Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V30: Photoluminescence and Polarons in 2D Materials |
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Sponsoring Units: DCMP DMP Chair: Kathy McCreary, Naval Research Laboratory Room: 293 |
Thursday, March 16, 2017 2:30PM - 2:42PM |
V30.00001: Many-Body Theory of Trion Absorption Features in Two-Dimensional Semiconductors Dmitry K. Efimkin, Allan H. MacDonald Recent optical studies of monolayer transition metal dechalcogenides have demonstrated that their excitonic absorption feature splits into two widely separated peaks at finite carrier densities. The additional peak is usually attributed to the presence of trions, bound states of two electrons and a hole or an electron and two holes. Here we argue that in the density range over which the trion peak is well resolved, it cannot be interpreted in terms of weakly coupled three-body systems, and that the appropriate picture is instead one in which excitons are dressed by interactions with a Fermi sea of excess carriers. This coupling splits the exciton spectrum into a lower energy attractive exciton-polaron branch, normally identified as a trion branch, and a higher energy repulsive exciton-polaron branch, normally identified as an exciton branch. We have calculated the frequency and doping dependence of the optical conductivity and found that: (i) the splitting varies linearly with the Fermi energy of the excess quasiparticles; (ii) the trion peak is dominant at high carrier densities; (iii) and the trion peak width is considerably smaller than that of the excitonic peak. Our results are in good agreement with recent experiments. [Preview Abstract] |
Thursday, March 16, 2017 2:42PM - 2:54PM |
V30.00002: Excitonic potential engineering and trion confinement in two-dimensional transition metal dichalcogenides Kristiaan De Greve, Luis Jauregui, Ke Wang, Andrey Sushko, Alexander High, You Zhou, Giovanni Scuri, Dominik Wild, Philip Kim, Hongkun Park, Mikhail Lukin Optically active, two-dimensional van der Waals materials, such as the transition metal dichalcogenides (TMDCs), have recently emerged as an interesting platform for novel optoelectronic devices and device physics. Many of the attractive properties of the TMDCs can be attributed to a combination of their large excitonic binding energy, the ability to stack multiple layers into a van der Waals heterostructure, and the large spin-orbit coupling that gives rise to spin-valley locking. The two-dimensional nature and large excitonic binding energy allow for interesting ways to explore novel quantum optical effects in TMDCs. In this work, we demonstrate recent results in manipulating excitons and trions (charged excitons). By controlling the dielectric environment of the TMDCs, excitonic potential landscapes can be created that could be exploited in future, quantum coherent excitonic devices. Similarly, by exploiting the large excitonic binding energy, trions can be manipulated and confined electrostatically to create hybrid, electrically and optically active quantum dots. [Preview Abstract] |
Thursday, March 16, 2017 2:54PM - 3:06PM |
V30.00003: On demand generation of photoluminescence on thick MoS$_{\mathrm{2}}$ flakes via laser thinning and gold nanoparticle self-assembly Lili Gong, Chorng Haur Sow Recently, the photoluminescence (PL) property of monolayer and few layer transition metal dichalcogenides (TMDs) has aroused tremendous attention. However, the difficulty in synthesizing large area monolayer and few layer TMDs limits its application. Here we show that photoluminescent patterns with arbitrary shapes can be produced via a simple but effective method based on thick MoS$_{\mathrm{2}}$ flakes. By scanning focused laser beam on thick MoS$_{\mathrm{2}}$ flakes, few layer MoS$_{\mathrm{2}}$ and thick edges can be fabricated arbitrarily. After the laser treated MoS$_{\mathrm{2}}$ being immersed in AuCl$_{\mathrm{3}}$ solution, gold nanoparticles are selectively grown on the few layer MoS$_{\mathrm{2}}$ and thick edges where the particle size on edges (from 100nm to 400nm) are much larger than that on few layer MoS$_{\mathrm{2}}$ (less than 150nm) The obtained PL spectra show that the PL intensity is largely enhanced (up to 20 fold) and an obvious blue shift can be observed after the gold growth. We also demonstrate that arbitrary fluorescent patterns can be produced by this laser thinning and gold deposition process. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V30.00004: Control of excitons in multi-layer van der Waals heterostructures Erica Calman, Chelsey Dorow, Michael Fogler, Leonid Butov, Sheng Hu, Artem Mishchenko, Andre Geim We present experimental studies of excitons in double quantum well van der Waals heterostructures composed of atomically thin layers of MoS$_2$ and hBN. We showed the control of emission of neutral and charged excitons by gate voltage, temperature, and both the helicity and the power of optical excitation [1]. We also observed low-energy emission line with orders of magnitude longer lifetime and energy controlled by voltage. We interpret this line in terms of the recombination of electrons and holes spatially separated in the direction perpendicular to the quantum well plane along which the electric field is applied. [1] E. V. Calman, C. J. Dorow, M. M. Fogler, L. V. Butov, S. Hu, A. Mishchenko, A. K. Geim. Appl. Phys. Lett. 108, 101901 (2016) [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V30.00005: Observation of the back-bending dispersion of waveguide exciton polaritons in MoSe$_{\mathrm{2}}$. Zhe Fei, Fengrui Hu We report on a real-space nanoimaging study of exciton polaritons in MoSe$_{\mathrm{2}}$ planar waveguides using the scattering-type scanning near-field optical microscopy. These polaritons are generated due to the coupling between waveguide photons with excitons in MoSe$_{\mathrm{2}}$ and they show sensitive dependence with the waveguide thickness and orientation. By measuring the mode wavelengths at various excitation energies, we were able to construct the mode dispersion relation, which shows a back-bending feature close to the exciton. Our theoretical analysis indicates that such an anomalous dispersion is the signature of polaritons subject to damping. The mode damping drops rapidly at lower energies away from the excitons, where the waveguide modes can propagate tens of microns. [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V30.00006: Fermi polaron-polaritons in MoSe2 Meinrad Sidler, Patrick Back, Ovidiu Cotlet, Atac Imamoglu The truly 2D nature of transition metal dichalcogenides (TMDs) as well as their large electron mass infer strong Coulomb interactions which imply strong exciton binding energies of order 500 meV. The resulting small Bohr radius ensures a strong coupling of the exciton to light. Using a fiber microcavity, we measure cavity spectroscopy of gate-tunable monolayer MoSe2 in the weak as well as in the strong coupling regime. In the weak coupling regime, we observe two resonances whose relative intensities change with the electron density. We find that both resonances show a sizable normal mode splitting which rules out their usual identification as exciton and trion but demonstrates that the elementary optical excitations in this new material system are attractive and repulsive polarons. Our findings completely revamp the current paradigm used to describe the ever-growing number of experiments based on TMD monolayers. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 3:54PM |
V30.00007: Theory of Fermi polaron polaritons in transition metal dichalcogenide monolayers embedded in a cavity Imamoglu Atac, Ovidiu Cotlet, Eugene Demler We present a theoretical study of Fermi polaron polaritons in monolayer transition metal dichalcogenides [1]. The Fermi polaron has received considerable attention in cold atom systems, where it results from the interaction of an impurity with a fermionic bath. We show that a similar theoretical framework can be used to understand the interaction of excitons with conduction band electrons in transition metal dichalcogenides monolayers. We show that, by embedding the monolayer into a cavity, the emerging quasi particles, which we call Fermi polaron polaritons, can also be understood using the tools developed for understanding Fermi polarons. To this end we first analyze the microscopic exciton-electron interaction in these materials and show that it can be approximated by a contact interaction. Then, we show that truncating the Hilbert space to a single electron hole pair, the many-body problem can be solved analytically and quantitative agreement with experiments can be obtained. We briefly comment on different methods that can be used to solve the Fermi polaron problem. and on the generalization of our results to different physical systems, like the excitons formed in GaAs quantum wells. [1] M. Sidler, et. al. Nat. Phys. 2016, doi:10.1038/nphys3949 [Preview Abstract] |
Thursday, March 16, 2017 3:54PM - 4:06PM |
