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 B57: 2D Semiconductors: Photonics and OptoelectronicsFocus Live
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Sponsoring Units: DMP Chair: Nicholas Borys, Lawrence Berkeley National Laboratory |
Monday, March 15, 2021 11:30AM - 12:06PM Live |
B57.00001: Optical and plasmonic spectroscopy of 2D semiconductor heterostructures Invited Speaker: John Schaibley Over the past decade, there has been significant interest in the optical and electronic physics of semiconducting transition metal dichalcogenide (TMD) monolayers and heterostructures. TMD monolayers are known to host strongly bound excitons at the K and –K valleys on the edge of the Brillouin zone, which exhibit a strong nonlinear optical response. In this presentation, I will discuss how TMD monolayers can be integrated in plasmonic waveguide structures to realize a coupled exciton-surface plasmon polariton that exhibits an enhanced nonlinear response, including coherent population oscillations of excitons. Furthermore, two different TMD monolayers, such as MoSe2 and WSe2, can be stacked together to realize a type-II heterojunction that hosts interlayer excitons. The interlayer exciton transitions are optically bright if the heterostructure twist angle is near 0° or 60°; however, these two types of heterostructures (i.e., near 0° vs. near 60°) exhibit different interlayer K valley alignments. I will discuss the impact of the band alignments on the moiré physics of the heterostructure and the interlayer exciton dynamics. These effects are probed with low temperature photoluminescence and nonlinear optical spectroscopy. |
Monday, March 15, 2021 12:06PM - 12:18PM Live |
B57.00002: Propagation of dark excitons in monolayer semiconductors at cryogenic temperatures Koloman Wagner, Edith Wietek, Jonas Ziegler, Takashi Taniguchi, Kenji Watanabe, Jonas Zipfel, Alexey Chernikov Two-dimensional transition metal dichalcogenides offer an excellent platform to study the propagation of tightly-bound exciton quasiparticles. While the exciton diffusion has been largely explored at ambient conditions, at cryogenic temperatures the rapid decay of bright excitons portrays a major limitation. We therefore take advantage of the long-lived reservoirs of dark states that emit via phonon-assisted processes and emerge as characteristic sidebands in the low-temperature luminescence spectra. This allows us to monitor the dynamics and transport properties of these dark states at cryogenic conditions in hBN-encapsulated WSe2 monolayer using time- and spatially-resolved optical measurements. Already at 5 K we find highly efficient diffusion of dark excitons that is intrinsically limited by scattering with the bath of acoustic phonons. No influence of localization from residual disorder is detected. Interestingly, with increasing temperature we find a pronounced decrease of the diffusion coefficient. This deviates from the semi-classical expectation from independently determined scattering rates and challenges the established description of the exciton transport in monolayer materials. |
Monday, March 15, 2021 12:18PM - 12:30PM Live |
B57.00003: Efficient GW calculations in two dimensional through the interpolation of the screened potential Alberto Guandalini, Andrea Ferretti, Pino D'Amico, Daniele Varsano The GW self-energy approximation is able to accurately predict quasiparticle (QP) properties of several classes of materials. However, the calculation of the QP band structure of 2D semiconductors is challenging due to the sharp q-dependence of the dielectric function in the long-wavelength limit (q→0). In this case, a very dense q-sampling of the Brillouin zone is usually needed to obtain properly converged quantities. In this work, we assess the possibility to drastically improve the convergence of the QP corrections of 2D semiconductors with respect to the q-sampling, by combining Monte Carlo integration techniques and interpolation schemes of the screened potential. We test our method by computing the bandgap for three prototypical materials: a wide bandgap insulator (hBN), a transition metal dichalcogenide (MoS2), and an anisotropic semiconductor (phosphorene). A speed-up of at least two orders of magnitude is found for all the systems considered. |
Monday, March 15, 2021 12:30PM - 12:42PM Live |
