics, as well as the intercorrelations of PD-L1 and PD-1 expression are shown in p = 0.023 and p = 0.004, respectively). PD-L1 expression on TC was not prognostic. Similar results were found for TTR and are shown in Kaplan-Meier analyses of the prognostic impact of PD-L1 expression on TC and TIC and PD-1 expression on TIC are shown in p = 0.041), whereas a borderline significant association was observed between high PD-1 expression and a shorter OS (p = 0.053) in esophageal cancer. Of note, both squamous cell carcinoma (SCC) and adenocarcinoma are included in the esophageal cancer dataset in TCGA.Kaplan-Meier analyses of OS according to PD-L1 and PD-1 mRNA expression in 354 cases of gastric cancer and 161 cases of esophageal cancer in TCGA are shown in In the entire cohort, 14 (8.1%) primary tumours had dMMR status. The distribution of these cases according to anatomical subsite was; 3 (7.5%) in the lower third of the esophagus including esophago-gastric junction (Siewert type 1), 3 (13.0%) in the true cardia (Siewert type 2), 1 (11.1%) in the subcardial stomach (Siewert type 3), 1 (50.0%) in fundus, 3 (8.3%) in corpus, 2 (8.7%) in antrum and 1 (20.0%) in pylorus.p = 0.001) and lower N stage (p = 0.012). Kaplan-Meier analyses of the prognostic impact of MMR status are shown in Associations of MMR status with tumour and patient characteristics are shown in + T cells with PD-L1 expression on TIC in primary tumours in the entire cohort are shown in + T cell density and categories of in particular PD-L1, but also PD-1, expression, on TIC. The associations between CD8+ T cells and PD-L1 on TC were less evident.The associations of CD8This is one of few studies that has investigated the intercorrelation between PD-L1 and PD-1 expression in primary tumours and paired lymph node metastases, as well as the relationship between MMR status and prognosis, in esophageal adenocarcinoma.High expression of both PD-L1 and PD-1 on TIC was significantly associated with a prolonged OS, the former independently of other prognostic factors and MMR status. These findings are partly in line with the study by Böger et al. on gastric cancer wherein patients with high expression of PD-L1 on TIC had a significantly better OS in both univariable and multivariable analysis and patients with high expression of PD-L1 on TC had a significantly improved survival in univariable analyses. However, patients with high PD-L1 expression on both TC and TIC had the best OS . Of noten = 119) wherein PD-1 expression on CD8+ T cells was found to be an unfavourable prognostic factor, but no other types of immune cells were explored regarding PD-1 expression (n = 354), PD-1 positivity correlated with increased mortality, but not after adjusting for other prognostic factors  and primary tumours (n = 119) than in primary tumours, TIC not investigated (n = 112), paired lymph node metastases (n = 55) and distant metastases (n = 17), and concluded that the expression in the different entities “does not necessarily correlate” (n = 101) PD-L1 expression was compared between primary triple negative breast tumours and axillary lymph nodes (n = 101), whereby PD-L1 expression was found to be significantly higher in both lymphocytes and TC of the axillary lymph node metastases compared to the primary tumours (n = 205) and paired lymph node metastases (n = 127) in cervix squamous cell carcinoma and adenocarcinoma no significant difference was found , wherein PD-L1 positivity (≥ 1%) on both TC and TIC was more frequently observed in dMMR tumours  demonstrated that patients with positive (>5%) PD-L1 TC expression and high CD8+ T cell infiltration was associated with an improved OS, and positive PD-L1 status correlated with high CD8+ T cell infiltration. PD-L1 expression on TIC was not investigated  wherein the PD-L1 TC positive/TIL positive TME subgroup, was found to be more frequent in EBV-positive and MSI-H gastric tumours and also to be associated with a favourable prognosis , lung biopsies and corresponding tumours were compared regarding PD-L1 expression, and the expression was found to be lower in the biopsy compared to the resected tumour in all cases, in particular the expression on TIC (n = 550) also demonstrating lower expression in TMA cores compared to the whole tissue blocks, although the use of two cores, one central and one from the invasive front, was thought to minimize the heterogeneity issue (n = 156) is comparable to the 3 FDA-approved clones 22C3, 28.8 and SP142 , wherein 5.0% of all gastric adenocarcinomas were found to be EBV-associated  is well in line with a study by Böger et al. , the Helsinki Declaration on ethical principles for medical research involving human subjects, and the EU Council Convention on human rights and Biomedicine. Approval for the study was obtained from the Ethics committee of Lund University (ref no 445/07), whereby the committee waived no need for consent other than the option to opt out.KJ conceived and designed the experiments. MS, CZ, and AM performed the experiments. MS, KJ, CZ, and AM analysed the data. DB, CH, AM, MU, BN, and KL contributed reagents, materials, and analysis tools. MS and CZ prepared the figures and tables. MS and KJ wrote the paper.