Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F16: Exciton Dynamics in 2D SemiconductorsFocus
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Sponsoring Units: DMP Chair: Xiaobo Yin, University of Colorado, Boulder Room: 315 |
Tuesday, March 15, 2016 11:15AM - 11:27AM |
F16.00001: Interlayer Exciton Lifetimes in MoSe2/WSe2 Heterostructures Kyle Seyler, Pasqual Rivera, Hongyi Yu, John Schaibley, Jiaqiang Yan, David Mandrus, Wang Yao, Xiaodong Xu Semiconductor heterostructures of two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as an exciting new platform for novel device engineering and physics. A fundamental question for the field is how the strong Coulomb interactions, electronic structure, and underlying valley physics affect the optoelectronic response. While researchers have made significant progress in understanding intralayer exciton dynamics in monolayer TMDs, there is comparatively little understanding of the interlayer excitons that form in their heterostructures. In this talk, we will report on time-resolved photoluminescence experiments of interlayer excitons in MoSe2/WSe2 vertical heterostructures, which show wide tunability with gate and variability with emission energy. We will also discuss the underlying mechanisms for this behavior and show how it can be utilized to generate long-lived valley excitons. [Preview Abstract] |
Tuesday, March 15, 2016 11:27AM - 11:39AM |
F16.00002: Coupled spin-valley-dynamics in singe-layer transition metal dichalcogenides Gerd Plechinger, Philipp Nagler, Christian Sch\"uller, Tobias Korn Single layers of transition metal dichalcogenides (TMDCs) like MoS$_2$ and WS$_2$ can be produced by simple mechanical exfoliation. Offering a direct bandgap at the K-points in the Brillouin zone, they re\-present a promising semiconductor material for flexible and transparent optoelectronic applications. Due to inversion symmetry breaking together with strong spin-orbit-interaction, the valley and spin degrees of freedom are coupled in monolayer TMDCs. Via circularly polarized optical excitation, an efficient polarization of the $K^+$ or the $K^-$ valley can be generated. Here, we investigate the dynamics of these coupled spin-valley polarizations in monolayer MoS$_2$ and WS$_2$ by means of photoluminescence spectroscopy and time-resolved Kerr rotation (TRKR). The results indicate a maximum achievable spin-valley-lifetime in these materials exceeding one nanosecond at low temperatures. Furthermore, we extract the dependence of the spin-valley lifetime on temperature. By varying the excitation energy, we reveal the excitonic resonances as well as the spin-polarized bandstructure around the K valleys common to monolayer TMDCs. [Preview Abstract] |
Tuesday, March 15, 2016 11:39AM - 11:51AM |
F16.00003: Measurements of ultrafast luminescence dynamics in transition metal dichalcogenide monolayers Cedric Robert, Gang Wang, Delphine Lagarde, Andrea Balocchi, Thierry Amand, Pierre Renucci, Fabian Cadiz, Bernhard Urbaszek, Xavier Marie We report time resolved photoluminescence (PL) measurements using a synchro-scan streak camera system with sub-ps time resolution, the fastest detector currently available for PL. The strong electron-hole Coulomb interaction in monolayer (ML) transition metal dichalcogenides results in excitons with high binding energies and oscillator strength. Therefore very short intrinsic radiative lifetimes can be expected. Here measurements with few ps time resolution are crucial. In our experiment we excite the ML sample with a fs laser pulse in a cryostat (T=4–300 K). In the model system ML MoSe2 we can separate spectrally the neutral and the charged exciton and perform detailed time-resolved PL studies on each complex. For the neutral exciton we resolve a PL emission time as short as 2ps, previous measurements were limited by the detector time-resolution. This short time depends on the experimental conditions such as temperature and applied external fields. We will discuss the different competing relaxation and recombination mechanisms, such as the intrinsic radiative recombination, the escape from the light cone through phonon scattering, the interplay between bright and dark exciton states and the possible transfer from the neutral to the charged exciton at lower energy. [Preview Abstract] |
Tuesday, March 15, 2016 11:51AM - 12:03PM |
