Chalcogenide glass for mid-infrared photon device for femtosecond laser direct writing and tuning method

Abstract

The invention belongs to the technical field of optical materials and photonic devices, and discloses chalcogenide glass for a mid-infrared photonic device for femtosecond laser direct writing and a tuning method. According to the invention, chalcogenide glass GeSbSNa is used as a substrate, and femtosecond laser beams are focused in the glass substrate; and controlling the central wavelength of the femtosecond laser to be 1030 nm, the pulse duration to be 100 fs to 1 ps, the repetition frequency to be 100 kHz and the scanning speed to be 1 mm / s, so that the laser energy density of the focusing region is enough to induce a modification region with the refractive index change delta n greater than 0.032 in the chalcogenide glass. The invention systematically evaluates the influence of composition and laser parameters on refractive index variation, phase offset and thermal stability. Through combination of a quantitative phase microscope and a Raman spectrum, optical modification and underlying structure transformation are associated, and an insight is provided for adjustability and toughness of GeSbSNa glass in MIR photon integration.

Description

Chalcogenide Glass and Tuning Method for Femtosecond Laser Direct-Writing Mid-Infrared Photonic Devices Technical Field

[0001] This invention belongs to the field of optical materials and photonic devices, particularly chalcogenide glass for mid-infrared photonic devices written with femtosecond lasers and its tuning method. It specifically relates to a sodium-doped germanium-antimony chalcogenide glass suitable for mid-infrared photonic devices written with femtosecond lasers, its internal refractive index tuning method, and the photonic device prepared therefrom. Background Technology

[0002] Femtosecond (fs) laser direct writing (FLDW) has become a revolutionary tool for fabricating compact, three-dimensional (3D) photonic structures within transparent materials. By utilizing ultrashort laser pulses, FLDW achieves local and permanent modification in bulk media through nonlinear optical ionization, reaching extreme energy densities on the order of terawatts per square centimeter. This capability provides unprecedented control over material structures at the microscale, facilitating the direct embedding of waveguides, gratings, and lenses into optical substrates. Consequently, applications such as in-line holography using a unified compressed phase retrieval framework, multi-channel optical data storage or birefringence patterning, and the realization of low-loss waveguides or fiber Bragg gratings in the mid-infrared range become feasible.

[0003] Among materials suitable for FLDW (Flexible Digital Light Wave) applications, chalcogenide glasses (ChGs) have attracted significant attention due to their unique combination of properties: wide infrared (IR) transmission windows (up to 28 μm), high Kerr nonlinearity, high refractive index, and remarkable photosensitivity compared to oxide glasses (such as heavy metal oxides or SiO2 glasses). These properties make ChGs ideal for integrated photonics in MIR (Multi-Infrared Resonance) applications, particularly in stimulated Brillouin scattering, chemical sensing, waveguides, nonlinear optics, and broadband supercontinuum generation.

[0004] Ge-Sb-S-based chalcogenide glasses stand out for their wide transmission window (0.5–10 μm), strong optical nonlinearity, and low thermal refractive index, making them superior to conventional oxide glasses in MIR photonic devices. Despite growing interest, little is known about the response of these glasses (especially when doped with mesh modifiers such as sodium) to femtosecond laser irradiation.

[0005] Based on the above analysis, the problems and defects of the existing technology are as follows: (1) The photoresponse mechanism of specific material systems is unclear: Although Ge-Sb-S-based chalcogenide glass is recognized as a potential material for mid-infrared photonics due to its comprehensive optical properties, the specific photoresponse behavior, modification mechanism and key influencing factors under femtosecond laser action are still unclear when a grid modifier such as sodium (Na) is introduced. The existing technology lacks a systematic revelation of the quantitative relationship between "composition (especially Na content and S / Ge ratio) - laser parameters - optical modification effect (such as refractive index modulation depth)".

[0006] (2) Lack of quantitative realization and verification of high refractive index modulation: Although existing technologies have confirmed that chalcogenide glasses have "significant photosensitivity", there is a lack of sufficient experimental data to support whether sodium-doped GeSbS glasses can achieve high and repeatable refractive index contrast (Δn) through femtosecond laser direct writing. In particular, whether modification effects of Δn>0.032 or even higher can be obtained has not been reported or verified. This leads to doubts about its application potential in devices requiring strong optical field confinement or high integration (such as high-contrast waveguides and microlenses).

