Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J41: Optical Phenomena in 2D MaterialsLive
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Sponsoring Units: DCMP Chair: Kathleen McCreary, US Naval Research Lab |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J41.00001: Many-Body Theory of Nonlinear Response in Dirac Materials Habib Rostami, Emmanuele Cappelluti We show that the standard concepts of nonlinear response to electromagnetic fields break down in two dimensional Dirac systems, like graphene, in the quantum regime close to the Dirac point. Using a conserving Baym-Kadanoff approach, we present a fully compelling theory of nonlinear optical and transport response of a Dirac system to electric fields in the presence of disorder scattering. We show that the nonlinear terms are strikingly ruled by the appearance of a dominant two-photon vertex which is absent at the bare level and finite even in the weak-coupling limit. Such two-photon vertex self-generation [1] highlights the crucial role of the frequency and field dependence of the scattering rates in the nonlinear regime. Moreover, we surprisingly predict four- and five-photon incoherent features in the third order optical response of the system. Our study reveals a novel many-body mechanism in the nonlinear response of Dirac materials whose effects are predicted to be observable in nonlinear dc transport and terahertz third-harmonic generation of graphene. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J41.00002: Layer-dependent photoresponse of atomically thin WSe2/WS2 heterojunctions Zhuangen Fu, Jifa Tian, Caleb Matthew Hill, Joshua Ward Hill Two-dimensional (2D) transition metal dichalcogenides (TMDs) have created new opportunities for novel electronic and optoelectronic devices due to their distinctive electrical and optical properties. Van der Waals (VdW) heterostructures based on the 2D TMD atomic layers provide a unique way to fabricate p-n junctions at the atomic scale, exhibiting completely different charge transport behaviors than bulk heterojunctions. Despite the recent progress of photoresponse in p-n junctions made of atomically thin TMDs, a systematical study of the layer-dependence of the photovoltaic effect and photodetections in these vdW heterostructures is lacking. Here, we report the fabrication of high-quality WSe2/WS2p-n junctions and a systematical study of their layer-dependent photoluminescence and photovoltaic effect using Scanning Electrochemical Cell Microscopy (SECCM). A strong enhancement of the photocurrent has been realized as the thickness of the compositing layer increases. We further applied the first-principle calculation to understand our observations. Our work may pave a way for making nanoscale optoelectronic devices using the TMDs heterostructures. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J41.00003: Evolution of Reststrahlen-like reflection in the optically thin limit Eric Yue Ma, Lutz Waldecker, Jenny Hu, Tony Heinz Reststrahlen bands, extended spectral bands with near-unity reflectance from the surface of a bulk material, are known to occur in ionic crystals near their optical phonon frequencies in the mid-infrared. They are manifestations of strong light-matter interaction near sharp and isolated resonances in solids. Although Reststrahlen bands in bulk materials have been extensively studied, their behavior in the optically thin limit has not been examined systematically. Here we present a study of Reststrahlen-like reflection bands when the material transitions from optically thick to optically thin. We take advantage of the unique layered structure of 2D van der Waals materials, which allows preparation of films with precise thicknesses across several orders of magnitudes, with essentially no change to the dielectric function, down to few atomic layers. We describe, experimentally and theoretically, the evolution of the reflectance spectra and their relation to radiative and nonradiative rates, considering hexagonal boron nitride as a test case. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J41.00004: Room temperature superfluorescence from a single nanocuboid John Philbin, Joseph Kelly, Lintao Peng, Igor Coropceanu, Dmitri Talapin, Eran Rabani, Xuedan Ma, Prineha Narang Single-photon superradiance arises when a collection of identical emitters are spatially separated by distances much less than the wavelength of the light they emit and results in the formation of a superradiant state that spontaneously emits light with a rate that scales linearly with the number of emitters. This collective phenomena has only been demonstrated in a few nanomaterial systems, none of which have used quasi-2D nanoplatelets as the emitter. By combining molecular dynamics, atomistic electronic structure calculations, and model Hamiltonians methods, we show that quasi-2D nanoplatelets oriented along each face of a “nanocuboid” can serve as the (nearly) identical emitters required to observe both superradiant and subradiant phenomena. And we demonstrate single-photon superfluorescence via single-particle time-resolved photoluminescence measurements at room temperature. These findings open the door to ultrafast single-photon emitters and may provide an avenue to entangled multi-photon states via superradiant cascades. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J41.00005: Temperature- and layer- dependent spin-orbit coupling in atom-thick transition metal dichalcogenides Akm Newaz, Garrett Benson, Viviane Z. Costa, Shirin Jamali, Kentaro Yumigeta, Mark Blei Blei, Sefaattin Tongay, Bin Wang, Santosh KC, Andrew Ichimura Atomically thin semiconducting transition metal dichalcogenides (TMDs) are emerging as a new platform for exploring two-dimensional exciton physics. These excitons play a crucial role in determining the light-matter interactions of a semiconducting material. Understanding the origin of valence band splitting is important because it governs the unique spin and valley physics in TMDs. We have explored the effects of temperature and number of layers on spin−orbit coupling (SOC) in MoS2 and WS2 (monolayers to 6 Layers and bulk~100 layers) by determining the difference between A and B excitonic peak energy (Δ=EB-EA) in photocurrent spectra at different temperatures from 77 K to 300 K. We have found few important characteristics of the SOC coupling. First, MoS2 and WS2 demonstrate very different temperature-dependent SOC behavior. Second, SOC strength is the lowest for monolayer. Third, coupling increases as we increase the layer thickness. Third, the rate of change of the coupling strength with respect to temperature (m=δΔ/δT) depends on the layer thickness for MoS2. Our study sheds light on the distinctive behaviors about SOC coupling in layered MoS2 and WS2. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J41.00006: Theory of the absorption lineshape in monolayers of transition metal dichalcogenides Frank Lengers, Tilmann Kuhn, Doris Reiter Monolayers of transition metal dichalcogenides are attractive materials for optoelectronics due to their strong exciton-light interaction. At the same time exciton-phonon interaction is exceptionally strong in these materials which is visible in optical spectra. In this contribution we compare different theoretical methods for the description of linear absorption spectra in 2D semiconductors [1]. To be specific, we consider the spectra of MoSe2 using either a correlation expansion in 2nd or 4th Born Approximation (2BA or 4BA) or a time convolutionless master equation (TCL). Additionally we introduce a damped version of the 4th Born Approximation (4BA-D) partially accounting for contributions of even higher-order correlations. We find that the 2nd Born Approximation gives poor results for elevated temperatures due to the strong exciton-phonon interaction such that higher-order methods are necessary (4BA and 4BA-D). Surprisingly, the TCL provides very good results despite its simplicity when compared to higher-order correlation expansion. We show that the TCL is very accurate and allows for an easy interpretation of the underlying non-Markovian effects. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J41.00007: Structure and Optical Anisotropy of Two-Dimensional Tetracene Crystals Seonghyun Koo, Sunmin Ryu Quantum size effects have long been observed in various semiconducting and metallic materials except for organic molecular solids. Because of weak intermolecular binding, well-defined and stable low-dimensional molecular crystals have been limited to large molecular building blocks or in-vacuo cryogenic environments that are technically incompatible with sophisticated laser spectromicroscopy methods. Here, we report the formation, structure and optical anisotropy of two-dimensional (2D) tetracene (Tc) crystals. Single and few-layer Tc crystals are sandwiched between two inorganic 2D crystals of graphene or hexagonal BN for ambient and photo-stability. The excitonic absorption and emission of 2D Tc consist of attenuated vibronic side bands and enlarged Davydov splitting because of reduced dielectric screening. The crystallographic domains and orientations mapped with polarized wide-field imaging and second-harmonic generation show distinctive registry between the 2D organic and inorganic crystals. The dimensional effects observed in 2D Tc will be universal for other 2D molecular crystals and useful in modifying their properties. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J41.00008: Quantum Geometric Exciton Drift Velocity Jinlyu Cao, Herb Fertig, Luis Brey Excitons are bound particle-hole excitations of an insulating state which can carry dipole moment, allowing for coupling to an external electric field. We present a new quantum geometric quantity relevant to excitons that can uniquely determines which we call the dipole curvature. In addition to the anomalous drift velocity given by the Berry’s curvature, this new quantity arises naturally in the semiclassical equation of motion of the exciton and leads to a drift velocity in an electric field. This drift is known to occur in strong magnetic fields, with drift velocity E/B, and we show that its origin can be understood in terms of the dipole curvature to be a quantum geometric effect. When the band environments of the electron and hole are included, we show that the effective magnetic field associated with the drift velocity may be considerably different than the bare magnetic field, and in many circumstances will be present even if no real field is applied. We present estimates of the exciton dipole curvature for some van der Waals heterostructure systems. