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Abstract
Chalcogenide glass has achieved great success in manufacturing axial-type infrared gradient refractive index (IR-GRIN) lenses. However, studies on radial-type IR-GRIN lenses, which are more ideal for optical design, remain rare. The present study introduces what we believe to be a new method for preparing radial IR-GRIN lens by creating high refractive index (n) In2S3 nanocrystals within a 65GeS2-25In2S3-10CsCl (GIC, in molar percentage) glass matrix. Upon introduction of multi-temperature field manipulation, we have successfully achieved central crystallization and simultaneous gradient attenuation spreading toward the edge within GIC glass, providing a radial GRIN profile with Δn over 0.1 while maintaining excellent IR transparency. In addition, the optical and structural properties of the GIC GRIN samples were characterized. The relationship between Raman intensity and the n of glass ceramics at different heat treatment temperatures was investigated, thereby enabling the indirect confirmation of the presence of radial gradient crystallization within the prepared GIC GRIN samples through Raman intensity. Multiple experimental results have shown that this approach has excellent reproducibility and potential for large-scale productions.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Over the past several decades, rapid advancements in infrared (IR) imaging technology have been driven by progress in critical fields, such as medicine, biology, aerospace, and environmental monitoring [1,2]. Next-generation IR imaging systems focus on improving size, weight, power, and cost (SWaP-C) and developing lighter weight, smaller optical systems [3]. However, traditional IR imaging systems consist of expensive non-planar lenses and complex lens components, causing difficulty to achieve high-level integration and lightweight design [4]. Consequently, the SWaP-C goals cannot be achieved. Gradient refractive index (GRIN) lenses are optical components with a gradually varying refractive index (n), providing more degrees of flexibility for optical designers [5]. Materials with GRIN characteristics allow for the correction of aberrations or chromatic aberrations of the lenses in the optical system, achieving lightweight and miniaturization by reducing the number of optical components compared with conventional optical systems [6,7]. Therefore, IR-GRIN, involving the creation of a GRIN profile within IR materials, could be an effectual route to realize the SWaP-C goals of new IR imaging systems [8]. However, the fabrication technology of IR-GRIN lenses presents considerable challenges [9], which is currently unable to satisfy the practical requirements for mass production and large-scale commercial applications.
In recent years, the study of IR materials applicable for IR-GRIN lens preparation has received considerable attention [10–12]. Chalcogenide glass (ChG) has emerged as a promising candidate for thermal imaging and IR optical lenses due to its wide IR transmission range (2–18 µm), excellent thermal and chemical stability, simple preparation, and the flexibility to tailor compositions for desired optical properties [13,1]. Therefore, ChG holds significant potential in manufacturing IR-GRIN lenses. Several protocols have been developed to fabricate ChG-based IR-GRIN lenses, including stack diffusion [14–16], laser-induced controlled crystallization [17] or vitrification [3], electrospray printing [18] and ion exchange [19]. While these approaches have yielded satisfactory IR-GRIN distribution results, they typically require complex multistep processes, and the majority of methods are only applicable to the preparation of axial-type GRIN lenses, while reports on radial-type GRIN preparation methods, which are more desirable for optical design, are still very limited [20].
Spatial gradient temperature-controlled crystallization (SGTC) method is an emerging technique for fabricating IR GRIN materials by precipitation of high or low n nanocrystals in IR-transparent ChG [21,22]. The method involves the precise control of a temperature gradient field within the ChG. Consequently, nanocrystals precipitate and align in a gradient crystal growth manner along the temperature gradient direction, resulting in a smooth, quantitatively accurate GRIN profile with customized optical properties. Therefore, the SGTC method is currently among key approaches for efficiently exploring the preparation of large ChG radial GRIN lenses. Lavanant et al. successfully achieved radial spatial crystallization in 80GeSe2-20Ga2Se3 glass matrix under a designed temperature gradient field [23], and the radial spatial crystallization initiated from glass edge and spread toward center, with a maximum Δn of 0.03 at 10.6 µm. To the best of our knowledge, this article represents the only study that reports about the preparation of ChG radial IR-GRIN lenses employing SGTC method.