[0007] (3) The thermal stability of the modified region has not been systematically evaluated: For integrated photonic devices for practical applications, the thermal stability of their laser-written structures is crucial. Existing technologies lack systematic evaluation (such as the determination of the 90% erasure temperature) and comparative data with other commercial glasses on whether the induced refractive index change of GeSbSNa glass after femtosecond laser modification can remain stable in the mid-temperature range (e.g., close to or above 200°C). This makes it impossible to predict the reliability of the devices under slightly harsh environments.

[0008] (4) Lack of correlation between microstructure evolution and macroscopic properties: Current understanding of femtosecond laser-induced modification of chalcogenide glasses is mostly limited to changes in macroscopic optical properties. For GeSbSNa glasses, there is a lack of direct experimental evidence (such as Raman spectroscopy mapping) and mechanistic correlation analysis regarding the specific reorganization of the microstructure after laser irradiation (such as the transformation of the connection mode of GeS4 tetrahedra from corner sharing to edge sharing), and how this reorganization dominates the refractive index change and thermal stability.

[0009] In summary, the lack of a systematic study on sodium-doped GeSbS, a promising material system, from controllable femtosecond laser processing to quantitative optical performance characterization, thermal stability assessment, and microscopic mechanism explanation, hinders the advancement of this type of material towards a predictable, designable, and highly reliable mid-infrared integrated photonics platform. Summary of the Invention

[0010] To overcome the problems existing in related technologies, the present invention discloses embodiments of chalcogenide glass for mid-infrared photonic devices and a tuning method for direct writing with femtosecond lasers. The technical solution is as follows: The present invention is implemented as follows: a chalcogenide glass for mid-infrared photonic devices for direct writing with femtosecond lasers, the composition of which includes germanium (Ge), antimony (Sb), sulfur (S) and sodium (Na), and a modification region with a refractive index change Δn greater than 0.032 is induced inside by femtosecond laser irradiation.

[0011] Furthermore, the ratio of the number of sulfur (S) to germanium (Ge) atoms (S / Ge) is 2.6 to 4.2.

[0012] Furthermore, the sodium (Na) content is from 0.5 at.% to 5.5 at.% on an atomic percentage basis.

[0013] Furthermore, the modified area is a Type I modification.

[0014] Furthermore, after the modified region undergoes a temperature treatment not exceeding 290°C, the induced refractive index change remains at least 10% of the original change; for example, GeSbSNa-DNa2 (S / Ge=4.2, Na=2 at.%) glass.

[0015] Furthermore, the Raman spectrum of the modified region shows that the characteristic peaks of the edge-shared (ES)GeS4 tetrahedral structure are relatively enhanced compared to the unmodified region.

[0016] Another objective of this invention is to provide a femtosecond laser-induced refractive index tuning method for GeSbSNa glass. The method uses the chalcogenide glass as a substrate and includes the following steps: focusing a femtosecond laser beam inside the glass substrate; controlling the femtosecond laser parameters so that the laser energy density in the focused region is sufficient to induce a modified region inside the chalcogenide glass with a refractive index change Δn greater than 0.032.

[0017] Furthermore, the center wavelength of the femtosecond laser is 1030-1060nm, the pulse duration is between 100 fs and 3 ps, the repetition frequency is 10kHz-1MHz, and the scanning speed is 1mm / s.

[0018] Another objective of this invention is to provide a method for manufacturing a gradient refractive index optical element. The method uses the aforementioned chalcogenide glass as a substrate and includes the following steps: employing the femtosecond laser-induced GeSbSNa glass refractive index tuning method, performing femtosecond laser direct writing on the interior of the chalcogenide glass substrate according to a predetermined three-dimensional pattern scanning path; wherein, by controlling the femtosecond laser direct writing process, a modified region with a spatially varying refractive index distribution is formed inside the chalcogenide substrate, thereby obtaining a gradient refractive index optical element.

[0019] Another object of the present invention is to provide a gradient refractive index optical element, wherein the substrate material of the optical element is the chalcogenide glass, and the interior contains a modified region with a spatially varied refractive index distribution formed by femtosecond laser direct writing, wherein the refractive index variation Δn of the modified region is greater than 0.032.

[0020] Combining all the above technical solutions, the beneficial effects of this invention are as follows: First, femtosecond laser direct writing (FLDW) enables precise three-dimensional (3D) structural processing within transparent materials, providing a multifunctional platform for integrated photonics. This invention explores the photoresponse of sodium-doped GeSbS (GeSbSNa) chalcogenide glasses under femtosecond laser irradiation. This system is of great significance for mid-infrared (MIR) applications, but has not been fully explored previously. Using quantitative phase microscopy, this invention extracts the refractive index change as a function of composition and laser pulse energy, achieving a phase shift of up to 62 radians and a refractive index contrast ratio (Δn) exceeding 0.05, one of the highest values ​​reported in chalcogenide glass systems. Raman spectroscopy reveals local structural reorganization, particularly the transformation from corner-sharing to edge-sharing GeS4 tetrahedrals, consistent with laser-induced densification. This invention also evaluates the thermal stability of the modification through isochronous annealing and compares it with commercial glass benchmarks. Experimental results demonstrate that this invention positions GeSbSNa glass as a promising candidate material for MIR photonic integration, where high photosensitivity and moderate thermal durability are crucial.