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J41.00009: Geometric photon-drag effect and nonlinear shift current in centrosymmetric crystals Li-kun Shi, Dong Zhang, Kai Chang, Justin Song The nonlinear shift current, also known as the bulk photovoltaic current generated by linearly polarized light, has long been known to be absent in crystals with inversion symmetry. Here we argue that a non-zero shift current in centrosymmetric crystals can be activated by a photon-drag effect. Photon-drag shift current proceeds from a “shift current dipole” (a geometric quantity characterizing interband transitions) and manifests a purely transverse response in centrosymmetric crystals. This transverse nature proceeds directly from the shift-vector’s pseudovector nature under mirror operation and underscores its intrinsic geometric origin. Photon-drag shift current can be greatly enhanced by coupling to polaritons and provides a new and sensitive tool to interrogate the subtle interband coherences of materials with inversion symmetry previously thought to be inaccessible via photocurrent probes. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J41.00010: Evidence for Incommensurate Composite Behaviour in Bi-2212 Crystals by Brillouin Light Scattering Spectroscopy Brad McNiven, James P. F. LeBlanc, G. Todd Andrews Room-temperature phonon dynamics in crystals of the high-Tc superconductor Bi-2212 were probed using Brillouin light scattering spectroscopy along the incommensurate c-axis. The spectra contain peaks due to the Rayleigh surface acoustic mode, two quasi-transverse bulk acoustic modes, and two quasi-longitudinal bulk acoustic modes. The presence of two distinct quasi-longitudinal modes, instead of the usual one, is interpreted as evidence of the incommensurate nature of Bi-2212, and is consistent with the so-called composite model for incommensurate materials. Furthermore, a weak, higher-energy peak was observed in Brillouin spectra at a frequency shift of ~95 GHz and may be another manifestation of the incommensurate character of Bi-2212. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J41.00011: Superradiant phase transition in two-dimensional materials Daniele Guerci, Pascal Simon, Christophe Mora Light-matter interaction is at the core of many fascinating and recent developments at the crossroad of quantum optics and condensed matter physics. In this talk we will present for the first time a general theory for the superradiant quantum phase transition in materials with arbitrary electron-electron interaction. We find that the no-go theorem presented in [1] is circumvented by taking into account the finite wave vector exchanged between light and matter [2]. The superradiant ground state is characterized by the spontaneous emergence of a magnetic flux state, which gives rise to a spatially modulated orbital ferromagnet [2]. Interestingly, we discover that the spontaneous breaking of time-reversal is responsible for novel topilogical properties. Our results open new perspectives on the interpretation of the superradiant phase transition and envision twisted bilayer graphene as a natural material hosting this hybrid phase of light and matter. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J41.00012: Multistable excitonic Stark effect in a nanophonotonic cavity Ying Xiong, Mark Rudner, Justin Song The excitonic Stark effect, wherein off-resonant irradiation continuously tunes the exciton transition to higher frequencies as light intensity increases, arises from coherent light-matter interaction in excitonic systems. Here we argue that coupling to nanophotonic cavities can enable the excitonic Stark effect to become highly nonlinear, displaying multi-valued and hysteretic Stark shifts that depend on the history of the irradiating light. This multistable Stark effect (MSE) emerges from a (Stark-effect induced) feedback between cavity mode occupation and exciton population. In readily available transition metal dichalcogenides coupled to photonic cavities, we find that the MSE can exhibit discontinuous Stark shift jumps of order meV even for very dilute exciton concentrations at modest pump intensities. Strikingly, the MSE can persist even in the single exciton limit and provides new means to engineer coupled states of light and matter. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J41.00013: Experimental determination of the complex third-order nonlinear optical susceptibility in graphene Daiki Inukai, Takeshi Koyama, Kenji Kawahara, Hiroki Ago, Hideo Kishida Research of the electronic process governing optical third-harmonic generation (THG) in single-layer graphene is essential for understanding high-harmonic generation or other nonlinear optical responses of graphene-related materials. In this research, we investigated the electronic transition dominating THG in single-layer graphene from a spectral measurement of THG. To determine absolute values and phases of third-order nonlinear susceptibility χ(3) quantitatively, we adopted the Maker fringe method. As a result of the experiment, we obtained the χ(3) spectrum of graphene in the range from 0.55 eV to 1.15 eV in fundamental photon energy. Here, as we analyzed the fringe patterns, we determined the phases of χ(3). The phase spectrum of χ(3) reveals the resonance processes and the electronic processes governing THG in graphene. On the basis of the experimental results, we discuss the electronic processes of the nonlinear optical response related to a Dirac-cone type electronic state. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J41.00014: Relaxation dynamics of photoexcited carriers in hBN-encapsulated graphene Matthew Yeung, Tianyi Han, Takashi Taniguchi, Kenji Watanabe, Long Ju
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