This work can serve as a supplemental explanation of the charge transfer during the photocatalytic process at the polar ZnO/graphene composite surface.A graphene coated hexagonal ZnO (HZO@Gr) with enhanced activity in photocatalysis was synthesized. However, the photoinduced charge transfer behavior and the beneficial role of graphene in promoting photocatalytic reactions have not been sufficiently investigated experimentally. In this paper, the surface potentials of the ±(0001)-polar plane of HZO (Zn-polar plane and O-polar plane), graphene, graphene/Zn-polar plane and graphene/O-polar plane were measured using Kelvin probe force microscopy (KPFM). On the basis of the KPFM results, the respective Fermi levels were calculated and the internal electric field (IEF) of HZO was confirmed. Taking the IEF of HZO into consideration, the three-dimensional band diagrams of the HZO@Gr composites in methyl blue (MB) solution in the dark and under UV-visible irradiation after equilibrium were proposed. Accordingly, it is found that there could emerge different interactions between graphene and HZO at the ±(0001)-polar plane of HZO. Furthermore, the photogenerated holes and electrons tend to migrate to opposite directions. With the participation of graphene and IEF, the composites show a decrease in possibility of charge recombination. As a result, the active groups, namely ˙OH and ˙O The Fermi levels and three-dimensional band diagrams of the synthesized HZO@Gr composites in methyl blue (MB) solution before and after equilibrium were assumed. It is found that an increase of polar Zn (0001) or O (0001̄) facets leads to a significant enhancement of photocatalytic activity, whereas the area of nonpolar  planes has negligible influence on the photocatalysis.5,6 The periodic density functional theory (DFT) demonstrates each layer of HZO contains all positive Zn2+ ions or all negative O2− ions in the [0001] direction and an internal electric field (IEF) thus is generated between Zn–HZO (0001) and O–HZO (0001̄) planes due to the spontaneous polarization.7 When HZO particles are irradiated by UV-visible light, photogenerated electrons (e−) and holes (h+) migrate to positive polar Zn–HZO (0001) planes and negative polar O–HZO (0001̄) planes, respectively, under the IEF, thus promoting the separation of photogenerated electron–hole pairs.8,9 In addition, since the graphene was coming into a hot topic of researching, the graphene–semiconductor composites have attracted increasing interests for researchers so that to improve the efficiency of the photocatalytic processes.10–13 Hence, the photocatalytic performance of ZnO can be obviously enhanced by combining with graphene, which possesses a unique sp2 hybrid carbon network and excellent mobility of charge carriers at room temperature.14 It is reported that photogenerated electrons tend to transfer to graphene for further charge separation, which could obviously decrease the recombination possibility.15–17Photocatalysis is a light induced catalytic process that can effectively degrade organic molecules in the water through oxidation and reduction reactions. As one of the most important semiconductor photocatalysts, ZnO has attracted considerable interest because of its high photosensitivity and stability.18 characterization of dopant profile of semiconductors in a nanometer resolution,19etc. Lin et al.20 have studied the charge transfer in Au nanoparticle–nonpolar ZnO photocatalysts by measuring the surface potential of the composites using KPFM, thus obtaining the surface-potential-derived band diagram, which exhibited a particular method to study the charge transfer of the photocatalysts in the photocatalysis process.The transfer behavior of electrons at the interfaces of composite is determined by the work function of respective species. Although the photocatalysis mechanism of ZnO/graphene composites has been explained in detail, and the existence of IEF for hexagonal structure ZnO has been demonstrated by theoretical calculations, the photocatalysis mechanism of ZnO/graphene still needs to be further analyzed by characterizing surface electrical properties with the help of more powerful experiment. The atomic force microscope (AFM) based Kelvin probe force microscopy (KPFM) technique has already been demonstrated as a powerful tool for measuring electrostatic forces and electric potential distribution with nanometer resolution. The technique has been applied to materials science applications such as: work function mapping,(aq) and HZO@Gr/MB(aq) interfaces were suggested in the dark and under UV-visible irradiation. On the basis of the obtained conclusions, the three-dimensional band diagrams of HZO/Gr, HZO/MB(aq) and HZO@Gr/MB(aq) interfaces were proposed for a comprehensive understanding of the charge transfer in the HZO–MB(aq), HZO@Gr–MB(aq) systems when photodegradation process took place. The results show that Zn-polar planes and O-polar planes could undertake different degradation reactions under IEF.Although the mechanism that the combination of graphene can enhance the photocatalytic efficiency has been proposed theoretically, there are still lack of experimental supports. To explore the transfer behaviors of photogenerated charges before and after combination of graphene, and the degradation mechanism of MB under UV-visible light, the KPFM method was applied in this paper. Different surface potential of Zn-polar planes and O-polar planes was found and thus the IEF of HZO was estimated. Surface potential of Zn-polar plane, O-polar plane, graphene, Gr/Zn-polar plane and Gr/O-polar plane was measured. Accordingly, the relevant Fermi levels were calculated and the band diagrams of interfaces, namely Gr/Zn-polar plane and Gr/O-polar plane in the dark and under UV-visible irradiation were proposed. According to the energy potential of the corresponding species of MB solution reported in literatures, the band diagrams of HZO/MB2.21ϕtip and ϕsample are the work function of the tip and the sample, respectively, and q is the elementary charge. If the work function of selected probe tip is constant, the difference of the work function between two samples is the negatively numeric equivalent of the difference between CPD. Therefore, by loading the samples onto a substrate whose work function is known, we can estimate the work function of the samples, thus referring the location of relevant Fermi levels.KPFM is similar to noncontact-mode AFM. It measures electrostatic force in addition to atomic force. By measuring the contact potential difference (CPD) which minimizes the electrostatic force between probe tip and sample surface, the work function difference between tip and surface is estimated. The CPD between probe tip and sample is defined as3.3.1N-methyl-2-pyrrolidone (NMP) were mixed in a conical breaker. Then 4.0 g zinc acetate dehydrate (Zn(Ac)2·2H2O) was added into the breaker. At the same time, the mixed solution was stirred continually and heated to 95 °C. After 5 hours, the acquired turbid solution was filtrated and the precipitate was washed with water at least 3 times to remove the remaining NMP. Finally, the product was drying at 80 °C for 6 hours to get the HZO powder.The polar-plane dominant HZO was synthesized using a facile method. First, 75 ml deionized water and 75 ml 3.22). First, GO was synthesized using the modified Hummer's method, which was reported elsewhere.22 The exfoliated GO was dispersed in deionized water and centrifuged at 8500 rpm for 15 min. Then the supernatant was diluted to the concentration of 0.1 mg ml−1 for storage. The HZO surface was amine-functionalized by mixing with 2.5% amino-propyl-trimethoxysilane (APTMS)/ethanol solution and meanwhile stirring for 8 hours at 70 °C. Then the turbid solution was washed with ethanol 3 times and the obtained precipitants were dried at 80 °C for 3 hours. Subsequently, 0.5 g acquired HZO-NH2 was dispersed in 70 ml deionized water by ultrasonic dispersion and then mixed with 8 ml GO solution mentioned above. The mixed solution was magnetically-stirred at 55 °C for 8 hours. After that, the turbid solution was washed 3 times with water by centrifugation to get the precipitate and subsequently drying for 5 hours at 80 °C. Finally, the obtained HZO@GO powder was thermal-reduced in Ar for 2 hours at 450 °C to get the HZO@Gr composite.Generally, the HZO@Gr composite was obtained by self-assembly of the graphene oxide (GO) and amine-functionalized HZO  equipped with energy-dispersive spectrometry , field emission SEM  and high-resolution transmission electron microscope . The KPFM measurements were conducted with atomic force microscope . UV-visible absorption spectra were measured using a UV-Vis-NIR spectrophotometer .3.4−1). The mixed suspensions were magnetically stirred for 1 h under dark in ambient conditions to get the absorption–desorption equilibrium before irradiation. Then the suspensions were exposed to the light irradiation produced by a 300 W Xe lamp positioned at a 36 cm distance away from the vessel. At certain time intervals, 3 ml of the mixed suspensions were extracted and centrifuged to remove the photocatalyst. The degradation MB solution was analyzed by recording changes of the absorption at the maximum band using a UV-vis absorption spectrometer.The photodegradation process of methyl blue (MB) was tracked based on the absorption spectroscopic technique. 50 mg samples were dispersed in 100 ml MB aqueous solution  with silicon cantilevers coated with PtIr and Sb with a nominal resonance frequency of 75 kHz were used in this experiment. The substrate, namely highly oriented pyrolytic graphite (HOPG) was used as reference sample. The graphene sample was obtained by spin-coating diluted GO solution onto HOPG at 500 rpm and subsequent annealing in Ar at 450 °C for 2 h. HZO and HZO@Gr were dispersed in ethanol and spin coated onto HOPG at 500 rpm to acquire HZO and HZO@Gr samples.4.4.123 which was based on the electrostatic-force-induced self-assembly process. In brief, the surface of HZO was amine-functionalized by APTMS as the The mechanism for the formation of HZO@Gr was demonstrated in our previous work,4.2C0 and Ct are the initial concentration after the equilibrium adsorption and the residual concentration of MB, respectively. The concentration of MB at each period is correlated with the maximum absorption peaks of MB in the UV-visible spectra, which were shown in ESI  the introduction of graphene can eliminate the surface defects, and (2) the graphene can accept the photo-induced electrons assuredly and improve the charge separation.27The photocatalytic experiments were carried out in order to study the influence of graphene on photocatalytic properties of as-prepared HZO. I Fig. S1. The resI Fig. S1 show tha2− and Zn2+ ions stack alternatively along the c-axis results in positive charged Zn-terminated (0001) and negative charged O-terminated (0001̄) planes.8,28 An IEF is thus generated between the two polar planes due to the spontaneous polarization.7 Because it is difficult to measure both surface potential of a HZO particle, we consider the difference between the VCPD is the potential difference of the HZO particle, thus the IEF of HZO isVCPD-Zn and VCPD-O are surface potential of Zn-polar plane and O-polar plane, h stands for the thickness of HZO particle. Hence, the IEF of particle A and B are estimated 9.9 × 104 and 5.2 × 104 V cm−1. For the high electron mobility of ZnO, it is reasonable to believe numerous photogenerated electrons and holes will be driven to [0001] and [0001̄] direction. On the other hand, the work function of Zn-(0001) and O-(0001̄) planes can be calculated by the measured CPD of each plane. What's more, the work function of HOPG in air is regarded as 4.65 eV.29 By taking the average value of respective potential as the CPD between tip and the surface of the sample, the work function of Zn-(0001) and O-(0001̄) was calculated to be 4.18 and 4.93 eV, which are approximated with the reported value (4.25 and 4.95 eV).30 Since the work function is correlated with Fermi level of each surface, when taking the vacuum level as 0 eV in this article, the Fermi level of Zn-(0001) and O-(0001̄) polar planes are −4.18 and −4.93 eV.To explore the photocatalytic mechanism of HZO@Gr, the AFM topography and KPFM CPD  of HZO, graphene and HZO@Gr were measured. 