[0021] Secondly, this invention explores femtosecond laser-induced modification of sodium-doped GeSbS (GeSbSNa) glasses, a system previously unexplored in this context. This invention systematically evaluates the effects of composition and laser parameters on refractive index variation, phase shift, and thermal stability. By combining quantitative phase microscopy and Raman spectroscopy, this invention links optical modification to underlying structural transformations, providing insights into the tunability and resilience of GeSbSNa glasses in MIR photonic integration. Attached Figure Description

[0022] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure; Figure 1 is a flowchart of a femtosecond laser-induced refractive index tuning method for GeSbSNa glass provided in an embodiment of the present invention; Figure 2 is a schematic diagram of the range of structural modifications induced by femtosecond laser irradiation in various GeSbSNa glasses provided in an embodiment of the present invention; Figure 3 is a graph showing the evolution of phase shift and optical thickness in different GeSbSNa glass compositions as a function of laser pulse energy provided in an embodiment of the present invention; wherein, (a) includes a constant S / Ge ratio (2.6) and a varying sodium content ( Figure 4 shows the ANa series (Na0, Na0.5, Na1), and Figure 5 shows the BNa and DNa series with fixed sodium content but different S / Ge ratios (4.2 and 3, respectively). Figure 6 shows the maximum and average refractive index changes as a function of S / Ge ratio in the GeSbSNa glass series provided in the embodiments of the present invention. Figure 7 shows the quantitative phase analysis results of the femtosecond laser-written structure in the GeSbSNa glass provided in the embodiments of the present invention. Among them, (a) shows the spatial phase distribution, and (b) shows the horizontal phase profile. Figure 8 shows the 90% erasure temperature of laser-induced phase shift as a function of glass transition temperature (T). g Figure 7 is a function curve of the function; it is a normalized Raman spectrum obtained from the center of the femtosecond irradiation region and the original region in GeSbSNa-ANa1 glass provided in the embodiment of the present invention. Detailed Implementation

[0023] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0024] Example 1: This embodiment of the invention provides a chalcogenide glass for mid-infrared photonic devices written directly by a femtosecond laser. The chalcogenide glass is composed of germanium (Ge), antimony (Sb), sulfur (S) and sodium (Na), and a modified region with a refractive index change Δn greater than 0.032 is induced inside by femtosecond laser irradiation.

[0025] Furthermore, the ratio of the number of sulfur (S) to germanium (Ge) atoms (S / Ge) is 2.6 to 4.2.

[0026] Furthermore, the sodium (Na) content is from 0.5 at.% to 5.5 at.% on an atomic percentage basis.

[0027] Furthermore, the modified region is a Type I modification. The Type I modification of this invention refers to a laser-induced modified region that has no macroscopic cracks or porous structures when observed under an optical microscope, and whose continuous phase change can be measured by a quantitative phase microscope, as shown by the pink rhombus in Figure 2. Figures 2, 3, and 5 all show the results of Type I modification.

[0028] Furthermore, after the modified region undergoes a temperature treatment not exceeding 290°C, the induced refractive index change remains at least 10% of the original change; for example, GeSbSNa-DNa2 (S / Ge=4.2, Na=2 at.%) glass.

[0029] Furthermore, the Raman spectrum of the modified region shows that the characteristic peaks of the edge-shared (ES)GeS4 tetrahedral structure are relatively enhanced compared to the unmodified region.

[0030] Example 2, as shown in Figure 1, provides a femtosecond laser-induced refractive index tuning method for GeSbSNa glass. The method uses the chalcogenide glass as a substrate and includes the following steps: S1, focusing a femtosecond laser beam inside the glass substrate; S2, controlling the femtosecond laser parameters so that the laser energy density in the focused area is sufficient to induce a modified region with a refractive index change Δn greater than 0.032 inside the chalcogenide glass.

[0031] Furthermore, the center wavelength of the femtosecond laser is 1030-1060nm, the pulse duration is between 100 fs and 3 ps, the repetition frequency is 10kHz-1MHz, and the scanning speed is 1mm / s.