F16.00004: Exciton Dynamics on Suspended and Substrate-supported MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ Monolayers. Chao Xu, Yiling Yu, Yifei Yu, Andy Barrette, Linyou Cao, Kenan Gundogdu MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ monolayers are promising atomic-scale platform for novel light emission devices, however, despite perfect surface passivation and strong exciton binding energy, their luminescence efficiencies are very low. Here, through the observation of exciton dynamics by ultrafast transient reflection, we revealed that the substrate can affect the exciton dynamics on MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ monolayers, by facilitating the non-radiative recombination pathways, thus reducing the luminescence efficiency. Furthermore, strong many-body interactions such as exciton-exciton annihilation, are enhanced on suspended MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ monolayers, whereas the defects in substrates may efficiently quench excitons thus mitigate those effects. [Preview Abstract] |
Tuesday, March 15, 2016 12:03PM - 12:15PM |
F16.00005: Observation of excitonic Floquet states in a monolayer transition metal dichalcogenide Edbert J. Sie, Xi Ling, Yi-Hsien Lee, Liang Fu, Jing Kong, Nuh Gedik Quantum systems that are driven by a time-periodic potential can form a series of new states that are called the Floquet states. In solids, the realization of Floquet states is rare and fascinating because it contains novel features that could be hybridized to create a new phase of matter. To date, such observation is limited to a recent experiment on a topological insulator (Y. H. Wang et al., Science 342, 453 (2013)). In this talk, we will show the observation of Floquet states from the excitons in a monolayer transition metal dichalcogenide that is performed using ultrafast optical spectroscopy. We will discuss the generation mechanism of these Floquet states as well as the significance of these observations. [Preview Abstract] |
Tuesday, March 15, 2016 12:15PM - 12:27PM |
F16.00006: Optical polarization and intervalley scattering in single layers of MoS$_{\mathrm{2}}$ and MoSe$_{\mathrm{2}}$ Berend Jonker, George Kioseoglou, Aubrey Hanbicki, Marc Currie, Adam Friedman We probe the valley population dynamics in MoSe$_{\mathrm{2}}$ and MoS$_{\mathrm{2}}$ by selectively populating the K and K' valleys with circularly polarized light while systematically varying the laser excitation energy. For both systems, the difference in the excitation energy and photoluminescence emission energy, d$E = \quad E_{pump}_{\mathrm{\thinspace }}$-- $E_{\mathrm{PL}}$, governs the depopulation of carriers in each valley. Adding more energy above a distinct threshold characteristic of the longitudinal acoustic (LA) phonon for each material enables inter-valley scattering and produces a sharp decrease in the observed circular polarization. LA phonons in these two systems have different energies (30 meV for MoS$_{\mathrm{2}}$ and 19 meV for MoSe$_{\mathrm{2}})$, and we show that the threshold for the excess energy required to initiate the depolarization process clearly reflects the material specific phonon energy. In addition, our results show that independent of how many carriers are excited, i.e. whether you create neutral or charged excitons, the scattering process is the same. We find that the key parameter for the depolarization process is the extra kinetic energy of the exciton -- depolarization is due to intervalley scattering that begins to occur when the exciton energy exceeds a threshold corresponding to twice the LA phonon energy. [Preview Abstract] |
Tuesday, March 15, 2016 12:27PM - 1:03PM |
F16.00007: Coherent Exciton Dynamics in Atomically Thin Semiconductors Invited Speaker: Xiaoqin (Elaine) Li The near band-edge optical response of an emerging class of semiconductors, known as the transitional metal dichalcogenides (TMDs), is dominated by tightly-bound excitons and charged excitons (i.e. trions). A fundamental property of these quasiparticles (excitons and trions) is quantum decoherence time, which reflects irreversible quantum dissipation arising from system (excitons and trions) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Dephasing time is also equivalent to the intrinsic homogeneous linewidth of exciton resonance. In