31 It might be caused by the remaining negatively charged oxygen-containing groups or the destroy of the structure during annealing that form the localization of the sp2 area and decrease the density of free electron on the whole graphene sheet. The KPFM images of Zn-(0001) and O-(0001̄) plane of HZO are showed in EF,Gr), Zn-polar plane , O-polar plane , Zn–graphene surface , O–graphene surface  were acquired  of O-polar surface tend to transfer to the Fermi level of graphene due to the as-established Schottky contact even Ohmic contact in the dark. Hence, graphene is serving as electron traps, which is beneficial to charge separation, causing the decreasing of PL intensity. Nevertheless, there exists a Schottky barrier at the interface of Zn-polar plane and graphene, which may hinder some photoelectrons with low energy transferring to the graphene. However, it can still be predicted that most of the photoelectrons could reach the lowest required energy level (E′CB) and thus transferring to the graphene. On the one hand, the photoelectrons transferring above the E′CB from valance band by absorbing demanded light energy could then transfer to the graphene. On the other hand, besides the photoelectrons excited from the surface, most photoelectrons transferring to the surface may be excited from the interior of HZO. Zhan et al. have demonstrated that the effective depth of UV-visible-light absorption of ZnO is around 100 nm,33 which is corresponding to the thickness of HZO. Since amounts of photoelectrons are generated in the interior of HZO and immediately exposed to the powerful inter electric field, the photogenerated electrons and holes will be accelerated under the electric field to the opposite directions quickly, thus promoting the separation of electron–hole pairs. For the particles of A and B in E′CB through accelerating and then transfer to the graphene. It can be concluded that most of the photogenerated electrons have the possibility to transfer to graphene and the excited electron–hole pairs can be separated quickly under inter electric field, which will decrease the possibility of recombination of electron–hole pairs. In addition, under the effects of UV-visible-light irradiation and inter electric field, the Zn-polar plane and O-polar plane could change to electron-enriched and hole-enriched plane, leading to different main chemical reactions during the photodegradation process.On the basis of the measured Fermi levels of HZO, graphene and HZO@Gr, the proposed band diagram of HZO and graphene in the dark before contact formation is sketched in gy bands . Fig. 7c(aq) and HZO@Gr–MB(aq) when using HZO@Gr composites as the photocatalysts for the degradation of MB solution. (aq) interfaces before they make contact with MB(aq). It can be regard the energy potential of MB(aq) as the standard redox potential of H2O  for the extremely low concentration of MB (10 ppm). When the O-polar plane and Zn-polar plane are contacting with MB solution, there respective Fermi levels tend to align with the corresponding energy level of E0redox:H2O until equilibrium is achieved.16 As a result, both of the two planes will form a Schottky barrier at the interfaces of HZO and MB(aq), as shown in EF,Zn and E0redox:H2O is larger than EF,O and E0redox:H2O, the Schottky barrier at the Zn-polar plane will be higher. When UV-visible light illuminates the HZO particles, the electrons in valence band (VB) were excited to CB both in the interior and at the surface of HZO, forming photogenerated electron–hole pairs. The energy levels of OH−/˙OH and H2O/˙OH, H+ shown in vs. NHE at 25 °C). Some of the photogenerated holes migrated to the surface from interior of HZO may react with the adsorbed H2O and OH− to form ˙OH radicals to degrade the MB molecules due to the suitability of the energy level for charge transfer. Based on the conclusion that the O-polar plane is abundant in the migrated holes, the main chemical reactions for the O-polar plane are.On the basis of the conclusions discussed above, it is necessary to discuss the interfaces of HZO–MB2(aq), causing the accumulation of electrons in the CB, which may lead to increasing recombination possibility. Taking the energy level of O2/˙O2− as −4.22 eV (vs. NHE at 25 °C), the electrons with enough energy will transfer to O2(aq) to form the ˙O2 radicals which may degrade MB as follows:15,37While for the electron-enriched Zn-polar plane, the higher Schottky barrier will resist more photogenerated electrons to react with O−4.22 eV   phonons, respectively.38,39 Generally, the quality of nanostructured carbon-based materials can be reflected by the ratio of the intensities (ID/IG) of the two bands D and G. Compared with GO and HZO@GO, HZO@Gr exhibits obviously increased ID/IG ratio, which means after thermal reduction, the oxygen functional groups in GO was removed and the conjugated graphene network (sp2) will be re-established.40 As a result, the average size of the sp2 domains is decreased and the Fermi level of each fragments could be considered localized, which may be gradually lifted due to photoelectrons transferred to graphene and then stored inside the localized graphene. Further, once the localized Fermi level of graphene is higher than the potential of O2/˙O2−, the reaction to generate ˙O2− radicals may occur and lead to the degradation of MB(aq) as elaborated in via the Schottky or Ohmic contact of the interface or transferring to Zn-polar plane under IEF. As the excited electrons are relatively scarce at/near the O-polar plane, the electrons may be not so enough as to lift the localized Fermi level of graphene over the potential of O2/˙O2−. While compared with the HZO/MB(aq) interface of O-polar plane side, the valence band could emerge a Schottky barrier at the surface of graphene/O-polar plane. The excess holes could also move laterally to the vicinity of the graphene-uncovered HZO surface for its lower potential energy at HZO/MB(aq) interface. The generated holes would transfer to energy levels of OH−/˙OH and H2O/˙OH, and contribute to the generation of ˙OH radicals and the following degradation of MB. Furthermore, once the holes transferred to graphene, they could easily recombine with the surrounded electrons, which is unfavorable for the generation of ˙OH. Therefore, the exposed O-polar surface of HZO@Gr composites plays a crucial role in the photodegradation of MB solution. However, although bare HZO has lager MB/O-polar plane interfacial area, its photocatalytic activity seems to be lower, which can be ascribed to the inefficient charge separation. The incomplete coverage and breakage or defects of graphene offer large area of exposed HZO in order to provide a channel for the reaction of the migrated holes and MB solution. As a result, by efficiently separating the photogenerated electron–hole pairs under IEF, the HZO@Gr composites show enhanced photocatalytic performance.Hence, the produced hydroxyl radicals are the direct strong oxidant to decompose the MB molecule. However, it seems that the photoelectrons will accumulate at the Zn-polar plane of pure HZO particles, which may lead to the high possibility to recombine with the holes. In contrast, the three-dimensional band diagrams of HZO and HZO@Gr composite in the dark and under UV-visible irradiation are shown in graphene . The sam5.4 and 5.2 × 104 V cm−1. By calculating their respective work function and Fermi levels, the charge transfer behaviors and the readjustment of energy levels in the dark and under UV-visible irradiation were supposed. According to the energy potential of the corresponding species of MB solution, the photodegradation processes of Zn- and O-polar plane were made clear. The electrons and holes can be generated at the surface and in the interior of HZO. Under IEF, the e− and h+ were accelerated to gain extra energy and aggregated at/near Zn- and O-polar plane, respectively. For bare HZO, the high Schottky barrier at the MB(aq)/Zn-polar plane could hinder most electrons and thus the excess electrons would increase the possibility of charge recombination. Meanwhile the holes could transfer to the energy levels of OH−/˙OH and H2O/˙OH because of the Schottky contact at the interface of MB(aq)/O-polar plane. As to HZO@Gr, most of the photogenerated electrons have the possibility to transfer to graphene under IEF, which ensured the effective photocarriers separation under illumination. Some of the transformed electrons on graphene could possess enough energy to transform O2(aq) to ˙O2−. The excess holes, which were laterally transferred to graphene-uncovered HZO, produced ˙OH on the surface. Hence, Zn-polar plane and O-polar plane could mainly generate ˙O2− and ˙OH radicals respectively, which are the main species to degrade the MB molecules. The results suggest that the IEF and combination of graphene are the factors to the effective separation of photocarriers, which may improve the photocatalytic activity and decide the generation of intermediate radicals at different polar plane.We have synthesized hexagonal ZnO (HZO) and graphene-coated HZO (HZO@Gr) photocatalysts by solution method. The HZO@Gr composite exhibited enhanced photocatalytic performance and low photogenerated electron–hole recombination possibilities. In order to investigate the effect of graphene and IEF on the photocatalytic process of ±(0001)-polar plane, KPFM method was used to characterize the surface potential of ±(0001)-polar plane, graphene, Gr/Zn-polar plane and Gr/O-polar plane. For HZO particle of A and B, the values of IEF were estimated 9.9 × 10There are no conflicts to declare.RA-008-C7RA12037K-s001