[0032] Example 3: This embodiment of the invention provides a method for manufacturing a gradient refractive index optical element. The method uses the aforementioned chalcogenide glass as a substrate and includes the following steps: using the femtosecond laser-induced GeSbSNa glass refractive index tuning method, performing femtosecond laser direct writing on the interior of the chalcogenide glass substrate according to a predetermined three-dimensional pattern scanning path; wherein, by controlling the femtosecond laser direct writing process, a modified region with a spatially varying refractive index distribution is formed inside the chalcogenide substrate, thereby obtaining a gradient refractive index optical element.

[0033] Example 4: This embodiment of the invention provides a gradient refractive index optical element. The substrate material of the optical element is the chalcogenide glass, and the interior contains a modified region with a spatially varying refractive index distribution formed by femtosecond laser direct writing. The refractive index change Δn of the modified region is greater than 0.032.

[0034] To further demonstrate the positive effects of the above embodiments, the present invention conducts the following experiments based on the above technical solutions.

[0035] 1. Experimental Procedure; 1.1 Glass Synthesis and Characterization; Chalcogenide glasses (ChGs) were prepared using high-purity elements Ge, Sb, and S (Alfa Aesar, 99.999%). Anhydrous sodium sulfide was used as a sodium dopant. Based on the stoichiometric composition of Ge... 25 Sb 10 S 65 Various samples were synthesized by changing the S / Ge ratio (i.e., increasing the sulfur content) or by keeping the ratio constant while introducing sodium. The nominal composition and corresponding labels of the different glasses are shown in Table 1. Two series of glasses were synthesized: (i) samples A, B, and D, with the sodium content fixed to analyze the effect of sulfur concentration; (ii) samples Na0, Na0.5, Na1, Na2, and Na5.5, with the S / Ge ratio fixed to explore the effect of sodium content.

[0036] Table 1. Label and batch composition and composition ratio of the GeSbSNa glass used in this invention. The raw materials were weighed in a glove box under a nitrogen atmosphere, then packed into quartz ampoules and evacuated to 10°C. -2 mbar and sealed with an oxygen-methane torch. A gyratory furnace was used during melting to ensure homogeneous mixing. The temperature was increased to 850°C at a rate of 1°C / min and held for 12 hours for homogenization. Then, gyratory heating was stopped, and the temperature was gradually reduced to 750°C for quenching. The melt was quenched in water and at the glass transition temperature (T0). g Anneal at 10°C for 6 hours. g Measurements were taken using differential scanning calorimetry (DSC) and differential thermal analysis (DTA) with a Netzsch Pegasus 404 apparatus. The glass block was placed in a platinum dish and heated at a rate of 10 °C / min, with a measurement accuracy of ±2 °C. The Tg values ​​of different glasses were measured. g The value is between 250-350℃.

[0037] Raman spectra were collected using an XploRA PLUS (Horiba) spectrometer equipped with a 785 nm excitation laser, focused onto the sample surface through a 50x objective lens. Raman mapping was performed on an 80 × 60 μm sample. 2 The scan was performed over a region using a high-resolution 100x objective lens (numerical aperture NA=0.9), achieving a lateral spatial resolution of approximately 2 μm during the scan.

[0038] 1.2 Characterization of Femtosecond Laser Irradiation and Laser Writing Modification Regions; Each sample was double-sided polished to optical quality before femtosecond laser irradiation using a Satsuma laser system (Amplitude Systemes Ltd., Pessac, France). The laser center wavelength was 1030 nm, repetition rate was 100 kHz, pulse duration was 800 fs, writing speed was 1 mm / s, corresponding to a pulse density of 100 pulses / μm. Linearly polarized light was used, with polarization along the x-axis (defined by the laser compressor plane). The beam was focused 300 μm below the surface (in air) using a 0.6 NA aspherical lens (focal length 4 mm). Pulse energy ranged from 0.001 to 1 μJ. Disk patterns were written using a helical scanning motion with a spacing of 1 μm, and biaxial displacement was achieved using a high-precision XYZ stage.

[0039] After irradiation, isothermal annealing (in a furnace) is performed in 30-minute increments, with the temperature gradually increased from room temperature to approximately 1.2 × T. g Optical characterization after each annealing step was performed using an Olympus BX51 microscope in transmission mode under natural light. In addition to conventional optical microscopy, quantitative phase microscopy (QPM, Iatia) was used to measure the optical phase shift (φ, unit radians) induced in the illuminated structure relative to the original glass. Due to the high optical nonlinearity of chalcogenide glasses, self-focusing and filamentation can occur, resulting in significant thicknesses of the modified regions—up to 200 μm in GeSbSNa glasses. All reported phase shifts were measured at or near the center of the written feature, where Δφ was the largest. The corresponding refractive index change (Δn) was calculated using the relation: Δn = Δφλ / 2πd, where λ = 750 nm is the illumination wavelength used in the QPM, and d is the thickness of the modified region.