V30.00008: Transport of polaritons in transition metal dichalcogenide monolayers embedded in a cavity Ovidiu Cotlet, Eugene Demler, Atac Imamoglu We present a theoretical study of the transport properties of Fermi polaron polaritons in monolayer transition metal dichalcogenides, which have been recently realized [1]. Exciton polaritons, resulting from the hybridization of excitons and photons, share the properties of both excitons and photons. It was shown that putting exciton polaritons in contact with a two dimensional Fermi system, the polariton can bind an electron-hole pair from the Fermi sea to form a quasi particle with new properties: the Fermi polaron polariton. Using a Chevy ansatz we investigate the response of this quasi particle to an external low frequency electric field. Although this particle is charge neutral we show that one can associate to it an effective charge. This is because the hole in the Fermi sea, which has a positive charge, has a negative mass: therefore it will move in the same direction as the electron under the influence of the electromagnetic field. We analyze the validity of the Chevy ansatz under the presence of an electric field. We also calculate the effect of disorder on this quasi particle. An interesting question is to what extent the ultra small mass of polaron polaritons influence transport properties. [1] M. Sidler, et. al. Nat. Phys. 2016, doi:10.1038/nphys3949 [Preview Abstract] |
Thursday, March 16, 2017 4:06PM - 4:18PM |
V30.00009: Near Infrared Emission from Defects in Few-Layer Phosphorene Shahriar Aghaeimeibodi, Je-Hyung Kim, Edo Waks Atomically thin films of black phosphorus have recently received significant attention as low dimensional optical materials with a direct exciton emission whose wavelength is tunable by controlling the number of layers. In addition to this excitonic emission, recent work has revealed emission from defect states and reported new methods to manipulate them. Monolayer phosphorene exhibits emission from localized defect states at wavelengths near 920 nm. Increasing the number of layers should shift defect emission to longer wavelengths, enabling the material to span a broader spectral range. But defect emission from few-layer phosphorene has not yet been reported. Here, we demonstrate a new class of near infrared defects in few layer phosphorene. Photoluminescence measurement shows a bright emission around 1240 nm with a sublinear growth of emission intensity with linear increase of excitation intensity, confirming the defect nature of this emission. From time-resolved lifetime measurements we determine an emission lifetime of 1.2 ns, in contrast to exciton and trion lifetimes from few layer phosphorene previously reported to be in the range of a few hundred picoseconds. This work highlights the potential of bright defects of phosphorene for near infrared optoelectronic applications. [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V30.00010: Tunable room-temperature single photon emission from atomic defects in hexagonal boron nitride Gabriele Grosso, Hyowon Moon, Benjamin Lienhard, Dmitri Efetov, Marco Furchi, Pablo Jarillo-Herrero, Sajid Ali, Michael Ford, Igor Aharonovich, Dirk Englund Two-dimensional van der Waals materials have emerged as promising platforms for solid-state quantum information processing devices with unusual potential for heterogeneous assembly. Recently, bright and photostable single photon emitters were reported from atomic defects in layered hexagonal boron nitride (hBN), but controlling inhomogeneous spectral distribution and reducing multi-photon emission presented open challenges. We demonstrate that strain control allows spectral tunability of hBN single photon emitters, and material processing sharply improves the single-photon purity. Our sample fabrication process relies on ion irradiation and high temperature annealing to isolate individual defects for single photon emission. Spectroscopy on this emitter reports high single photon purity of g$^{\mathrm{(2)}}$(0)$=$0.07, and high count rates exceeding 10$^{\mathrm{7}}$ counts/sec at saturation. Furthermore, these emitters are stable to material transfer to other substrates, including a bendable beam that allows us to controllably apply strain. Our experiments indicate a maximum tuning of 6 meV and emission energy dependencies ranging from -3 to 6 meV/{\%}. High-purity and photostable single photon emission at room temperature, together with spectral tunability and transferability, opens the door to scalable integration of high-quality quantum emitters in photonic quantum technologies. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V30.00011: Atomically thin single-photon emitting diodes Matteo Barbone, Carmen Palacios-Berraquero, Dhiren M. Khara, Xiaolong Chen, Ilya Goykhman, Alejandro R.