The spin and valley physics in 2-dimensional van der Waals materials provides a unique platform for novel applications in spintronics and valleytronics. 2H phase transition metal dichalcogenides (TMD) monolayers possesses broken inversion symmetry and strong spin-orbit coupling, leading to a coupled spin and valley physics that makes them better candidates for these applications. For practical device applications, spin and valley Hall effect (SVHE) is a good way of charge to spin and charge to valley conversion, making the electrical generation of spin and valley polarization possible. While SVHE has been observed via optical measurements at cryotemperatures below 30 K, the behavior at elevated temperatures and thorough understanding of the data are still lacking. In this work we conduct spatial Kerr rotation (KR) measurements on monolayer tungsten diselenide (WSe2) field effect transistors and study the electrical control and temperature dependence of SVHE. We image the distribution of the spin and valley polarization directly and find clear evidence of the spin and valley accumulation at the edges. We show that the SVHE can be electrically modulated by the gate and drain bias, and the polarization persists at elevated temperatures. We then conduct four-port electrical test reflection spectra measurement and use a drift-diffusion model to interpret the data and extract key parameters. A lower-bound spin/valley lifetime is predicted of 40 ns and a mean free path of 240 nm below 90 K. The spin/valley polarization on the edge is calculated to be ~4% at 45 K. WSe2-on-hBN samples are prepared as well, and the KR measurements on these samples are discussed.
Transition metal dichalcogenides (TMDs) have garnered considerable interest in recent years owing to their layer thickness-dependent optoelectronic properties. In monolayer TMDs, the large carrier effective masses, strong quantum confinement, and reduced dielectric screening lead to pronounced exciton resonances with remarkably large binding energies and coupled spin and valley degrees of freedom (valley excitons). Coherent control of valley excitons for atomically thin optoelectronics and valleytronics requires understanding and quantifying sources of exciton decoherence. In this work, we reveal how exciton-exciton and exciton-phonon scattering influence the coherent quantum dynamics of valley excitons in monolayer TMDs, specifically tungsten diselenide (WSe2), using two-dimensional coherent spectroscopy. Excitation-density and temperature dependent measurements of the homogeneous linewidth (inversely proportional to the optical coherence time) reveal that exciton-exciton and exciton-phonon interactions are significantly stronger compared to quasi-2D quantum wells and 3D bulk materials. The residual homogeneous linewidth extrapolated to zero excitation density and temperature is ~1:6 meV (equivalent to a coherence time of 0.4 ps), which is limited only by the population recombination lifetime in this sample.
The interplay between disorder and Coulomb interactions ubiquitously affects the properties of condensed matter
systems. We examine its role in the nonlinear optical response of semiconductor quantum wells. In particular, we
investigate the coherent coupling strength between exciton resonances that are spectrally split by interface fluctuations.
Previous studies yielded conflicting results. In light of rising interest in semiconductor devices that rely on spatial and/or
temporal coherence, we revisit this problem by applying a newly developed spectroscopy method: electronic two-dimensional
Fourier transform spectroscopy (2DFTS). 2DFTS is a powerful technique for revealing the presence of
coupling and for distinguishing the (coherent or incoherent) nature of such coupling, especially in complex systems with
several spectrally overlapping resonances. Even the most basic information about such complex systems, including the
homogeneous and inhomogeneous linewidths of various resonances, cannot be extracted reliably using conventional
spectroscopic tools. In these new 2DFTS measurements, we did not observe any clear cross peaks corresponding to
coherent couplings between either heavy-hole or light-hole excitons. These measurements allow us to place a
quantitative upper bound on the possible coupling strength in this prototypical system. A modified mean-field theory
reveals a simple yet important relation that determines how the coherent coupling strength depends on the disorder
correlation length and Coulomb interaction length.
Conference Committee Involvement (11)
Ultrafast Phenomena and Nanophotonics XXIX
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Ultrafast Phenomena and Nanophotonics XXIV
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Ultrafast Phenomena and Nanophotonics XX
15 February 2016 | San Francisco, California, United States
Ultrafast Phenomena and Nanophotonics XIX
8 February 2015 | San Francisco, California, United States
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