Abstract
Time-dependent responses of materials to an ultrashort optical pulse carry valuable information about the electronic and lattice dynamics; this research area has been widely studied on novel two-dimensional materials such as graphene, transition metal dichalcogenides (TMDs) and topological insulators (TIs). We report herein a time-resolved and angle-resolved photoemission spectroscopy (TRARPES) study of WSe2, a layered semiconductor of interest for valley electronics. The results for below-gap optical pumping reveal energy-gain and -loss Floquet replica valence bands that appear instantaneously in concert with the pump pulse. Energy shift, broadening, and complex intensity variation and oscillation at twice the phonon frequency for the valence bands are observed at time scales ranging from the femtosecond to the picosecond and beyond. The underlying physics is rich, including ponderomotive interaction, dressing of the electronic states, creation of coherent phonon pairs, and diffusion of charge carriers – effects operating at vastly different time domains.
Highlights
Optical signal processing holds the key to developing ultrafast electronics
Using a probe beam (84-fs pulses, 28-eV photons, 1-kHz rate), a spectrum obtained without application of the pump pulses (Fig. 2a) shows VB1 and VB2
Since the carrier relaxation time is much longer than 1 ms, the surface photovoltage reaches a steady state in the experiment
Summary
Optical signal processing holds the key to developing ultrafast electronics. A fundamental question is how a material responds and relaxes as a function of time after a delta excitation by an optical pulse. Our study focuses on the short-time behavior of optical excitation of WSe2, which is a member of a vast TMD family, many of which exhibit novel properties that have galvanized the attention of the condensed matter physics community[6,7,8]. Referring Brillouin zones shown, the material is an indirect semiconductor with a gap of 1.22 eV between the valence band maximum at Γ and the conduction band minimum at the bottom of a “valley” between Γ and K (Fig. 1d)[10]. 1.64 eV, is located at the K point, where the two top valence bands (VBs) VB1 and VB2, separated by a spin-orbit splitting of 0.5 eV11, each exhibit a maximum. The pump laser photon energy, 1.55 eV, is below the direct gap, and optical absorption by indirect excitation is weak
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