Many-body effects in nonlinear optical responses of 2D layered semiconductors

TL;DR

This study investigates many-body interactions in monolayer WSe2 using ultrafast spectroscopy, revealing exciton-exciton interactions and band gap renormalization that influence nonlinear optical responses, with a model fitting experimental data.

Contribution

The paper introduces a model that quantitatively analyzes nonlinear optical spectra of 2D semiconductors considering many-body effects, highlighting Coulomb interactions' role.

Findings

Exciton-exciton interaction causes a blue shift lasting several picoseconds.

Band gap renormalization results in a red shift with tens of picoseconds lifetime.

Free carrier density increases superlinearly with pump power, suggesting Auger recombination.

Abstract

We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2 near its exciton resonance. The observed differential reflectance signals exhibit signatures of strong many-body interactions including the exciton-exciton interaction and free carrier induced band gap renormalization. The exciton-exciton interaction results in a resonance blue shift which lasts for the exciton lifetime (several ps), while the band gap renormalization manifests as a resonance red shift with several tens ps lifetime. Our model based on the many-body interactions for the nonlinear optical susceptibility fits well the experimental observations. The power dependence of the spectra shows that with the increase of pump power, the exciton population increases linearly and then saturates, while the free carrier density increases superlinearly, implying that exciton Auger recombination could be the origin of these free carriers. Our model demonstrates a simple but efficient method for quantitatively analyzing the spectra, and indicates the important role of Coulomb interactions in nonlinear optical responses of such 2D materials.