-P. Montblanch, Andrea C. Ferrari, Mete Atature Integrating single-photon sources into on-chip optical circuits is a challenge for scalable quantum-photonic technologies. Despite a plethora of single-photon sources reported to-date, all-electrical operation has been reported for only a few. The attractiveness of single-photon sources in layered materials stems from their ability to operate at the fundamental limit of single-layer thickness, foreseeing high extraction efficiency and providing the potential to integrate into conventional and scalable high-speed optoelectronic device systems. We use light emitting devices realized by vertical stacking of graphene, hexagonal-BN few layers thick and mono- and bilayer transition-metal dichalcogenides (TMDs) and achieve charge injection from graphene into the TMD layer containing optically active quantum dots. We demonstrate that layered materials enable all-electrical single-photon generation over a broad spectrum. We demonstrate for the first time that quantum emitters reported in WSe$_{\mathrm{2\thinspace }}$can operate electrically, paving the way towards a new class of quantum light emitting devices. We further report all-electrical single-photon generation in the visible spectrum from quantum emitters in a new material, WS$_{\mathrm{2}}$. I will also discuss the potential for scalability and charge control to show that 2d materials are a platform for fully integrable and atomically precise quantum photonics device technologies. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V30.00012: Hot charge separation process in MoS2/WSe2 heterostructure based solar cell Zhenzhu Li We investigated the charge separation processes happened before the formation of tight bounded exctions in MoS2/WSe2 heterostructures, which is viewed as an essential step to facilitate the high quantum efficiency of solar cells based on such material geometry. In this work, we try to figure out whether it is the charge transfer process or energy transfer process that dominating the charge separation process and their corresponding time domain, which could be confirmed with experimental results. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V30.00013: Anisotropic ultrafast response of MoS$_{\mathrm{2}}$ on rippled substrates Eugenio Cinquanta, Andrea Camellini, Christian Martella, Carlo Mennucci, Alessio Lamperti, Giuseppe Della Valle, Margherita Zavelani Rossi, Francesco Buatier de Mongeot, Alessandro Molle, Salvatore Stagira TMDs represent one of the most promising option for new devices characterized by high performances for opto- and nanoelectronics applications. Top-down schemes have been fruitfully exploited for the tuning of TMDs physics by stain engineering in exfoliated flakes. We propose an original bottom-up strategy based on the CVD growth of MoS$_{\mathrm{2}}$ on anisotropic substrates and its characterization by means of pump-probe spectroscopy. The ultrafast response of the rippled MoS$_{\mathrm{2}}$ reveals strongly anisotropic. While the transient absorption emerges as independent from the orientation of the pump beam polarization, the angle between the probe beam polarization and the ripples induces remarkable effects. Within an orthogonal geometry, both the overall intensity of the transient spectrum and the el-ph scattering decay time are halved while the photo-bleaching at 450 nm is blueshifted with respect to the parallel orientation case. Our results demonstrate that the coupling of TMDs with anisotropic substrates is a promising way for the integration of TMDs photonics devices. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V30.00014: Anisotropic optical properties of semimetal WTe$_2$ Alex J. Frenzel, Christopher C. Homes, Quinn D. Gibson, Yinming Shao, Kirk W. Post, Aliaksei Charnukha, Robert J. Cava, Dimitri N. Basov WTe$_2$ is a semimetallic transition metal dichalcogenide which exhibits extreme magnetoresistance and is expected to be a type-II Weyl semimetal. Its orthorhombic crystal structure comprises planes of distorted hexagons in which tungsten atoms form quasi-one-dimensional chains, leading to strong anisotropy of its electronic properties. We measured the $ab$-plane optical conductivity of single crystals of WTe$_2$ for light polarized parallel and perpendicular to the W-chain axis over a broad range of frequency and temperature. At low temperatures and far-infrared frequencies, we observe a striking dependence of the reflectance edge on light polarization, corresponding to anisotropy of the plasma frequency. We quantitatively study the temperature dependence of the plasma frequency, yielding insight into the effective mass anisotropy. We also find strongly anistropic interband transitions persisting to high photon energies. These results are analyzed by comparison with $ab$ $initio$ calculations. [Preview Abstract] |
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