Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A34: Optical Spectroscopic Measurements of 2D Materials IIndustrial
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Sponsoring Units: FIAP Chair: Nicholas Borys, Lawrence Berkeley National Laboratory Room: Room 226/227 |
Monday, March 6, 2023 8:00AM - 8:12AM |
A34.00001: Exploring new optical resonances in TMDs beyond the bright exciton Gabriele Grosso, Saroj B Chand, John M Woods, Enrique Mejia The rich band structures of transition metal dichalcogenides (TMDs) give rise to several transitions beyond standard bright excitons, including intervalley momentum-forbidden excitons (with electrons and holes located in two different valleys) and spin-forbidden excitons (with electrons and holes having opposite spin). Moreover, the malleability of TMDs allows for the manipulation of their atomic structure that opens exciting opportunities to tailor their optical response at the atomic scale. Here we report on the control of different exciton species in TMDs for optoelectronic applications. We show evidence that the formation and emission of momentum-forbidden excitons are related to compressive strain due to a phonon-assisted intervalley scattering process that can be used as an ultrasensitive optical strain sensing mechanism. We also report on the repulsion-driven propagation of dark spin-forbidden excitons that, due to their permanent dipole, can travel for several micrometres transporting spin-valley information. Finally, we show that when the semiconducting and the metallic phase coexist in a monolayer of TMDs, additional excitonic resonances appear in the optical spectrum due to the creation of new band nesting effects occurring at the phase boundaries. |
Monday, March 6, 2023 8:12AM - 8:24AM |
A34.00002: Voltage-controlled GaSe single photon emitters Weijun Luo, Alex A Puretzky, Benjamin J Lawrie, Xi Ling The solid-state single photon emitter (SPE) is a critical building block for quantum information applications such as quantum communication, photonic quantum computing, and quantum metrology. SPEs in two-dimensional Van der Waals semiconductors have received extensive attention because of the potential for on-chip integration offered by monolayer materials with direct band gaps. Strain-induced quantum confinement has recently been used as a tool for deterministic localization of SPEs in multilayer GaSe. However, the engineering of GaSe SPEs’ brightness, photon purity and operating temperature remains a challenge. Here, we report on controlling the brightness and photon purity of strain localized GaSe SPEs through electrostatic doping. We show that the GaSe SPEs can operate up to 85 K with appropriate gating. Biexciton luminescence exceeds that of the exciton at higher temperatures, resulting in reduced single photon purity. Combining local strain control with electrostatic doping thus provides a flexible framework for the optimization of the brightness, photon purity and operating temperatures of GaSe SPEs. |
Monday, March 6, 2023 8:24AM - 8:36AM |
A34.00003: Strain tuning of linear and nonlinear optical properties of bilayer MoS2 Biswajit Datta, Saroj B Chand, Charanjot Singh, Gabriele Grosso, Vinod M Menon Naturally occurring Mo-based bilayers of Transition Metal Dichalcogenides (TMDCs) like MoS2 and MoSe2 have recently attracted interest due to interlayer excitons with substantial oscillator strength. Further, it has been shown that this interlayer exciton properties can also be controlled by external electric field. Here we show that strain can also be used to tune both linear and nonlinear optical properties of bilayer MoS2. We do a comparative study of A, B, interlayer (IE), and C excitons under strain to reveal how strain changes the band structures of bilayer MoS2 and how that affects the nonlinear optical properties. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A34.00004: Excitons or not excitons: Contrasting fates of photo-excitations in monolayer transition metal dichalcogenides Taketo Handa, Nicholas Olsen, Madisen A Holbrook, Luke N Holtzman, Lucas Huber, Hai I Wang, Mischa Bonn, Katayun Barmak, Abhay N Pasupathy, James C Hone, Xiaoyang Zhu Photo-excitation of atomically thin transition metal dichalcogenide (TMD) monolayers is commonly assumed to result in excitons with large binding energies. Here, we apply time-resolved THz photoconductivity spectroscopy to mechanically exfoliated, large-area monolayer WS2 and MoSe2 at excitation densities well below the exciton-Mott transition. We observe contrasting photoconductivity responses, depending on defect density. For monolayers with moderate defect densities, a positive THz conductivity emerges just after photoexcitation under both above-gap and resonant excitation conditions, with ≥ 20% of photogenerated excitons immediately dissociating into charge carriers, likely due to a trap-mediated process. In contrast, in monolayers with low defect density, a negative THz conductivity is observed following photo-excitation; this is consistent with the binding of photo-generated excitons with free carriers from intrinsic doping to form heavier exciton-carrier complexes commonly known as trions. These contrasting fates of photo-excitation in monolayer TMDs reveal the complexity of exciton-carrier physics at the 2D limit. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A34.00005: Furthering nano-optical techniques by integrating tapping-mode AFM and TCSPC Kevin W Kwock, Thomas P Darlington, Emanuil S Yanev, Matthew Strasbourg, James C Hone, Nick Borys, P J Schuck Scanning tip-enhanced optical spectro-microscopy techniques are powerful tools for the investigation of optoelectronic properties of materials surfaces. However, the improved resolution comes at the significant expense of optical signal, requiring the use of higher excitation powers which can be detrimental to samples and nano-optical probes.1-2 This current experimental paradigm is heavily dependent on of fabrication of extraordinary nano-optical probes that can simultaneously tolerate high excitation densities and greatly enhance the signal of interest for a measurement’s duration. Methods to increase signal/noise in a system without high photon fluences on the sample/tip are thus needed for imaging weak optical interactions, including Raman and nonlinear optical (e.g. SHG) scattering signals. In this study, we present efforts to improve our nano-optical studies by incorporating lock-in detection of tip-enhanced signals using tapping-mode atomic force microscopy scanning. We correlate fast single photon sensitive avalanche photodiodes and time-correlated single photon counting electronics, following the system design first demonstrated by Gerton et al.3 We show successful imaging of the nano-PL from a nanobubble in monolayer WSe2 on Au, achieving resolutions ~ 10 nm. Our results show the viability of lock-in detection in tip-enhanced experiments and offer a potential path for high-sensitivity nano-optical measurements utilizing photon correlations. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A34.00006: Towards Compact Phase-Matched and Waveguided Nonlinear Optics in Atomically Layered Semiconductors Chiara Trovatello, Xinyi Xu, Fabian Mooshammer, Yinming Shao, Shuai Zhang, Kaiyuan Yao, Dmitri N Basov, Giulio Cerullo, P J Schuck Nonlinear frequency conversion provides essential tools for generating new colors and quantum states of light. Transition metal dichalcogenides (TMDs) possess huge nonlinear susceptibilities; further, 3R-stacked TMDs possess aligned layers with broken inversion symmetry, representing ideal candidates to boost the nonlinear optical gain with minimal footprint. Here we report the second-order nonlinear processes of 3R-MoS2 along the ordinary and extraordinary directions. Along the ordinary axis, by measuring the thickness-dependent second-harmonic generation, we present the first measurement of the second-harmonic-generation coherence length of 3R-MoS2 and achieve record nonlinear optical enhancement from a van der Waals material, >104 stronger than a monolayer. We characterize the full refractive-index spectrum and quantify its birefringence with near-field nanoimaging. Furthermore, along the extraordinary axis, we achieve broadly tunable second-harmonic generation from 3R-MoS2 in a waveguide geometry for the first time [Xu, X., Trovatello, C. et al. Nature Photonics,16, 698–706, 2022]. Our results highlight the potential of 3R-stacked TMDs for integrated photonics, providing critical parameters for designing highly efficient on-chip nonlinear optical devices including optical parametric oscillators and amplifiers, and quantum circuits. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A34.00007: Charge excitations in resonant Raman scattering due to inversion breaking spin-orbit coupling Saurabh Maiti, Surajit Sarkar, Girsh E Blumberg, Alexander Lee Raman spectroscopy has been used extensively to study spin and charge collective excitations of various quantum systems. The observed spectrum can be decomposed into excitations with symmetries described by the irreducible representations (irreps) of the point group of the system. One excitation of interest in the fully symmetric irrep is the plasmon which are longitudinal oscillations of charge density in the system. In the fully symmetric irrep, the plasmon only couples to the Raman probe with a weight of q2, where q in the momentum imparted by the photon to the system during the inelastic scattering. Due to this q2 suppression, one needed to couple plasmons to phonons (phonon-plasmon-polariton) or use gratings to induce larger quasi-momentum to observe the plasmons. In this work, we argue that for low density systems, with strong inversion breaking spin-orbit coupling, it is possible to couple to the plasmons without the q2 suppression if we tune the incident photon energy to resonate between bands that are split due to an inter-band SOC. We list the conditions under which this effect is observable and discuss the implications for the self-doped semiconductor BiTeI. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A34.00008: Direct observation of self-hybridized exciton-polaritons in tungsten disulfide membranes and their valley polarization Dong-Jin Shin, Su-Hyun Gong, Junghyun Sung Polaritons are quasiparticles with a hybrid nature of matter and light. Exciton-Polaritons (EPs) in transition metal dichalcogenides (TMDCs) have been extensively explored due to their large exciton binding energies. Owing to its strong excitonic behavior, EPs have been successfully demonstrated in a TMDCs monolayer integrated with various microcavities. Meanwhile, excitonic resonance in TMDCs is strong enough to realize inherent guided EPs in TMDCs membranes. However, unlike cavity EPs, guided EPs have been far less studied because of their non-radiative nature which makes it hard to be detected using far-field microscope techniques. For this reason, recent studies on guided EPs have been accomplished only by near-field scanning optical microscope (NSOM). Here, we report a direct measurement of optical dispersion relations for guided EPs in tungsten disulfide (WS2) membrane via near-field coupling method and angle-resolved spectroscopy. Using oil-immersion microscope objective lens for the near-field coupling, we observed the anti-crossing behavior of the dispersion relation near the exciton resonance in freestanding WS2, which is a signature of polaritonic dispersion. We show that WS2 membranes are capable of supporting EPs at a nanometerthickness limit. We also confirmed the pumping power dependence of polaritonic dispersions in WS2 membranes. Lastly, we first report the valley-polarized guided EPs in WS2 layers at room temperature. This valley polarization originates from strong coupling of photons with excitons in two different K and K’ valleys. We believe that our results pave a new way for valleytronic and nanophotonic applications. |
Monday, March 6, 2023 9:36AM - 9:48AM |
A34.00009: Ultrastrong Light-Matter Coupling with a single THz Split Ring Resonator and monolayer graphene Elsa Jöchl, Jerome Faist, Giacomo Scalari, María Barra Burillo, Shima Rajabali, Mattias Beck Getting a better understanding of ultrastrong light matter interaction is not only interesting regarding fundamental research, but also relevant to gain insights on new properties of matter, possible new different phase transitions, and for applications in Quantum Information Processing. For all these purposes, it is most beneficial to study the interaction at a single electron level. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A34.00010: Optical investigation of the band structure-dependent many-body interactions in monolayer and bilayer transition metal dichalcogenides Aidan J Campbell, Shun Feng, Mauro Brotons-Gisbert, Hyeonjun Baek, Kenji Watanabe, Takashi Taniguchi, Brian D Gerardot Transition metal dichalcogenide (TMD) heterostructures allow the investigation of many-body interactions between the exciton and a tuneable Fermi-sea, which can be described by the Fermi-polaron model. Experimental and theoretical work has mainly focused on direct bandgap monolayer TMDs, where carriers are doped into the conduction and valence band edges at the ±K valleys. However, there has been recent interest in natural homobilayer TMDs as a host of interlayer excitons and a platform for correlated physics. The band extrema in these systems are often predicted to be located at other high symmetry points (elsewhere to ±K) in the Brillouin zone. Despite this, there is a lack of optical characterisation of the doping-dependent properties in these systems. Here we perform reflection contrast measurements at 4 K on a high-quality dual-gate tuneable device that contains regions of monolayer and bilayer WSe2 and MoSe2. We explain the doping-dependent peak dispersions and magnetic properties of the attractive exciton-polarons that form in the bilayer regions through their distinctive band-structures. We use our observations from the natural bilayers to gain insight into the poorly understood feature that emerges at high carrier concentrations in monolayer WSe2 and other TMD systems, including monolayer MoS2 . Our results further the understanding of monolayer and bilayer TMDs which can help to engineer future optoelectronic devices using these materials. |
Monday, March 6, 2023 10:00AM - 10:12AM Author not Attending |
A34.00011: Engineering exciton emission in monolayer TMDCs by dielectric encapsulation Abhay A V S, Abin Varghese, Saurabh Lodha, ANSHUMAN KUMAR Monolayer TMDCs are direct band gap semiconductors having exciton emission in the visible spectrum. Exciton emission engineering in monolayer TMDCs is highly relevant for application in optoelectronics and valleytronics. Chemical vapour deposition and mechanical exfoliation are two established ways of obtaining monolayer TMDCs. In this work, we present the engineering of photoluminescence of MoS2 by dielectric encapsulation. It has been reported that the encapsulation of 2D material can improve the exciton emission properties, such as the linewidths, stability and saturation intensity. We investigate and compare the excitonic features of MoS2 encapsulated by van der waal dielectric versus non van der waal dielectric at low as well as room temperature. Gate voltage and excitation power dependent measurements further validate our findings. |
Monday, March 6, 2023 10:12AM - 10:24AM |
A34.00012: Second harmonic generation spectroscopy in van der Waals homo- and heterobilayers Ioannis Paradisanos, Delphine Lagarde, Andres Manuel Saiz Raven, Thierry Amand, Shivangi Shree, laurent Lombez, Pierre Renucci, Cedric ROBERT, Kenji Watanabe, Takashi Taniguchi, Andrea Balocchi, Iann C Gerber, Leonid Golub, Xavier Marie, Mikhail M Glazov, Bernhard Urbaszek The twist angle in transition metal dichalcogenide bilayers is a compelling degree of freedom that determines electron correlations and the period of lateral confinement of moiré excitons. We perform polarization-resolved second harmonic generation (SHG) spectroscopy of MoS2/WSe2 heterostructures. We demonstrate that by choosing suitable laser energies the twist angle between two monolayers can be measured directly on the assembled heterostructure. We show that the amplitude and polarization of the SHG signal from the heterostructure are determined by the twist angle between the layers and exciton resonances at the SH energy. For heterostructures with close to zero twist angle, we observe changes of exciton resonance energies and the appearance of new resonances in the linear and nonlinear susceptibilities. In inversion symmetric MoS2 homobilayers we demonstrate tuning of non-linear optical processes with a strong enhancement of the SHG signal in applied electric fields for pump energies resonant with interlayer exciton states. |
