Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Q12: Optical Spectroscopic Measurements of 2D Materials IRecordings Available
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Sponsoring Units: FIAP Chair: Nick Borys, FIAP Room: McCormick Place W-181C |
Wednesday, March 16, 2022 3:00PM - 3:12PM |
Q12.00001: Near-deterministic generation of defect-related emitters in GaSe Weijun Luo (Boston University); Alexander Puretzky (Oak Ridge National Laboratory); Qishuo Tan (Boston University); Zhuofa Chen (Boston University); Liangbo Liang (Oak Ridge National Laboratory); Xi Ling (Boston University); Weijun Luo Single-photon emission from atomic defects in two-dimensional (2D) van der Waals materials have recently shown the promising potential to serve the development of quantum information technology. We report the near-deterministic generation of defect-related quantum emitters (650 nm – 750 nm) in GaSe via localized strain engineering. We demonstrate the strain-dependent emission wavelengths and bandwidths via the photoluminescence mapping of individual pillars. We illustrate the emitters arising from defect-bound excitons via power-dependent photoluminescence measurements. Temperature-dependent measurements suggest that strain promotes the thermal quenching temperature of defect-related emitters from 120 K to 200 K. Our findings provide insights into the design of deterministic generation and control of defect-related GaSe emitters. |
Wednesday, March 16, 2022 3:12PM - 3:24PM |
Q12.00002: Polarization dynamics in black phosphorus-based van der Waals heterostructures Souvik Biswas, Meir Y Grajower, Kenji Watanabe, Takashi Taniguchi, Harry Atwater Polarization of light plays an essential role in a wide variety of classical and quantum optics-based applications. We demonstrate electronically reconfigurable, broadband polarization conversion at telecom wavelengths with atomically thin van der Waals heterostructures, using hBN-encapsulated trilayer black phosphorus (TLBP) integrated in a Fabry-Pérot cavity. The strong light-matter interaction of TLBP with the optical cavity mode further enhances the intrinsic material (BP) birefringence in the complex reflectivity plane and allows generation of polarization states over the entire Poincaré sphere via spectral and input azimuth tuning. In addition, the extreme susceptibility of the exciton binding energy and its Bohr-radius to electrical doping provides near-complete suppression of its dipole oscillator strength, enabling electrostatic control of the polarization state across nearly half the Poincaré sphere. Both linear to circular (tunable quarter-waveplate) and linear cross-polarization (tunable half-waveplate) dynamics, among numerous other trajectories, were achieved (for example at ~1444 nm) via electrical gating (up to ~8x1012/cm2) demonstrating superior control over the reflected ellipticity and azimuthal angles. Specifically, for the aforementioned two cases, the ellipticity was tuned from 0o to 44o and the azimuthal was tuned from 24o to 90o, outperforming most existing polarization modulators. We also discuss spatial polarization dependence through Stokes mapping of our devices, which reveals both dielectric disorder and stacking order differences in TLBP. |
Wednesday, March 16, 2022 3:24PM - 3:36PM |
Q12.00003: Uncovering polar domains in the Rashba semiconductor BiTeI through piezoresponse force and second harmonic generation microscopy Kevin W Kwock, Prashant Padmanabhan, Kaiyuan Yao, Jaewook Kim, Kai Du, Fei-Ting Huang, Sang-Wook Cheong, P J Schuck, Rohit P Prasankumar Nonlinear optics has proven to be a powerful probe for understanding the intrinsic crystal structure and phase of materials. Recently, it has been shown that the 2D class of vdW materials exhibits remarkable nonlinear optical properties. Among this exciting family of 2D materials, the Rashba family of semiconductors, including BiTeI, has emerged as an excellent candidate for studying the origins of optical nonlinearities in polar materials. BiTeI and related materials have a large Rashba spin-orbit coupled state, and their large polarity is expected to persist down to the monolayer limit. Here, we used second harmonic generation (SHG) microscopy to show that BiTeI is an extremely nonlinear material using layer-dependent studies and extracting its nonlinear coefficients. This then enabled us to exploit SHG microscopy to image polar domains in BiTeI and correlate SHG emission intensity with piezoresponse force microscopy (PFM), providing key insights into the origin of the domains. |
