Optical Stark shifts of hybrid light-matter states in two-dimensional  semiconductors

LaMountain, Trevor

doi:https://doi.org/10.21985/n2-8g1s-yh12

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Optical Stark shifts of hybrid light-matter states in two-dimensional semiconductors

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Light provides a high-speed and coherent medium for controlling quantum states. In semiconductors, coherent optical effects have been used extensively to lift spin degeneracy on ultrafast timescales and demonstrate high-fidelity control of quantum spin states. Extending this approach to novel pseudospins in monolayer transition metal dichalcogenides (TMDs), in this thesis we examine how the optical Stark effect can be used for selective control of energy levels in different valleys of momentum-space. First, valley-selective optical Stark shifts are studied in bare-TMD excitons. Time-resolved Kerr rotation is used to detect and quantify the effect. Compared to more conventional time-resolved reflectance techniques, we find that Kerr rotation provides a five-fold improvement in measurement sensitivity and a more precise estimate of the energy shift. Next we embed TMD monolayers in planar microcavities to generate strong coupling between excitons and on-resonance cavity photons. This results in the formation of hybrid light-matter exciton-polaritons with optically-addressable valleys. In the final sections of this thesis, we combine these two regimes of light-matter interaction to measure the optical Stark effect in TMD exciton-polaritons. We observe Stark-shifted polariton reflectance spectra with strong polarization contrast, constituting the first measurements of a valley-selective optical Stark shift in exciton-polaritons. The underlying physics of the effect are described using a modified Jaynes-Cummings Hamiltonian, and the measured spectral shifts are understood using a dielectric model of the microcavity structure. This demonstration of valley degeneracy-breaking at picosecond timescales establishes a new method for coherent control of valley phenomena in exciton-polaritons.

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