Probing ultrafast carrier dynamics and nonlinear absorption and refraction in core-shell silicon nanowires

TL;DR

This study explores ultrafast carrier relaxation, nonlinear absorption, and refraction in silicon nanowires with crystalline cores and amorphous shells, revealing complex dynamics influenced by structure and excitation density.

Contribution

It provides detailed insights into carrier dynamics and nonlinear optical properties of core-shell silicon nanowires using femtosecond spectroscopy and z-scan techniques.

Findings

Faster relaxation with increased carrier density.

Two-photon absorption coefficient ~3 cm/GW.

Nonlinear refraction coefficient -2.5x10^-4 cm^2/GW.

Abstract

We investigate the relaxation dynamics of photogenerated carriers in silicon nanowires consisting of a crystalline core and a surrounding amorphous shell, using femtosecond time-resolved differential reflectivity and transmission spectroscopy at photon energies of 3.15 eV and 1.57 eV. The complex behavior of the differential transmission and reflectivity transients is the mixed contributions from the crystalline core and the amorphous silicon on the nanowire surface and the substrate where competing effects of state filling and photoinduced absorption govern the carrier dynamics. Faster relaxation rates are observed on increasing the photo-generated carrier density. Independent experimental results on crystalline silicon-on-sapphire help us in separating the contributions from the carrier dynamics in crystalline core and the amorphous regions in the nanowire samples. Further, single beam z-scan nonlinear transmission experiments at 1.57 eV in both open and close aperture configurations yield two-photon absorption coefficient \$beta$ (~3 cm/GW) and nonlinear refraction coefficient \$gamma$ (-2.5x10^-4 cm2/GW).