Wednesday, March 16, 2022 3:36PM - 3:48PM |
Q12.00004: Probing gap mode plasmonic enhanced optical properties in the oxidized edge of WSe2 monolayer Junze Zhou, Edward S Barnard, Shaul Aloni, Adam Schwartzberg, Stefano Cabrini, Alexander Weber-Bargioni Driven by the recent investigations in quantum engineering on tuning the electrical and optical density of states of the Transition Dichalcogenide Metal (TMD) monolayer in nanoscale, this work focuses on taking nanoscale spectroscopic imaging of the WSe2 monolayer by the scanning near-field optical microscope (SNOM). For the results in this talk, the near-field enhancement was based on the strong plasmonic resonance within the few-nanometer size gap between the Gold coated sharp tip and the gold substrate surface. In order to have the gap plasmonic mode, we deposited the TMD monolayer on the top of an ultra-flat gold thin film. The near-field photoluminescence (PL) images and correlated topographic images reveal that the optical resolutions of the nano PL mappings are consistent with sizes of the near-field tips. Through spectral and nano-auger analysis, we also identified the oxidized feature along the sample edge. |
Wednesday, March 16, 2022 3:48PM - 4:00PM |
Q12.00005: Magnetic Proximity Effects in MoSe2/CrBr3 van der Waals Heterostructures Junho Choi, Christopher A Lane, Jian-Xin Zhu, Scott A Crooker 2D van der Waals heterostructures comprising ferromagnetic and semiconductor materials represent a unique platform to explore the magnetic exchange interactions that can introduce an effective magnetism in non-magnetic semiconducting layers. These interfacial magnetic proximity effects include electronic wavefunction overlap, spin-dependent charge transfer, and band hybridization. Here, in MoSe2/CrBr3 heterostructures, we employ magnetic circular dichroism (MCD) spectroscopy to observe valley Zeeman splitting of the monolayer MoSe2 excitonic resonances due to magnetic proximity effects from the underlying CrBr3. The effective exchange field, studied at both the A- and B-exciton resonances in MoSe2, is on the order of a few tesla. Imaging experiments reveal the presence of magnetic domains in the ferromagnetic CrBr3 layer. |
Wednesday, March 16, 2022 4:00PM - 4:12PM |
Q12.00006: Multivalley dynamics in monolayer TMDs at high pressures revealed by double-resonance Raman Luiz Gustavo Pimenta Martins, Bruno R Carvalho, Connor A Occhialini, Natália P Neme, Ji-Hoon Park, Qian Song, Mário S.C. Mazzoni, Pedro Venezuela, Matheus J.S. Matos, Jing Kong, Riccardo Comin Transition metal dichalcogenides (TMDs) possess unique spin-valley locked and spin-split K/K’ valleys which have led to many fascinating valley-related phenomena. Furthermore, TMDs also exhibit other conduction band minima with similar properties, the Q/Q’ valleys. Previous reports demonstrated exciting physical phenomena involving K-Q scattering, but those effects are usually hindered in monolayer TMDs because of the energy difference between K and Q valleys. To unlock new multivalley phenomena in monolayer TMDs, it is desirable to reduce that energy difference, while being able to sensitively probe the valley shifts and the multivalley scattering processes. Here, we compress monolayer MoS2 and WSe2 at high-pressures and probe K-Q crossing and multivalley scattering via double resonance Raman scattering for the first time. The ability to probe K-K’ and K-Q scattering processes as a function of strain shall shed light on different multivalley phenomena in TMDs such as superconductivity, intervalley quantum interference and valley transport. |
Wednesday, March 16, 2022 4:12PM - 4:24PM |
