# A New Approach Toward Extreme Thermal Stability of Femtosecond Laser Induced Modifications in Glasses

Imane Ktafi, Jing Kong, Maxime Cavillon, Bertrand Poumellec, Maureen Yembele, Renaud Valois, Mathieu Allix, Gang-ding Peng, Matthieu Lancry

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Imane Ktafi, Jing Kong, Maxime Cavillon, Bertrand Poumellec, Maureen Yembele, et al. A New Approach Toward Extreme Thermal Stability of Femtosecond Laser Induced Modifications in Glasses. Laser and Photonics Reviews, 2024, 19 (3), pp.2401086. 10.1002/lpor.202401086. [https://hal.science/hal-05466533v1](https://hal.science/hal-05466533v1)

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# A New Approach Toward Extreme Thermal Stability of Femtosecond Laser Induced Modifications in Glasses

Imane Ktafi, Jing Kong, Maxime Cavillon,* Bertrand Poumellec, Maureen Yembele, Renaud Valois, Mathieu Allix, Gang-Ding Peng, and Matthieu Lancry

Imprinting thermally stable transformations by femtosecond laser in glass would benefit the development of optical sensors dedicated to harsh environments including combustors, nuclear reactors, aircraft engines, or metal/ceramic manufacturing processes. While glass brings undeniable assets over refractory crystalline materials like shaping ability (e.g., optical fiber form), one key challenge is to prevent the erasure of induced transformations at high temperatures and for long periods. In this article, the role of glass composition and viscosity to achieve modifications stable at high temperatures is first reviewed, providing a comprehensive roadmap for engineers in optics and photonics. While silica appears to be the candidate of choice, it is revealed that binary aluminosilicates can compete and sometimes surpass it. The hypothesis is formulated and investigated that a hybrid glass-crystalline nano-structuring can imprint ultra-stable modifications inside glass. Laser-induced modifications in Al₂O₃-SiO₂ and ZrO₂-Al₂O₃-SiO₂ glasses reveal a partial crystallization, shaped into a lamellar structure and orientable with laser light polarization. These birefringent structures can withstand temperatures up to 1300 °C for 30 minutes. Even after erasure, a positive index contrast persists, up to 1650 °C for binary 60Al₂O₃-40SiO₂ (mol%). This is the first observation of this kind of persisting index contrast, paving the way to ultra-stable glass-based optical waveguiding.

## 1. Introduction

The use of optical sensing technologies to be deployed in harsh environments is a currently growing field of research and has drawn considerable attention over the past decade.[1-4] Harsh environments can include high radiation dose, temperature,

pressure, electromagnetic fields, or a combination thereof. Focusing on high temperatures (HT), there is a broad demand for HT sensing materials, for instance in turbine engines in trains, aircraft, and space vehicles,[5,6] manufacturing processes (3D laser additive manufacturing of metal or ceramic parts, steel and glass industry),[2,7] nuclear reactors (instrumentation of future reactors and tokamaks),[8] photonics (optics and micro-optics for continuous wave (CW) kW-level fiber lasers) and optical sensing (structural health monitoring and deep drilling).[4]

Using glass-based devices such as optical fibers to probe an HT environment brings remarkable advantages with respect to conventional thermocouples. Among them, we can cite compactness, lightness, flexibility, high transparency, chemical/radioactive/electromagnetic resistance, multiplexing ability, superior sensitivity, and high-speed processing/monitoring. Considering that most optical fibers currently employed are

telecom Ge-doped silica core optical fibers, they are readily available at competitive prices. Focusing on high-temperature sensing, that is >1000 °C, there exists a variety of fiber-based technologies/strategies available, as summarized in.[1] This includes blackbody optical fiber thermometers, fluorescence-based or interferometric sensors, Fiber Bragg Gratings (FBGs), or distributed sensors. Among these options, the use of

I. Ktafi, M. Cavillon, B. Poumellec, M. Lancry
Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO)
CNRS
Université Paris-Saclay
Orsay 91190, France
E-mail: maxime.cavillon@universite-paris-saclay.fr

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/lpor.202401086

© 2024 The Author(s). Laser & Photonics Reviews published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/lpor.202401086

J. Kong, G.-D. Peng
School of Electrical Engineering and Telecommunications
University of New South Wales
Sydney 2052, Australia

M. Yembele, R. Valois, M. Allix
Laboratoire Conditions Extrêmes et Matériaux: Haute Température et Irradiation (CEMHTI)
CNRS
Université d'Orléans
Orléans 45100, France

M. Yembele
Laboratoire commun Canopée
CNRS, Saint Gobain
Université de Lorraine
Vandoeuvre-les-Nancy 54500, France
femtosecond (fs) laser pulses to imprint permanent modifications inside a transparent material is one of the most promising as it enables fast and versatile 3D transformations inside a small focal volume (in the order of 10–100 µm³). The combination of telecom fibers and fs laser modifications inside them can yield thermal stability up to ≈1000°C for hundreds of hours.[³,⁴,⁹] Refractory crystalline oxide materials, as opposed to glass, can be used, including $Al_2O_3$, $ZrO_2$, YAG, directionally solidified eutectic ceramics, metal alloys, carbides/nitrides, or combined with glass-based devices, due to their high melting points, good resistance to oxidation and abrasion. Nevertheless, once these materials are employed, the intrinsic advantages of a glass system as listed above are at least partially lost.