B57.00004: Polaron-polariton interactions mediated by an electron gas Miguel Bastarrachea-Magnani, Arturo Camacho-Guardian, Georg M Bruun We study quasiparticle interactions between exciton-polaritons mediated by an electron gas in a two-dimensional semiconductor inside a microcavity. To this end, we employ a microscopic many-body theory combined with Landau’s quasiparticle framework. We show the presence of attractive interactions between polaritons that stem from the exchange of particle-hole excitations mediated by the electron gas. Besides, the excess of electrons can create bound-states, the so-called trions, that interact with the polaritons by the exchange of an electron, and can be either repulsive or attractive depending on the specific polariton branch. These mediated interactions are intrinsic to the quasiparticles, present in the absence of light, and can be tuned by either the light-matter detuning or the electron density. The combination of attractive and repulsing effects could pave the way to controllable non-lineal polaritonic components. |
Monday, March 15, 2021 12:42PM - 12:54PM Live |
B57.00005: Strained bilayer WSe2 with reduced exciton-phonon coupling Ozgur Burak Aslan, Minda Deng, Mark Brongersma, Tony Heinz We investigate excitonic absorption and emission in bilayer (2L) WSe2 under tensile strain. We observe a redshift of 110 meV in the energy of the A exciton absorption peak (direct gap) under 2.1% strain. The photoluminescence (PL) spectrum exhibits multiple peaks under 0% strain, corresponding to the indirect and direct gaps. Surprisingly, the spectral linewidth of the A exciton decreases by almost a factor of two under strain, from 70 to 36 meV at room temperature. We explain this effect as the result of the suppression of phonon-mediated exciton scattering channels. That suppression is associated with the relative shift under strain of the Q valley in the conduction band (involved in the indirect PL), which is nearly degenerate with the K valley (involved in A exciton). We analyze the strain-dependent absorption and PL spectra to determine the relative positions of those valleys and to calculate the intervalley scattering rates. Our model describes well the strain-induced linewidth decrease of both monolayer (1L) and 2L WSe2 and helps us understand what contributes to the linewidths. The results show that strain tuning can be employed to probe the band structures and the excitonic properties of 1L and 2L TMDCs. |
Monday, March 15, 2021 12:54PM - 1:06PM Live |
B57.00006: Valley polarization and the exciton-trion equilibrium in an atomically thin semiconductor. Joris Carmiggelt, Michael Borst, Toeno van der Sar Monolayer WS2 is a transition metal dichalcogenide (TMD) with two direct-bandgap valleys in its band structure that can be selectively addressed using circularly polarized light1. Its photoluminescence is characterized by excitons and trions that form a chemical equilibrium governed by the net charge density2. Chemical doping is a convenient method to achieve high charge densities in TMDs3, which we employ to drive the conversion of excitons into trions. We study the resulting valley polarization under valley-selective optical excitation at room temperature. After doping, the WS2 emission is dominated by trions with a strong valley polarization associated with rapid non-radiative recombination. Simultaneously, the doping results in strongly quenched but highly valley-polarized exciton emission due to the enhanced conversion into trions. We use a rate equation model to explain our observations and shed light on the important role of exciton-trion conversion on valley polarization4. |
Monday, March 15, 2021 1:06PM - 1:18PM Live |
B57.00007: Quantum plasmonic doping in bilayer 2D heterostructures Sharad Ambardar, Zachary Withers, Dmitri Voronine Two-dimensional (2D) semiconductors exhibit interesting opto-electronic properties due to their direct bandgap and strong light-matter interactions. Various approaches have been developed to enhance and tune photoluminescence (PL) of 2D transition metal dichalcogenides (TMDs) such as electrical and chemical doping. Here we demonstrate quantum plasmonic doping originating from hot electron tunneling between a gold-coated plasmonic tip and bilayer MoS2-WS2 heterostructure. We used optimized near-field spectroscopy in the quantum plasmonic regime to obtain both tip-enhanced Raman scattering (TERS) and tip-enhanced photoluminescence (TEPL) images of bilayer MoS2-WS2 heterostructures on Si/SiO2 substrate without the plasmonic gap mode. Simultaneously, contact potential difference (CPD) and capacitance mapping confirms the accumulation of charges. Quantum plasmonic doping is a new technique for tuning opto-electronic properties of 2D heterostructures with promising future applications. |
Monday, March 15, 2021 1:18PM - 1:30PM Live |
