" It is the mark of an educated mind to be able to entertain a thought without accepting it. "
- Aristotle
Jianbo Gao - Wednesday, March 1, 2017
When ultrafast meets ultrasmall: carrier dynamics in nanostructured materials
ABSTRACT: Understanding ultrafast carrier photophysics including photogeneration, recombination, transport, and energy transfer is the foundation of nanostructured material electronic and optoelectronic applications. Nanocrystals constitutes a major class of nanostructured material. They have unique physics property due to strong quantum confinement effect that leads to multiple exciton generation (MEG) effect where more than two pairs of exciton generated by absorbing one photon, and strong multiple exciton interactions that lead to Auger recombination. While the majority research groups rely on all optical spectroscopies to understand novel photophysics, I use a unique ultrafast photocurrent spectroscopy (sub-40 ps) by directly collecting photocurrent in situ devices. In this talk, I will demonstrate this unique ultrafast photocurrent spectroscopy (which can be developed to sub- 1ps) to bridge the gap between fundamental photophysics and applied devices research. In addition to nanocrystals, I will demonstrate carrier transport dynamics study in 2D materials of black phosphorus
Andreas Stier - Wednesday, March 8, 2017
Magneto-optical spectroscopy of atomically-thin semiconductors using pulsed magnetic fields to 65 Tesla
Los Alamos National Laboratory
ABSTRACT: In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality and so on). Due to heavy band masses and large exciton binding energies in the newly discovered class of 2D semiconductors, such as monolayer WSe2 or MoS2, low-magnetic field studies have to date not revealed the majority of these properties.
In this talk, I will describe our results on low-temperature, circularly polarized magneto-optical spectroscopy on atomically-thin semiconductors in pulsed magnetic fields to 65 Tesla [1, 2].
After a brief introduction of the field of 2D semiconductors, I will present our results on the valley Zeeman splitting of both the A and the B excitons in WS2. We find effective valley g-factors = -4.0 for both excitons. This unexpected and surprising result suggests that the valley Zeeman effect in these 2D semiconductors originates primarily from the atomic orbital magnetic moment alone – that is, the much-discussed Berry curvature in TMDs, appears to have minimal influence [1].
More importantly, the use of large magnetic fields allowed the first observation of the small quadratic diamagnetic shift of excitons in these materials. Diamagnetic shifts provide a direct experimental measure of the exciton size, and I will discuss how we can use this parameter to estimate the large exciton binding energies [1].
Lastly, I will discuss how we can tune the exciton size, and therefore the binding energy, in 2D materials by tuning the dielectric screening of the environment [2].
This work highlights how the dielectric screening of the environment influences 2D excitons and therefore aids in the smart design of novel optoelectronic devices that exploit the unique physics of van der Waals heterostructures.
[1] A. V. Stier et al., Nat. Comm. 7, 10643 (2016).
[2] A. V. Stier et al., Nano Lett. 16, 7054 (2016)
Euclides Almeida - Monday, March 13, 2017
Nanostructured Materials as the New Playing Field for Nonlinear Optics
ABSTRACT: In nonlinear optics, we study the material response to excitation by intense optical fields. Phenomena such as harmonic generation, ultrafast laser spectroscopy and super-resolution microscopy, have all emerged from fundamental understanding of the nonlinear material response.
In this talk, I will discuss how nanostructures provide a fertile ground for nonlinear optical interactions, and how "squeezing" light to the nanometric regime gives rise to unforeseen nonlinear phenomena and photonic devices with unmatched functionalities. By using modern nanofabrication tools such as focused ion beam and additive electron beam nanolithography, we can rationally design nanostructures to selectively enhance the nonlinear response and efficiently generate coherent optical beams at new optical frequencies. The nanoscale control over the phase of the optical waves leads to new implementation of the basic physical laws governing electromagnetic wave interactions, leading the way towards the realization of integrated photonics devices that can simultaneously generate and shape light beams. Using these ideas, we have recently demonstrated holograms that produce a "floating" visible image when illuminated by an invisible beam, and multi layered plasmonic devices with unique optical functionalities. Prospects for the incorporation of precise nonlinear phase control with state-of-the-art nanofabrication for fundamental studies and for practical device applications will be discussed.