Monday, March 6, 2023 10:24AM - 10:36AM |
A34.00013: Interlayer charge transfer induced robust spin-valley polarization in hBN/TMDC/PbI2 heterostructures in type I and type II configurations PRAHALAD K BARMAN, Saroj Poudyal, Bubunu Biswal, Ramesh Rajarapu, Abhishek Misra Achieving a high degree of spin-valley polarization is the key task for emerging quantum technology. In most cases, generation and manipulation of spin-valley polarization with a significant value have been recorded with a cryogenic temperature regime. Here we demonstrate the mechanism of having a high degree of spin-polarization at the room temperature regime by manipulation of charge transfer and reduction of carrier lifetime. We have observed spin-valley polarization 70% in PbI2 structure at RT limit. This intrinsic high spin polarization is due to high spin-orbit coupling in PbI2 layers. However, we are able to manipulate spin-valley polarization in the range of 50%-80% with the hBN/TMDC/PbI2 vdW heterostructure, at resonant excitation. With type I (MoS2/PbI2) band alignment, spin-valley polarization could be reached 80% by enhancing the PL emission. Whereas, with type II (WS2/PbI2) band alignment, it decreases to 50% with PL quenching. Helicity-resolved PL measurements (with CPexcitation) were performed to quantify the amount of spin-valley polarization. However, the effect of thickness, temperature, and excitation wavelength dependency has also been verified expensively. Here, polarized interlayer charge transfer could be the essential mechanism for high value of spin-polarization. Here the carrier lifetime is significantly lower in HS, whereas the spin relaxation lifetime is higher and this gives rise to having a high value of spin-polarization in the TMDC/PbI2 HS system. |
Monday, March 6, 2023 10:36AM - 10:48AM |
A34.00014: Near-field optical mapping of strong exciton-plasmon coupling in 2-dimensional semiconductor Junze Zhou, Fabrizio Ruminucci, Edward Barnard, Adam Schwartzberg, Javier Garcia de Abajo, Alexander Weber-Bargioni The strong interaction of emitters with plasmonic cavities has been studied intensively during the last decade. Much of the experimental works have focused on applying the far-field optical approaches to read out the coupled states, manifested by the separated peaks in the scattering or emission spectra. However, in the 2-dimensional (2D) semiconductors related strong coupling systems, the splitting effect in the emission spectrum is hidden by the far-field background read from the objective lens, since the plasmonic modulated coupling events occur in an optical volume much smaller than the diffraction limit spot size. This work proposes a unique near-field measurement configuration to read out the strong exciton-plasmon coupling from the 2D WSe2 placed on the plasmonic nano trenches. By hyperspectral mapping, we can precisely locate the positions of the strong coupling and study the difference in coupling strength. |
Monday, March 6, 2023 10:48AM - 11:00AM |
A34.00015: Fabrication of embedded plasmonic micropillars for nano-optomechanics and quantum light emission with 2D materials Joe C Stage, Wataru Nakagawa, Andrew Lingley, Nicholas Borys The intense light-matter interactions in 2D materials can be deterministically tuned with external stimuli such as localized strain. Further, the atomically thin structure of 2D materials allows them to be precisely positioned to nanophotonic antennas that are designed to manipulate and further enhance their light-matter interactions. Nano-optical antennas embedded in dielectric cavities present new opportunities for nanophotonic engineering and nano-optomechanics in 2D materials, with applications spanning from on-demand single-photon emitters to low-temperature, high Q-factor optomechanical resonators. Here, the fabrication of metallic micropillars embedded in an SiO2 dielectric cavity is described alongside photoluminescence and atomic force microscopy characterization of single- and few-layer WSe2 crystallites that have been transferred onto the micropillar. The proof-of-concept fabrication process demonstrates that the height of the embedded pillars can be tuned to either rise above or below the cavity with nanoscale precision and sets the stage for deeper miniaturization of the micropillar and the cavity using nanofabrication techniques. This precision will prove crucial for future investigations of optomechanical interactions between a plasmonic dipole tip and a 2D heterostructure with a graphene layer and the ability for such a system to modulate 2D semiconductor phenomena such as exciton funneling and quantum light emission. |
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