B57.00008: Unveiling the optical emission channels of monolayer semiconductors coupled to silicon nanoantennas Shivangi Shree, Jean-Marie Poumirol, Ioannis Paradisanos, Gonzague Agez, Xavier Marie, Cedric ROBERT, Nicolas Mallet, Peter R. Wiecha, Guilhem Larrieu, Vincent Larrey, Frank Fournel, Kenji Watanabe, Takashi Taniguchi, Aurelien Cuche, Vincent Paillard, Bernhard Urbaszek Monolayers (MLs) of transition metal dichalcogenides (TMDs) such as WSe2 and MoSe2 can be placed by dry stamping directly on broadband dielectric resonators, which have the ability to enhance the spontaneous emission rate and brightness of solid-state emitters at room temperature. We show strongly enhanced emission and directivity modifications in room temperature photoluminescence mapping experiments. By varying TMD material (WSe2 versus MoSe2) transferred on silicon nanoresonators with various designs (planarized versus non-planarized), we experimentally separate the different physical mechanisms that govern the global light emission enhancement. For WSe2 and MoSe2 we address the effects of Mie Resonances and strain in the monolayer. For WSe2 an important additional contribution comes from out-of-plane exciton dipoles. This paves the way for more targeted designs of TMD -Si nanoresonator structures for room temperature applications. |
Monday, March 15, 2021 1:30PM - 1:42PM Live |
B57.00009: Hyperbolicity on-demand in van der Waals Semiconductors Aaron Sternbach, Sang Hoon Chae, Simone Latini, Andrey Rikhter, Yinming Shao, Baichang Li, Daniel Rhodes, Brian Sae Yoon Kim, P. James Schuck, Xiaodong Xu, Xiaoyang Zhu, Richard Averitt, James Hone, Michael Fogler, Angel Rubio, Dmitri Basov Strong anisotropy is inherent to natrually layered van der Waals (vdW) materials. Consequentially, dipole active resonances can render the principle values of the dielectric tensor of opposite sign along orthogonal crystallographic directions within naturally occurring vdW materials. Chief among the resultant non-intuitive optical properties is the formation of sub-diffractional wavepackets that travel as conical rays with hyperbolic dispersion throughout their bulk. Here, I discuss our work on producing an on-demand hyperbolic response within the vdW semiconductor WSe2. By utilizing femtosecond photoexcitation to inject electron-hole pairs in WSe2 we dramatically altered its electronic response. Our time-resolved nano-imaging data reveals key signatures of hyperbolicity produced on-demand, which appear on the sub-picosecond timescale. |
Monday, March 15, 2021 1:42PM - 2:18PM Live |
B57.00010: First-principles spectroscopy of multiparticle excitations in low-dimensional materials Invited Speaker: Felipe Da Jornada The synthesis of low-dimensional materials, such as monolayer transition metal dichalcogenides (TMDCs), opened the door to the study of new classes of systems with spatial confinement, locked spin and valley physics, weak electronic screening, and enhanced many-electron interactions. Such systems host a variety of charged and neutral multiparticle excitations – such as plasmons, excitons, trions, biexcitons, and condensates – often displaying strong and well-defined spectroscopic signatures. We present here a first-principles formalism and calculations based on the interacting Green’s function to compute and understand these excitations and their dynamics. |
Monday, March 15, 2021 2:18PM - 2:30PM Not Participating |
B57.00011: Effect of Isotopes on Photoluminescence in Single Layer MoS2 Naseem Ud, Volodymyr Turkowski, Yu Yiling, Xiao Kai, Talat Rahman We have performed calculations based on DFT and TDDFT to elucidate the effect of Mo isotope mass on photoluminescence (PL) of single-layer MoS2. In particular, we have computed the electron and the phonon spectra of the system, as well as the strength of the electron-phonon coupling of the system with 92Mo, 95Mo and 100Mo. Contrary to most bulk semiconductors in which isotope shift of the PL peak results from electron-phonon and hole-phonon interactions induced change of the bandgap, we show that in single-layer MoS2 the observed PL frequency decreases with mass increasing is due to a renormalization of the exciton energy because of the exciton-phonon coupling. Such a strong effect of exciton-phonon coupling can be explained by the large excitonic binding energy in the system that corresponds to a strong interaction of a very stable “e-h dipole” with ionic vibrations, and to reduced effects of screening. The results for the isotope shift of the PL peak are in a good agreement with experimental data[1]. |
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