In this work, a novel approach has been proposed to improve the SGTC method by manipulating multi-temperature fields, thereby achieving crystallization in central and spreading toward edge within a ChG rod, which exhibits a crystallization trend opposite to that of the GRIN lens prepared by Lavanant et al. Here, a 65GeS2-25 In2S3-10CsCl (GIC, in molar percentage) pseudo-ternary system which possesses controlled crystallization behavior of In2S3 phase [24] was selected as the matrix material for radial crystallization. After specific gradient thermal treatment, the GRIN profile of Δn above 0.1 was formed by the central gradient crystallization of In2S3 phase within GIC ChG. In addition, the optical and structural properties of this radial GIC IR-GRIN lens have been characterized.
2. Glass synthesis and characterization
The GIC base glass was synthesized using the conventional melt-quenching method. High-purity raw materials of Ge (5N), In (5N), S (5N), and CsCl (4N) were weighed and transferred into 10 mm-diameter quartz tube, which was sealed under 10−5 Pa. Then, the sealed tube was heated in a rocking furnace at 970 °C for 12 hours (h) and subsequently quenched with cold water. To reduce internal stress, the samples were annealed at 310 °C, which was 25 °C below the transition temperature (Tg = 335 °C), for 4 h and then slowly cooled to room temperature. Finally, the obtained GIC ChG rod was sliced into 2.5-millimeter-thick glass pieces, and glass ceramics (GCs) were prepared by heat treatment (HT) at various temperatures above its Tg for 5 h.
Thermal parameters of the base glass were tested using a differential scanning calorimeter (DSC, TA Q2000, USA) with a heating rate of 10 K/min. Transmission spectra in the range of 400–2500 nm and 2.5–15 µm were recorded using a spectrophotometer (Lambda950, PerkinElmer, USA) and a Fourier-transform infrared spectrometer (FTIR, Nicolet381, Nicolet, USA), respectively. The crystallization of GIC GCs was identified by X-ray diffraction (XRD; AXS D2 PHASER, Bruker, Germany) with Cu Ka radiation at 30 kV, 10 mA, and a step width of 0.02°. Refractive dispersion curves of glasses were measured using an IR spectroscopic ellipsometer (J.A. Woollam IR-Vase II, USA) from 2.5 µm to 12 µm, with a refractive index measurement accuracy of ±0.0005.
Raman spectra of the samples were obtained using a laser confocal Raman spectrometer (InVia, Renishaw, UK) in back scattering configuration with an excitation wavelength of 785 nm at room temperature. During Raman line scans on the GIC GRIN sample, the laser was focused on the sample surface to a spot diameter of approximately 1 µm. The laser power on the sample was maintained at approximately 0.2 mW to prevent damage, and the laser scanning started at the center of the sample and moved radially at 100 µm intervals until the edge was reached.
3. Results and discussion
3.1 Characterization of GIC base glass and GCs
In this section, we thoroughly characterized the GIC base glass and GCs after HT at different temperatures to provide basic data for comparison with subsequently prepared GIC GRIN samples. Figure 1(a) shows the DSC curve of the GIC base glass, where Tg is 335 °C and two evident exothermic peaks onset at Tx1 = 423 °C and Tx2 = 462 °C are observed. According to previous studies on GIC GCs, the first peak is related to the crystallization of In2S3 phase, whereas the second is related to GeS2 phase [24–26]. The thermal parameter ΔT = Tx - Tg is 88 °C, which is lower than 100 °C, indicating that the GIC glass matrix is suitable for temperature-controlled crystallization. According to the DSC data, the GIC base glasses were subjected to HT process at temperatures of 25 °C, 40 °C, 55 °C, and 65 °C above its Tg for 5 h to induce crystallization in the glass network. Figure 1(b) shows the XRD patterns of the GIC glass samples heat-treated at different temperatures, demonstrating that crystal growth has started at 360 °C and intensified with increasing HT temperature. The Refs. [24,26] demonstrate the formation of In2S3 phase (PDF No. 84-1385). When the HT temperature reached 400 °C, precipitation of other crystals belonging to GeS2 phase (PDF No. 30-597), in addition to In2S3 phase, can be observed [24]. The inset of Fig. 1(b) illustrates that the color of the GCs gradually changed from transparent orange to reddish-brown as the temperature increased, consistent with the growth pattern of the crystals within.
