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

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.

Exciton mediated self-organization in glass driven by ultrashort light pulses

“…This could explain phase transition from isotropic refractive index increase to birefringent modification reported by several groups. 22,23 Self-organization of exciton-polaritons is qualitatively similar to the exciton pattern formation effect observed in cold exciton gases: 24 in both cases the instability in the exciton (exciton-polariton) gas appears due to dipole-dipole interactions between its constituents. The difference is in the sign of interaction, which is repulsive in Ref.…”

Towards fast femtosecond laser micromachining of fused silica: The effect of deposited energy

“…The hypothesis that v/f << w nla is a fair assumption considering our experimental conditions. Note that w nla is dependent on the pulse energy as can be clearly seen from scanning thermal microscopy of laser-affected zones [20].…”

“…Each single pulse induces structural modifications in the material. These structural modifications can be of various kinds: defects but also formation of smaller SiO 4 -ring structures (as suggested by Raman observations [19,20]) that lead to a gradual densification effect. The hypothesis of laserinduced densification is supported by nano-indentations experiments [21].…”

Femtosecond versus picosecond laser machining of nano-gratings and micro-channels in silica glass

“…The etching rate of fs-laser irradiated regions has been shown to depend upon the stress and densification in the glass in the low fluence regime and by the nanogratings' orientation in the high fluence regime [9,27]. Hence, the formation of nanogratings by 8 ps-pulses and the concurrent build-up of stress in the regions surrounding the laser tracks will have an impact on the two-step machining by laser irradiation and chemical etching.…”