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Here we solve this long-standing paradox and demonstrate that, rather than χ(3)-based refraction, a complex phenomenon which we call saturable photoexcited-carrier refraction is at the heart of nonlinear-optical interactions in graphene such as self-phase modulation. Saturable photoexcited-carrier refraction is found to enable self-phase modulation of picosecond optical pulses with exponential-like bandwidth growth along graphene-covered waveguides. Our theory allows explanation of these extraordinary experimental results both qualitatively and quantitatively. It also supports the graphene nonlinearities measured in previous self-phase modulation and self-(de)focusing (Z-scan) experiments. This work signifies a paradigm shift in the understanding of 2D-material nonlinearities and finally enables their full exploitation in next-generation nonlinear-optical devices.Graphene is considered a record-performance nonlinear-optical material on the basis of numerous experiments. The observed strong nonlinear response ascribed to the refractive part of graphene’s electronic third-order susceptibility  Graphene enables extraordinary nonlinear-optical refraction, far exceeding predictions based on conventional nonlinear-susceptibility theory. Here, Vermeulen et al. show that rather than the nonlinear susceptibility, a complex saturable refraction process is central to graphene’s unusual behavior. These measurements mostly pointed at an extremely high effective third-order susceptibility 19 in the density-matrix framework at the single-particle level21. The origin of this large discrepancy between theory and experiments has so far remained unknown, hindering the full exploitation of graphene in nonlinear-optical applications. Several recent experiments also revealed that the sign of graphene’s effective 14, but again without conclusive theoretical support. Hence, both the negative sign of the experimentally observed nonlinearity and its large quantitative discrepancy as compared to the theoretically predicted response have unclear origins at this point. This holds not only for graphene experiments with free-space optical excitation13 but also for the cases where graphene on a photonic chip is examined using waveguided excitation beams14. The latter configuration is particularly interesting from a practical point-of-view as it allows exploring the potential of graphene for next-generation on-chip nonlinear-optical devices  opening up new applications in optical telecommunication, biomedical imaging, absorption spectroscopy, and many other domains26.Over the past several years numerous experiments have been carried out to study the response of graphene in parametric nonlinear-optical processes like four-wave mixing2 for the larger part, since the nonlinearity of SiO2 is orders of magnitude smaller than that of other materials typically employed for photonic chip fabrication 27 and because its dielectric nature provides electrical isolation of the graphene.To investigate graphene’s intrinsic nonlinear physics, one should ideally be working with a substrate free from any nonlinear-optical effects that could interact with those of the 2D material. In case of graphene-covered photonic chips, the core of the on-chip waveguides should preferably be made of SiO2-core waveguides. We observe an extraordinary spectral broadening behavior, and demonstrate through an in-depth theoretical and experimental analysis that the underlying nonlinear physics does not depend on refraction induced by graphene’s electronic χ(3), but instead on a much more intricate phenonemon that we refer to as saturable photoexcited-carrier refraction (SPCR).In this paper, we solve the long-standing theory-versus-experiments paradox in graphene nonlinear optics by investigating SPM of optical pulses in graphene-covered SiO2-core waveguides employed in our experiments, here also referred to as hybrid waveguides, feature a cross-section as illustrated in Fig. 12 cm−2 as a result of unintentional doping28, and is patterned with plasma etching29 to create sections with variable length L between 220 and 1100 μm on the waveguides as shown in Fig. λ = 1563 nm in the telecom band . The input can be described by a pulse amplitude P0 up to 2.7 W, a temporal shape U = sech(t/T0), a full-width-at-half-maximum pulse duration TFWHM,0 = 1.76 T0 = 3 ps  divided by the corresponding value at the input (z = 0), i.e., μ2(zout)/μ2(0) with L. We observe a linearly increasing effective loss along graphene length with a slope αeff = 4605 m−1 . This process relies on the χ(3)-induced nonlinear part of the refractive index χ(3)-based SPM obeys the following well-established expression31 (after introducing τ = t/T0 and K = γ = f(χ(3)) being the medium’s nonlinear parameter γ function of χ(3) (ref.32) or function of weighted χ(3) contributions in case of hybrid waveguides33, with Θ1 = 1 (the latter function is introduced to facilitate comparison with other chirp formulas further on). The resulting spectral width thus varies linearly with input power P0 and with effective length31, leveling off starting from It is widely accepted that spectral broadening of optical pulses in a nonlinear material typically originates from SPML = 1100 μm -dependent SPM, we have to reconsider the very basics of the process at work and verify whether it is effectively based on chirping or rather on absorption-induced pulse shaping.As the observed spectral broadening in the graphene-covered waveguides cannot be explained quantitatively nor qualitatively by conventional φ of the pulse envelope would not