Ultrafast bandgap narrowing and cohesion loss of photoexcited fused silica

Abstract

Coupling and spatial localization of energy on the timescale of the excitation pulse in ultrashort laser irradiated dielectric materials are key elements for enabling processing precision beyond the optical limit. We apply a first-principles model to determine dynamic distortions of energy bands following the rapid increase in the free-carrier population in an amorphous dielectric excited by an ultrashort laser pulse. Fused silica glass is reproduced using a system of (SiO4)4- tetrahedra, where DFT extended to FT-DFT reproduces ground and photoexcited states. Triggered by electronic charge redistribution, a bandgap narrowing of more than 2 eV is shown to occur in fused silica under geometry relaxation. Calculations reveal that the bandgap decrease results from the rearrangement of atoms altering the bonding strength. Despite an atomic movement impacting strongly the structural stability, the observed change of geometry remains limited to 7% of the interatomic distance and occurs on the femtosecond timescale. This structural relaxation is thus expected to take place quasi-instantly following the photon energy flux. Moreover, under intense laser excitation, fused silica loses its stability when an electron temperature of around 2.8 eV is reached. Further increase of the excitation energy leads to the collapse of both structure and bandgap.

First-principles study of ultrafast bandgap dynamics in laser-excited $$\alpha$$-quartz

“…The bandgap narrowing observed in this work is caused purely by the change of electronelectron interaction at high electron temperatures. Additional bandgap decrease is expected from excitation-induced atomic displacements [23,24] but might happen at longer time delays. Moreover, the laser-dressed electronic structure can experience a bandgap collapse, although this process is transient and is not maintained after the end of the laser pulse [25].…”