Consequently, when it comes to choosing optical sensing HT materials, two principal options emerge: i) silica and silica-rich glass compositions, or ii) crystalline materials (e.g., Sapphire, YAG). The former choice is motivated by silica being the most stable glass material when it comes to high temperatures as it owns one of the highest glass transition temperatures, only overcome by oxynitrides or oxycarbides,[¹⁰] and the lowest crystallization rate (that could weaken the fiber). The drawback is that in a fiber a dopant must be added to the silica core (or cladding) to make the fiber optically guiding, ultimately lowering the glass viscosity and degrading its thermal performance. Possible hollow core or photonic crystal fibers (all silica) can be employed, but the latter are not only costly but also more sensitive to deformation and breaking. The second choice, corresponding to crystalline materials (Sapphire, YAG, etc.), is an exciting alternative. Provided a structure can be imprinted into them, they can operate theoretically up to the melting temperature of the given structure. However, such structures are usually air-clad and therefore heavily multimode, remain rigid (rod-like, as opposed to glass-based optical fibers), and the material is overly expensive.

Consequently, a combination of the advantages brought by the glass and the thermal stability of refractory materials appear as a route to develop a new generation of optical fibers fully dedicated to sensors operating in high-temperature environments. In the first section, we review and provide a context of thermal stability based on the glass matrix composition, and the modifications imprinted inside it using fs-laser. We discuss the ability to photo-induce thermally stable structural changes, including oxide decomposition and glass ceramics made of refractory nanocrystals, inside high-viscosity silicate glass matrices. Following this, we investigate the thermal stability of nanostructures inscribed in a new generation of HT glasses including binary aluminosilicates and ternary zirconium aluminosilicates (compositions provided in the Experimental Section). We demonstrate, through isochronal annealing experiments, that these new glass materials can present excellent thermal stability. An index contrast from the laser-modified region can persist up to 1650 °C, which is 400 °C higher than current state-of-the-art values (silica). This exceptional thermal stability is discussed considering their hybrid glass-ceramic compositions post-irradiation and becomes a serious alternative to all-crystalline materials.

## 2. State-of-the-Art Glass Matrices for High Temperature Applications

The thermal performance of various glass families is tentatively summarized in **Figure 1**. In this figure, the horizontal axis corresponds to an increase in the temperature of operation. It must immediately be emphasized that the thermal stability of a device is related to the kinetics of a given property evolution (e.g., refractive index contrast, anisotropic index changes, photo-darkening). Therefore, both time and temperature (t,T) are equally important, since it is a matter of activation energy. However, here we chose to represent only “refractive index changes-based results” as a function of temperature since the annealing protocol varies from one publication to another (isothermal, isochronal, cycling experiments, etc.).

The color code of Figure 1 regroups glass families as follow: silica and germanosilicates (e.g., fiber core composition of telecom fibers) are shown in blue; binary aluminosilicates (i.e., $Al_2O_3-SiO_2$) are displayed in green; multicomponent silicate glasses (e.g., commercial glasses) are shown in purple; mid-IR glasses such as chalcogenides, fluorides and heavy oxides, all exhibiting relatively low glass transition temperatures, are presented in orange. Crystalline sapphire is provided in black, as a means of comparison with the glass systems. The reported temperature or temperature intervals mostly correspond to the temperature of erasure/instability of the modified region during the thermal treatment. More details can be found in the references, all listed in the caption of Figure 1. The modifications inscribed in the materials are of several kinds, all specified in the figure. In the literature, Type I corresponds to the formation of an isotropic index change modification, Type II to a birefringent optical modification typically taking the form of nanogratings (NGs), and Type III to the formation of voids/cavities.[11] Other types of transformations, being of interest, are still presented in the figure (dash boxes). This includes stress induced by Type II in silica, precipitation of mullite nanocrystals in aluminosilicate, and fs-laser irradiation in sapphire. Finally, the inscriptions and thermal treatments were either performed on bulk glasses, optical fibers, or optical fiber preform glasses, labeled *b*, *of*, and *ofp*, respectively. The thermal stability of Type I and II has been intrinsically related to the thermal stability of the glass host, i.e., its temperature-dependent viscosity profile.[12–16] Consequently, silica, germanosilicates, and aluminosilicate glasses exhibit a higher thermal stability relative to the multicomponent, heavy oxide, chalcogenide, and fluoride glasses, as the latter own comparatively lower glass transition temperatures. However, exceptions exist, as for a borgermanosilicate fiber core (20 mol% $B_2O_3$, 33 mol% $GeO_2$) from [17] where thermal sensing could be performed up to 1295 °C. This was possible using a microheater and a cycling thermal treatment (time step not mentioned), which is a rather restrictive condition, with a very brief time at the highest temperatures. Nevertheless, it highlights the possibility to use silicate core glasses for high-temperature sensing provided the associated time of operation remains short (e.g., for fire alarm or structural health monitoring).