[0040] 2. Experimental Results; Figure 2 shows the range of structural modifications induced by femtosecond laser irradiation in various GeSbSNa glasses. The observed mechanisms include: no modification, Type I, porous structures, spatially broadened structures + porous structures, and spatially broadened structures only. These mechanisms are represented by the distinct symbol shown in the figure legend.

[0041] Typically, the Type I modification threshold is slightly influenced by the glass chemical composition, sometimes attributed to structural relaxation and rapid quenching. In this invention, the deviation of the Type I formation threshold between GeSbSNa-BNa5.5 (~0.02 μJ) and other compositions (~0.001 μJ) remains within one order of magnitude. This indicates that inducing structural modification in higher alkali content glasses requires slightly higher energy, as shown in Figure 2. The energy window for Type I modification ranges from approximately 0.001 to 0.2 μJ across all compositions.

[0042] Interestingly, GeSbSNa-DNa2 and GeSbSNa-BNa5.5 glasses exhibit porous structures and space-enlarged structures plus porous structures in the intermediate energy ranges (0.3–0.6 μJ and 0.5–0.7 μJ, respectively). These features tend to appear in glasses with higher concentrations of network modifiers (such as Na) and lower proportions of network-forming agents (such as Ge).

[0043] At higher pulse energies or high repetition rates, another modified structure emerges—referred to as the space-widening structure—marked by a green asterisk in Figure 2. This mechanism exhibits a Type I morphology, originating from inter-pulse cumulative thermal deposition. Due to its ability to induce significant phase changes and refractive index alterations, it holds particular research value in integrated photonics applications such as waveguides. Among all the compositions studied, GeSbSNa-BNa5.5 consistently exhibits a slightly higher energy threshold for all types of laser-induced modification. This behavior may be attributed to the influence of alkali metal ions, which act as mesh modifiers, altering the glass structure and its laser sensitivity.

[0044] Distinct structural responses were identified: unmodified (black cross), Type I (pink rhombus), porous (blue circle), space-amplified and porous (purple circle), and space-amplified only (green asterisk). The data highlight the effect of glass composition on the energy threshold of the modified structure, with sodium-rich glass showing a slightly higher threshold. All samples were irradiated with femtosecond laser pulses (1030 nm, 800 fs, 100 kHz) at a writing speed of 1 mm / s.

[0045] The photosensitivity of transparent glass at laser wavelengths makes it possible to induce local phase changes via FLDW. Previous studies have reported high phase changes—typically in the 6–8 radian range—for Type I modification of heavy metal oxide glasses (such as barium gallium germanate BGG) under similar femtosecond laser exposure conditions. Furthermore, Lepicard et al. demonstrated the generation of phase changes in GeSbSNa glass via microscale thermoelectric imprinting, where sodium content was shown to significantly affect surface topology, density shrinkage, and volume displacement, all of which led to local refractive index changes. Adding sodium to the parent GeSbS glass resulted in an increase in refractive index (e.g., from 2.181 to 2.207), due to the Na+ content. + Ion departure to maintain charge neutrality leads to local density variations. Recently, Yang et al. reported the fabrication of gradient refractive index microlens arrays in sulfide glasses using microthermal polarization, achieving phase shifts from 0.35λ to 1.19λ under monochromatic illumination (λ = 632.8 nm). These results highlight the crucial role of glass composition and processing methods in controlling refractive index modulation.

[0046] Against this backdrop, Figure 3 illustrates the evolution of phase shift and optical thickness with laser pulse energy in several GeSbSNa glass compositions. Figure 3(a) includes the ANa series with a constant S / Ge ratio (2.6) and varying sodium contents (Na0, Na0.5, Na1), while Figure 3(b) shows the BNa and DNa series with a fixed sodium content but different S / Ge ratios (4.2 and 3, respectively).

[0047] Typically, the phase shift increases sharply with pulse energy, rising to approximately 0.09 μJ, and then gradually increasing at higher energies. The phase shift values ​​range from approximately 23 radians (plateau region) to 62 radians (maximum value), corresponding to relatively high refractive index variations, competing with values ​​reported for similar glass systems in the literature.

