Scanning thermal microscopy and Raman analysis of bulk fused silica exposed to lowenergy femtosecond laser pulses

Y Bellouard,E Barthel,A A Said,M Dugan,P Bado

doi:10.1364/oe.16.019520

Abstract

Low energy femtosecond laser pulses locally increase the refractive index and the hydro-fluoric acid etching rate of fused silica. These phenomena form the basis of a direct-write method to fabricate integrated glass devices that are of particular interest for optofluidics and optomechanical applications. Yet the underlying physical mechanism behind these effects remains elusive, especially the role of the laser polarization. Using Scanning Thermal Microscope and Raman spectrometer we observe in laser affected zones, a localized sharp decrease of the thermal conductivity correlated with an increased presence of low-number SiO(2) cycles. In addition, we find that a high correlation exists between the amount of structural changes and the decrease of thermal conductivity. Furthermore, sub-wavelength periodic patterns are detected for high peak power exposures. Finally, our findings indicate that, to date, the localized densification induced by femtosecond laser pulses remains well below the theoretical value achievable in mechanically densified silica.

Highlights

Femtosecond laser pulses interact in unusual ways with matter
Low energy pulses applied on fused silica can (i) locally increase the material refractive index [1], and (ii) increase its etching rate when exposed to hydrofluoric acid (HF) [2]
Using a high-resolution Scanning Thermal Microscope (SThM), we recently discovered that femtosecond laser pulses leave thermal footprints in the material [13]

Summary

Introduction

Femtosecond laser pulses interact in unusual ways with matter. In particular, low energy pulses applied on fused silica can (i) locally increase the material refractive index [1], and (ii) increase its etching rate when exposed to hydrofluoric acid (HF) [2]. The enhanced etching rate is strongly polarization dependent [5] These phenomena are of particular interest for waveguides fabrication [3] and for optofluidics and optomechanical device manufacturing [4]. Taylor et al and thoroughly discussed in [6]) attributes the enhanced etching rate to the presence of oriented cracks that form capillaries, which speed the acid progression inside the glass. In this model, different polarization states lead to different crack orientations. Regime II is associated with moderate pulse energy (typically above 200nJ) and/or longer pulse duration (above 200 fs) which produce what has been labeled by Shimotsuma et al [7]

Methods

Results

Discussion

Conclusion