Origin of the refractive-index increase in laser-written waveguides in glasses

Abstract

We present firm evidence that the mechanism for the refractive-index increase in fused silica caused by irradiation with ultrafast intense laser pulses is the densification of glass. This conclusion is based on the correlation observed between the refractive-index values in waveguides in silica produced by focused femtosecond laser pulses and the shift of the central frequency of ω4 (TO) band (Si–O stretching mode) in micro-Raman spectra. These data were compared with the relation of the Raman shift to density and to refractive index changes in glasses modified by high pressure or irradiation. We conclude that the measured refractive-index increase in silica waveguides can be explained by densification of glass and exclude other hypothesis such as fictive-temperature effect, color center formation, etc.

Laser-induced structural changes in pure GeO2 glasses

“…The TO (857 cm − 1 ) band position decreases by only 2 cm − 1 . As it was observed on SiO 2 [13], GeO 2 samples densified or with increasing fictive temperature show a similar evolution of the band position and intensity but with very distinct amplitudes. For instance the TO (857 cm − 1 ) mode position decreases by 30 cm − 1 for a densified glass at 15.4 GPa and decreases by 2 cm − 1 if the fictive temperature is raised by 140°C.…”

“…Its origin was elucidated using Raman spectroscopy [13] and is linked to high pressure transformations induced during the laser treatment. It was confirmed by X-ray diffraction experiments [14], that strong changes of the Si-Si and O-O pair correlation distances occur due to the irradiation by fs-laser pulses.…”

Refractive index and dispersion control of ultrafast laser inscribed waveguides in gallium lanthanum sulphide for near and mid-infrared applications

“…This variation beyond the area modified by the laser is attributed to residual stresses induced in the bulk by the ULI technique. The main mechanisms considered responsible for the refractive-index increase, observed in femtosecond-laser-written waveguides are densification [38] and stress [39] induced by the laser in the glass substrate. Therefore the area modified by the laser will inevitably impose a certain amount of stress to its adjacent regions even though unmodified by the laser, thus causing the refractive index variation to extend beyond the modified region.…”

Fabrication of Micro-optical Components in Glass

“…The mechanism responsible for increasing the refractive index of glass during femtosecond laser irradiation has not been fully determined and is still being investigated. The current consensus is that refractive index modification can most likely be attributed to a combination of color-center formation and densification of glass; however, a quantitative model has yet to be developed [9,10].…”