Exciton-enhanced electrooptic effect in GaN: a time-dependent GW study*

 The Franz-Keldysh effect refers to the change in the optical absorption with an electric field applied to a semiconductor. However, previous interpretations of this phenomenon often overlook the significant role played by excitonic effects (electron-hole interactions), especially near the absorption spectrum edge. Therefore, an accurate incorporation of electron-hole interactions (excitonic effects) is essential for a comprehensive understanding of the Franz-Keldysh phenomenon. In this work, based on an ab initio time-dependent adiabatic GW approach, we investigate the modifications in the optical absorption induced by a DC electric field in gallium nitride (GaN), which is a wide bandgap semiconductor with numerous practical applications, particularly in power devices. Our findings show that the prominent variations at the spectral edge of the optical absorption of GaN are dominated by excitonic effects, which cannot be captured by the independent-particle interpretation of the Franz-Keldysh effect. Additionally, we study second harmonic generation in the system and discuss potential applications.