Q12.00007: Enhanced multiexciton formation by an electron-hole plasma in 2D semiconductors Matthew Strasbourg, Nicholas Borys, Thomas P Darlington, James C Hone, P. James Schuck, Emanuil Yanev Transition metal dichalcogenide semiconductors are layered van der Waals materials that exhibit exceptional optoelectronic properties in monolayer form. Their atomically thin nature and reduced long-range dielectric screening make them ideal systems in which to study a rich suite of many-body electronic states that emerges from intense coulomb interactions between quantum-confined charge carriers in a truly 2D system. Using photoluminescence action spectroscopy of monolayer WSe2, we find an enhancement of multiexciton formation with increasing excitation energy. This enhancement is attributed to the formation of excitons from a high-energy electron-hole plasma and generates 200% more multiexciton states than lower-energy excitation. By measuring the enhancement effect in multiple samples with varying doping levels, we observe increased enhancement of the charged biexciton with increased doping and observe a point of maximal charged biexciton generation at an elevated temperature, which we hypothesize arises from donor ionization. Furthermore, the onset of the enhancement coincides with the energy of the quasiparticle bandgap, corroborating the role of the electron-hole plasma and highlighting how the formation of excited states can be uniquely manipulated in 2D semiconductors. Understanding these formation and relaxation dynamics of the rich manifold of exciton states is critical for leveraging this new class of 2D semiconductors for advanced technologies. |
Wednesday, March 16, 2022 4:24PM - 4:36PM |
Q12.00008: Structure-property relationships of strain-induced ultrabright nanoscale emitters in a hybrid 2D semiconductor metal system Mohammad T Soroush Nanoscale solid-state light sources with tailorable properties which can be integrated into photonic systems are important for technologies ranging from sensing to the quantum information sciences. Here, we report the multimodal characterization of nanoscale light sources that are embedded in a hybrid system composed of single-layer (1L) WS2 bound to a pristine gold surface. The emitters are ultrabright and correspond to localized “nanobubbles” of the 1L-WS2 that have lateral sizes smaller than 100 nm. Structure-property relationships of the nanobubbles are investigated by correlating atomic force microscopy characterization with the corresponding properties of their photoluminescence (PL), including brightness, saturation, and emission spectra. Power-dependent PL spectroscopy shows a blue shift of the emission energy with increasing power, which is attributed to the state-filling of the localized strain regions within the nanobubbles. At low excitation power, excitons concentrate in the higher strain areas and emit photons with lower energies. With increasing power, excitons fill these states and recombine in less-strained areas and emit at higher energies. Spatial imaging of the PL emission of the nanobubbles at different powers confirms this funneling picture as the emission profile gets larger with increasing power. These strain-tailored nanoscale emitters open new possibilities in optoelectronic applications like single-photon emission at cryogenic temperature. |
Wednesday, March 16, 2022 4:36PM - 4:48PM |
Q12.00009: High Field Terahertz Spectroscopy of Optically Excited Transition Metal Dichalcogenides Alden N Bradley, Spencer G Thorp, Yue Zhang, Arend M van der Zande, Matthew Graham 2D transition metal dichalcogenides (TMDs) with semiconducting bandgaps exhibit strong optical responses including exciton resonances and spin-dependent photocarrier dynamics due to their unique band structure. We investigate the ultrafast dynamics of optically excited carriers in CVD grown large-grain MoS2 and WSe2 monolayer and multilayers in the presence of strong terahertz (THz) fields, employing time-resolved THz-control/optical-probe and optical-pump/THz-probe spectroscopy. We generate free carriers via optical excitations above the A and B exciton transitions in MoS2 and WSe2, and observe the relaxation dynamics with strong single-cycle THz pulses. The photocarriers have a long