[0048] The optical thickness of the modified region also increases with pulse energy, indicating a volumetric increase in the laser-affected zone. Cross-sectional images reveal structures extending tens of micrometers along the laser propagation direction, consistent with the formation kinetics of Type I modification. Importantly, above the critical pulse energy (approximately 0.2 μJ in GeSbSNa), secondary non-divergent traces appear outside the main modified structure, marking the beginning of a more complex propagation mechanism beyond the linear focus. This effect is attributed to filamentation, stemming from a balance between self-focusing due to the increase in nonlinear refractive index (n²) and defocusing due to the formation of free electron plasma. These observations highlight the strong dependence of femtosecond laser interaction dynamics on composition (Na content, S / Ge ratio) and energy mechanisms, confirming that controlling these parameters is crucial for precise and repeatable refractive index modification in chalcogenide-based photonic materials.

[0049] The refractive index change (Δn) was calculated using the relationship Δn = Δφλ / 2πd, where Δφ is the measured phase shift (in radians), λ is the probe wavelength, and d is the optical thickness of the modified region. Figure 4 shows the maximum and average refractive index changes as a function of the S / Ge ratio in the GeSbSNa glass series. A clear decreasing trend was observed: the maximum Δn decreased from approximately 0.05 for GeSbSNa-ANa0.5 to approximately 0.032 for GeSbSNa-DNa2. However, these values ​​are still significantly higher than those typically reported for oxide-based glasses (such as BGG or SiO2), which are known for their highly interconnected tetracoordinate glass networks.

[0050] This remarkable refractive index tunability indicates a decrease in glass connectivity with increasing sulfur content, a trend previously explained by a combination of hydrodynamic expansion and thermomechanical relaxation following laser exposure. Furthermore, Raman spectroscopy revealed the emergence of new vibrational bands (such as at 152 cm⁻¹) with increasing sulfur content. -1 219cm -1 and 474cm -1), corresponding to the same polar SS bond. At high S / Ge ratios, femtosecond lasers may form the sulfide ring (S8) based on thermodynamic effects, which is the most stable and abundant known structure.

[0051] Another contributing factor to the observed trend is the gradual dilution of germanium (a strong network agglomerate) with increasing S / Ge ratio. The reduced Ge content weakens the glass's structural framework while simultaneously promoting the formation of sulfur-rich tetrahedral units (such as GeS). 4x These units may stabilize excess free sulfur and result in less significant laser-induced densification.

[0052] Finally, the mean refractive index change (which provides a more reliable measure from an engineering perspective) follows the same decreasing trend. On average, the mean Δn value is about 20% lower than the corresponding maximum, reflecting the spatial average of local modifications and providing insights into the repeatability and uniformity of femtosecond-induced transitions.

[0053] The observed decreasing trend with increasing sulfur content indicates a reduction in glass connectivity and densification efficiency under femtosecond laser exposure. The colored ellipses highlight the trends in the maximum (yellow) and average (purple) Δn values ​​within the sodium-doped series. These results confirm the crucial role of composition in determining the photoresponse of chalcogenide glasses.

[0054] Figure 5 illustrates the quantitative phase analysis of a femtosecond laser-written structure in GeSbSNa glass, namely a gradient refractive index lens structure (referred to as a GRIN lens). Compared to its conventional counterpart, the GRIN dispersive objective offers significant advantages, including a more compact design, a smaller diameter, and greatly enhanced measurement range and resolution. Therefore, it enables more accurate and easier measurements in practical applications. The spatial phase distribution, as shown in Figure 5(a), reveals a symmetrical circular pattern centered on the laser-modified region, corresponding to a spiral scan with a spacing of 1 μm (inset). The maximum phase shift, reaching ~20 radians, is observed near the center of the structure.

[0055] As shown in Figure 5(b), the horizontal phase profile—extracted along the dashed line and after baseline subtraction—confirms the smooth and locally gradient refractive index distribution of the induced optical phase change. The laser-modified region is clearly bounded, as indicated by a sharp phase drop (~-5 radians) at the edge, with a diameter of approximately 100 μm. This high phase contrast indicates a significant refractive index change at the center accompanied by a negative change around the periphery, consistent with the tensile stress field characteristics that inevitably develop in the laser-irradiated region. This type of quantitative phase information directly provides insights into the degree of structural and optical modification and supports the potential for femtosecond laser writing to achieve high-contrast, locally photonic microstructures in chalcogenide glasses, for example, for developing broadband and low-dispersion 3D integrated optics. As shown in Figure 5, a GRIN lens was fabricated on a chalcogenide sample using the method of this invention, and its phase distribution demonstrates the successful realization of a spatially varied refractive index distribution.