These distinct properties enable the construction of practical optoelectrical sensor arrays. However, contact engineering for MoS2 films is not still optimized. In this work, we inserted a graphene interlayer between the MoS2 films and Au contacts (graphene/Au) via the wet-transfer method to boost the device performance. Using graphene/Au contacts, outstanding electrical properties, namely field-effect mobility of 12.06 cm2/V∙s, on/off current ratio of 1.0 × 107, and responsivity of 610 A/W under illumination at 640 nm, were achieved. These favorable results were from the Fermi-level depinning effect induced by the graphene interlayer. Our results may help to construct large-area photonic sensor arrays based on 2D materials.Two-dimensional (2D) molybdenum disulfide (MoS Many stonducted ,5,6. Dasred SBHs . Howeverred SBHs ,8. To erred SBHs ,10. The contacts ,11,12. Ooperties ,11,13. Holiation ,15,16, a2 films grown by chemical vapor deposition (CVD) and Au contacts (graphene/Au contacts) to induce Fermi-level depinning leading to the enhancement in device performance of the resultant MoS2 device arrays. To investigate the effect of the graphene interlayer, we also compare the performance of conventional Au contacts in terms of electrical and photoelectrical properties.Here, we inserted a graphene interlayer between large-area MoS2 film via CVD using precursor powders, including MoO3  and sulfur . Each precursor was placed into two ceramic crucibles. A MoO3 crucible was placed at the center of a 1 inch quartz tube, and a sulfur crucible was positioned upstream of the MoO3 crucible. A Si/SiO2 substrate was loaded downstream of the MoO3 crucible. Then, the furnace was heated to 650 °C and maintained at this temperature for 40 min with a flow of Ar (50 sccm) under 0.3 Torr. After finishing the reaction, natural cooling was performed with 200 sccm of Ar flow. Graphene monolayer film was also synthesized on Cu foil  via identical CVD. The specific synthetic process is explained in our previous report [We first synthesized MoSs report .2 devices. MoS2 films were first prepared on Si/SiO2 substrate. To construct patterned graphene, first, photolithography and oxygen plasma etching treatment were performed. Once the patterned graphene was prepared, it was transferred onto MoS2 film via the wet-transfer method, where we used a buffered oxide etch solution to delaminate graphene. The second photolithography was carried out on patterned graphene electrodes, followed by metal deposition (50 nm-thick Au). To minimize the misalignment, the size of the Au electrodes was slightly bigger (200 × 200 μm) than that of graphene interlayers (150 × 150 μm). To prevent current leakage, channel definition was performed with a length of 14 μm and width of 40 μm using the third photolithography. Finally, the uncovered MoS2 area was etched using SF6 plasma treatment. We also fabricated MoS2 devices with Au-only contacts via the same procedure without a graphene layer to investigate the effect of the graphene interlayer.We used a standard photolithography method to fabricate MoS2 and graphene film grown by CVD. Transmission electron microscopy (TEM) measurement was performed to investigate the crystal structure of the obtained MoS2 film. Electrical and photoelectrical properties were characterized using a semiconducting parameter analyzer  with a 10−6 Torr vacuum at 80–300 K and light emitting diode (LED) visible light at 640 nm.Atomic force microscopy  and Raman/photoluminescence (PL) spectroscopy  equipped with a 532 nm laser were utilized at room temperature to explore the physical properties of MoS2 film synthesized via CVD. Overall, the obtained film shows good uniformity with no remarkable voids or cracks.  and on/off current ratio of 1.01 × 107(±2.52 × 106), compared with those of Au-contacted devices (mobility: 2.1 cm2/V∙s (±0.75) and on/off current ratio: 1.0 × 106 (±1.34 × 105)), as shown 2 devices with graphene/Au contacts while preserving off-state current levels. This enhancement presumably results from the decreased SBHs of the resultant devices and the Fermi-level depinning effect induced by the graphene interlayer, providing an efficient charge transfer between the graphene/Au contacts and MoS2 channels [Using the fabricated MoSeristics a,b. Notechannels ,21.T is the temperature (K), I0 is the saturated current level, q is the electronic charge, k is the Boltzmann constant, σ is the standard deviation of the Gaussian function of the SBH, A is the device area, A* is the Richardson constant, and Bφ is the barrier height. The Richardson constant is strongly associated with the effective mass of the material. It is estimated as A* = 4πqm*k2/h3, where m* is the effective mass of MoS2 and h is Planck’s constant [Rc) with respect to that of the Au-only–contacted devices I0T2)−q2σk2T2=ln, defined as R = Iph/(AP), where Iph is the difference between dark and photocurrents, A is the device area, and P is the light power density  than those of Au-contacted devices . This faster photosensor performance of the graphene/Au-contacted device is due to the efficient carrier separation behavior and low recombination rate caused by the improved carrier mobility [We finally assessed the photoelectrical properties of MoSt 640 nm a,b. Unde density c. Both d density . Furthermobility .2 device arrays with graphene/Au contacts. These graphene/Au-contacted devices have noticeable electrical performance, including a field-effect mobility of 12.06 cm2/V∙s and on/off current ratio of 1.0 × 107. This improvement can be attributable to the Fermi-level depinning effect caused by the graphene interlayer between Au and MoS2 channels, whereby the SBH and contact resistance are reduced. Furthermore, they show a visible photoresponse, achieving the responsivity of 610 A/W under illumination at 640 nm. This value is significantly comparable to that of TMDC photodetectors. Fabricating large-area MoS2 device arrays using graphene films constructed via CVD rather than exfoliated flakes offers insight into practical large-scale optoelectronic applications based on 2D TMDC materials.This report demonstrates the construction of large-area MoS


Opioid related overdose deaths in the United States claimed over 100,000 thousand lives during the 12-month period ending in April 2021, an increase of 28.5% from the previous period. Syringe services programs (SSPs) are an evidence-based harm reduction strategy that have been shown to be effective in reducing opioid overdose deaths and infectious complications and increasing rates of entry into recovery programs. Ignoring this evidence, South Carolina (SC) and several states have yet to legalize SSPs. In the absence of full legalization, the operation of SSPs in SC faces many barriers. Despite these barriers, Challenges Inc. has been successful in playing a critical role in preventing opioid overdoses through naloxone and fentanyl test strip distribution, reducing infectious complications by providing clean needles, treating individuals with hepatitis C and HIV, and helping patients remain in sustained recovery from opioids. In order for SSPs to function at their full potential to curb the rising tides of opioid overdose deaths and related health complications, policymakers in SC and similar states need to urgently legalize them. During the 12-month period ending in April 2021, there were a record high over 100,000 overdose deaths in the United States, an increase of 28.5% from the previous period . SyringeAccording to the North America Syringe Exchange Network (NASEN) there are 370 SSPs in the United States as of 2021. Sixty-nine percent of those SSPs are in urban regions and 63% provided mobile exchange. Services provided range from only HIV and HCV testing to onsite HIV and HCV testing and treatment combined with programs that provide MOUD. As of 2019, thirty-one states and the District of Columbia have laws that explicitly legalize SSPs; nine have laws that reduce barriers to SSPs such as not prohibiting free distribution of syringes; and twelve do not have laws that reduce barriers to SSPs implementation, making SSPs illegal [While SC does not have legislation that authorize the use of SSPs, it also does not have laws that specifically prohibit it. Syringe services programs are not illegal in SC because SC drug paraphernalia law does not specifically mention syringes despite intentionally calling attention to other tools such as bongs and cocaine spoons , 11. HowFounded in 2017 by M.B. (second listed author), Challenges Inc. is a volunteer-run program that has been successful in providing harm reduction services in seven counties in SC. The counties are primarily suburban and rural and have a total population of approximately 1.4 million people . ThroughAn SSP participant is asked about getting point-of-care HIV testing during a regular visit to get clean syringes and naloxone. He tested positive, and the SSP care coordinator provided comfort and education on the new HIV diagnosis and made a warm handoff to the HIV treatment provider. He was started on antiretroviral treatment the next day, and 30 days later his HIV viral load is undetectable.A second SSP participant who has come in regularly for clean needles, naloxone, fentanyl test strips and other supplies for the last 2 years decided he wants to stop using drugs. He was referred to the co-located clinic which provides MOUD. He received financial assistance to cover his first month of treatment, which included medication and psychosocial therapy, and came in the following week to start treatment. He is now stable on buprenorphine and is actively searching for employment.A third SSP participant who lives in a transition house was told in the past he may have HCV. He heard about the SSP’s free HCV testing. He came and got tested and is positive for HCV. He was referred to a local primary care provider who provides on-site HCV treatment. He tolerated oral antivirals for hepatitis C well, and he achieved a sustained viral response.As the success stories with Challenges Inc. illustrate, an SSP can help promote safer drug use behaviors, reduce the onward transmission of infectious diseases including HIV and HCV, increase the rates of entry into MOUD programs, and connect individuals to primary care, even in a state where its existence is not explicitly legal. Syringe services programs have had an even more profound effect in Indiana. In 2015, Austin (Scott County), Indiana experienced the largest concentrated HIV outbreak in the United States . At thatTo prevent further loss of lives from the opioid epidemic and related complications including HIV and HCV, SC and the remaining 18 states without legalized SSPs need to urgently legalize and fund SSPs. While the SC legislature has attempted to authorize SSPs in the past, both attempts have stalled in committees , 16. The