Recently, and as can be viewed from Figure 1, the use of aluminosilicate materials provides higher thermal stability compared
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Selected results of laser-induced modifications withstanding as a function of temperature. The color code is set for different classes of glasses or materials: Crystalline Sapphire (black), silica or germanosilicates (blue), binary aluminosilicates (green), multicomponent silicates (purple), heavy oxide, fluoride, and chalcogenide glasses (orange). The dashed boxes emphasize specific cases (discussed in the text). References (from (a) to (o)) given at the end of each box are the following ones: (a) = [14]; (b) = [15]; (c) = [18]; (d) = [13]; (e) = [3]; (f) = [19]; (g) = [20]; (h) = [21]; (i) = [22]; (j) = [16]; (k) = [23]; (l) = [17]; (m) = [24] (n) = [25]; (o) = [26]. Complementary information is provided in the text.</figcaption></figure>

to silica glass. In fact, when the Al₂O₃ concentration increases up to ≈50 mol%, both in bulk and optical fiber cores,[15,18,19] the laser-modified nanostructures (Type II) can withstand temperatures as high as 1225 °C under 30 min step isochronal annealing conditions. This is comparatively higher than most pure silica(s) (Suprasil CG, Herasil, Homosil, Suprasil F300) and higher than germanosilicate fiber cores such as SMF28 (1075–1100 °C) under similar isochronal annealing conditions.[14,24] Only Type II in Infrasil or Spectrosil erases at higher temperatures, i.e., 1350 and 1300 °C.[14] Moreover, Type III transformations in SMF28 show a thermal withstanding up to ≈1150 °C,[26] which is closer to the erasure temperature of Type II in aluminosilicate glasses. It is worth pointing out that Type III in aluminosilicate glasses has not been investigated yet. Moving to higher temperature sensing (>1200 °C) and hours/day range timescale, the use of crystalline materials, or partially crystallized modifications, appears to be a prerequisite. For instance, the formation of mullite crystals in an aluminosilicate fiber core organized as a Fabry-Perot device can in principle operate at temperatures up to 1600 °C (for at least 10 min, and 6 h stability at 1200 °C),[20] which is a promising candidate for crystalline-amorphous composite for high-temperature sensing. Finally, the highest thermal stability is achieved through the use of crystalline materials, like Sapphire or YAG. In principle, the device can operate up to the material melting temperature (≈2050 °C for crystalline Al₂O₃), although FBGs typically operate at lower temperatures (<1900 °C).[21,27,28] How-

ever, as discussed in the introduction, the advantages brought by the glass material as a host are undeniably lost when considering a purely crystalline structure.

Consequently, a hybrid “glass-ceramic” transformation by fs-laser to imprint refractory structures inside refractory glasses is possible. Some glasses have shown crystallization upon laser irradiation including, but not limited to, Li₂O-Nb₂O₃-SiO₂ (LiNbO₃ crystals - SiO₂ glass),[29] Na₂O-SiO₂ (Na₂Si₄O₉ crystals - SiO₂ glass),[30] Al₂O₃-Dy₂O₃ (Dy₃Al₅O₁₂ and DyAlO₃ crystals),[31] La₂O₃-Ta₂O₅-Nb₂O₅,[32] or Li₂O-Al₂O₃-SiO₂ (sporadene nanocrystals).[33] Recently, irradiation of a 60% Al₂O₃–40% SiO₂ glass revealed the formation of a 3Al₂O₃:2SiO₂ multilete phase, identified by Raman spectroscopy and scanning electron microscopy. Additionally, this glass presents a relatively high glass transition temperature (T_g = 824 °C, estimated from Sciglass software), making it a refractory material and therefore a promising candidate for high-temperature operation. Moreover, the addition of ZrO₂ is investigated as well since it has a melting temperature of ≈2700 °C and is well known to promote crystallization serving as a nucleating agent. For completeness, moving away from SiO₂ can provide additional flexibility in terms of targeted properties. Aluminosilicates containing large amounts of Al₂O₃ exhibit intrinsically low nonlinear properties, including reduced Brillouin or Raman gain coefficient facilitating laser power scaling,[34,35] and high resistance to fracture due to enhanced elastic properties.[36]
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) Kinetics of retardance writing in SiO₂, H-ZrAlSi, L-ZrAlSi, and AlSi samples as a function of laser pulse energy (conditions: λ = 1030 nm, τp = 300 fs, RR = 100 kHz, v = 10 µm s⁻¹, NA = 0.6, polarization parallel to the writing direction (Xx). b) Normalized retardance (Rₙorm) and phase shift (ΔΦₙorm) as a function of annealing temperature under an isochronal step of 30 min per temperature data point, for the sample samples as in a) (conditions similar to a), but with a fixed Eₚ = 2 µJ). The full lines for Rₙorm(T) are guides for eyes and correspond to a combination of sigmoidal functions.</figcaption></figure>

