Theory of coherent phonons coupled to excitons

TL;DR

This paper develops a microscopic theory describing how coherent phonons interact with bright excitons, predicting polaronic energy shifts and oscillations, validated by simulations in MoS2 monolayers.

Contribution

It introduces a new theoretical framework for exciton-coherent phonon coupling, highlighting the role of bare electron-phonon matrix elements and providing explicit coupling expressions.

Findings

Predicts exciton energy red-shift due to polaronic effects.

Shows oscillations in exciton energy depend on coupled optical modes.

Validates theory with numerical simulations in MoS2 monolayers.

Abstract

The interaction of excitons with lattice vibrations underlies the scattering from bright to dark excitons as well as the coherent modulation of the exciton energy. Unlike the former mechanism, which involves phonons with finite momentum, the latter can be exclusively attributed to {\it coherent phonons} with zero momentum. We here lay down the microscopic theory of coherent phonons interacting with resonantly pumped bright excitons and provide the explicit expression of the corresponding coupling. The coupling notably resembles the exciton-phonon one, but with a crucial distinction: it contains the bare electron-phonon matrix elements rather than the screened ones. Our theory predicts that the exciton energy features a polaronic-like red-shift and monochromatic oscillations or beatings, depending on the number of coupled optical modes. Both the red-shift and the amplitude of the oscillations are proportional to the excitation density and to the square of the exciton-coherent-phonon coupling. We validate our analytical findings through comparisons with numerical simulations of time-resolved optical absorbance in resonantly pumped MoS$_{2}$ monolayers.