Excitation and relaxation dynamics in ultrafast laser irradiated optical glasses

Abstract

We discuss the dynamics of ultrashort pulsed laser excitation in bulk optical silica-based glasses (fused silica and borosilicate BK7) well-above the permanent modification threshold. We indicate subsequent structural and thermomechanical energy relaxation paths that translate into positive and negative refractive index changes, compression and rarefaction zones. If fast electronic decay occurs at low excitation levels in fused silica via self-trapping of excitons, for carrier densities in the vicinity of the critical value at the incident wavelength, persistent long-living absorptive states indicate the achievement of low viscosity matter states manifesting pressure relaxation, rarefaction, void opening and compaction in the neighboring domains. An intermediate ps-long excited carrier dynamics is observed for BK7 in the range corresponding to structural expansion and rarefaction. The amount of excitation and the strength of the subsequent hydrodynamic evolution is critically dependent on the pulse time envelope, indicative of potential optimization schemes.

An excitonic model for the electron–hole plasma relaxation in proton-irradiated insulators

“…This behaviour must be contrasted against the case of sapphire (Al 2 O 3 ) for which a much longer (about 100 ps) free electron-hole pair relaxation time has been reported [53], along with a much smaller activation energy for detrapping, between 0.04 an 0.51 eV [54]. The case of borosilicate crown glass BK7 is also relevant because long (exceeding 10 ps) free electron-hole pair relaxation times-longer than for a-SiO 2 -have been reported both using optical [15] and proton [24,[26][27][28] probes. Borosilicate glasses present more non-bridging oxygen atoms than a-SiO 2 because boron is a trivalent cation (B 3+ ) which disrupts the silica tetrahedral network and can create localised soft modes [55].…”

“…These groundbreaking femtosecond pump-probe experiments found that free electron-hole pairs relax into STEs in about 150 fs. Subsequent measurements indicate a relaxation time between 50 and a e-mail: l.stella@qub.ac.uk (corresponding author) 220 fs, suggesting a longer free electron-hole pair relaxation time as the laser intensity is increased [15,16]. Grojo et al [17] also observed an abrupt increase of the free electron-hole pair relaxation time in a-SiO 2 as the charge density exceeds 10 20 cm −3 .…”

Volume photoinscription of glasses: three-dimensional micro- and nanostructuring with ultrashort laser pulses

“…Two regions of interest emerge; a low carrier density region preceding the focal point and the focal region. It is interesting to note that in the low density areas preceding the focal region, the carrier decay is sub-ps in fused silica and few ps in BK7, while in the focal region the carrier lifetime seems significantly larger [68]. The fast electronic decay is accompanied by fast increase of the index in the low carrier density regions, and by a negative index change in the high carrier density regions.…”

A Strategy for Achieving Smooth Filamentation Cutting of Transparent Materials with Ultrafast Lasers

“…Until these times have elapsed, it can be assumed that the shock wave and any related destruction do not propagate into the surrounding material from a highly heated filament or from the space between neighbour filaments. When substituting into Equation 3the characteristic value of the sound velocity in the material cs ≈ 6 × 10 5 cm/s [47], the submicron radius of the filaments used in this method rf = 0.25-0.5 μm and, for example, s1≈ 2.5-5 μm, one gets the estimates: tP1 ≈ 0.08-0.17 ns and tP2 ≈ 0.4-0.8 ns. As seen, teq << tP1, tP2.…”

“…(6) Here te-h is a characteristic relaxation time of bound electrons in the electron-hole plasma, which is obtained at relatively low fluences or VED, i.e., electrons located at the electronic levels of the material excited by laser photons [21]. Numerically te-h ranges from values close to teq (i.e., from 0.15 -2 ps mentioned above) to few tens of picoseconds, when the formation of laser-induced self-trapped excitons and related defects is taken into account [47,48]. Formation of other long-living states can also contribute.…”