recombination lifetime exceeding 1 ns, while increasing optical pump intensity opens additional relaxation pathways such as carrier-carrier and carrier-lattice scattering, leading to fast carrier relaxation (~100 ps) at the early stage. Unlike 3D semiconductors such as Si and GaAs, in which strong THz fields enhance transparency by driving hot electrons into sidebands via intervalley scattering, MoS2 and WSe2 exhibit no pronounced signatures of field-driven intervalley scattering. Analyzing the THz waveforms transmitted through the optically excited MoS2 and WSe2, we obtain the temporal evolution of the complex conductivity spectra as photocarriers decay. The THz conductivity of optically excited MoS2 and WSe2 exhibit subtle differences, indicating photocarriers undergo distinct relaxation processes. This optical and THz study lends to future applications of TMDs in photonic and optoelectronic devices. |
Wednesday, March 16, 2022 4:48PM - 5:00PM |
Q12.00010: Highly non-linear interlayer exciton-polaritons in bilayer MoS2 Biswajit Datta, Mandeep Khatoniar, Prathmesh Deshmukh, Rezlind Bushati, Simone D Liberato, Stephane K Cohen, Vinod M Menon Planar semiconductor microcavities in the strong light-matter coupling regime have recently emerged as a platform to realize strong optical nonlinearities in low-density limits. The 2D nature of transition metal dichalcogenides makes them ideal candidates to be integrated into a microcavity that can host exciton-polaritons. Among different excitations supported by different TMDCs, a prime candidate is the charge transfer excitons that form in heterobilayer TMDCs. Due to the spatial separation of the electron and holes into different layers, they have a permanent dipole moment which is expected to form dipolar polaritons with very high nonlinearities. However, due to a pure vertical dipole moment, the oscillator strength is small and hence can not be strongly coupled to microcavity photons. In principle, these problems can be circumvented in a suitable bilayer interlayer exciton system. Here, we report the strong coupling of interlayer excitons hosted by bilayer MoS$_2$ with the photonic modes supported by a microcavity. This is possible because the interlayer exciton in bilayer MoS$_2$ is an admixture of the charge transfer exciton and intralayer B exciton. Due to the mixing with B exciton, the interlayer exciton has a sizable oscillator strength which is enough to strongly couple it with the cavity photon. We observe a 10 fold increase in nonlinearity for the interlayer exciton compared to the A exciton-polariton in low-density limit. We also separate the contributions of the exciton-exciton interaction and saturation due to phase space filling from the observed nonlinearity. |
Wednesday, March 16, 2022 5:00PM - 5:12PM |
Q12.00011: The Reststrahlen effect in the optically-thin limit Eric Y. Ma, Jenny Hu, Lutz Waldecker, Kenji Watanabe, Takashi Taniguchi, Fang Liu, Tony F Heinz The Reststrahlen effect of extended spectral bands in the mid-infrared with near-unity reflectance has been known for many bulk materials since the 19th century. The case for finite thickness, however, has not been treated systematically. Here we describe the evolution of reflectance spectra near the phonon resonance in hexagonal boron nitride (hBN) across more than 5 orders of magnitude in thickness, down to a monolayer. We show that the Reststrahlen band evolves to a single peak in the optically-thin limit, within which two regimes can be identified: a strong-response regime dominated by coherent radiative decay and a weak-response regime dominated by damping. This evolution clearly links fast radiative decay with strong coherent optical response and represents an extended-state analog of superradiance. |
Wednesday, March 16, 2022 5:12PM - 5:24PM |