This study aimed to investigate the prevalence of and risk factors for adhesive capsulitis in older adults from Germany. The one-year and lifetime prevalence of adhesive capsulitis were assessed in adults aged ≥65 years with at least one visit to one of 1207 general practices in Germany in 2021. Associations between physical and psychiatric conditions and adhesive capsulitis were further assessed in older patients diagnosed for the first time with adhesive capsulitis in general practices in 2010–2021 (index date) and matched (1:5) to patients without adhesive capsulitis using a propensity score based on age, sex, and the index year. In adults without adhesive capsulitis, the index date was a randomly selected visit date in 2010–2021. The one-year and lifetime prevalence of adhesive capsulitis in older adults in 2021 were 0.4% and 2.4%, respectively. In 8439 patients with and 42,195 patients without adhesive capsulitis, 12 conditions were positively and significantly associated with adhesive capsulitis. Effect sizes were strongest for other and unspecified osteoarthritis (OR = 1.93), injury to the shoulder and upper arm (OR = 1.85), and injury to the thorax (OR = 1.47). Based on these findings, adhesive capsulitis can occur at older age, and osteoarthritis and injuries are major risk factors. Adhesive capsulitis of the shoulder  is a chronic shoulder condition characterized by pain, loss of range of motion, and decreased upper arm function . UsuallyThe prevalence of adhesive capsulitis of the shoulder is between 3% and 5% in the general population . The majTherefore, the present retrospective study aimed to investigate the prevalence of and risk factors for adhesive capsulitis of the shoulder in patients aged ≥65 years from general practices in Germany.This study used data from the Disease Analyzer database (IQVIA). Details on the methodology of the database have been published elsewhere . BrieflyTwo distinct populations were included in the present retrospective study. The first population was used to estimate the one-year and lifetime prevalence of adhesive capsulitis of the shoulder in older adults from general practices in Germany in 2021. This population included all individuals aged ≥65 years with at least one visit to one of 1207 general practices in 2021. The second population was used to identify physical and psychiatric conditions significantly associated with adhesive capsulitis. This population included all patients aged ≥65 years with an initial diagnosis of adhesive capsulitis of the shoulder (ICD-10 code: M75.0) between January 2010 and December 2021 . Additional inclusion criteria were at least five years of observation prior to the index date and at least one visit each year in the five years prior to the index date. After applying similar inclusion criteria, patients aged ≥65 years without adhesive capsulitis were matched (1:5) to those with adhesive capsulitis using a propensity score based on age, sex, and index year. In participants without adhesive capsulitis of the shoulder, the index date was a randomly selected visit date between January 2010 and December 2021. The flow diagram of the second study population is displayed in Physical and psychiatric conditions were documented in the five years prior to the index date. These conditions were selected based on previous literature and included cancer (ICD-10 codes: C00-C97) , disordep-values < 0.001 were considered statistically significant. All analyses were performed using SAS 9.4.The one-year prevalence of adhesive capsulitis of the shoulder was estimated as the proportion of individuals diagnosed at least once with adhesive capsulitis in 2021 among patients with at least one visit to their general practice in 2021. Similarly, the lifetime prevalence of adhesive capsulitis corresponded to the proportion of individuals diagnosed at least once with adhesive capsulitis in their whole medical history among patients with at least one visit to their general practice in 2021. Demographic characteristics of patients without adhesive capsulitis in 2010–2021 matched to those with adhesive capsulitis in 2010–2021 were compared using the Wilcoxon signed-rank test for continuous age, the Stuart–Maxwell test for categorical age, and the McNemar test for sex. Finally, the associations between physical and psychiatric conditions (independent variables) and the diagnosis of adhesive capsulitis of the shoulder in 2010–2021 (dependent variable) were studied using an adjusted logistic regression model. All physical and psychiatric diseases were simultaneously included in the logistic regression model. Results of the logistic regression model are displayed as odds ratios (ORs) and 95% confidence intervals (CIs). The one-year and lifetime prevalence of adhesive capsulitis of the shoulder in the overall sample and by sex are displayed in After 1:5 matching of patients with adhesive capsulitis of the shoulder with those without adhesive capsulitis using a propensity score based on age, sex, and index year, there were 8439 participants in the adhesive capsulitis group and 42,195 participants in the no adhesive capsulitis group. Mean (standard deviation) age at the index date was 75.3 (6.8) years, while the prevalence of women was 57.4% .The results of the adjusted logistic regression model are shown in The one-year and lifetime prevalence of adhesive capsulitis of the shoulder in adults aged ≥65 years from general practices in Germany in 2021 were 0.4% and 2.4%, respectively. The adjusted logistic regression model conducted on 8439 adults with and 42,195 adults without adhesive capsulitis further revealed that 12 physical and psychiatric conditions were positively and significantly associated with the diagnosis of adhesive capsulitis in 2010–2021. These conditions were other and unspecified osteoarthritis (OR = 1.93), injury to the shoulder and upper arm (OR = 1.85), injury to the thorax (OR = 1.47), polyosteoarthritis (OR = 1.44), palmar fascial fibromatosis , gout (OR = 1.36), reaction to severe stress and adjustment disorders (OR = 1.32), gastro-esophageal reflux disease (OR = 1.29), somatoform disorders (OR = 1.19), obesity (OR = 1.15), ischemic heart diseases (OR = 1.13), and disorders of the thyroid gland (OR = 1.11). Finally, there was a negative and significant relationship between adhesive capsulitis and Parkinson’s disease (OR = 0.71) and hypertension (OR = 0.81). To the best of the authors’ knowledge, this is only the second study to investigate the epidemiology of adhesive capsulitis in the older population, while this research is the first to analyze the effects of a large number of physical and psychiatric disorders on adhesive capsulitis, particularly in older age.In this retrospective study of older patients followed in general practices in Germany, the one-year prevalence of adhesive capsulitis of the shoulder was 0.4%. The finding was corroborated in the sex-stratified analyses. This finding is in line with the sole study on the epidemiology of adhesive capsulitis in older adults, which reported a one-year prevalence of 0.35% in the United States . Given tThere was a positive and significant association between several physical and psychiatric conditions and adhesive capsulitis of the shoulder. Osteoarthritis  was strongly associated with adhesive capsulitis. Although it was not possible in this study to differentiate shoulder osteoarthritis from osteoarthritis of other regions of the body , it is likely that the observed associations are largely explained by shoulder osteoarthritis. Interestingly, in a population-based nested case–control study of 24,414 individuals with and 97,656 individuals without adhesive capsulitis from Taiwan, osteoarthritis was associated with a 4.27-fold increase in the odds of adhesive capsulitis after adjusting for multiple confounding variables [p-value = 0.068), the lack of statistical significance potentially being related to the small sample size [Palmar fascial fibromatosis, also known as Dupuytren’s disease, was positively and significantly associated with adhesive capsulitis. Palmar fascial fibromatosis, which corresponds to the benign proliferation of fibroblasts and myofibroblasts, involves nodules and contractures on the palmar crease . A case–ple size . In addiple size .The present retrospective study from Germany further showed a significant association between gout and adhesive capsulitis of the shoulder. Findings pointing in the same direction were obtained in a Taiwanese nationwide population-based matched-cohort study of 117,282 adults aged 40–70 years, as gout led to a 1.71-fold increase in the adjusted risk of adhesive capsulitis . Gout isThis research also showed that participants with obesity were more likely to be diagnosed with adhesive capsulitis of the shoulder than their counterparts without obesity. This result is in line with another study of 4380 patients from the United States, as obesity was significantly more frequent in people with than in those without adhesive capsulitis . AlthougIn terms of psychiatric conditions, two disorders  were positively and significantly associated with adhesive capsulitis of the shoulder. This finding is relatively new and should be interpreted with caution. Interestingly, previous research has reported a high prevalence of psychiatric conditions in patients with adhesive capsulitis compared with the general population . The preFinally, there was a negative and significant association between Parkinson’s disease, hypertension, and adhesive capsulitis, while no statistically significant relationships were observed for diabetes and cerebrovascular diseases. The negative Parkinson’s disease–adhesive capsulitis relationship is unexpected, as a previous review article has obtained opposite findings . Given tBased on these findings, adhesive capsulitis of the shoulder affects not only middle-aged but also older adults. Conditions the most strongly associated with adhesive capsulitis were osteoarthritis and injuries to the shoulder, upper arm, and thorax. When these conditions are present, shoulder pain, range of motion, and function should be regularly assessed by general practitioners. The first-line treatment of adhesive capsulitis is physiotherapy, and several physiotherapy interventions  are recommended for the management of this shoulder condition . CorticoStrengths of the study are the large sample size, the use of data collected in general practices, and the inclusion of a wide range of physical and psychiatric conditions. Nonetheless, the study findings should be interpreted in light of several limitations. First, the diagnosis of adhesive capsulitis exclusively relied on the ICD-10 classification. More data on adhesive capsulitis  would have allowed more detailed analyses. Second, adhesive capsulitis may have been diagnosed in rheumatology practices, and the prevalence of this shoulder condition may have been underestimated. Third, there was no information on prolonged immobilization of the shoulder and HLA-B27 status, and given that these factors may be associated with adhesive capsulitis of the shoulder ,37, resiThis retrospective study, including older adults followed in general practices in Germany, found that the one-year and lifetime prevalence of adhesive capsulitis of the shoulder were 0.4% and 2.4%, respectively. Multiple physical and psychiatric conditions were significantly associated with adhesive capsulitis, and effect sizes were the strongest for osteoarthritis and injuries to the shoulder, upper arm, and thorax. More studies on the epidemiology of adhesive capsulitis in older adults are needed to confirm the present results.


