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

Optical devices fabricated by femtosecond (fs) laser within the Type II regime are of interest for high temperature applications (> 800 °C). fs-Type II regime is characterized by the formation of self-organized nanogratings, which are composed of regularly spaced porous nanolayers with nanopores having a typical size of a few tens of nm. In this work, we first investigate the evolution of the nanopore size distribution as a function of fs-laser writing speed and pulse energy, as well as a function of annealing temperature after fs-laser irradiation. Then, the thermal stability of such nanopores is numerically investigated through the use of the Rayleigh–Plesset (R–P) equation, and is compared with experimental data. The R–P equation provides insights into the temperature range at which the nanopores would ultimately collapse, serving as a design tool for future high temperature fs-Type II based devices. The key role of glass viscosity and nanopore diameter on the overall thermal stability is also discussed.

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Erasure of nanopores in silicate glasses induced by femtosecond laser irradiation in the Type II regime

https://hal.science/hal-03857133/document

Link between Medium and Long-range Order and Macroscopic Properties of Silicate Glasses and Melts

Femtosecond (fs) laser written fiber Bragg gratings (FBGs) are excellent candidates for ultra-high temperature (>800 °C) monitoring. More specifically, Type II modifications in silicate glass fibers, characterized by the formation of self-organized birefringent nanostructures, are known to exhibit remarkable thermal stability around 1000 °C for several hours. However, to date there is no clear understanding on how both laser writing parameters and glass composition impact the overall thermal stability of these fiber-based sensors. In this context, this work investigates thermal stability of Type II modifications in various conventional glass systems (including pure silica glasses with various Cl and OH contents, GeO2-SiO2 binary glasses, TiO2- and B2O3-doped commercial glasses) and with varying laser parameters (writing speed, pulse energy). In order to monitor thermal stability, isochronal annealing experiments (Δt⁓ 30 min, ΔT⁓ 50 °C) up to 1400 °C were performed on the irradiated samples, along with quantitative retardance measurements. Among the findings to highlight, it was established that ppm levels of Cl and OH can drastically reduce thermal stability (by about 200 °C in this study). Moreover, GeO2 doping up to 17 mole% only has a limited impact on thermal stability. Finally, the relationships between glass viscosity, dopants/impurities, and thermal stability, are discussed.

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

The period (
 $$\wedge $$
 ) of nanograting in sapphire varied from 320 to 398 nm with increasing the laser fluence, which is similar to the change trend of period of the near-subwavelength ripples previously observed on the material surface (
 $$0.4<\wedge / \lambda <1$$
 ) (Huang et al. in ACS Nano 3:4062, 2009). The result shows that the interference of the incident laser with the plasma could take place at the high-excited state of internal modified interface and leads to a spatial modulation of the local energy (fluence) distribution. The initial plasma–laser interference and the subsequent nanocrack-assisted plasma–laser coupling were used to explain the growth of nanograting what we have observed experimentally. Using a finite-difference time-domain method, we simulated the redistribution of laser fluence about the nanocracks, which derived from the pre-induced periodic refractive index changes in the focal volume after the acid etching. The experimental result and theoretical simulation can be in good agreement. In addition, we realized the erasing and rewriting of nanograting by using two beams of orthogonally polarized femtosecond laser pulses. This study can provide new proof for the physical mechanism of laser-induced nanograting and offer a reference for the fabrication of nanodevices in the substrate of sapphire.

Evolution of self-organized nanograting from the pre-induced nanocrack-assisted plasma–laser coupling in sapphire

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Miss R. Cicconi

Papers

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