Pulse-duration dependence of laser-induced modifications inside silicon.

Amlan Das,Olivier Uteza,David Grojo,Andong Wang

doi:10.1364/oe.398984

Abstract

The advent of ultrafast infrared lasers provides a unique opportunity for direct fabrication of three-dimensional silicon microdevices. However, strong nonlinearities prevent access to modification regimes in narrow gap materials with the shortest laser pulses. In contrary to surface experiments for which one can always define an energy threshold to initiate modifications, we establish that some other threshold conditions inevitably apply on the pulse duration and the numerical aperture for focusing. In an experiment where we can vary continuously the pulse duration from 4 to 21 ps, we show that a minimum duration of 5.4 ps and a focusing numerical aperture of 0.85 are required to successfully initiate modifications. Below and above thresholds, we investigate the pulse duration dependence of the conditions applied in matter. Despite a modest pulse duration dependence of the energy threshold in the tested range, we found that all pulse durations are not equally performing to achieve highly reproducible modifications. Taken together with previous reports in the femtosecond and nanosecond regimes, this provides important guidelines on the appropriate conditions for internal structuring of silicon.

Highlights

The emergence of ultrafast laser writing in narrow bandgap materials offers the possibility for directly creating microdevices with 3D architectures for application fields as important as micro-electromechanics and silicon photonics
This leads to important dependences to the various experimental parameters making hard to make comparisons between experiments where the pulse duration is changed. This strong interplay is causing an increased sensitivity to fluctuations in the experimental conditions. It is the subsequent lack of repeatability in the observations that has likely limited the conclusions of a first attempt in investigating the pulse duration dependence of this problem in the picosecond regime [23]
The beam is injected into a stretcher arrangement consisting of two aluminum Littrow gratings (GR50-0616, Thorlabs) having 600 groove lines/mm to introduce negative group delay dispersion that results in chirping the laser pulses

Summary

Introduction

The emergence of ultrafast laser writing in narrow bandgap materials offers the possibility for directly creating microdevices with 3D architectures for application fields as important as micro-electromechanics and silicon photonics. The investigations for internal modification of silicon have been focused on the short-wave infrared region of the spectrum (SWIR) and very modest dependence to the wavelength parameter is found as long as multiphoton absorption initiates localized energy deposition near the focus in an originally transparent Si target [8,9] This is very similar to how visible or near-infrared femtosecond pulses are used for 3D writing applications inside dielectrics. As we will show in this paper, the controversy persists because the space-time localization of light inside Si-bulk rely on a stronger interplay between nonlinear processes across the focus than for dielectrics [8,21,24,25] This leads to important dependences to the various experimental parameters making hard to make comparisons between experiments where the pulse duration is changed. We deal only with pulses of high temporal contrast, the latter being another critical parameter [26]

Methods

Results

Conclusion