Abstract
Selective breaking of degenerate energy levels is a well-known tool for coherent manipulation of spin states. Though most simply achieved with magnetic fields, polarization-sensitive optical methods provide high-speed alternatives. Exploiting the optical selection rules of transition metal dichalcogenide monolayers, the optical Stark effect allows for ultrafast manipulation of valley-coherent excitons. Compared to excitons in these materials, microcavity exciton-polaritons offer a promising alternative for valley manipulation, with longer lifetimes, enhanced valley coherence, and operation across wider temperature ranges. Here, we show valley-selective control of polariton energies in WS2 using the optical Stark effect, extending coherent valley manipulation to the hybrid light-matter regime. Ultrafast pump-probe measurements reveal polariton spectra with strong polarization contrast originating from valley-selective energy shifts. This demonstration of valley degeneracy breaking at picosecond timescales establishes a method for coherent control of valley phenomena in exciton-polaritons.
Highlights
Selective breaking of degenerate energy levels is a well-known tool for coherent manipulation of spin states
Even in GaAs quantum wells, polarization-selective optical Stark shifts of polaritons have not yet been demonstrated. Expanding these observations to polaritons in different materials like transition metal dichalcogenides (TMDs) is in itself an important step in verifying that polariton optical Stark effect (OSE) physics can be broadly applied to describe the effect in polaritons with larger Rabi splitting, faster decay rates, and larger inhomogeneous broadening
Since TMD polaritons inherit the polarizationdependent selection rules from their excitonic component[16,17,18], a circularly-polarized pump will induce a Stark shift of polaritons in only one valley, leaving polaritons in the opposite valley unperturbed (Fig. 1b). This valley-selective shift manifests as a change in the polariton reflectance spectrum: while the subresonant light is present, the upper polariton (UP) and lower polariton (LP) peaks shift to higher energy and change amplitude (Fig. 1c)
Summary
Selective breaking of degenerate energy levels is a well-known tool for coherent manipulation of spin states. Ultrafast pumpprobe measurements reveal polariton spectra with strong polarization contrast originating from valley-selective energy shifts This demonstration of valley degeneracy breaking at picosecond timescales establishes a method for coherent control of valley phenomena in exciton-polaritons. Using circularly-polarized laser pulses, the optical Stark effect (OSE) can provide large effective magnetic fields for breaking spin degeneracy on sub-picosecond timescales, offering an all-optical alternative that enables more precise control of quantum states[1,2]. Because optical transitions in each valley are governed by polarization-sensitive selection rules, ultrafast optical Stark shifts can be used to break valley degeneracy[4,5] and control valley pseudospin in excitons[6], suggesting applications in quantum information. The different electronic and optical properties of TMDs can enable additional features of polariton optical Stark effects, like the previously unobserved polarization-selective polariton Stark shift
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