[1] E. Almeida and Y. Prior. Scientific Reports 5, 10033 (2015)
[2] E. Almeida, G. Shalem and Y. Prior. Nature Communications 7, 10367 (2016)
[3] E. Almeida, O. Bitton and Y. Prior. Nature Communications 7, 12533 (2016)
[4] O. Avayu*, E. Almeida*, Y. Prior and Tal Ellenbogen. Nature Communications, in press (2017)
Akashdeep Kamra - Monday, March 20, 2017
Non-integer-spin bosonic excitations in untextured magnets
ABSTRACT: Recent experimental advances in generation and detection of pure spin currents have opened up new avenues for exploiting magnets for technology as well as for exciting fundamental physics. Exotic quasiparticles have been observed in complex spin systems exhibiting spin ice rules, skyrmions etc. In this talk, I will discuss emergence of novel quasiparticles, mediated by magetic dipolar interactions, that have been hiding in simpler spin systems with uniformly ordered ground states.
Amongst other properties, these quasiparticles exhibit spin ranging from zero to above 1. These exotic excitations can be interpreted as quantum coherent conglomerates of magnons, the eigen-excitations when the dipolar interactions are disregarded. Of particular interest is our finding that the eigenmodes in an easy-axis antiferromagnet are spin-zero quasiparticles instead of the widely believed spin 1 magnons. The latter re-emerge when the symmetry is broken by a sufficiently large applied magnetic field. The spin greater than 1 is accompanied by vacuum fluctuations and may be considered a weak, non-geometrical form of frustration.
Michael Shlesinger - Monday, March 27, 2017
The Glass Transition: Stretched Times and Diverging Time Scales
David Pile - Wednesday, March 29, 2017
How to publish at Nature Photonics
Senior Editor, Nature Photonics
Download PDF 12:00 pm 12:15 pm
ABSTRACT: Nature journals in the physical sciences will be discussed with particular emphasis on Nature Photonics. Historic data of submissions and acceptances will be shown, showing some strong geographic trends. The importance of cover letters, what makes a useful referee report, how to appeal (and how not to appeal), and other types of opportunities for getting published in Nature Photonics (Reviews, Correspondence, Commentary, etc.) will be covered. Bring all of your tough questions.
Diederik Wiersma - Thursday, March 30, 2017
Photonics Walking Up a Human Hair
National Institute for Metrology Research (INRIM) European Laboratory for Non-linear Spectroscopy (LENS) and Univ. of Florence, Italy
ABSTRACT: I will report on a new research line in which nano photonics is combined with deformable micro structures. In particular, I will discuss how liquid crystal elastomers can be used to make deformable photonic components, where light induces structural deformations that feed back on the optical properties of the structure. Also I will show how such concepts can be used to make functional microscopic structures that can perform simple tasks and move around (walk/swim), using the environmental light as energy source. Nature is a source of inspiration for many of the designs used in this context.
Mirco Milletari - Monday, April 3, 2017
Quantum theory of the extrinsic spin-Hall effect in graphene
ABSTRACT: Spintronics, the science that aims at utilising the spin degrees of freedom in addition to the charge of electrons for low-power operation and novel device functionalities, has seen rapid developments in the past decade. In particular, the Spin Hall (SH) effect, i.e. the emergence of a transverse spin current in response to an applied longitudinal electric field, has attracted much interest for the possibility of building all-electric Spin manipulation devices [1]. The effciency of the Spin current generation is measured by the SH angle. While in semiconductors the SH angle is quite small (0.0001-0.001) [2], it was shown that a giant SH conductivity can be achieved in Graphene decorated with small doses of resonant, Spin orbit active adatoms [3]. It was argued that in this case, the e_ect is mostly due to the semiclassical Skew scattering mechanism, where electrons of di_erent spins are scattered asymmetrically.
In this talk, I will present present a rigorous microscopic theory of the extrinsic spin Hall effect in disordered graphene based on a nonperturbative quantum diagrammatic treatment incorporating skew scattering and anomalous {impurity concentration-independent{ quantum corrections on equal footing [4, 5]. Our self-consistent approach {where all topologically equivalent noncrossing diagrams are resummed - unveils that the skewness generated by spin-orbit-active impurities deeply inuences the anomalous component of the SH conductivity, even in the weak scattering regime. This seemingly counterintuitive result is due to the symmetry structure induced by spin-orbit coupling, for which the commonly Gaussian white noise approximation is generally invalid. Our treatment shows that it is possible to experimentally access regions in parameter space where anomalous quantum contributions to the SH conductivity are dominant.