**3. Results and Discussion**

**3.1. Optical Retardance and Thermal Stability of the Laser-Inscribed Structures**

The retardance (R) as a function of pulse energy in the investi-
gated samples is presented in **Figure 2a**. Under the conditions
of this work, an onset of retardance characteristic of the forma-
tion of NGs is found between 0.2 and 0.4 µJ, typical of what
is expected in silicate glasses and corresponding to the forma-
tion of nanogratings.[³⁷] The measured birefringent response of
the laser tracks could be oriented with respect to the laser po-
larization orientation, with a slow axis perpendicular to it. This
was confirmed using Xx, X45°, and Xy irradiation configurations
(see Experimental Section), which is one typical feature of Type
II nanostructuring. The progressive increase of R as a function
of energy has already been investigated in silica and binary alu-
minosilicates (20%, 30%, 40%, 50%, and 60% of Al₂O₃),[¹⁴,¹⁵,³⁸]
which is consistent with the present work. Following this, the
samples were annealed under isochronal conditions (time step
of 30 min) and the retardance was monitored after each temper-
ature step, at room temperature. The results for an Eₚ of 2.0 µJ
are provided in Figure 2b. The retardance R(T, Δt = 30 min) was

normalized relative to its value at room temperature R(20 °C, t =
0 min), to better compare the overall samples.

For temperatures typically up to 1200 °C, the samples showed
a behavior similar to silica, i.e., a progressive decay down to
1200 °C or so. The somewhat erratic values of retardance at tem-
peratures below 1000 °C for H-ZrAlSi and L-ZrAlSi samples are
due to greater uncertainties in the measurements since initial re-
tardance values are ≈4 times lower than the one of SiO₂ and the
laser tracks less homogeneous under optical microscopy obser-
vation. Moreover, other effects such as defects bleaching, stress,
and densification relaxation can cause important variations of re-
tardance below the glass transition temperature.[³⁹] It is worth
pointing out that, under similar conditions, the L-ZrAlSi sam-
ple fully erased at 1300 °C, i.e., 75 °C higher than the erasure
temperature of silica glass. For SiO₂, the retardance erasure was
correlated to an absence of index contrast inside the glass. How-
ever, a refractive index contrast persisted for the other samples
(AlSi, H-ZrAlSi, and L-ZrAlSi), visible from optical microscopy.
This index contrast was characterized by QPM and corresponds
to a positive phase shift (ΔΦ) of 0.97, 0.22, and 0.12 rad for
AlSi, L-ZrAlSi, and H-ZrAlSi, respectively. Uncertainties are pro-
vided in Figure 2. This phase shift was then normalized with
respect to this first measured value (ΔΦₙorm), and then mon-
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Full erasure temperature (i.e., optical property monitored decreased by 95%) as a function of pulse energy under an isochronal step for SiO₂, H-ZrAlSi, L-ZrAlSi, and AlSi compositions (conditions: λ = 1030 nm, τp = 300 fs, RR = 100 kHz, ν = 10 µm s⁻¹, NA = 0.6, polarization parallel to the writing direction (Xx). The colored domains are guide-to-the-eye to distinguish persistent birefringent signature (Birefringent modifications, delimited by filled-symbols) and persistent phase shift (isotropic refractive index changes, delimited by hollow symbols) during isochronal (Δt = 30 min) annealing experiments. The inset corresponds to a microscope image (top view, transmission mode) of the laser tracks in AlSi after annealing at 1600 °C.</figcaption></figure>

itored as the temperature is further increased, continuing the isochronal annealing process initially used to measure R<sub>norm</sub>. As shown in Figure 2b, this index contrast persisted upon very high temperatures: up to 1300 °C for H-ZrAlSi, 1450 °C for L-ZrAlSi, and 1700 °C for AlSi. This is much beyond the state-of-the-art results (1350 °C for pure and bulk SiO₂ Infrasil with low Cl and OH, or 1050–1100 °C for a conventional SMF28, fiber form).

