Gain without inversion and enhancement of refractive index via intervalley quantum coherence transfer in hybrid WS2 -metallic nanoantenna systems

Abstract

We theoretically study formation of gain without inversion and resonant enhancement of refractive index with suppressed absorption in a system consisting of a monolayer of transition metal dichalcogenide (${\mathrm{WS}}_{2}$) and a metallic nanoantenna. We show that these processes happen when the hybrid system interacts with a right-circularly polarized laser beam, allowing quantum coherence transfer to happen from the K to ${\mathrm{K}}^{\ensuremath{'}}$ valley. Our results show that the quantum coherence transfer can convert the ${\mathrm{K}}^{\ensuremath{'}}$ valley into a conjugated system wherein the real and imaginary parts of its polarization become similar to the imaginary and real parts of the K valley polarization, respectively. This allows the maximum refractive index in the ${\mathrm{K}}^{\ensuremath{'}}$ valley to occur as the absorption becomes zero. We characterize the conjugated valley system by inspecting the evolution of its Bloch vector as the incident laser frequency varies. The theoretical approach includes density matrix formalism while considering a pure dephasing rate that represents the fast decay of exciton valley polarization in transition metal dichalcogenide monolayers.