Optical Characteristics and Nanoscale Energy Transport in Thin Film Structures Irradiated by Nanosecond-to-Femtosecond Lasers

Abstract

Extensive numerical simulations are rigorously conducted for conductive and radiative heat transfer characteristics in thin silicon structures irradiated by nano-to-femtosecond pulsed lasers. The two-temperature model is used to calculate the carrier and lattice temperatures, respectively. The wave interference effects on reflectivity and absorption are considered by using the thin film optics and the electromagnetic theory. The radiation property of silicon is expressed in terms of lattice temperature and carrier density. The reflectivity of thin film structure exhibits different patterns with variation of laser pulse durations. In femtosecond laser irradiation, the energy transfer between carriers and lattice phonons mostly takes place after laser irradiation is over and then it rapidly heats the ions to much higher temperatures, compared to the long pulse cases. For nanosecond pulse lasers, the carrier and lattice temperature distributions do not show wavy patterns, whereas for subpicosecond pulse lasers, the spatial carrier and lattice temperature distributions appear to be periodic in space because of shorter pulse duration than diffusion time.