The thermal annealing investigations from Figure 2 were performed under 0.5–2.0 µJ energy range conditions. The results are summarized in **Figure 3**, where the erasure temperature of the modified structures is shown for all samples investigated in this work, as a function of pulse energy. In this figure, the two colors distinguish the birefringent (green, hatched) – isotropic refractive index contrast (light yellow) regimes. At higher temperatures, only isotropic index modifications (positive index changes) are observed. This is exemplified with an inset showing a persisting index contrast from irradiated rectangles observed under optical microscopy (transmission mode) at 1600 °C in the heat-treated AlSi sample. Additionally, characteristic erasure temperatures are provided for complementary information from Type II modifications in an SMF-28 (optical fiber, typical state-of-the-art value).[24] For completeness, Type II in Infrasil (low OH and Cl silica glass, but made by natural quartz fusion in an electric furnace), owns the highest thermal stability under isochronal conditions to date, up to 1350 °C.[14]

## 4. Internal Structure of Irradiated Zirconium Aluminosilicate Glasses

While Figure 2a showed somewhat similar trends for all the samples, except for the amplitude of R, the nature of the NGs is different. For silica, the NGs are composed of alternate layers of porous glass, containing nanopores being a few tens of nm in size,[40–42] and this is for a very wide range of laser parameters, including / close to the ones of this study. However, for the AlSi sample, the NGs structure takes the form of Al₂O₃-rich crystalline nanolayers (identified as mullite phase with 3:2 ratio) surrounded by a SiO₂-rich amorphous phase.[38] Finally, ZrAlSi internal structure was investigated as well. For this purpose, Raman spectroscopy and elemental distribution investigation (EDS) of one laser track of the H-ZrAlSi glass were performed subsequently to femtosecond laser direct writing. Details of irradiation conditions are provided in the caption of **Figure 4** and in the Experimental Section section.

The Raman spectroscopy of the pristine glass next to the laser track in Figure 4a revealed the amorphous nature of the pristine glass, with broad overlapping bands typical of aluminosilicate glasses.[15] However, the Raman spectrum from the laser track center revealed some sharp peaks, indicative of laser-induced 3D crystallization. These peaks at 147, 280, 320, 447, and 648 cm⁻¹ all correspond to tetragonal ZrO₂ (t-ZrO₂),[43–45] which is a high temperature and metastable polymorph of ZrO₂, typically formed
<figure><img src="image_6.png" /><figcaption>**Figure 4.** a) Raman spectroscopy of H-ZrAlSi inside and outside the laser track (conditions: λ = 1030 nm, τ<sub>p</sub> = 300 fs, RR = 100 kHz, E<sub>p</sub> = 2.4 μJ, ν = 10 μm s<sup>-1</sup>, NA = 0.6, polarization parallel to the writing direction (Xx). b) Dark field mode imaging and chemical analysis (ESD, count map, colored) of a TEM foil prepared by FIB (lift out technique) showing chemical migration (E and k stand for laser polarization and beam direction). c–e) TEM imaging evidencing a crystallized area including atomic planes and diffraction pattern, and f) compound concentration profile (taking Al<sub>2</sub>O<sub>3</sub>, SiO<sub>2</sub>, ZrO<sub>2</sub> compounds) across the fs-laser modified region.</figcaption></figure>

in the 1200–2400 °C range at ambient pressure.[46] This phase
has also been observed in ternary ZrO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> glasses from
rapid quenching.[47,48] Complementary to this analysis, a ≈30 nm
thin foil extracted from the laser track center was subsequently
analyzed by STEM-TEM, and EDS was performed on it. Figure 4b
provides a STEM image in dark field (DF) mode, along with
an EDS elemental analysis (count map). The DF mode shows
a contrasted image, with the dark part corresponding to en-
hanced electron diffraction efficiency relative to its surround-
ings, which may be attributed to crystalline t-ZrO<sub>2</sub>. Figure 4c–e
provides TEM imaging revealing unambiguous crystallized area
(along with planes and diffraction pattern) on the dark regions
observed in DF mode. Moreover, elemental mapping revealed
a chemical contrast, showing regions locally enriched in ZrO<sub>2</sub>,
Al<sub>2</sub>O<sub>3</sub>, and SiO<sub>2</sub>. The ZrO<sub>2</sub>-rich phases are elongated perpen-
dicularly to the laser polarization orientation and overlap with
the dark regions of the DF mode imaging. A quantitative chem-
ical analysis demonstrating the chemical composition fluctua-
tion (in mol% of compounds) is provided in Figure 4c along the
line shown in Figure 4b. It revealed that the ZrO<sub>2</sub> concentra-
tion is very high (>90 mol%) in some cases. This corresponds
to nearly pure ZrO<sub>2</sub> and agrees with the Raman spectrum from
which t-ZrO<sub>2</sub> phase was identified. The remaining signal might
come from the glass environment surrounding the crystallites.
The Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> ratio along the laser track center in the polariza-
tion orientation was found to be equal to 2.33 ± 0.66, for an initial
batch ratio of 1.99. Finally, the average composition along the line