Fig. 1. (a) DSC curve of the GIC base glass at a heating rate of 10 K/min. (b) XRD patterns of GIC base glass and GCs were obtained after HT at 360 °C, 375 °C, 390 °C, 400 °C for 5 h, with the inset displaying a photograph of polished samples.
The precipitation of crystals can impact the basic optical properties of the GIC glass. The VIS-IR transmittance spectra in Fig. 2(a) clearly illustrate that as the HT temperature increases from 360 °C to 390 °C, the short-wavelength cutoff edge experiences a red-shifting trend, attributed to band-gap narrowing caused by precipitation of the In2S3 crystals and scattering losses by their growth. In addition, as displayed in Fig. 2(b), the IR transmission window of GIC GCs remains unchanged in the range of 2.5–12 µm, indicating that the In2S3 crystals are uniformly distributed in the glass matrix and in nanoscale sizes.
Figure 3 depicts the n dispersion curves of the GIC base glass, as well as three GCs heat-treated at 360 °C, 375 °C, and 400 °C. The precipitation of In2S3 crystals significantly elevates the n value of the base glass, particularly in the 3–12 µm wavelength range, and the maximum Δn (compared with n value of the base glass) of 0.1204 is obtained at 5.78 µm. In addition, the precipitation of GeS2 crystals at 400 °C decreases the n value, consistent with the previous research results [27]. This finding indicates that the detrimental GeS2 crystals should be avoided in the GIC-based GRIN lens preparation.
Fig. 3. Dispersion curves of GIC base glass and GCs heat-treated at 360 °C, 375 °C and 400 °C for 5 h.
Characteristic structural changes of GIC base glass and GCs were investigated by Raman spectroscopy. Figure 4(a) shows their Raman spectra, and two prominent peaks were observed at wavenumbers of 307 and 342 cm−1. The peak at 307 cm−1, attributed to the symmetric stretching vibration of [InS4] units [28,29], gradually intensifies with increasing HT temperature. In addition, the band located at approximately 342 cm−1, which is attributed to the symmetric stretching vibration of [GeS4] tetrahedra [28,29], is also gradually enhanced with the increasing HT temperature. The enhanced Raman intensity of the GIC GCs indicates an increasing degree of internal structural ordering due to crystallization, which is also associated with the presence of In2S3 crystals. Figure 4(b) depicts the Raman intensities of the peaks at 307 and 342 cm−1 versus the n value at 5.78 µm, and a proportional relationship between the Raman intensity and n value can be obtained. In other words, the Raman intensity shows a linear correlation with the n value as the HT temperature increases. Thus, such correlation can be applied to indirectly determine the n variation profile of GIC GRIN lenses according to structural evolution.
Fig. 4. (a) Raman spectra of GIC base glass and GCs heat-treated at 360 °C, 375 °C and 390 °C for 5 h. (b) The n value (at 5.78 µm) varies as a function of Raman intensity at 307 and 342 cm−1 at different HT temperatures, with the solid lines are linear fitting.
3.2 Preparation of GIC GRIN lenses
The above section has demonstrated that controllable crystalline states of In2S3 phase in GIC base glass can be achieved by HT process at various temperatures between 360 °C and 400 °C. Therefore, a fundamental protocol for creating a radial GRIN profile within a GIC glass rod is provided by manipulating spatially gradient crystallization through multi-temperature fields using the SGTC method, thereby achieving radial gradient crystallization. Figure 5(a) details the process of utilizing such protocol to create radial GIC GRIN materials. A cylindrical GIC glass rod (10 mm in diameter, 50 mm in length) is prepared and placed into a gradient furnace customized with three independent temperature zones, each equipped with thermal insulating layers which are 20 mm thick ceramic fibers to minimize temperature interference. Then, the three zones of the gradient furnace are set to high-temperature zone at the top, medium-temperature zone in the middle, and low-temperature zone at the bottom (all above Tg but not exceeding 400 °C) to reduce temperature interference, and nitrogen is introduced during the HT process to prevent oxidation of the glass surface. The GIC glass rod moves back and forth at a constant speed in three different temperature zones, with the annealing time controlled by drawing the glass rod at a constant speed through the gradient furnace. Low thermal conductivity of the ChG glass, combined with reciprocating motion, prevents homogenization of temperature inside the glass rod. Particularly, passing through low-temperature zone from high-temperature zone accelerates heat dissipation at the edge of the glass rod, thereby resulting in higher temperatures in the middle of the glass rod and generating the desired radial temperature distribution. Figure 5(b) illustrates the expected temperature distribution inside glass rod, where surface is cooled as it passes through the cold zone while the center maintains a higher temperature. Consequently, the expected radial distribution of crystallization should align with the temperature field, ultimately resulting in a GRIN profile.