[0056] The thermal stability of laser-induced modified glass is a critical parameter for practical applications, especially in devices exposed to high temperatures, such as optical sensors and integrated components in harsh environments. To evaluate this property of GeSbSNa glass, Figure 6 plots the 90% erasure temperature of the laser-induced phase shift as a function of the glass transition temperature (T). g The erase temperature is a function of the type I modification in transparent materials. This erase temperature serves as a key metric for evaluating the thermal durability of Type I modifications.

[0057] This invention compares data obtained on GeSbSNa glasses (such as DNa2, BNa2) with various MIR-compatible glasses, including As2S3, SF10, BGG, ZBLAN, and commercial oxide glasses such as Corning 9754. A general trend is observed: higher T g The glass exhibits greater thermal durability with laser-written modifications, following a quasi-linear relationship marked by the dashed trend line. This is consistent with previous reports on the properties of Type I modifications, which are known to be thermally erasable and associated with defect formation and glass densification.

[0058] The GeSbSNa samples of this invention belong to the low to medium thermal stability category, with wipe-off temperatures ranging from ~230 to ~290 °C, depending on composition. While these values ​​are lower than those of oxide glasses (such as BGG or Corning 9754, exceeding 600 °C), they are still significantly higher than those observed in classical chalcogenide systems such as As2S3. This positions GeSbSNa glasses advantageously for applications requiring a balance between MIR transmittance and moderate thermal robustness. In particular, the GeSbSNa-DNa2 sample, after annealing at 290 °C for 30 minutes, exhibits a refractive index change Δn retention of over 90%, indicating a wipe-off temperature of approximately 290 °C, demonstrating its excellent thermal stability.

[0059] GeSbSNa-DNa2 glasses exhibit improved thermal resistance compared to GeSbSNa-BNa2, likely due to their higher sulfur content (from 67 at.% to 72 at.%). The increased sulfur content is associated with the formation of thermally stable S8 rings, which dominate the glass structure at elevated temperatures and are reported to be the most abundant species. In contrast, ZBLAN glasses, despite their widespread use in femtosecond writing, deviate from this trend, possibly due to their fluoride-based composition and poor thermal toughness.

[0060] Overall, while GeSbSNa glasses fall into the low to medium thermal stability category compared to robust oxide systems, they offer a promising trade-off between wide MIR transmittance and laser-written photosensitivity. These results highlight the importance of compositional tuning in optimizing optical performance and thermal durability in femtosecond laser-written photonic devices.

[0061] As is well known, the phase shifts or refractive index changes observed in femtosecond laser-irradiated glasses originate from structural rearrangements, such as local volume changes, element migrations, or bond configuration reorganizations. To analyze these modifications, Figure 7 shows the normalized Raman spectra obtained from the center of the femtosecond-irradiated region and the original region in GeSbSNa-ANa1 glass.

[0062] The spectrum exhibits characteristic bands associated with Ge-S bond stretching vibrations, as well as a band near 304 cm⁻¹ from the SbS₃ cone unit. -1 The Sb-S telescopic mode. 338 and 415cm -1 The peaks at 377 and 444 cm⁻¹ can be attributed to the angle-shared (CS) GeS4 configuration, while those at 377 and 444 cm⁻¹ are attributable to the angle-shared (CS) GeS4 configuration. -1 The peaks at [location] represent edge-sharing (ES) arrangements of GeS4 tetrahedra. These features are consistent with the known vibrational fingerprints of the Ge-Sb-S chalcogenide network. A subtle but distinct difference is observed between the two spectra, particularly in the high-frequency region. To better resolve these variations, the differential spectrum (irradiated-raw) is shown in the inset. This reveals a net increase in ES vibrational features following laser exposure, at a slight cost to the CS GeS4 tetrahedra.

[0063] Slight growth in ES pattern (typically between 370-480cm) -1 (Observed within the range) This indicates localized compaction or increased structural ordering within the irradiated volume, as ES connectivity implies a more tightly packed local network. Meanwhile, the CS mode at 338 cm⁻¹... -1 The reduction in the vicinity (after femtosecond irradiation) may reflect subtle reorganization or clustering of GeS4 units, potentially promoting an increase in refractive index by enhancing polarizability. Peak fitting and integral calculations of the spectrum revealed that after laser modification, the characteristic peak of ES-GeS4 (~377 cm⁻¹) was significantly reduced. -1 With ~444 cm -1 The joint integral intensity of ) and the characteristic peak of CS-GeS4 (~338 cm⁻¹) -1 The ratio of the integral intensity of the modified region to that of the unmodified region was increased by 10%–25%. No significant displacement was observed in the SbS3 mode, indicating that the antimony coordination remains relatively stable under femtosecond laser conditions. This implies that the structural response to laser irradiation is primarily driven by the germanium-sulfur grid, which controls short-range order and optical properties. These findings support the hypothesis that Type I modification in GeSbSNa glasses is accompanied by local rearrangement of the grid structure (particularly involving GeS4 units)—i.e., glass densification, rather than bond breaking or crystallization, consistent with the amorphous nature of the modified structure.