from Al₂O₃, SiO₂, and ZrO₂ is 47.6, 21.6, and 30.8 mol%, respec-
tively (±0.5 mol%). These values only slightly deviate from the
pristine glass composition. Moreover, the structuring observed
by electron microscopy did not reveal a layering like Type II in
silica or aluminosilicates. Instead, it appears as the formation
of slightly elongated Zr-rich nanoscale phases. This resembles
the formation of spodumene crystals (LiAlSi₂O₆) in lithium alu-
minosilicate glasses (LAS) under irradiation.[33] This anisotropy,
attributed in the previous reference as initially induced by the
asymmetric growth of the nanoplasma, induces a birefringence
of a few tens of nm depending on the irradiation conditions.

**5. Exceptional Thermal Stability of Laser-Induced Transformations**

The results from this work demonstrated that birefringent struc-
tures in low silica (<42.5 mol%) glass compositions could exhibit
similar, or improved, thermal stability with respect to silica glass
(Suprasil CG or even Infrasil) or conventional telecom fibers
(SMF28). Recently, it has been shown that the thermal stability of
porous NGs, like the ones observed in silica glass, is intrinsically
related to the glass viscosity, η(T),[12,13,24] where the nanopores
progressively erase as the temperature increases. In fact, the turn-
ing point is typically when the temperature reaches the glass an-
nealing temperature (T<sub>a</sub>, set when log(η, Pa.s) = 10<sup>12</sup>) or the glass
transition temperature (T<sub>g</sub>, close to T<sub>a</sub>) under 30 min isochronal
annealing experiments.[13] More specifically, from the previous
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Literature review summarizing erasure temperature of fs-laser induced modifications under isochronal annealing conditions, as a function of glass annealing or transition temperatures ($T_a$, $T_g$). Symbols provide information on transformation types (I, Ic, II, IIc, bulks, optical fibers, optical fiber preforms). Color code is established based on glass family types. References are given here as family types: Silica(s) and Ge-doped silica: [14,24,39]; Binary aluminosilicates: [15,18]; Multicomponent silicates: [3, 12–14]; Chalcogenides, heavy oxides, fluorides: [16] Results from this work are framed with the dashed red line. Type I: isotropic index contrast after irradiation; Type II: birefringent porous nanostructure after irradiation; Type IIc: birefringent partly crystallized nanostructure after irradiation; Type Ic: isotropic index contrast remaining after erasure of Type IIc; Stress: Induced stress by Type II post irradiation.</figcaption></figure>

reference using 50 nm nanopore diameter and a constant surface tension of 0.3 J·m⁻², the temperature is found for log(η, Pa·s) = 10.1 instead of $T_a$ or $T_g$; nevertheless we use the latter values are easily available. Therefore, **Figure 5** summarizes the literature data for erasure temperature, $T_{era}$, of fs-laser-induced modifications as a function of $T_a$ or $T_g$ under such isochronal annealing conditions. In this figure, several glass families, laser-induced modification types, and glass geometries are reported (see Figure 5 legend and caption for complete information), and $T_{era}$ was defined for an erasure of 95% of the optical property.

For the trend to hold, the data points must be situated close to the yellow line, corresponding to a 1 to 1 ratio between $T_{era}$ and $T_a$ or $T_g$. Interestingly, this trend reasonably holds despite the wide temperature range (≈1000 °C span from $As_2S_3$ to Infrasil), the plurality of the transformations (Types I, II, Ic, IIc, stress), and glass compositions investigated. It is worth pointing out that in Figure 5 the thermal erasure coincides rather well with the glass annealing temperature, as mentioned before. This is valid because 30 min isochronal annealing steps are used.[13] However, $T_{era}$ would be shifted to lower (higher) temperatures if longer (shorter) time steps were used. An interested Reader is referred to ref. [13] for complementary information regarding the link between thermal treatment protocol (coupled time and temperature), glass viscosity, and $T_{era}$.

However, this trend breaks down for the samples investigated in this work, since aluminosilicates exhibit lower $T_g$ than silica or germanosilicates, but improved thermal performance. In AlSi, the glass transition temperature ($T_g$) is estimated at $T_g$ = 824 °C (Sciglass) with $T_{era}$(IIc) = 1225 °C and $T_{era}$(Ic) = 1700 °C, while for silica it is $T_g$ = 1203 °C (Sciglass), and $T_{era}$(II, suprasil CG) = 1175 °C. For ZrAlSi glasses, the $T_g$ is in the range of 920 °C. In fact, as observed in previous work (ref. [38]) and here for H-ZrAlSi glass, the irradiated glasses are composed of refractory crystalline domains, i.e., aluminosilicates (3:2 mullite) or t-ZrO₂. Under a thermal treatment, they do not evolvelike nanopores. They are stable around $T_g$ as they exhibit large melting temperatures i.e. $T_m$(3:2 mullite) ≈1850 °C; $T_m$(cubic-ZrO₂) ≈2700 °C.

