Femtosecond laser induced density changes in GeO_2 and SiO_2 glasses: fictive temperature effect [Invited]

Abstract

Density changes of GeO 2 and SiO 2 glasses subjected to irradiation by tightly focused femtosecond pulses are observed by Raman scattering. It is shown that densification caused by the void formation in GeO 2 glass is very similar to the changes under hydrostatic pressure. In contrast, the experimental observations in SiO 2 glass could be explained by pressure effect or by the fictive temperature anomaly, i. e., a resultant smaller specific volume of the glass (a denser phase) at a high thermal quenching rate. Density changes of GeO 2 and SiO 2 glasses are opposite upon close-to-equilibrium heating; this gives new insights into the mechanisms of densification under highly non-equilibrium conditions: fs-laser induced micro-explosions, heating and void formation. The pressure and temperature effects of glass modification by ultra-short laser pulses are discussed considering applications in optical memory, waveguiding, and formation of micro-optical elements.

Bragg diffraction gratings formed in bulk fused silica by femtosecond Bessel beams

“…It should also be noted, that forming of modification lines was also sensitive to incident beam polarization and polarization along the direction of writing was chosen for the final grating. For modifications formed by Bessel beams there was no observed birefringence or writing polarization sensitivity and grating performance indicated the refractive index modulation to be 0.001 and is close to those achieved for isotropic modifications by other authors [3]. Table 1 outlines the parameters chosen for each type of grating after evaluating gratings of different geometries as well as main measured characteristics which fit well theoretic predictions [4].…”

Femtosecond laser induced phenomena in transparent solid materials: Fundamentals and applications

“…Depending on the regime of laser interaction, particularly on the incident fluence, pulse duration, wavelength, laser polarization, focusing conditions and scanning speed, femtosecond laser pulses can induce either positive isotropic refractive index changes (Δn ~ 10 -4 -10 -3 ) or void-like rarefaction regions of lower density with compression shells, or self-organized nanoscale layered structures resulting in formation of birefringence with overall negative index changes [86,95,96,290,[296][297][298]. Several mechanisms are considered to be responsible for index changes: (i) thermal mechanisms (fictive temperature model), when higher density local structures are created after fast quenching from a high temperature melt [297,299], (ii) non-thermal mechanisms, when index changes originate from the generation of color centers [66,86,98,157,300,301], (iii) density changes originating from defects induced structural network reorganization (defects induced densification) [86,297,302], and (iv) mechanical contributions, when compaction and rarefaction of material result from the pressure wave release [53,54,303,304].…”

“…In addition, both the sign of index change and the mechanism underlying such change are also dependent on the glass compositions [299,310]. Bressel et al report that densification caused by the void formation in GeO 2 glass is responsible for the changes of refractive index, which is very similar to the changes under hydrostatic pressure.…”

Correlation of the refractive index change at the surface and inside phosphate glass upon femtosecond laser irradiation

“…The presence of Newton rings in irradiated phosphate glass implies a sharp interface, which suggests that the plasma relaxes and transfers energy to the lattice, thereby melting the material. The sharp interface is formed upon rapid solidification into a less dense structure, which indicates a normal glass transition [17,18]. A different mechanism takes place in fused silica at similar plasma densities (n e 1.4 × 10 22 cm −3 ), where surface depression and lattice densification is observed [8].…”

“…A different mechanism takes place in fused silica at similar plasma densities (n e 1.4 × 10 22 cm −3 ), where surface depression and lattice densification is observed [8]. This behavior is generally attributed to the anomalous glass transition in fused silica, which increases the material density upon rapid solidification [17,18]. Yet the absence of Newton rings in the densified region in fused silica indicates the absence of a sharp interface, therefore questioning the presence of melting and opening the possibility of plasma-induced densification [8].…”