RD-DsRed folding reporter. All four LMDS compounds reduced tau aggregation and reactive oxygen species. Among them, LMDS-1 and -2 reduced caspase-1, caspase-6 and caspase-3 activities and promoted neurite outgrowth, and the effect was significantly reversed by knockdown of TRKB. Treatment of ERK inhibitor U0126 or PI3K inhibitor wortmannin decreased p-CREB, BDNF and BCL2 in these cells, implying that the neuroprotective effects of LMDS-1/2 are via activating TRKB downstream ERK, PI3K-AKT and CREB signaling. Furthermore, LMDS-1/2 demonstrated their ability to quench the intrinsic fluorescence of tryptophan residues within the extracellular domain of TRKB, thereby consolidating their interaction with TRKB. Our results suggest that LMDS-1/2 exert neuroprotection through activating TRKB signaling, and shed light on their potential application in therapeutics of Alzheimer’s disease/tauopathies.Hyperphosphorylation and aggregation of the microtubule binding protein tau is a neuropathological hallmark of Alzheimer’s disease/tauopathies. Tau neurotoxicity provokes alterations in brain-derived neurotrophic factor (BDNF)/tropomycin receptor kinase B (TRKB)/cAMP-response-element binding protein (CREB) signaling to contribute to neurodegeneration. Compounds activating TRKB may therefore provide beneficial effects in tauopathies. LM-031, a coumarin derivative, has demonstrated the potential to improve BDNF signaling in neuronal cells expressing pro-aggregated ΔK280 tau mutant. In this study, we investigated if LM-031 analogous compounds provide neuroprotection effects through interaction with TRKB in SH-SY5Y cells expressing ΔK280 tau Neurodegenerative tauopathies, including Alzheimer’s disease (AD), frontotemporal dementia, progressive supranuclear palsy and corticobasal syndrome, are characterized by accumulation of misfolded tau proteins . These mMAPT, tau plays an important role in axonal transport, microtubule dynamics and assembly Tand [D]Tare the total concentration of TRKB-ECD-His protein and test compound, respectively.The interaction between TRKB-ECD-His and LM-031, LMDS-1 or LMDS-2 was determined by tryptophan fluorescence titration. 7,8-DHF (Sigma-Aldrich) was included as a positive control . Tryptopu et al. :KD = [P]t-test or one-way analysis of variance (ANOVA) was performed, with Tukey’s post hoc test where appropriate. p values < 0.05 were considered as statistically significant.The presented data are shown as mean ± SD of three independent experiments in two or three biological replicates. To compare the differences between groups, two-tailed Student’s RD SH-SY5Y cells is compromised, resulting in BCL2 and BDNF down-regulations, BAX up-regulation, ROS overproduction and neurite outgrowth impairments. Binding to TRKB, activating its downstream ERK, PI3K-AKT and CREB signaling pathways and reducing caspase activity, LMDS-1/2 exert neuroprotective effects in these cells. Blockage of either ERK or PI3K-AKT signaling pathways counteracted their neuroprotection against tau aggregation, indicating both signaling pathways are indispensable to protect neurons against tau-mediated neurotoxicity. Given that the effects of LMDS-1/2 were only examined in cell models, future studies in animal models of tauopathies or other neurodegenerative diseases will be necessary to validate their potential in treating patients with AD and tauopathies.In the present study, we show that the TRKB signaling pathway in ΔK280 tau


Collective oscillations of massless particles in two-dimensional (2D) Dirac materials offer an innovative route toward implementing atomically thin devices based on low-energy quasiparticle interactions. Strong confinement of near-field distribution on the 2D surface is essential to demonstrate extraordinary optoelectronic functions, providing means to shape the spectral response at the mid-infrared (IR) wavelength. Although the dynamic polarization from the linear response theory has successfully accounted for a range of experimental observations, a unified perspective was still elusive, connecting the state-of-the-art developments based on the 2D Dirac plasmon-polaritons. Here, we review recent works on graphene and three-dimensional (3D) topological insulator (TI) plasmon-polariton, where the mid-IR and terahertz (THz) radiation experiences prominent confinement into a deep-subwavelength scale in a novel optoelectronic structure. After presenting general light-matter interactions between 2D Dirac plasmon and subwavelength quasiparticle excitations, we introduce various experimental techniques to couple the plasmon-polaritons with electromagnetic radiations. Electrical and optical controls over the plasmonic excitations reveal the hybridized plasmon modes in graphene and 3D TI, demonstrating an intense near-field interaction of 2D Dirac plasmon within the highly-compressed volume. These findings can further be applied to invent optoelectronic bio-molecular sensors, atomically thin photodetectors, and laser-driven light sources. Graphene and topological insulators are promising platforms for near-field nanotechnology on a subwavelength scale. Authors present the highly confined nature of Dirac plasmon-polaritons in graphene, topological insulator, and hybrid systems. Because the Bloch Hamiltonian in a unit cell is inversion symmetric, the gapless-linear band dispersion appears at the corner of the Brillouin zone, namely K and K′ points. Strikingly, the many-body interactions in graphene can be described based on the (2 + 1)-dimensional Dirac Hamiltonian. A range of optoelectronic measurements has demonstrated that graphene’s optical responses are different from those in the conventional two-dimensional semiconductor structures, primarily because of the 2D Dirac dispersion. The early studies on graphene focused on the transport measurements in a strong magnetic field2 verifying the half-integer quantum Hall (QH) conductivity σxy = (4e2/h)(N + 1/2), where N is integer. While the conventional 2D electron gas presents the standard integer QH state σxy ∝ (e2/h)N, the QH spectra in monolayer graphene are shifted by a half-integer, which can be elucidated by the (2 + 1)-dimensional Dirac Hamiltonian in a strong magnetic field. The gapless nature of graphene bandstructure also hosts the Dirac-fluid; the co-existence of electron and hole in the charge-neutral point has shown viscous charge flow5.Graphene consists of hexagonal arrangements of carbon atoms in two-dimension (2D). The unit cell of graphene contains two equivalent carbon atoms, where the nearest-neighbor hopping interaction is based on the sp9. The light-induced particle-hole polarization of graphene can be calculated using random-phase approximation (RPA)14. The calculation results show that the graphene polarization is determined by two independent contributions of the intraband and interband polarization, where the Fermi energy distinguishes the two regimes. At the long-wavelength limit, the interband polarization promises a constant absorbance ~2.3% independent of light frequency15. In contrast, the intraband polarization explains the free carrier response against electromagnetic radiation18. Notably, light coupling with dipole momentum q exceeding photon qc can excite the graphene plasmon-polariton (q/qc ~ 102). Here, the relationship of plasmon frequency ω versus the plsmon momentum q follows the conventional plasmon mode n) dependence allows us to distinguish the 2D Dirac plasmon (ω ∝ n1/4) from the conventional 2D plasmon (ω ∝ n1/2)19. A truly intriguing aspect of monolayer graphene is that the interparticle interaction is weak compared to conventional semiconductors, which is attributed to the linear dispersion of the gapless bandstructure11. The RPA result can precisely describe the 2D Dirac plasmon’s density-dependence in this weakly interacting system.Plasmonics, the collective response of light-matter interaction, also shed light on the unique properties of graphene22. Nonetheless, the scattering dynamics of Dirac fermions mainly limit the lifetime of 2D Dirac plasmon25. The momentum relaxation mechanism at low carrier density is primarily attributed to the impurity scatterings24, although the electron-phonon interaction still affects the carrier lifetime at high carrier density25. On the other hand, monolayer graphene encapsulated with hexagonal boron nitride (hBN) has shown high carrier mobility μ ~ 105 cm2V−1 s−1 because of uniform interfaces of the van der Waals heterostructure26. With the low density of interface impurity, the hBN/graphene/hBN structures provide a sensitive tool to investigate the quasiparticle interactions. For instance, 2D Dirac plasmon can be coupled with phonon-polaritons29, where the plasmon energy is split into several branches near the phonon-polariton energy. The high mobility of monolayer graphene also enables us to detect an acoustic plasmon mode34, which strongly confines the plasmon momentum into a deep-subwavelength limit. Notably, the compressed graphene polarization unravels electron-electron interactions at low carrier density31.The near-field distribution arising from the charge oscillations in monolayer graphene can be coupled with novel optoelectronic structures2 hybridization for the crystal bonding, the gapless-linear dispersion appears at the K and K′ points in the Brillouin zone. In reality, however, the spin–orbit coupling can open up the bandgap via the σ-π bond hybridizations37. The induced bandgap is a tiny perturbation ~μeV compared to the typical Fermi-energy scale observed in graphene; thereby, it is safe to use the ideal 2D Dirac dispersion to describe the light-matter interactions. Nonetheless, the bulk bandgap arising from the spin-orbit coupling produces highly non-trivial results at the graphene boundary: the bandgap is replaced by a spin-dependent one-dimensional (1D) conduction channel. A significant aspect of the 1D boundary state is that the spin direction is orthogonal to the direction of particle momentum. The spin-filtered transport promises dissipationless conduction as long as the spin-orbit coupling sustains the bulk bandgap in monolayer graphene. The possible existence of the non-trivial boundary state could be experimentally confirmed in other crystals with strong spin-orbit interaction39. Crucially, the boundary state of three-dimensional (3D) crystal can host 2D conduction at the surface, where the spin direction is orthogonal to surface normal vector and momentum direction. 3D topological insulators (TI), Bi2Se3, Bi2Te3, and Sb2Te3, are known to show the non-trivial boundary state on the surface41. Essentially, the surface state of 3D TI follows 2D Dirac dispersion similar to the case of graphene, except that the spin- and valley-degeneracy are absent on the 3D TI surface. Therefore, the 3D TI surface hosts 2D Dirac plasmon44, while the bulk inside TI spatially separates the opposite surface states, screening the near-field emanating from the plasmon-polariton.For the ideal case of graphene, which assumes spThis review provides a comprehensive analysis of 2D surface plasmon-polariton (SPP) in the gapless Dirac-band dispersions of graphene and 3D TI. The SPP in graphene and 3D TI uncover the unique light-matter interaction in the associated 2D Dirac-band. The electric field oscillations from SPP are confined up to a few atomic layers, which could amplify Coulomb interactions with neighboring materials. The following section addresses general aspects of 2D Dirac plasmon in terms of dispersion relations calculated from RPA. After presenting the dispersion relation in a monolayer 2D Dirac-band, we proceed with the interactive systems incorporating quasiparticle excitations. Furthermore, we introduce basic experimental techniques to couple electromagnetic waves with plasmon-polariton, presenting novel methods to control the 2D Dirac plasmon in a deep-subwavelength scale. The electrical and optical modulations are achieved in a graphene field-effect device and graphene-integrated 3D TI structure. As to provide future outlooks for the 2D Dirac plasmon, we list state-of-art applications of graphene plasmon at the end of this review.q → 0 of 2D Dirac plasmon dispersion follows the sublinear relationship 31 from the conventional SPP dispersion. When q is nested at the Fermi surface, additional photon energy is required to excite SPP above the Fermi energy, which results in the blue-shift in the SPP dispersion. Recent studies