Upon annealing, the glass will progressively crystallize, changing the amplitude of the birefringence as nucleation and growth take place. In binary aluminosilicate glasses at 1200 °C (mol %: ≈57Al₂O₃-43SiO₂ and ≈36Al₂O₃-64SiO₂), the volume amount of crystallization can reach 50% within seconds to minutes, but then saturates over time.[49] This means that the glass does not necessarily fully crystallize upon isochronal heating. It is also worth pointing out that, in AlSi or ZrAlSi samples, the laser tracks are partly crystallized after irradiation. Therefore, upon progressive heating, the glass around the crystalline parts, as in ref. [38] for AlSi, will eventually recrystallize. Consequently, a decrease in the
retardance magnitude is expected as the refractive difference between the birefringent layers would be reduced. Finally, heterogeneous nucleation and growth are anticipated primarily in and around the laser track during the annealing, ultimately destroying the birefringent structure.

Unexpectedly, once the birefringence had been fully erased, we noticed a persisting refractive index contrast for the investigated samples except for SiO₂. In the latter, the nanopores erase, and since there is no chemical migration nor crystallization associated to it, no index contrast is left afterward. However, for AlSi and ZrAlSi glasses, the local crystallization and large chemical migration imprint a permanent refractive index modification that remains even though the birefringent contrast is lost and eventually up to 1675 °C like in Figure 3, which is quite stimulating. This is a new type of transformation, that we call Type Ic in Figure 5, c standing for “crystalline” and to distinguish it from typical Type I. In our case, the phase shift ΔΦ(λ) translates to an index contrast using the relationship Δn(λ) = (ΔΦ·λ)/(2πL), with λ = 546 nm, L ≈50 µm (typical laser track length, verified from SEM and optical imaging, for E<sub>p</sub> = 2 μJ and the three glasses investigated). This yields a positive Δn(λ) of 1.7·10<sup>-3</sup>, 3.8·10<sup>-4</sup>, and 2·10<sup>-4</sup> for AlSi, L-ZrAlSi, and H-ZrAlSi, respectively, and could be exploited for various optical applications, including waveguides, splitters, or integrated interferometers. For completeness, one must bear in mind that L can vary with respect to laser parameters, including laser pulse duration, energy, or focusing conditions, and possible refractive index tuning can be anticipated.

The observed index contrast ultimately erases starting at 1600 °C for AlSi, while it is at lower temperatures (1250–1350 °C) for the ZrAlSi family. The first hypothesis is to consider the formation of a liquid phase, which is expected to facilitate elemental re-migration and ultimately index contrast vanishing. In fact, in the SiO₂-Al₂O₃ binary system, there exists a solidus ≈1600 °C for composition lower than the 3:2 mullite one,[50] which agrees rather well with the experimental observations. However, for the ZrAlSi structure, the first partial melting is expected to be ≈1550–1600 °C considering a ZrO₂ – (xAl₂O₃·(100-x)SiO₂) phase diagram.[51] Therefore, a second hypothesis for the ZrAlSi case would be a global crystallization and possibly solid-solid phase transformations upon annealing,[51–53] ultimately suppressing the index contrast. Additional studies are requested to fully elucidate how the index contrast in this ternary system erases.

As a final note, the AlSi composition is close to the ≈50 mol% Al₂O₃ content reached in the core of optical fibers.[18,19,34] Consequently, further research in the development of low SiO₂ silicate core compositions including ZrO₂ and Al₂O₃ refractory compounds is expected to transfer the current exceptional thermal stability observed from bulk to optical fiber forms. Moreover, a further increase in the Al₂O₃/SiO₂ ratio could lead, from the phase diagram, to a laser imprinted mullite and corundum solid solution with a solidus at T > 1800°C, provided such glasses can successfully be made.

## 6. Conclusion

This work investigates the formation of thermally stable nanos Structures in aluminosilicate glasses with low SiO₂ content. We reveal that the three investigated samples (AlSi, H-ZrAlSi, L-

ZrAlSi) present a partially crystalline birefringent structure, stable up to ≈1300 °C for L-ZrAlSi under isochronal annealing experiments. This value is higher than most silica(s) glasses. Moreover, a new type of thermally stable modification (Ic), corresponding to a positive index persisting post-Type II erasure, is revealed. For H-ZrAlSi and L-ZrAlSi glasses, Type Ic respectively erases at 1325 and 1425 °C, which is among the highest values reported to date. However, AlSi is found to erase at 1675 °C, which is the highest temperature value reported for a fs-laser-induced modification initially irradiated in a glass sample. Finally, a possible prospect is the direct imprinting of fine nanocrystals like mullite or zirconia in these refractory binary and ternary glasses but without post heat-treatment to prevent degradation of the host material. This could be achieved through precise control of chemical migration/phase separation, engineered by the time-temperature control of the laser irradiation process (e.g., heat deposition, repetition rate, or burst mode).