Finally, we assess the role of quantum interference corrections by evaluating an important subclass of crossing diagrams, considered only recently in the context of the anomalous Hall effect [6]. We show that diagrams encoding quantum coherent skew scattering events, display a strong Fermi energy dependence, dominating the anomalous spin Hall component away from the Dirac point. Our findings open up the intriguing prospect of measuring quantum interference fingerprints in nonlocal spin signals.
[1] J. Sinova, S. O. Valenzuela, J. Wunderlich, C. H. Back, T. Jungwirth, Rev. Mod. Phys. 87 (2015).
[2] Y. K. Kato, R.C. Myers, A.C. Gossard and D. D. Awschalom, Science 306, 1910 (2004).
[3] A. Ferreira, T. G. Rappoport, M. A. Cazalilla, and A. H. Castro Neto, Phys. Rev. Lett. 112, 066601 (2014).
[4] M. Milletar__ and A. Ferreira, Phys. Rev. B 94, 201402(R) (2016)
[5] M. Milletar__ and A. Ferreira, Phys. Rev. B 94, 134202 (2016)
[6] A. Ado, I.A. Dmitriev, P. M. Ostrovsky and M. Titov, EPL 111, 37004 (2015).
Diana Bratu - Monday, April 24, 2017
Visualizing mRNA transport during Drosophila melanogaster oogenesis
ABSTRACT: Research efforts to better understand the regulation of mRNA are afoot worldwide, with one of the key challenges being the visualization of mRNA and how it interfaces with these proteins to influence the expression of important genes. We are contributing to this field through a gamut of unique biophotonic methods, from probe design to advanced imaging approaches, thus improving upon the detection and accuracy of mRNA visualization and its co-localization with trans-acting proteins important for the normal function of processes within a cell.
Using Drosophila melanogaster (the fruit fly) as a model organism, my research group employs fluorescent probes and spinning disc confocal microscopy to track the movement of mRNA and proteins throughout the egg chamber. We employ genetically encoded fluorescent proteins and short molecular probes (i.e. molecular beacons) allowing for the detection of various endogenous proteins and mRNAs, thus enabling real-time tracking of these molecules as they are transported within the cell. While this has obvious implications for the research of oogenesis, these studies act as a biologic proof of principle to guide other researchers¹ studies examining mRNA transport in other systems.
Miquel Rude* - Wednesday, April 26, 2017
Tunable Micro- and nano- structured optical devices using phase change materials
ICFO - The Institute for Photonic Sciences
ABSTRACT: Phase-change materials (PCMs) are a group of chemical compounds that exhibit two stable phases with large contrast in their optical and electrical properties. Moreover, reversible transitions between the two phases can be easily triggered using optical or electrical pulses. These properties make PCMs interesting to implement new applications in photonics. In the first part of the talk we will show an optical switch in a Si ring resonator covered with GST as well as control of surface-plasmon propagation in Au/SiO2 plasmonic waveguides. In the second part we will explain how to combine GST with thin-film multilayer structures, showing how it can be used to tune EOT resonances in periodic arrays of nanoholes drilled in metallic films, achieving large shifts (385 nm) in the resonance wavelength after crystallization. Finally using thin-film interference effects and exploiting the high absorption of GST we demonstrate broadband perfect absorbers in the visible and narrowband absorbers in the NIR.
*Miquel Rude is a graduate student at ICFO
Felix Izrailev - Monday, May 1, 2017
The temperature of a single chaotic eigenstate
BUAP, Mexico and Michigan State University
ABSTRACT: The onset of thermalization in a closed system of randomly interacting bosons, at the level of a single eigenstate, is discussed. We focus on the emergence of Bose-Einstein distribution of single-particle occupation numbers and give a local criterion for thermalization. We show how to define the temperature of an eigenstate, provided that it has a chaotic structure in the basis defined by single-particle states. The analytical expression for the eigenstate temperature as a function of the inter-particle interaction and energy is complemented by numerical data. The relation of thermalization to the many-body localization transition is discussed.