change along the propagation path, and when injecting different pulses that share the same input amplitude |A|, the evolution in |A| along the propagation path would be the same as well for these pulses. In that case, when performing broadening experiments for two different input pulses p1 and p2 with opposite input chirp parameters A| , the resulting spectral widths z. Dividing Eq. , altering the refractive index34 Δncarriers ∝ Nc and yielding pulse chirping 38. Yet, in the specific case of graphene-covered waveguides, FCR-induced chirping turns out to be much more complex than in the 3D-semiconductor-based waveguides where it has been examined so far. The reason is that here the time evolution of Nc along the pulse propagation path strongly depends on the possible occurrence of saturation  in the graphene due to Pauli blocking39, and therefore we refer to the resulting chirping as ‘saturable photoexcited-carrier refraction (SPCR)’-induced chirping. To model this new phenomenon, we employ the following phenomenological equations for the propagation of the optical pulses in the hybrid waveguides and for the time evolution of the free-carrier density:While the optical pulses are traveling through the graphene-covered waveguides, they experience graphene-induced optical absorption and as such give rise to photoexcited free carriers with a density σFCR represents the graphene-induced FCR coefficient, and αeff. In Eq. . As such, 37, and also opposite to conventional SPM featuring only a fixed-shaped chirp profile31. The determining factors for this novel behavior are the short τc for photoexcited carriers in graphene and the saturability of the 2D material.In the cross-section of the electrically isolating SiOg in Eq.  becomes d in Eq. , and η1P in Eqs.  and 5)  2-core wces Fig. , the pulces Fig. , and satτc ≈ T0:Θ2 = 1/(1 + x), where χ(3)-based SPM  takes up the role of proportionality constant K. The most striking features of this quantity are its negative sign and its dependence on graphene’s FCR efficiency rather than on the electronic χ(3).Finally, comparing Eq.  with Eq.14, we investigated spectral broadening in graphene-covered semiconductor (namely silicon (Si)) waveguides and excited with negatively chirped ps pulses that were quasi-identical to those used here. The graphene-induced broadening in graphene-on-Si presented there shows a very different tendency as compared to that seen here for graphene-on-SiO2: it exhibits a linear effective-length dependence in line with the conventional-SPM model of Eq. . The use of spatially averaged carrier densities stems from the often employed semiconductor-based waveguide description as outlined in ref.32, and also implies implementing an average decay time value incorporating the decay times of the different constituents and mechanisms in the waveguide cross-section. Due to the nanosecond-scale carrier lifetime in the underlying Si waveguide42, we here have a much larger decay time2τc than when the carriers remain confined to the graphene . As shown in the Supplementary Table Although the broadening in graphene-on-Si is in line with the conventional-SPM model, the actual physical process taking place in the graphene is again SPCR and can be described by the model of Eqs.  and 5)..5). It smentclass2pt{minimK. This quantity produces both the negative sign and the order of magnitude for the extracted effective γgr-on-Si. At the same time, it is one order of magnitude smaller than the quantity K = −σFCRαeffτc /(ℏω) for the graphene-covered SiO2 waveguides (Eq. (2 counterparts discussed above. Further information on the SPCR modeling results for the hybrid Si waveguides can be found in Supplementary Note By deriving from Eqs.  and 5)  5) an apmentclass2pt{minimχ(3). Sophisticated free-space Z-scan techniques were developed over the years for distinguishing the electronic χ(3) contribution from two-photon-absorption-generated free-carrier contributions in 3D semiconductors45; their application to graphene with one-photon-absorption-induced carriers is yet to be explored.The impact of our SPCR analysis goes beyond the process of spectral broadening in graphene-covered waveguides: it actually shows that graphene’s nonlinear phase modulation ability measured in any kind of free-space or waveguide-based experiment depends on its free-carrier generation and refraction efficiencies, rather than on its electronic Z-scan measurement results (or Z-scan-like phase modulation results) for graphene on dielectric substrates and excited with ~100 fs pulses can be fitted using the macroscopic formula Δn ∝ χeff(3)|A|2 along the conventional-SPM notation12. In these experiments τc(∝ps) Z-scan experiments consistently is of the order of 10−7 esu12.The currently available standard Z-scan results obtained with longer, ps-scale pulse lengths T0 ≈ τc yield a −6 esu)11, but no conclusive physical explanation for the difference in Z-scan nonlinearities measured in the two lifetime/pulse-length scenarios was given in those works. Yet, when looking at our results for the ps-pumped graphene-covered SiO2 waveguides featuring T0 ≈ τc, an effective graphene nonlinearity γgr-on-Si where Z-scan experiments with free-space light beams propagating across atomically thin graphene sheets. Finally, it is interesting to note that for Z-scan measurements with fs-scale pulse excitation graphene’s nonlinearity is expected to increase with increasing