Q12.00012: Switchable intervalley excitons with strong two-phonon scattering in bilayer WSe2 Mashael M Altaiary, Erfu Liu, Ching Tarng Linang, Fu Chen Hsiao, Jeremiah van Baren, Takashi Taniguchi, Kenji Watanabe, Nathaniel M. Gabor, Yia Chung Chang, Chun Hung Lui Abstract: We report the observation of QΓ intervalley exciton in bilayer WSe2 devices encapsulated by boron nitride. The QΓ exciton resides at ~18 meV below the QK exciton. The QΓ and QK excitons exhibit different Stark shifts under an out-of-plane electric field due to their different interlayer dipole moments. By controlling the electric field, we can switch their energy order and control which exciton dominates the luminescence of bilayer WSe2. Remarkably, both QΓand QK excitons exhibit unusually strong two-phonon replicas, which are comparable to or even stronger than the one-phonon replicas. By detailed theoretical simulation, we reveal the existence of numerous ( 14) two-phonon scattering paths involving (nearly) resonant exciton-phonon scattering in bilayer WSe2. To our knowledge, such electric-field-switchable intervalley excitons with strong two-phonon replicas have not been found in any other two-dimensional semiconductors. They make bilayer WSe2 a distinctive valleytronic material with potential novel applications. |
Wednesday, March 16, 2022 5:24PM - 5:36PM |
Q12.00013: Electronic Raman scattering of repulsive and attractive Fermi-sea dressed polarons in monolayer MoSe2 Yueh-Chun Wu, Jun Yan, Takashi Taniguchi, Kenji Watanabe In this study, we report the electronic Raman scattering (ERS) corresponding to electronic transition between repulsive polaron (neutral exciton) and attractive polaron (charge exciton) in monolayer MoSe2. The ERS is confirmed by photoluminescence excitation (PLE) study and gate-dependent response, where the wavenumber shift is in consistent with the binding energy of attractive polaron as Fermi level changes. Further circular-polarized PL investigation in magnetic field demonstrates the valley selection rule of ERS signals in co- and cross-circular channels with distinct g factors. The spatial PL mapping across the sample reveals possible structural non-uniformity in associate with the ERS feature. We discuss plausible mechanisms for the ERS and the implications of up- and down- conversion between excitonic states. |
Wednesday, March 16, 2022 5:36PM - 5:48PM |
Q12.00014: Ultrafast interlayer charge transfer probed by subcycle contact-free THz nanoscopy Fabian Mooshammer, Markus Plankl, Paulo E Faria Junior, Thomas Siday, Martin Zizlsperger, Fabian Sandner, Felix Schiegl, Simon Maier, Markus A Huber, Martin Gmitra, Jaroslav Fabian, Jessica L Boland, Tyler L Cocker, Rupert Huber Tunneling is one of the most fundamental phenomena in quantum mechanics, and a hallmark of interlayer exciton formation in transition metal dichalcogenide heterostructures. Here, we introduce a new contact-free nanoscopy technique centered around probing the local polarizability of photogenerated electron–hole pairs with evanescent terahertz fields. This concept is even capable of tracing ultrafast charge carrier dynamics in insulating materials. Thus, we are able to resolve the interlayer tunneling process in a heterobilayer comprising atomically thin sheets of WSe2 and WS2 with <50nm spatial and subcycle temporal resolution. Nanoscale strain and changes in atomic registry give rise to pronounced variations of the formation and annihilation dynamics of interlayer excitons. Our results demonstrate the potential of this technique for revealing how ultrafast tunneling governs the properties of a large variety of solid-state systems. |
Wednesday, March 16, 2022 5:48PM - 6:00PM |
Q12.00015: The bright and dark exciton coherent coupling enabled by the external magnetic fields. Varun Mapara, Arup Barua, Minh Tuan Trinh, Christopher E Stevens, Fang Liu, Xiaoyang Zhu, Stephen McGill, Volodymyr Turkowski, Denis Karaiskaj Magnetic field and polarization-dependent measurements on monolayer WSe2 were performed using time-integrated four-wave mixing spectroscopy. Magnetic fields up to 25T were applied parallel to the monolayer plane, and the partial brightening of the dark excitonic state was observed as an increase in dephasing time. The polarization-dependent study shows the coherent quantum beating between the states when higher magnetic fields were applied. The results were then modeled using time-dependent density functional theory. |
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