## 7. Experimental Section

Considering the introductory remarks, we selected several bulk glass samples with low SiO₂-high (Al₂O₃ + ZrO₂) concentration to investigate their thermal stability. The specific sample compositions are, given in molar percentage: 60%Al₂O₃- 40%SiO₂, 46.8%Al₂O₃- 23.5%SiO₂- 29.7%ZrO₂ and 42.5%Al₂O₃- 42.5%SiO₂-15%ZrO₂. They are thereafter labeled AlSi, H-ZrAlSi, and L-ZrAlSi, respectively. H and L distinguish high (H) and low (L) ZrO₂ concentrations for the two zirconium aluminum silicates. H-ZrAlSi composition corresponds to a eutectic composition. Finally, a pure silica sample (Suprasil CG) was included as a reference for this work. The glasses were fabricated using the CO₂ laser-based aerodynamic levitation technique, and more details on the technique can be found in ref. [15, 54, 55]. Glass beads (2–3 mm diameter depending on the sized of the beads synthesized) were thus double side optically polished into disks and subsequently irradiated using a fs-laser (Satsuma, Amplitude Système, France) in volume at a focal depth of approximately 300 µm under the glass surface. The laser has a central wavelength (λ) of 1030 nm and a focusing aspheric lens with a numerical aperture (NA) of 0.6 was employed. The inscription geometry was set as rectangles composed of 10 lines of 100 µm length, separated by a 2 µm line gap, with a forward-backward inscription scheme. The rectangles were inscribed 25 µm apart from each other. Finally, the laser irradiation conditions were chosen in order to induce nanogratings in the glass. Pulse durations (τ<sub>p</sub>) were set as 300 fs, the repetition rate (RR) was fixed at 100 kHz, the writing speeds (v) taken as 10 µm s<sup>-1</sup>, and the pulse energies (E<sub>p</sub>) were varied from 0.1 to 2 µJ. The laser linear polarization was varied from perpendicular (Xy), 45 degrees (X45), and parallel (Xx) to the writing direction (labeled X).

Following laser inscription, polarization-oriented optical retardance, characteristic of the formation of NGs, was measured using the Olympus BX51 polarizing optical microscope with De Sénarmont compensator, at a wavelength of 546 nm. Finally, we conducted annealing experiments for these four samples to test their thermal stability. They were isochronally heat treated with a time step of 30 min, starting from 200 °C and up to the temperature achieving full inscription erasure. After each annealing step, these samples were carefully moved out of the furnace and naturally cooled down to room temperature, after which a new optical retardance measurement was taken. With the continuous increase of annealing temperatures, retardance finally decreases and disappears, but not the index contrast, still visible by optical microscopy. Therefore, quantitative phase microscopy (QPM) was used to quantitatively monitor the phase shift,[56] proportional to the mean refractive index (isotropic), until full erasure.

Another series of irradiations was also performed to investigate the glass structure using high-resolution transmission electron microscopy, and
Raman microspectroscopy. For this purpose, 100 µm lines were irradiated in the H-ZrAlSi sample with varying energies from 1.2 to 3 µJ, using the following conditions (RR = 100 kHz, v = 10 µm s⁻¹, τₚ = 300 fs, polarization condition = Xy). The Eₚ = 2.4 µJ was selected for the structural analysis since it was a good compromise between homogeneity and size of the laser track. Raman spectroscopy was performed using a Labram HR Evolution spectrometer equipped with a Peltier-cooled CCD, illuminated under a Coherent MX 488 nm solid-state laser focused through a 50 × Olympus microscope objective. The electron microscopy analysis was performed on the laser track after a foil was extracted and thinned down using a focused ion beam coupled with a scanning electron microscope (SEM-FIB, Helios Nanolab 660 dual-beam), following the conventional lift-out method. Observations and elemental characterizations were performed on a corrected FEI TITAN3 G2 transmission electron microscope (ThermoFischer) operating at 300 kV. Observations were performed in transmission electron microscopy (TEM) and in scanning TEM (STEM) using bright field (BF), annular dark field (ADF), and high-angle annular dark field (HAADF) detectors. Moreover, elemental mapping was performed using energy-dispersive X-ray spectroscopy (EDS). This procedure is similar to what has been done in previous work.[³⁸]

## Acknowledgements

Funding from the Agence Nationale de la Recherche (project RE-FRACTEMP no. ANR-22-CE08-0001-01) is acknowledged.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

birefringence, femtosecond laser, glass, optical properties, thermal stability

Received: July 12, 2024

Revised: September 23, 2024

Published online:

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