pulse length T0, as can be derived from our graphene-on-Si results with In contrast, χ(3)-based predictions and the experimental observations for graphene’s nonlinear phase-modulation ability in SPM and Z-scan investigations. Rather than relying on graphene’s electronic χ(3), the measured phase modulation effects arise from a more complex phenomenon we refer to as saturable photoexcited-carrier refraction. Depending on the ratio between the carrier lifetime and the pulse length used, the SPCR-induced physics can either mimic the tendencies of conventional χ(3)-based phase modulation, or produce a totally different behavior. A striking example of the latter is the exponentially growing spectral broadening we have seen for ps pulses propagating in graphene-covered SiO2-core waveguides. This behavior relies on both up-chirping and down-chirping along variable chirp profiles and is totally different from the earlier studied FCR-induced spectral broadening in 3D-semiconductor waveguides. The determining factors for this exceptional behavior are the short τc for photoexcited carriers in graphene and the saturability of the 2D material. This exponential broadening growth opens up new routes to exploiting graphene for on-chip spectral broadening of laser signals and, in case of an optimized balance between the one-photon absorption efficiency and the SPCR efficiency, for actually creating new wavelengths in on-chip frequency-comb and supercontinuum light sources. Furthermore, as the optical pulse power turns out to have a limited impact on the broadening efficiency, one could even envisage frequency-comb and supercontinuum generation at record-low input power levels. In other words, our findings thoroughly change how nonlinear-optical devices could be operated when enhanced with graphene. The SPCR model presented here provides a qualitative and quantitative description for both the cases where such extraordinary broadening behavior is produced and the cases where conventional χ(3)-based broadening is mimicked. It also reveals the physical origin of the negative sign of graphene’s effective Z-scan experiments where mimicked χ(3)-based behavior was produced in the regime of 7 and 2D materials in general49 by means of free-carrier dynamics modeling as we used here. We note that for some of those experiments  nonlinearity magnitudes below 10−7 esu were reported, but also for those measurements the insights we have obtained here regarding, e.g., the impact of the pulse duration on the nonlinearity strength in Z-scan experiments can be relevant. A more in-depth understanding of the fundamental 2D-material nonlinearities will finally allow the full exploitation of their great potential in next-generation (on-chip) nonlinear-optical devices.In conclusion, we have revealed the origin of the long-standing quantitative discrepancy between the theoretical https://www.lionix-international.com). The spot-size converters allow edge coupling with flat-cleaved fibers placed at the waveguides’ input and output facets, yielding a fiber-chip-fiber coupling efficiency around −10 dB. The cross-sectional waveguide geometry consists of a SiO2 core region (thickness around 500 nm) with above and below a stripe of Si3N4 (stripe thickness around 170 nm). The foundry also provided a top oxide layer above the upper Si3N4 stripe  of methane in a commercially available Black Magic Pro system. The time and flow rate settings were optimized such that monolayer graphene growth was ensured. Afterwards a 200 nm-thin layer of poly(methyl methacrylate) (PMMA) was spincoated on top of the graphene. After electrochemical delamination of the CVD-grown graphene film in a 1 M aqueous solution of potassium chloride, it was deposited on top of the photonic chip along the wet-transfer method14. To determine the appropriate fiber length for achieving a given chirp modification, one can make use of our pulse-chirp control method based on the generalized-length formalism50. The chirped pulses were then coupled into and out of the hybrid waveguides by means of flat-cleaved fiber probes positioned horizontally at the waveguides’ end facets, and their peak power was varied using a variable optical attenuator. A 99:1 coupler was inserted in front of the incoupling fiber probe to split off 1% of the input pulse power towards a power meter (Newport 2832-C with 818-SL detector head) and an optical spectrum analyzer (Yokogawa AQ6370D) for monitoring both the input power and spectrum. Along the same approach the output power and spectrum were measured just behind the outcoupling fiber probe using a second power meter and optical spectrum analyzer. Prior to the optical measurements, the input pulse duration and phase/chirp profile were characterized by means of a frequency-resolved optical gating (FROG) instrument (Coherent Solutions HR150).The pulsed laser source used to excite the graphene-covered waveguides is an optical parametric oscillator OPO (APE Levante IR) generating picosecond pulses at a repetition rate of 80 MHz. To create input pulses with either a positive or negative chirp, we made use of the chirp modification that the OPO pulses experience when injected in sufficiently long fiber sections32.The quasi-TE mode in the graphene-covered waveguide as shown in Fig. The data that support the findings of this study are available from the corresponding author upon reasonable request.Supplementary Information