Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN: Effect of Excitation-induced Dephasing

TL;DR

This study investigates the slow rise in differential reflectivity spectra of excitons in GaN, attributing it to excitation-induced dephasing effects through combined experimental and theoretical analysis.

Contribution

It introduces a numerical model incorporating nonlinear dephasing to explain the slow spectral rise observed in GaN excitons.

Findings

Slow rise in DRS due to excitation-induced dephasing

Beats between A-B excitons only for positive delay

Model confirms density-dependent dephasing explains observations

Abstract

Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found \textit{only for positive time delay} in both PP and FWM experiments. The rise time at negative time delay for the differential reflection spectra was much slower than the FWM signal or PP differential transmission spectroscopy (DTS) at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing (EID), that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.