on graphene highlight the significance of non-local response resulting from the strongly-confined plasmon wavelength (λp)31. Figure ω and wavevector q are normalized by the Fermi energy EF and Fermi wavevector kF, respectively. Here, we employed the full description of the non-local graphene polarization P derived from the RPA (Eq. (9) in ref. 10), finding zeros of the dielectric function ε = 1−υqP, where υq = e2/2κ0κq is the Fourier-transformed 2D Coulomb interaction. Figure κ changes, where the screening effect of the surrounding medium shifts the plasmon dispersion toward the lower frequency regime. Although the local approximation ωlocal obtained from Plocal (Eq. (12) in ref. 10) follows the non-local SPP  at the long wavelength limit (q → 0), it deviates when the q is comparable to kF, implying that the SPP is nested at the Fermi surface. SPP can excite interband particle-hole pairs when entering the single-particle excitation (SPE) region. The intrinsic SPP decay rate is characterized by the Landau damping γLandau, which becomes significant at the deep-subwavelength regime. The rest this section embraces the non-local P to calculate the SPP dispersion with an average dielectric constant κ = 4 of the surrounding medium. This is because graphene is surrounded by dielectric materials in general. Besides, the RPA calculation is appropriate for weakly interacting particles, which requires the screening effect of the surrounding medium. The Wigner-Seitz radius rs is a brief measure of the interparticle interactions rs = (α/κ)(c/vF)(4/gsgv)1/2, where rs ≪ 1 implies weak particle interactions11. The screening factor κ = 4 lowers the rs value to ~0.5, where we assume the fine-structure constant α = 1/137, the graphene Fermi velocity vF = 1 × 106 m/s, and spin- and valley-degeneracies gsgv = 4.The energy and momentum dispersions of SPP characterize the longitudinal oscillations of free carriers on metallic films. The local approximation mentclass2pt{minimmentclass2pt{minimωSO)47, where the SPP dispersion is split into upper and lower branches at the vicinity of ωSO. The energy splitting is characterized by the coupling constant αsp = ε∞[(ε∞ + 1)−1−(ε0 + 1)−1], where ε∞ (ε0) is the high-frequency (static) dielectric constant. Figure 13d = 0.5 nm is the distance between the graphene and substrate. Here, we assumed the graphene EF = 0.4 eV above the silicon oxide SO-phonon energies of 100 meV and 145 meV. Clearly, αsp increases the energy splitting in the SPP dispersion. The lowest branch of the phonon-coupled SPP is damped upon entering the SPEintra region, which requires extreme confinement of λp and the large plasmon-phonon coupling. Yan et al.45 reported that the graphene optical phonon (~0.2 eV) assists the plasmon damping into intraband particle-hole pairs, where the inelastic damping manifests as the broadened SPP resonance. On the other hand, the hyperbolic phonon-polariton (HPP) can propagate inside a dielectric material with anisotropic phonon-polariton resonances49. Thin films of hBN host the HPP energy bands at 90–100 meV (type I) and 175–200 meV (type II). The plasmon-phonon coupling inside the HPP bands is different from the SO phonon-plasmon coupling because the HPP mode also disperses with the polariton wavevector. These considerations are discussed elsewhere50.Charge oscillations in an atomically thin layer experience strong Coulomb interactions when the dielectric constant rapidly changes as a function of frequency. This effect is pronounced near the surface optical (SO) phonon frequencies  along with the normal axis of the optical (acoustic) plasmon mode. At the long-wavelength limit q → 0, the dispersion of the optical mode ω ∝ q). Besides, the distinct charge distribution imposes a potential limit to the light coupling method. The far-field radiation mainly excites the optical plasmon mode since the distance of the double-layer graphene is generally far shorter than the photon wavelength in free space54. Recently, the acoustic plasmon mode in graphene has been demonstrated in a novel dual-gate structure31, where the asymmetric charge distribution is driven by the image charge reflected on the metallic gate side. As indicated in Fig. d decreases. Strikingly, the decrease in d also curbs the acoustic mode’s damping rate, presumably because the out-of-plane dipole moment becomes significant in close proximity. On the other hand, the SPP response in 3D TI is analogous to the case of the double-layer graphene because the 3D TI surface states always come in a pair, where the TI bulk spatially separates the surface states. Notably, a general expression of dynamic screening function for the double-layer structure can be found in ref. 53, where the ε includes different values of κ inside and outside of the 2D Dirac layers.The decrease in Te above 1000 K, creating the hot Dirac plasma57. Figure kBTe exceeds EF, where we introduce a characteristic constant η = kBTe/EF to compare the intrinsic plasmon to the cold plasma case (η = 0). The intrinsic plasmon dispersion can be obtained from the zeros of the dielectric function 1 − υqP = 0 with the polarization14μ(Te) is the chemical potential following the charge conservation D(E) = gsgvE/2πℏ2vF2. When η > 1, the interband thermal excitation increases the sum of the electron and hole density following ω ∝ n1/4, it is evident that the increase in ω arises from the thermal population of the 2D Dirac bands14. However, at the intermediate state (η = 0.6), ω is somewhat decreased compared to the cold plasma case, implying that the 2D Dirac plasmon non-monotonically depends on the temperature. Unlike the conventional 2D electron gas, D(E) in 2D Dirac-bands is proportional to E. As a result, the intraband thermal excitation always accompanies the decrease in μ(Te), which explains the red-shift in ω at the intermediate state. Figure ω as a function of η. Meanwhile, the SPP encounters Landau damping γLandau before entering the SPE region because of the free carriers smeared from the Fermi surface , where the strong confinement is ascribed to the slow Fermi velocity of graphene (vF/c = 1/300). One intuitive way to acquire an additional momentum q = 2π/λp is to fabricate a nanoribbon structure58, where the period P gives rise to the plasmon momentum q = 2π/P. Figure 54. The electric field perpendicular to the nanoribbon confines the polarization in the nanoribbon width W upon the laser irradiation. As shown in Fig. W (increase in q) increases the plasmon frequency, following the sublinear dispersion of the 2D Dirac plasmon This part introduces experimental techniques to excite plasmon-polariton in graphene and 3D TI. In general, the plasmon wavelength 60 and TI plasmon62, tailoring the phase and propagation of the excited SPP64. Figure λ0 = 11.28 μm), confining the photon wavelength into the deep subwavelength scale. The SPP launched by the AFM-probe spreads out away from the source, which experiences reflection at the edge of the graphene flake. A prominent light scattering occurs when the reflected SPP reaches the AFM-probe again, producing the near-field modulations depending on the AFM-tip position  allows us to acquire the near-field profiles of the graphene plasmonion Fig. . Sweepin23, electron–impurity scatterings24, electron–phonon scatterings25, and dielectric losses of environment materials, imposing an intrinsic limit to the SPP lifetime. The highest Qp value of 130 is reported in the graphene plasmon using the s-SNOM measurement65, circumventing various scattering sources by lowering the temperature (~50 K) and encapsulating the graphene with hBN flakes. At the low temperature, the electron-phonon scattering is largely reduced, remaining the dielectric loss as a residual scatterer against the graphene SPP. As reported in ref. 28, capping the graphene with hBN plays a vital role in reducing the impurity scattering, where the Qp can be increased by five times when transferring the graphene from Si/SiO2 (Qp ~ 5) to hBN substrate (Qp ~ 25). Besides, the impurity scattering can be reduced by increasing the free carrier density of graphene. On the other hand, the fabrication of graphene nanostructure may contribute to the plasmon damping, which takes place at the edge of graphene nanoribbon via the elastic processes45. Similar effects are extensively studied in metallic nanoparticles66.Understanding the dissipation pathway of graphene plasmon is essential to develop graphene-based optoelectronic applications. In general, the scattering event of graphene SPP involves a range of sources: electron–electron scatterings69. Figure fpump−fprobe = fSPP) and momentum (kpump + kprobe = kSPP) are conserved during the difference frequency generation (DFG). Here, the transverse magnetic (TM) polarizations of the pump and probe allow the efficient coupling of the evanescent electric field and the graphene SPP at the interface. The excitation of graphene SPP manifests in the transient increase of the probe-intensity since the pump-photon is converted to the probe-photon through the DFG process. Here, the extracted fSPP and kSPP from the transient change in the probe spectra follow the 2D Dirac plasmon dispersion. Proper tuning of photon energies yields the nonlinear conversion efficiency of 6 × 10−5 W−1 in the optical waveguide69  of ~50, greater than the value in graphene nanoribbons (Qp ~ 5).Counter-propagating electromagnetic waves inside a nonlinear medium can generate a difference-frequency of the input frequencies. Such a nonlinear process can be used to excite the SPP in graphenee69 Fig.  and 6 × ace Fig. 68. The s70, photovoltaic71, and bolometric effects72. Whether the plasmonic excitation affects the photoresponse of graphene is of particular interest in the field of graphene-based optoelectronic applications73. Lundeberg et al.21 reported that the 2D Dirac plasmon can be employed as a thermoelectric photodetector in a dual-gated graphene field-effect device. Figure λ0 = 10.6 μm) into the photocurrent I2. Here, the AFM-tip position (xtip and ytip) modulates the I2 amplitude, indicating that the plasmonic excitation affects the graphene photoresponse. In fact, the AFM-tip position changes the absorption cross-section of the device; the light absorption can be enhanced by the constructive interference between the AFM-tip and the SPP reflected from the graphene edge, otherwise destructive interference reduces I2. The sixfold change in the I2 sign indicates that the thermoelectric effect is responsible for the photoresponse in the dual-gate device ∆T, where SR(L) is the Seebeck coefficient of the right (left) junction, and ∆T is the light-induced increase in the junction temperature. The gate voltage independently controls the carrier density of the right and left side of the junction. A closer look into the photocurrent map reveals that the I2 signal decays away from the junction with a decay length of ~250 nm  was less than the I2 decay length (xtip-axis) at all carrier densities, indicating that the thermoelectric action is driven by the thermal diffusion rather than the direct transport of SPP into the junction.Photoresponse in doped graphene is based on thermoelectric30. For this reason, the dual-gate graphene field-effect device can launch the acoustic plasmon mode, given that the graphene-metal distance d is minimal compared to λp. The frequency dispersion extracted from the near-field modulations can verify the acoustic plasmon mode. Figure λ0 ~ 100 μm), which far exceeds the distance d = 42 nm of the device. As seen from Fig. λp/λ0 ~ 1/66) in the dual-gate structure. Previously, we have addressed that the phase velocity vp = ω/q of the acoustic plasmon mode approaches the Fermi velocity (vF = 106 m/s in graphene) when d is decreased. As demonstrated in ref. 31, vp are suppressed up to vF when the d decreases from 27 nm to 5.5 nm in the dual-gate device  deviates from the long-wavelength limit (dashed lines), explaining the non-local response at the low carrier density.The dispersion relation of 2D Dirac plasmon can be altered by surrounding electrostatic properties. In particular, an image charge of the graphene SPP can be reflected on a neighboring metallic film. As displayed in Fig. ice Fig. . The den55 reported that the electron temperature of graphene reaches 3200 K when using the ultrashort pulses (40 fs) of the near-infrared (IR) radiation (λ = 1.56 μm). In Fig. λp induced by the near-IR pump, where the