[0064] In summary, this invention demonstrates the significant tunability of the refractive index and structural properties of sodium-doped GeSbS (GeSbSNa) chalcogenide glasses under femtosecond laser irradiation. By varying the sodium content and S / Ge ratio, this invention identifies key component parameters controlling photosensitivity, refractive index contrast (Δn up to 0.05), and thermal durability of laser-written features. Quantitative phase microscopy and Raman spectroscopy reveal that the observed structural modifications originate from localized densification and tetrahedral rearrangement of GeS4, while causing minimal disturbance to the glass matrix. Importantly, moderate thermal stability (erasure temperature up to ~290°C) positions GeSbSNa glasses in a practical position among transparent MIR materials. These findings position GeSbSNa glasses as a strong candidate for 3D MIR photonic integration, particularly in applications requiring high refractive index contrast and miniaturized designs, such as on-chip laboratory spectroscopy, nonlinear MIR optics, or thermally stable waveguide components.

[0065] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention and within the spirit and principles of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A chalcogenide glass for femtosecond laser direct writing of mid-infrared photonic devices, characterized in that, The chalcogenide glass is composed of germanium, antimony, sulfur and sodium. When irradiated with a femtosecond laser, a modified region with a refractive index change Δn greater than 0.032 is induced.

2. The chalcogenide glass for mid-infrared photonic devices written by femtosecond laser according to claim 1, characterized in that, The ratio of sulfur to germanium atoms is 2.6-4.

2.

3. The chalcogenide glass for mid-infrared photonic devices written by femtosecond laser according to claim 1, characterized in that, The sodium content is 0.5 at.% to 5.5 at.% (atomic percentage).

4. The chalcogenide glass for mid-infrared photonic devices written by femtosecond laser according to claim 1, characterized in that, The modified area is a Type I modification.

5. The chalcogenide glass for mid-infrared photonic devices written by femtosecond laser according to claim 4, characterized in that, After the modified region is subjected to a temperature treatment not exceeding 290°C, the induced refractive index change remains not less than 10% of the original change.

6. The chalcogenide glass for mid-infrared photonic devices written by femtosecond laser according to claim 1, characterized in that, The Raman spectrum of the modified region shows that, compared with the unmodified region, the characteristic peaks of the shared GeS4 tetrahedral structure are relatively enhanced.

7. A method for femtosecond laser-induced refractive index tuning of GeSbSNa glass, characterized in that, This method is used to adjust the refractive index of chalcogenide glass for mid-infrared photonic devices written by femtosecond lasers according to any one of claims 1-6. The method includes the following steps: providing a chalcogenide glass substrate; focusing a femtosecond laser beam inside the glass substrate; and controlling the femtosecond laser parameters so that the laser energy density in the focused region is sufficient to induce a modification region with a refractive index change Δn greater than 0.032 inside the chalcogenide glass.

8. The femtosecond laser-induced refractive index tuning method for GeSbSNa glass according to claim 7, characterized in that, The femtosecond laser has a center wavelength of 1030-1060nm, a pulse duration between 100 fs and 3 ps, a repetition frequency of 10kHz-1MHz, and a scanning speed of 1mm / s.

9. A method for manufacturing a gradient refractive index optical element, characterized in that, The method includes the following steps: providing a chalcogenide glass substrate as described in any one of claims 1-6; using the femtosecond laser-induced GeSbSNa glass refractive index tuning method as described in claim 7 or 8, performing femtosecond laser direct writing on the interior of the chalcogenide glass substrate according to a predetermined three-dimensional pattern scanning path; wherein, by controlling the femtosecond laser direct writing process, a modified region with a spatially varying refractive index distribution is formed inside the chalcogenide substrate, thereby obtaining a gradient refractive index optical element.

10. A gradient refractive index optical element, characterized in that, The substrate material of the optical element is a chalcogenide glass as described in any one of claims 1-6, and the interior contains a modified region with a spatially varying refractive index distribution formed by femtosecond laser direct writing, wherein the refractive index variation Δn of the modified region is greater than 0.032.

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