Fig. 5. (a) Schematic diagram of the multi-temperature fields manipulated STGC method. Three furnaces from top to bottom represent the high, middle, and low temperature zones, respectively. (b) Expected radial crystallization in GIC GRIN glass.
The capability to fabricate GIC GRIN lenses relies on establishing a stable gradient temperature field within the glass rod. However, variations in the movement speed and the temperatures of three independent zones of the gradient furnace, as the glass rod reciprocates, can significantly impact the temperature field within the glass rod. Figure 6 presents four physical samples produced under identical temperature conditions (high, middle, and low temperature zones at 400 °C, 380 °C, and 360 °C, respectively) as well as the image of a metallic grid captured using an IR camera in the 8–12 µm waveband. The figure clearly demonstrates the impact of movement speed on the radial gradient crystallization distribution within the glass rod. At 80 mm/min, rapid movements prevent crystallization due to frequent transitions through the low-temperature zone, and the metal grid remains flat and continuous. At 70 mm/min, a GRIN profile begins to form in the glass rod as indicated by the slight compression of metal grid in the central, but it is weak because the crystallinity is low. When movement speed reaches 60 mm/min, notable crystallization behavior (darker color) can be observed in the sample, indicating that crystallization may have occurred at the center rather than at the edge. The metal grid beneath this sample is remarkably compressed and deformed in the central region and edge due to the presence of gradient n variation along the radial direction. Conversely, when the movement speed is reduced to 40 mm/min, over-crystallization occurs as a result of prolonged exposure to the high-temperature zone, and no GRIN behavior can be observed. Therefore, determining the optimal movement speed is critical in the present IR-GRIN preparation technology. Moreover, the effective speed for the preparation of GIC-based GRIN lens is in the range between 40 and 70 mm/min.
Fig. 6. Physical images of the samples prepared by spatial gradient temperature-controlled crystallization at different speeds in the same temperature condition. Images of the metal grids through the prepared samples at different speeds captured using an IR camera.
Figure 7 depicts the GIC base glass and three GIC GRIN samples prepared by the same movement speed of 60 mm/min but different temperature condition. The thermal images illustrate that all the three samples exhibit GRIN behavior, but the spreading range and intensity of gradient crystallization are different. The sample subjected to relatively higher HT temperatures demonstrates notably increased crystallinity, manifested by a darker color and larger range in the center. Thus, it exhibits evident GRIN behavior, displaying a more distorted metal grid in the image. Therefore, higher HT temperature is more beneficial for the preparation of such IR-GRIN lens preparation.
Fig. 7. Physical images of the GIC base glass and GRIN samples prepared under gradient HT at different temperature conditions (A, B and C); images of the metal grids through the GIC base glass and GRIN samples captured using an IR camera.
3.3 Characterization of GIC GRIN lenses
These experimental results and fabrication protocol indicate that an expected GIC GRIN lens should display darker color (high crystallinity) in the center, with a lighter color (low crystallinity) toward the edges, as well as high transparency in the visible and IR ranges. Figure 8(a) presents two typical physical GIC GRIN samples produced at the temperature condition C, as shown in Fig. 7, with movement speed of 60 mm/min (C1) and 65 mm/min (C2). The XRD patterns in Fig. 8(b) reveal the absence of crystallization at the edge, whereas a distinct crystalline peak (In2S3 phase) is observed in the center, verifying the above speculation. It should be noted that no GeS2 crystals precipitated in the samples, which is attributed to the vertical reciprocating motion and the relatively low thermal conductivity of the glass unable to meet the growth condition of such crystals. In addition, Fig. 8(c) presents the transmission spectra obtained at three points from the center to the edge of sample C1. This finding indicates that excellent IR transparency in the range of 2.5–12 µm was maintained after crystallization, but the overall transmittance decreased slightly from the non-crystallized edge to the crystallized center, due to the scatting loss from n mismatch between GIC glass matrix and In2S3 crystals.
