Femtosecond laser direct writing in SiO₂‐Al₂O₃ binary glasses and thermal stability of Type II permanent modifications

Abstract

We investigate the potential of fabricating thermally stable refractive index contrasts using femtosecond (fs) near-infrared (IR) radiation in aluminosilicate glasses. A set of pure SiO 2-Al 2 O 3 glasses are manufactured, characterized (density and Raman) and investigated after being irradiated by fs laser within the Type II regime. The formation of nanogratings is identified and studied using quantitative birefringence measurements. Their thermal stability is then investigated through 30min step isochronal annealing (up to 1250°C). For both SiO 2 and 50SiO 2-50Al 2 O 3 compositions, the normalized birefringence does not decrease when tested up to 1100°C, while for the 4,6 mol% GeO 2-SiO 2 erased for 20% at 1000°C.

Thermal Stability of Type II Modifications by IR Femtosecond Laser in Silica-based Glasses

“…Similar behavior of the birefringence has been observed in SiO 2 [9,24], 50SiO 2 -50Al 2 O 3 pellet [24] and slightly Ge-doped optical fibers [8]. Briefly, it is assumed that this observed unexpected increase is related to some contribution of residual part of Type I (defects, densification, and related stress), which erased at low temperature, resulting in a higher birefringence response [24]. As the annealing temperature increases, the induced stress relaxes gradually, lessening the refractive index and further decreasing retardance.…”

“…As we can see, at the beginning, there is a slight increase of the retardance value from 600 • C to 800 • C, and it is more obvious for germanosilicate glasses with less GeO 2 content. Similar behavior of the birefringence has been observed in SiO 2 [9,24], 50SiO 2 -50Al 2 O 3 pellet [24] and slightly Ge-doped optical fibers [8]. Briefly, it is assumed that this observed unexpected increase is related to some contribution of residual part of Type I (defects, densification, and related stress), which erased at low temperature, resulting in a higher birefringence response [24].…”

“…Such index change demonstrates thermal stability at 1000 ºC for a few hours [9], which is a critical advantage for its high temperature application. According to previous research, fs-Type II modifications of SiO2 (Suprasil CG ® ) happen above an energy threshold of 0.4 μJ for laser polarization parallel to the laser scanning (λ = 1030 nm, Δtp = 300 fs, RR = 100 kHz, and NA = 0.6) [24]. Therefore, in order to study the writing kinetic of type-II modifications, we performed the experiments by varying pulse energy from 0.3 μJ to 4 μJ.…”

“…At higher temperatures, the decrease in R norm (T) is accelerated and neither retardance nor refractive index contrast can be found at 650 • C for Borofloat33 ® and at 1025 • C for ULE ® , respectively. The studies of mechanisms leading to decay in retardance at high temperature is exemplified in Reference [24] and is attributed to the erasure of point defects below 500 • C. Then there are at least two additional contributions: the first one (intermediate temperature range, typ. 0.8T a to T a ) is related to the stress-induced birefringence generated inside and outside the nanolayers; the second one (sharp final decay above T a ) is related to the nanotexturation itself (porosity and periodicity) leading to form birefringence.…”

An Overview of the Thermal Erasure Mechanisms of Femtosecond Laser‐Induced Nanogratings in Silica Glass

“…There are numerous studies on the thermal stability of optical properties within the type II regime, i.e., retardance in bulk samples, [ 10,12,13,15–19 ] but also Bragg wavelength shift [ 20 ] and/or reflectivity of FBGs. [ 21–29 ] However, there are only few studies on the erasure kinetics of the underlying mechanisms supporting these optical properties as we have summarized in the previous paragraphs.…”