mid-IR pulses (~200 fs) are resonantly coupled with the AFM-tip, leaving the near-field fringes as a function of the tip position. As displayed in Fig. s is pronounced at the temporal overlap of near-IR pump and mid-IR probe pulses, i.e. at the zero pump-probe delay. The fringe intervals of s (black dashed lines) reveal that the pump-induced λp is inversely proportional to the mid-IR frequency ω, which characterizes the 2D Dirac plasmon dispersion under the optical excitations. Notably, the derivatives of scattering signal (ds/dx) shown in Fig. s. The pump-induced signal diminishes at the probe delay of 2 ps . The large bandgap in Bi2Se3 (~0.3 eV) allows us to access the TSS response, hosting the 2D Dirac plasmon at the THz band. In Fig. 2Se3 bandstructure is projected onto the Γ point of the surface Brillouin zone, where the TSS Fermi velocity is ~6 × 105 m/s according to the angle-resolved photoemission measurement74. The molecular-beam-epitaxy (MBE) synthesizes 30 quintuple layers  of Bi2Se3 on (Bi0.5In0.5)2Se3 buffer layer, preventing the impurities arising from the lattice mismatch75. In general, pristine 3D TI films are degenerately doped because of vacancy-defect sites at the surface and crystalline defects in the bulk. With the decrease in the defects via interface engineering, a truly bulk-insulating status of 3D TI is obtained. The microribbons of Bi2Se3 provide the plasmon momentum k = π/L0, which is coupled with the THz-probe electric-field perpendicular to the ribbon direction. On the other hand, the THz-pump with an electric-field intensity ~ 0.4 MV/cm is incident parallel to the ribbon direction  from the phonon-dressed plasmon spectra (solid red line) by employing the conventional plasmon-phonon coupling model77.Having identified the thermal effect on the graphene plasmon, we now present the 2D Dirac plasmon on the topological insulator (TI) surface, where an intense terahertz (THz) radiation drives the non-equilibrium plasmonic dynamicsion Fig. . The THz2Se3 bulk bandgap, the optical response in TSS is entangled with the bulk state. The low photon energy of the THz-pump (~4 meV), on the other hand, can avoid the unwanted carrier excitation in bulk, achieving the thermal distribution of the TSS with Te above ~1400 K. As we have shown in Fig. kBTe exceeding the Fermi energy EF increases the plasmon frequency νpl due to the interband thermal excitations. Hence, the small value of Bi2Se3EF ~ 75 meV allows the interband thermal excitation when kBTe ~ 120 meV is provided. Figure L0 value decreases, similar to the result observed in graphene  decreases when Te increases, following the charge conservation law. Nonetheless, the strong interband thermal excitation increases νpl at all k values, demonstrating the intrinsic TSS plasmon under the THz excitations  can trigger photoexcitation inside bulk, increasing the free carrier density transiently. According to the time-resolved THz spectroscopy82 and angle-resolved photoemission spectroscopy83, the recombination of the photoinduced bulk carriers shows a similar timescale to the cooling dynamics of thermal energy. For this reason, THz-pump as a driving source is indispensable for measuring the intrinsic plasmon dynamics in Bi2Se3, because the low photon energy ~4 meV does not contribute to the interband photoexcitation. In contrast, when the 1.55 eV optical-pump drives the bulk photoexcitation, the time-resolved THz spectroscopy reveals a large blue-shift in the plasmon frequency84, revealing the density-dependence of the conventional 2D plasmon.As we have discussed in this section, irradiation of laser pulses can drive the ultrafast dynamics of 2D Dirac plasmons, where the transient increase of 81 showed that the TSS scattering dynamics are different from the bulk scatterings under the THz-pump excitation, where the pump-induced TSS scattering suppresses the THz conductivity. On the other hand, a resonant coupling of THz-pump and the Bi2Se3 phonon at ~2 THz displaces the lattice ions from the equilibrium85, leaving coherent oscillations in the second-harmonic-generation of probe pulses. Recently, Schmid et al.86 observed the high-harmonic (HH) generation of intense THz photon below the Bi2Te3 bulk bandgap, where the HH orders was continuously controlled by the carrier-envelop phase of the THz pulse. The non-integer multiples of the HH generation occur when the Dirac electrons traverse Dirac point ballistically, allowing to discern between the TSS dynamics and the bulk response.It is instructive to note that the nonlinear kinetics of TSS under the intense THz radiation is of general interest in the field of Dirac materials. Indeed, Luo et al.2Se3 at low frequency (κTI = 100) prevents the electrostatic doping, effectively screening the gate-induced electric field. Previously, thin layers of Bi2Se3 (~10 nm) and molecular charge transfers are used to demonstrate the ambipolar field-effect of the TSS88, otherwise the gate-voltage VG barely charges the Bi2Se3 carrier density. Nonetheless, one can electrically control the TSS plasmon by integrating a monolayer CVD-grown graphene on top of Bi2Se3 TI microribbons, where the VG can tune the graphene chemical potential μG, changing the dielectric environment of the surrounding Bi2Se3. Indeed, the dynamic screening effect of monolayer graphene modulates the SPP response in Bi2Se3  device and the field-effect, where the transparent ionic-gate controls μG. While the longitudinal resistance ρxx of Bi2Se3 remains at ~2.5 kΩ, the ambipolar field-effect on G-Bi2Se3 illuminates that the monolayer graphene is in a p-doped state before applying VG. To appreciate the SPP response in a doped G-Bi2Se3, we performed simulations using the 3D-EM simulator (High-Frequency Structure Simulator), where the impedance boundary conditions Z(ω) = 1/σ(ω) incorporate the Drude sheet conductance σ(ω) = D/(Γ − iω). Here, the Drude weight D = (e2/4πℏ2)gsgvμTSS/G and the scattering rate Γ determine the density-dependent reactance −Im[Z(ω)] .Next, we explore the electrostatic modulation of the TSS plasmon integrated with graphene. Unlike the case of graphene, the large dielectric constant of BiSe3 Fig. . Figure ω)] Fig.  with theμTSS of 250 meV and the gate-dependent μG ranging from 0 meV to 200 meV, we can reproduce the density-dependent wave extinction E(ν) = 1 − t/t0 spectrum of the G-Bi2Se3  relative to the undoped G-Bi2Se3 (VG = 1.5 V) to isolate the plasmonic resonance from the phonon resonance at ~ 2 THz, where the spectral blue-shift becomes evident at the higher carrier density. The bare plasmon extinction (gray dashed line) can also be extracted from the fit function (red line). The experimental νpl shows an excellent agreement with the simulated νpl when we assume μG = 200 meV at VG = −1 V, which corresponds to the gate capacitance ~1.2 × 1012 cm−2. Figure νpl values are almost identical at all doped status of G-Bi2Se3. One intriguing aspect of G-Bi2Se3 is that the density-dependence in νpl is different from the standard 2D Dirac plasmon μTSS is independent of VG in the device, the density-dependence in G-Bi2Se3 is written as μTSS arises from the top and bottom TSS. For this reason, the dynamic polarization of graphene on top of Bi2Se3 can efficiently control the SPP response, achieving the νpl modulation ~50% within the VG range.Using Se3 Fig. , where t VG Fig. . We plot89, gas molecules90, and polar phonons46. Figure −1 and 1660 cm−1 provide the vibrational fingerprints of the protein. The sensitive molecular detection of the graphene plasmon is ascribed to the strong confinement of near-field distribution up to a few nanometers from the surface. In contrast, the near-field distribution of gold plasmons reaches hundreds of nanometers at the same resonance frequency, resulting in a lower sensitivity to the vibrations of the molecular band.The extraordinary near-field interaction of graphene plasmon and molecular vibrations can be used to develop highly efficient molecular sensors detecting proteins51 demonstrated a mid-IR photodetector based on the graphene nanodisk stacks  of hBN enhance the mid-IR responsivity of the graphene field-effect device. Figures −1 inside the HPP band at both TM and TE pump polarizations  but intrinsically distinguished since the spin texture itself constructs the surface state in the 3D TI. The spin conversion efficiency in the TI surface is superior to the conventional Rashba state in 2DEG because the TI Fermi surface consists of a single helical spin texture. In contrast, the Fermi energy in 2DEG intersects with co-centric double Fermi contours with opposite spin helicity. Meanwhile, the light-matter interaction brings significant aspects to the Dirac surface states. The obliquely-incident photon helicity can be coupled with the in-plane spin texture of the 3D TI surface99, where the corresponding spin depletion leads to the transverse charge current. As a result, the helicity-dependent photocurrent captures the unique helical spin texture of the 3D TI surface. In a ferromagnet and 3D TI bilayer, light illumination on the ferromagnetic layer injects spin into the 3D TI surface100. The spin is efficiently converted into charge current via the inverse Edelstein effect, where the surface charge current manifests itself in the broadband THz radiation.From the viewpoint of spintronics, the 3D TI helical spin texture provides promising tools to investigate the spin-charge coupled phenomena2Se3. Integrating the graphene and Bi2Se3, we could control the Bi2Se3 plasmon frequency by changing the graphene Fermi energy, revealing the extraordinary response of near-field interactions in close proximity. The outstanding properties of 2D Dirac plasmon can be employed to develop future mid-IR photodetectors, bio-molecular sensors, THz detectors, and light sources.Versatile optoelectronic applications of 2D Dirac plasmon stem from the highly confined near-field distribution on the surface at the wide range of IR wavelength. Employing the non-local 2D Dirac polarization, we have shown that the plasmon dispersion undergoes remarkable modifications under the plasmon-phonon coupling, plasmon-plasmon coupling, and interband thermal excitations. Both far- and near-field detections of 2D Dirac plasmon have been performed at IR wavelength, allowing us to characterize the comprehensive plasmonic response for device applications. For instance, the thermoelectric current in a novel dual-gate device could detect the acoustic mode of graphene plasmon-polariton, confining the plasmon wavelength into the deep-subwavelength scale. Meanwhile, intense optical and THz pulses can trigger non-equilibrium thermal distribution at the 2D Dirac-band, changing the plasmon dispersion of graphene and 3D TI Bi


