Ionisation processes and laser induced periodic surface structures in dielectrics with mid-infrared femtosecond laser pulses

George D Tsibidis,Emmanuel Stratakis

doi:10.1038/s41598-020-65613-w

George D Tsibidis, Emmanuel Stratakis

Open Access

https://doi.org/10.1038/s41598-020-65613-w

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Abstract

Irradiation of solids with ultrashort pulses and laser processing in the mid-Infrared (mid-IR) spectral region is a yet predominantly unexplored field with a large potential for a wide range of applications. In this work, laser driven physical phenomena associated with processes following irradiation of fused silica (SiO2) with ultrashort laser pulses in the mid-IR region are investigated in detail. A multiscale modelling approach is performed that correlates conditions for formation of perpendicular or parallel to the laser polarisation low spatial frequency periodic surface structures for low and high intensity mid-IR pulses (not previously explored in dielectrics at those wavelengths), respectively. Results demonstrate a remarkable domination of tunneling effects in the photoionisation rate and a strong influence of impact ionisation for long laser wavelengths. The methodology presented in this work is aimed to shed light on the fundamental mechanisms in a previously unexplored spectral area and allow a systematic novel surface engineering with strong mid-IR fields for advanced industrial laser applications.

Highlights

Irradiation of solids with ultrashort pulses and laser processing in the mid-Infrared spectral region is a yet predominantly unexplored field with a large potential for a wide range of applications
These results demonstrate the need to describe in more detail the ultrafast dynamics following excitation with mid-IR sources as they can potentially influence the morphological changes on the material[43]
It is known that while an extensive research has been conducted towards elucidating laser-induced growth of damage for irradiation of SiO2 with IR pulses, little is known about the effects of electron excitation with longer wavelength pulses

Summary

Introduction

Irradiation of solids with ultrashort pulses and laser processing in the mid-Infrared (mid-IR) spectral region is a yet predominantly unexplored field with a large potential for a wide range of applications. A systematic analysis of the role of excitation levels for conditions directly related to formation of sub-wavelength LIPSS through the investigation of surface plasmon excitation-based mechanisms revealed significantly different effects, such as surface plasmons (SP) with smaller confinement, longer lifetime and larger decay lengths for mid-IR pulses compared to irradiation with lower wavelengths[43]. These results demonstrate the need to describe in more detail the ultrafast dynamics following excitation with mid-IR sources as they can potentially influence the morphological changes on the material[43]. An interesting question that needs to be explored is whether conditions for the formation LSFLǁ and LSFL ⊥ can, be achieved following irradiation of SiO2 with mid-IR sources

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