addition, excitons in TMDs are localized at the corners of the Brillouin zone and provide a convenient way to optical manipulate the valley degree of freedom, which may act as a useful information carrier analogous to electronic charge or spin. Direct measurement of valley coherence time is challenging because it corresponds to a non-radiative coherence between two degenerate states. Using ultrafast multi-dimensional optical spectroscopy, we investigate the intrinsic homogeneous linewidth of excitons, exciton valley coherence as well as coupling between excitons and trions. Our studies reveal coherent electronic dynamics on the order of \textasciitilde 100 fs in monolayer TMDs. We gratefully acknowledge financial support from NSF, AFOSR, and the Welch Foundation. [Preview Abstract] |
Tuesday, March 15, 2016 1:03PM - 1:15PM |
F16.00008: Ultrafast Manipulation of the Valley Coherence in Monolayer WSe$_{\mathrm{2}}$ Ziliang Ye, Dezheng Sun, Tony Heinz The valley degree of freedom in solids has been proposed as a pseudospin carrier for the future quantum electronics. Valley polarization has been created in transition metal dichalcogenide monolayers using circularly polarized light and the existence of coherence in the valley exciton pseudospin has been experimentally demonstrated$^{\mathrm{1}}$. The degeneracy of the valley degeneracy has recently been lifted both by the application of magnetic fields and dynamically by the optical Stark effect with circularly polarized light$^{\mathrm{2-3}}$. Here we demonstrate the all-optical manipulation of valley exciton coherence, i.e., of the valley exciton pseudospin, by the optical Stark effect. Using below-bandgap circularly polarized light, we rotate the valley exciton pseudospin~on the femtosecond time scale. Both the direction and speed of the rotation can be optically controlled by tuning the dynamic phase of excitons in opposite valleys. In addition, by varying the time delay between the excitation and control pulses, we probe the lifetime of the intervalley coherence in monolayer WSe$_{\mathrm{2}}$. \newline 1. X. Xu, W. Yao, D. Xiao, and T. F. Heinz, Nature Physics \textbf{10}, 343 (2014). \newline 2. E. J. Sie, J. W. McIver, Y.-H. Lee, L. Fu, J. Kong, and N. Gedik, Nature Materials \textbf{14}, 290 (2014). \newline 3. J. Kim, X. Hong, C. Jin, S. F. Shi, C. Y. S. Chang, M. H. Chiu, L. J. Li, and F. Wang, Science \textbf{346}, 1205 (2014). [Preview Abstract] |
Tuesday, March 15, 2016 1:15PM - 1:27PM |
F16.00009: Probing ultrafast valley dynamics in 2D semiconductors via time-resolved Kerr rotation Jiani Huang, Thang Hoang, Tian Ming, Jing Kong, Maiken Mikkelsen Monolayer transition metal dichalcogenides (TMDCs) offer a tantalizing platform for controlling spin and valley degrees of freedom, enabling future optoelectronic devices with enhanced and novel functionalities. Here, we experimentally probe the valley dynamics in monolayer MoS$_2$ and WSe$_2$ using time-resolved Kerr rotation (TRKR) from $T=10\;\mathrm{K}$ to $300\;\mathrm{K}$. This pump-probe technique offers sub-picosecond temporal resolution, providing insight into ultrafast valley dynamics inaccessible by polarized and time-resolved photoluminescence spectroscopy. Bi-exponential decay dynamics were observed for both materials at low temperatures. Strong long-range exchange interactions between the K valleys led to a rapid exciton valley depolarization time ($<$ 10 ps), while the valley polarization of the trion and defect states decays within several tens of ps. Moreover, spatial distributions of the TRKR amplitude across monolayer flakes indicated weaker valley polarizations near the edges of MoS$_2$, which is likely associated with the Mo- or S-zigzag terminations at the boundaries. These temporal and spatial TRKR measurements reveal insight into the complex dynamics of valley excitonic states in monolayer TMDCs. [Preview Abstract] |
Tuesday, March 15, 2016 1:27PM - 1:39PM |