The terahertz wave modulation properties of graphene were investigated using an external 975 nm continuous wave laser with different power. Upon excitation laser, the transmission and modulation depth was measured using terahertz time-domain spectroscopy. The experimental results showed that the modulation depth of monolayer graphene and 3-layer graphene was 16% and 32% under the 1495 mW excitation power. Further, we analyzed the graphene modulation mechanism based on the Drude model and the thin-film approximation. Both theoretical analysis and calculation results showed that the terahertz wave could be modulated using graphene with different excitation laser power. Terahertz wave modulation properties of graphene are investigated using an external 975 nm continuous wave laser. The modulation depth of monolayer graphene and 3-layer graphene was 16% and 32% under 1495 mW excitation power. However, the modulation of terahertz wave technology has developed slowly and is restricting the development of terahertz technology. Thus, modifying the terahertz wave is a popular area of study.The terahertz (THz) wave is an electromagnetic wave that falls between the infrared and microwave wavelengths. Because of its strong penetration, low energy, spectral fingerprint, coherence, and large communication bandwidth,2 hybridized carbon atoms. Due to its excellent optical, electrical, and magnetic properties, graphene has crucial applications in field-effect transistors, liquid crystal displays, nanocomposites, photodetectors, light-emitting devices, microwave transistors, ultrafast lasers, photovoltaic cells, sensing and energy storage materials.17,18 Zhang et al.19 reported the first graphene light modulator; a graphene layer was placed on top of the optical waveguide in the modulator, and a driving voltage was used to control the graphene layer’s Fermi energy level. To modulate terahertz waves, Sensale-Rodriguez et al.20,21 designed a transmissive and reflective terahertz modulator. The terahertz wave was modulated by adjusting the gate voltage. The transmissive and reflective terahertz modulators had maximum modulation depths of 15% and 64%, respectively. Wu et al.22 developed a graphene-based terahertz modulator by filling an ionic gel material between two monolayers of graphene. This structure could enhance the electric field modulation effect on graphene’s Fermi energy level. The modulation depth was 83% when the voltage was 3 V. The modulation methods described above are based on the electrically-driven mode. However, the influence of the charging and discharging process severely restricts the response speed of the electrically controlled terahertz wave. The issue of constrained response speed can be resolved by modulating the terahertz wave in an optically-driven mode. Weis et al.23 reported using femtosecond laser excitation to achieve a wide band tunability of the THz transmission. They observed a maximum amplitude modulation depth of 99%. However, there was only a maximum 18% difference between the transmission through bare silicon and the sample. Compared with pulsed femtosecond lasers, continuous wave (CW) lasers are more convenient and cost-effective for implementing all-optical terahertz modulators. Wen et al.24 demonstrated an all-optical terahertz modulator based on monolayer graphene on germanium, which could be driven by a 1.55 μm CW laser. The maximum difference between the transmission through the germanium and the sample was 30%. Fu et al.25 fabricated a THz modulator consisting of monolayer graphene on a silicon substrate. The experimental results showed that the modulation depth of the transmission could reach 74% for the proposed modulator under an external 450 nm continuous-wave laser photo-excitation. And the difference in transmission between the sample and the silicon substrate can reach a maximum of 49.3%. Du et al.26 reported an active broadband THz wave modulator based on the graphene-silicon hybrid structure under photoexcitation of a 532 nm CW laser. Previous reports have discussed the optically-driven modulation properties of monolayer graphene. However, there are rare reports about optically-driven multilayer graphene.Graphene is a typical two-dimensional material that consists of spIn this paper, we analyzed the THz modulation properties of monolayer and 3-layer graphene. We used a 975 nm CW laser as the excitation source. The penetration depth of the silicon substrate is different under different exciting wavelengths. The excitation source used in this study had a larger penetration depth in the silicon substrate than in the previous reports. With 975 nm CW laser illumination, the transmission and modulation depth was measured using terahertz time-domain spectroscopy (THz-TDS) under different excitation power. We observed significant transmission attenuation and increased modulation depth when the excitation laser power was tuned. And then, we revealed the graphene modulation mechanism using the Drude model and the thin-film approximation.22.1The experimental system contained a THz-TDS and an external 975 nm CW laser as shown in The ultrafast femtosecond laser emitted a central wavelength of 780 nm pulse, of which the pulse width was less than 100 fs. Using a beam splitter (CBS), the pulse laser was divided into two paths with different energy. The laser with higher energy served as the pump beam, irradiating the GaAs photoconductive antenna and generating a terahertz beam. The generated terahertz beam was focused on a test sample through the parabolic mirror. Due to the interaction between the test sample and the terahertz beam, the terahertz wave penetrated the sample and carried the sample information. And then, the terahertz beam carrying information from the sample struck the ZnTe electro-optic crystal. On the other hand, the laser with lower energy was used as the probe beam, which met the terahertz beam on the ZnTe electro-optic crystal after the lens collimation and focus. Throughout the ZnTe electro-optic crystal, the probe beam containing the terahertz information of the sample was changed from isotropic polarization to anisotropic polarization. And then, the probe beam was divided into two perpendicular polarized beams using a Wollaston prism. A differential photodetector (PD) collected the two perpendicular polarized beams. At last, the terahertz time-domain spectrum was obtained by the PD collected signal. Moreover, a 975 nm CW laser was selected as an excitation laser.2.2The monolayer and 3-layer graphene samples were purchased from Shenzhen Six Carbon Technology Co., Ltd. We measured the spectral transmission of the graphene samples by standard THz-TDS. We used a 975 nm CW laser for photo-doping the graphene samples. The laser beam was obliquely incident on the sample (from graphene to the silicon substrate) with an angle of 45°, as shown in 33.1The time-domain spectra of silicon, monolayer graphene, and 3-layer graphene, without external photoexcitation, are shown in Taking silicon as a reference, the terahertz amplitude decreased after it penetrated the graphene. In particular, at two peaks (12 ps and 14.5 ps) and one trough (13.4 ps), the terahertz signal decreased to 86.2%, 86.5%, and 86.2% of the reference signal amplitude after passing through 3-layers graphene, respectively. This indicates that graphene absorption of the terahertz signal is subsequently enhanced with increasing layers.The THz response monolayer graphene and 3-layer graphene under an external excitation of a 975 nm CW laser irradiation is shown in 3.2Atrans(ω), Aref(ω) are the amplitude values of the sample and the reference (silicon) after Fourier transform, respectively.The frequency-domain spectra were obtained using a fast Fourier transform of the time-domain signals to quantify the transmission of terahertz waves at different frequencies. The transmission was defined as in 27,28 For silicon, the average transmission decreased from 100% without optical excitation to 96% with a pump power of 1495 mW. Upon photo-doping of graphene, the transmission of the THz wave decreased. With a pump power of 1495 mW, the average transmission of monolayer graphene decreased to 62%, and the average transmission of 3-layer graphene decreased to 38%. Compared with the previous reports, we obtained a higher maximum value difference between the transmission through the silicon and graphene using a 975 nm CW laser pump. The maximum difference between the transmission through the silicon and the monolayer graphene or 3-layer graphene, was 34% and 58%, respectively. This was because the more photo-generated carriers could be excited using a longer wavelength laser. Furthermore, the THz beam attenuation was significantly enhanced compared to silicon at equivalent power levels of the photo-doping graphene. This showed that graphene was dominant in the THz wave modulation compared with silicon.The dependence of the average transmission on the excitation laser power is shown in Aoff(ω) and Aon(ω) denote the terahertz amplitudes without and with excitation laser, respectively.Furthermore, the modulation depth was defined as follows.The modulation depth of monolayer and 3-layer graphene at 1.14 THz is depicted in 3.3When the excitation laser was incident from the graphene side, a small portion of the excitation laser was absorbed by graphene. It caused photo-generated free carriers to arise from the graphene. The nonabsorbed transmitted portion of the excitation laser penetrated the silicon substrate. Most of the energy was completely absorbed within the penetration length defined by the absorption coefficient. Because the amount of absorbed energy of silicon was larger than in graphene, the major fraction of photo-generated free carriers were generated in silicon. The illuminated area and the penetration length of the excitation laser in silicon defined the free carriers within a volume. And then, the photo-generated free carriers diffused into graphene and accumulated at the interface of the silicon substrate and graphene until a steady state was attained. These free carriers arriving in graphene experienced much higher mobility than their silicon counterparts. This resulted in a more significant change of conductivity than in silicon. The free carriers induced a change in the electric conductivity, which was inherently correlated with decreased THz transmission through the material. Thus, the THz transmission and modulation depth depended on the conductivity change.29e, ℏ, ω, vF, Γ, and Nc were the elementary charge, Planck’s constant, angular frequency, Fermi velocity, carrier scattering probability and carrier concentration, respectively. Moreover, randomly stacked multilayer graphene can be treated as multiply electronically decoupled monolayer graphene.30,31 The silicon prohibited bandwidth was 1.12 eV, while the silicon cutoff wavelength for its intrinsic leap was 1107 nm. We used a 975 nm CW laser as an excitation source to generate photo-generated carriers. The free carriers induced a change in the electric conductivity. Different laser power could excite different amounts of photo-generated charge carriers. Thus, graphene could modulate the terahertz wave with different excitation laser power.The monolayer graphene conductivity in the terahertz range complied with the Drude model, as follows.32 to describe the experimental data. The frequency-dependent equivalent conductivity of graphene could be extracted from the measured transmission spectra using the following formula,n1 is the refractive index of air, n2 is the refractive index of the graphene substrate, Z0 = 377 Ω is the vacuum impedance, and Anlay(ω), Aref(ω) are the amplitude values of the sample and the reference after Fourier transform.Because the thickness of the graphene was far smaller than the center wavelength of the THz wave, we applied the thin-film approximationThe relation between excitation power and graphene conductivity is shown in 4In this paper, we investigated the terahertz wave modulation properties of graphene with different excitation CW laser power. Under excitation from a 975 nm CW laser in 10 gradients, the monolayer and 3-layer graphene terahertz transmission and modulation depth were measured using the THz-TDS system. Different from previous reports, we use a 975 nm CW laser as an excitation source. It had a larger penetration depth in the silicon substrate than in previous reports, which excited more photo-generated carriers. Thus, a higher maximum value difference between the transmission through the silicon and graphene than in previous reports was obtained. For the graphene, the THz transmission decreased with increasing layer number or excitation power. When the excitation power was 1495 mW, the modulation depth of monolayer graphene and 3-layer graphene was above 16% and 32%, respectively. The Drude model and standard thin-film approximation were employed to analyze the graphene modulation mechanism. The variation of graphene conductivity increased with increasing laser power, which influenced the modulation depth. Therefore, the terahertz wave could be modulated using graphene with different excitation laser power. Optically tuned graphene is a potential method for realizing a terahertz wave active amplitude modulator in future applications.Conceptualization – Shaohang Chen, Binyi Qin and Jianming Liu; data curation – Binyi Qin; formal analysis – Shaohang Chen, Binyi Qin, Yizhi Liang and Jincun Zheng; funding acquisition – Binyi Qin, Ruizhao Yang, Tinghui Li and Yanni Zhou; investigation – Ruizhao Yang, Tinghui Li and Yun Li; methodology – Shaohang Chen, Binyi Qin, Yanni Zhou and Jincun Zheng; project administration – Binyi Qin; resources – Binyi Qin, Ruizhao Yang and Tinghui Li; software – Yizhi Liang and Jianming Liu; supervision – Binyi Qin and Jianming Liu; validation – Yun Li, Tinghui Li and Yanni Zhou; writing original draft – Shaohang Chen, Ruizhao Yang and Yun Li; writing review and editing – Binyi Qin, Yizhi Liang and Jianming Liu. All authors have read and agreed to the published version of the manuscript.There are no conflicts to declare.