Fig. 8. (a) Physical image of two GIC GRIN samples, C1 and C2. (b) XRD patterns of sample C1 at the edge and center. (c) FTIR transmission spectra at 3 different places in sample C1 as noted in (a). (d) IR dispersion curves of sample C2 at the edge and center; the inset depicts the difference between the refractive index between the edge and center.
Figure 8(d) shows two n dispersion curves ranging from 2.5 µm to 12 µm, measured at the center and the edge of sample C2. The inset displays the corresponding Δn dispersion curve, indicating a general Δn above 0.07 in this IR region. The maximum Δn also presents at 5.78 µm, with the value of 0.1047, is more than three times the maximum Δn of 0.03 at 10.6 µm reported in a Ga2Se3 phase-based GRIN lens using 80GeSe2-20Ga2Se3 ChG as a substrate [23].
While XRD patterns and IR transmission spectra can confirm the presence of spatially distributed gradient crystallization in the prepared GIC GRIN lenses, tracing the internal GRIN profile of the lens remains challenging. According to the proportional correlation in Fig. 4(b), the internal structural changes manifested in the evolution of Raman spectra can be applied to identify the variations in the n value [30–32]. Line scanning Raman spectra along the radial axis of the sample C1 were conducted to structure the radial GRIN profile in the prepared lens. The scanning began at the center of the sample C1 and moved radially at intervals of 100 µm until the edge is reached. Figures 9(a)-(c) show the 50 collected radial Raman spectra and 3D patterns of the Raman peaks at 307 and 342 cm−1. The figures evidently show that the intensity of both Raman peaks varied with radial distance, with a summit intensity at the center and gradient decreasing toward the edge. The Δn profile at 5.78 µm obtained from the Raman intensity variation of peaks at 307 and 342 cm−1 is shown in Fig. 9(d). The results confirmed that a radial GRIN profile with Δn between canter and edge over 0.1 had formed in the prepared sample C1.
Fig. 9. (a) Line scan Raman spectra measured from edge to center in a GIC GRIN lens. (b) and (c) 3D patterns of Raman spectra from the line scan for InS4 and GeS4 peaks, respectively. (d) The gradient refractive index distribution inferred from the fit of the Raman intensity profile in (b) and (c).
While refractive index change is important when considering GRIN applications, optical designers typically rely on a material’s dispersion characteristics across the target spectral windows. Corsetti et al. [30] suggested that an achromatic doublet could be replaced by a singlet with the addition of a GRIN component. The greater magnitude of the deviation between the Abbe number of the substrate and the GRIN indicates higher potential for achromatic aberration correction in a GRIN singlet prism. Abbe number (V) serves as a vital parameter for quantifying the dispersion of a refractive index spectrum, and V expressions for conventional materials and GRIN lenses are shown as follows [31].
Table 1. Abbe number (V) of GIC base glass, GCs, GRIN lenses and other axial GRIN materials.
4. Conclusions
This article presents a novel method for fabricating radial IR-GRIN lenses using a SGTC technique manipulated by multi-temperature fields, considering GIC ChG as the substrate. A custom-designed gradient furnace with three independent temperature zones and automatic lifting platform was utilized, allowing the glass rod to move up and down through different heating states, thereby creating a radial temperature distribution with high temperature in center and low temperature at the edges of the substrate. A radial GRIN profile with Δn above 0.1 was obtained in GIC GCs with controlled crystallization of the In2S3 phase. The calculation of the Abbe number reveals significant differences in dispersion characteristics between the GIC base glass, GCs, and GRIN lenses. Particularly noteworthy, the Abbe number of the GIC GRIN lens can be negative in the mid-IR region (3–5 µm), rendering its ability for IR achromatic aberration. Therefore, the excellent repeatability, flexibility, and simplicity of such preparation technology demonstrate its potential for manufacturing large radial IR-GRIN lens.
Funding
National Natural Science Foundation of China (62075108); Natural Science Foundation of Ningbo (2023J016); K. C. Wong Magna Fund in Ningbo University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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