Ultra-short laser-induced high aspect ratio densification in porous glass

Tatiana E Itina,Oleg S Medvedev,Maxim M Sergeev,Sergey I Kudryashov,Roman A Zakoldaev,Vadim P Veiko,Hongfeng Ma

doi:10.1364/ome.9.004379

Abstract

Multiple ultra-short laser irradiation enabling direct writing of high aspect ratio barriers is used for structuring of nanoporous glass. Shape and morphology of laser-modified regions are examined, and high aspect ratio laser-induced material densification is founded. Experimental results are analyzed by modeling describing laser propagation, non-linear ionization and thermal effects. The role of laser focusing, laser energy and pulse number are examined. Several regimes are distinguished. Particularly, high-aspect ratio densified zones are obtained for the numerical aperture of 0.25, whereas either more symmetric densified regions or spherical cavities are shown to be formed for numerical aperture of 0.4. The resulting laser irradiation conditions required for deep and prolonged densification are explained by a lower ionization rate, leading to the under-critical free electron plasma density, longer filamentation and pulse-to-pulse elongation effects. Furthermore, filling of the porous glass with water is demonstrated to particularly extend the length of the densified region in depth. The presented study provides insights facilitating laser-based fabrication of barriers, membrane and patterns suitable for the environmental gas-phase analysis.

Highlights

Laser-induced formation of complex three-dimensional structures inside transparent optical materials [1,2,3,4] has attracted tremendous attention during last decade due to numerous emerging applications in photonics [1,5], optics [6], as well as in many other industrial [7] and medical fields [8]
These experiments demonstrated that the size of the laser-affected zone increases with laser pulse energy if laser repetition rate becomes larger than a certain critical value
Laser modification of PG samples was performed by a transverse volume scanning using femtosecond laser pulses with wavelength λ = 515 nm, pulse duration τ = 300 fs, maximum pulse energy Ep = 3μJ, operated in the TEM00 mode and at constant repetition rate of 500 kHz

Summary

Introduction

Laser-induced formation of complex three-dimensional structures inside transparent optical materials [1,2,3,4] has attracted tremendous attention during last decade due to numerous emerging applications in photonics [1,5], optics [6], as well as in many other industrial [7] and medical fields [8] Such structures as periodic volume nanogratings [9], lattices [10], voids [11] and channels [12] were fabricated by either single or multiple ultra-short laser irradiation. Ultra-short treatment of optical materials was shown to be promising in multi-pulse regime, both for surface structuring [17,18,19,20] and for material treatment in volume [21,22,23,24,25] These experiments demonstrated that the size of the laser-affected zone increases with laser pulse energy if laser repetition rate becomes larger than a certain critical value. Several effects of scanning speed and laser focusing conditions were discussed [23,27,29]

Methods

Results

Conclusion