F16.00010: Intervalley double resonance processes in MoS$_2$ Yuanxi Wang, Bruno Carvalho, Leandro Malard, Cristiano Fantini, Vincent Crespi, Marcos Pimenta Intervalley scattering plays a significant role in electronic energy dissipation in semiconductors. We investigate the intervalley scattering of monolayer and few-layer MoS$_2$, by combining density functional theory calculations and resonant Raman spectroscopy probed by up to 20 laser excitation energies. We observe that two Raman peaks within 420-460 cm$^{-1}$ are dispersive over a small range of laser energy, a clear signature of second-order processes involving intervalley scattering. Both modes involve LA and TA phonons at or near the K point. A third Raman peak at 466 cm$^{-1}$ shows a strong intensity dependence on the layer number and is assigned 2LA(M). Our results invalidate previous Raman peak assignment proposals and open up a better understanding of double resonance processes in transition metal dichalcogenides. [Preview Abstract] |
Tuesday, March 15, 2016 1:39PM - 1:51PM |
F16.00011: Interlayer Interaction Effects and Spin-Pseudospin Transfer in 2D MoSe$_{2}$-WSe$_{2}$ Heterostructures John Schaibley, Pasqual Rivera, Kyle Seyler, Hongyi Yu, Jiaqiang Yan, David Mandrus, Wang Yao, Xiaodong Xu Heterostructures composed of MoSe$_{2}$-WSe$_{2}$ monolayer semiconductors host spatially indirect interlayer excitons, which are bound states of an electron in the MoSe$_{2}$ layer and a hole in the WSe$_{2}$ layer. Interlayer excitons are observable in photoluminescence experiments as a low energy peak whose spectral position is consistent with the predicted type-II band alignment. The electron and hole which form the interlayer exciton are localized in momentum space valleys that occur at the (K and -K) corners of the Brillouin zone. To probe this interlayer valley physics, we perform two color pump-probe measurements to investigate the interactions between intralayer excitons in the heterostructure, resonantly pumping excitons in one layer and probing excitons in the other layer. We observe evidence of interlayer interaction effects in the nonlinear differential reflection spectra. Polarization dependent spectroscopy reveals evidence of interlayer spin-valley pseudospin transfer. [Preview Abstract] |
Tuesday, March 15, 2016 1:51PM - 2:03PM |
F16.00012: The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures. Han Wang, Junhyeok Bang, Yiyang Sun, Liangbo Liang, Damien West, Vincent Meunier, Shengbai Zhang The success of van der Waals (vdW) heterostructures made of graphene, metal dichalcogenides, and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that vdW heterostructues can exhibit ultra-fast charge transfer despite the weak binding of these heterostructures. Using time-dependent density functional theory molecular dynamics, we find that the collective motion of excitons at the interface lead to plasma oscillations associated with optical excitation. Furthermore, instability of these oscillations explain the rapid charge transfer across the interface and are shown to be a general feature of vdW heterostructures provided they have a critical minimum dipole coupling. Application to the MoS2/WS2 heterostructure yields good agreement with experiment, indicating near complete charge transfer within a timescale of 100 fs. [Preview Abstract] |
Tuesday, March 15, 2016 2:03PM - 2:15PM |
F16.00013: Layer- and Frequency-dependent Second-harmonic Generation from GaSe Atomic Crystals Yanhao Tang, John A. McGuire, Chih Wei Lai, Krishna C. Mandal GaSe is a layered semiconductor with an indirect bandgap at about 2.0 eV only $\sim$20 meV below the direct bandgap at room temperature. Atomically thin GaSe crystals are expected to exhibit an increasing bandgap. This can be probed through the strong nonlinear optical response of GaSe. We report optical second-harmonic generation (SHG) in reflection from GaSe atomic crystals of 1 to $>$ 100 layers on a Si substrate with a 90 nm SiO$_2$ layer. Room-temperature measurements were performed with fundamental photon energies of 0.85 to 1.4 eV as well as at 1.58 eV. By accounting for multilayer interference, the layer-dependent SHG intensity data are fit to obtain the magnitude of the second-order nonlinear optical susceptibility, $\chi^{(2)}$. For samples thicker than $\sim$7 layers, we find $|\chi^{(2)}|=80\pm 18$~pm/V, consistent with reported values for bulk GaSe. For samples $\leq$6 layers, $|\chi^{(2)}|$ is reduced compared to that in thicker samples and shows a minimum in trilayer samples. The frequency-dependence of the SHG response suggests that this reduction of $|\chi^{(2)}|$ in the few-layer region is due to increase of the direct bandgap. [Preview Abstract] |
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