David Petiteau - Wednesday, September 6, 2017
Probing the dynamics of microwave pulses in disordered 1D waveguides
Viviana Acquaviva - Monday, September 25, 2017
Understanding the Universe through distant galaxies
ABSTRACT: Understanding the physical properties of galaxies and their evolution through cosmic time means learning more about the Hubble expansion, gravity, and the physical mechanisms that regulate the growth of structures. My work focuses on developing and using better tools to extract maximal information from ongoing and future data from large galaxy surveys, such as CANDELS and LSST. I will present my efforts at improving our ability to determine galaxy properties through Spectral Energy Distribution (SED) fitting. I will introduce GalMC and SpeedyMC, the Markov Chain Monte Carlo algorithms for SED fitting I created, and show how they can be used to recover the age, mass, dust content, metallicity and star formation history of galaxies, as well as to jointly determine photometric redshifts and SED fitting parameters. If time allows it, I will describe the science goals of the Hobby Eberly Telescope Dark Energy eXperiment (HETDEX), which is set to discover several hundred thousand Lyman Alpha Emitting galaxies at 2 < z < 3.5 and use them to shed light on the behavior of dark energy and gravity in this largely unexplored redshift range, and summarize our recent efforts in optimizing the sample selection using Bayesian statistics and machine learning techniques.
Sarang Gopalakrishnan - Monday, October 2, 2017
Phase transitions in quasiperiodic systems
ABSTRACT: Electronic wave functions in quasiperiodic systems are intermediate between those in crystalline and random systems. Quasiperiodic systems exhibit Anderson localization, but the properties of the localized state and the localization transition are different from those in random systems. We explore various distinctive aspects of localization in quasiperiodic systems, including a multicritical point in quasiperiodic models with power-law hopping and a semimetal-to-metal phase transition in quasiperiodic Dirac materials.
Vipin Kerala Varma - Monday, October 16, 2017
Fractality and transport in boundary-driven (quasi) disordered chains
College of Staten Island, Graduate Center
ABSTRACT: In this talk we report on the response of (quasi)disordered spin-chains to boundary driving through reservoirs at its ends. In the nonequilibrium current-carrying states, anomalous transport rates of spins are shown to be harbored in noninteracting quasidisordered systems at criticality, and far from criticality in the interacting system; in addition, these steady states exhibit spatial fractality in many of its expectation values, opening an alternative route to experimentally probe a system's fractal properties in contrast to measuring quantum wavefunctions.
Alexander Granovsky - Monday, October 23, 2017
Modern problems of magnetism: from multifunctional magnetic materials to spintronics
Moscow State University
Stephen Arnold - Monday, October 30, 2017
Opto-mechanics: fabrication of nano and micro-optic sensors
Polytechnic Institute of New York University
Daniel Kabat - Wednesday, November 8, 2017
Gravity gauge duality in everyday life
Lehman College
Michael Mirkin - Monday, November 13, 2017
Electrochemical tools for probing structures and processes on the nanoscale
Andrew Kent - Monday, November 20, 2017
Physics and applications enabled by the discovery of spin-transfer torques
Roman Kezerashvili - Monday, November 27, 2017
Superfluidity in Two-Dimensional Nanomaterials
City Tech
ABSTRACT: A study the formation of a spatially indirect exciton as a pair of an electron and a hole in two layers of gapped graphene, or transition metal dichalcogenide (TMDC), or phosphorene separated by a dielectric is presented. A solution of two-body problem in these systems is discussed. We propose to observe the superfluidity and Bose-Einstein condensation for a quasi-two-dimensional gas of indirect excitons in these quantum heterostructures. The superfluidity of 2D spatially indirect excitons at low densities in TMDC double layers form a two-component weakly interacting gas of A and B excitons. We demonstrate that the mean field critical temperature for a two-component dilute weakly interacting Bose gas of excitons in a TMDC double layer is an increasing function of the factor Q, determined by the effective reduced mass of A and B excitons. We predict that a weakly interacting gas of dipolar excitons in a double layer of phosphorene exhibits superfluidity and show that the critical velocity of superfluidity, the spectrum of collective excitations and mean field critical temperature for superfluidity are anisotropic and depend on the direction of motion of dipolar excitons.