Transverse Ultrafast Laser Inscription in Bulk Silicon

TL;DR

This paper demonstrates a method for ultrafast laser inscription inside bulk silicon by optimizing spectral, temporal, and spatial parameters, overcoming nonlinear effects to produce controlled modifications.

Contribution

It introduces a triple-optimization approach using femtosecond pulses at 2 μm wavelength to enable reliable transverse laser inscription in silicon.

Findings

Successful in-volume modifications inside silicon using optimized femtosecond pulses.

Establishment of laws governing pulse-to-pulse modification growth.

Demonstration of various line morphologies depending on irradiation conditions.

Abstract

In-volume ultrafast laser direct writing of silicon is generally limited by strong nonlinear propagation effects preventing the initiation of modifications. By employing a triple-optimization procedure in the spectral, temporal and spatial domains, we demonstrate that modifications can be repeatably produced inside silicon. Our approach relies on irradiation at $\approx 2$-$\mu$m wavelength with temporally-distorted femtosecond pulses. These pulses are focused in a way that spherical aberrations of different origins counterbalance, as predicted by point spread function analyses and in good agreement with nonlinear propagation simulations. We also establish the laws governing modification growth on a pulse-to-pulse basis, which allows us to demonstrate transverse inscription inside silicon with various line morphologies depending on the irradiation conditions. We finally show that the production of single-pulse repeatable modifications is a necessary condition for reliable transverse inscription inside silicon.