The gradient refractive index (GRIN) lens is widely used in the visible band, but it is still elusive in the infrared band. In this paper, we propose a new method of fabricating chalcogenide GRIN by spark plasma sintering (SPS) technology based on powder stacking and sintering thermal diffusion. We replaced Se in Ge11.5As24Se64.5 glass with S and prepared several Ge11.5As24Se(64.5-x)Sx glasses as infrared transmission GRIN materials. The maximum refractive index difference (Δn) of the matrix glass is 0.18. The effects of heat treatment temperature and time on diffusion depth and concentration-dependent thermal diffusion coefficient were investigated. The diffusion depth of 100 µm was demonstrated under the condition of 400 °C-48 h by this method. The thickness of the glass layer can be well controlled by powder stacking. The obtained GRIN glass is highly transparent in the near- and mid-infrared wavelength region.
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Gradient refractive index (GRIN) lenses are optics where the refractive index varies uniformly in the axial, radial, and spherical dimensions . GRIN lenses with well-tailored optical properties provide great spatial design freedom for the optical system and reduce the system's requirements for the difficulty of chromatic aberration correction . Therefore, GRIN has received great attention in the fields of optical fiber communication, micro-optics, and precision medicine [3,4]. Nowadays, GRIN optical elements have been widely used in visible light systems owing to their excellent performance [5,6], while few ideal results have been obtained for the preparation of infrared (IR) GRIN [7–9].
At present, IR GRIN is mainly prepared by ionic exchange, glass stacking thermal diffusion, thermally-induced crystallization [10,11], and other methods. Typical work, such as Zhang Xianghua et al., reports an innovative way to prepare IR GRIN by ion exchange technology. A diffusion length of 2 mm and a refractive index change of Δn = 0.045 were demonstrated by this method . However, the diffusion depth is limited by the fixed ion concentration difference, so it is still a great challenge to prepare large-diameter chalcogenide glasses. A method was proposed by Richardson Kathleen et al. that the partial vitrification of chalcogenide glass-ceramics induced by femtosecond laser [12–14] can achieve the refractive index change of Δn∼0.062, and their work can generate a refractive index diffraction grating structure in which the refractive index is spatially modulated in a discrete manner. In 2014, Gibson et al. of the U.S. Navy Laboratory (NRL) used the chalcogenide glass stacking thermal diffusion method to prepare axial IR GRIN glass . By optimizing and developing 24 kinds of basic glass materials with different components (no specific components are disclosed), the refractive index is close to gradient change through long-time thermal diffusion of atoms between layers . However, due to the large glass thickness (mm level), the time required for uniform diffusion is quite long (30 days in theory).
The glass stacking thermal diffusion method is more universal and not limited to the size of the glass, but also easy to achieve Δn control compared with other infrared GRIN material preparation technologies. However, thick glass requires a longer thermal diffusion time, and it is difficult to obtain a sub-millimeter lens with the traditional mechanical cutting process. These two points contradict the requirements for the preparation of GRIN. For this respect, we extend the above idea and propose a new method of fabricating chalcogenide GRIN by spark plasma sintering (SPS) technology based on powder stacking. SPS is a novel pressure-assisted pulsed electric current sintering process utilizing ON-OFF DC pulse energizing. This technology reduces the sintering temperature considerably, provides a very fast heating rate, very short holding time, and improves the preparation efficiency of glass . Therefore, it has been widely studied in the preparation of chalcogenide glasses [18–20]. Powder particles smaller than micrometers can be obtained by mechanical grinding of glass blocks, which makes it possible to lay down sub-millimeter powder layers. The thinner powder layer can greatly shorten the thermal diffusion time and also reduce the refractive index gradient difference between layers by increasing the number of powder layers on the premise of controlling the thickness.
Chalcogenide glasses have a wide infrared transparency range, low temperature dependent refractive index (about 1/10 of germanium single crystal material), large-scale molding lens manufacturing, and continuously adjustable components, making it very promising for fabricating IR GRIN. Ge11.5As24Se(64.5-x)Sx was used as the matrix glass for the preparation of GRIN because its composition is continuously adjustable to meet the change of refractive index gradient . The gradient change of the refractive index was realized by replacing the Se element in the glass system with element S. The glass also has good infrared transmission performance and good thermal stability (MCN = 2.47), which is conducive to preventing glass crystallization due to long-time heat treatment.
In this paper, we proposed the fabrication of IR GRIN by powder stacking sintering thermal diffusion method. In this way, the thickness of the layer can be well controlled (hundred-micron level). SPS sintering process of multilayer chalcogenide glass powder was explored. The good transmission was measured in such a multilayer chalcogenide glass block. The feasibility of this method was proved. The thermal diffusion of the material was carried out at a certain temperature and time, and the atomic diffusion depth was measured by electron probe microanalysis (EPMA).
2.1 Preparation of basic glass
Ge11.5As24Se(64.5-x)Sx bulk glasses with x = 0, 4, 8, 12, 16, 24, and 32 mol% (labeled S0, S4, S8, S12, S16, S24, S32) were prepared from the elements using the conventional melt quenching technique. High purity (5 N) Germanium, Arsenic, Sulfur, and Selenium elements were weighed and loaded into a pre-cleaned quartz ampoule. The loaded ampoule was dried under vacuum (< 10−6 Torr) at 110 °C for 2 h to remove surface moisture from the raw materials. The ampoule was then sealed under vacuum using a gas-oxygen torch, and introduced into a rocking furnace to melt the contents at 750 °C for a period not less than 20 h. The ampoule was subsequently removed from the rocking furnace and being quenched. The resulting glass boule was annealed at a temperature of 30 °C below Tg to minimize residual stresses in the melt. After the annealing process, the glass rod was cut into a disc with a thickness of about 2 mm and polished to optical quality.
The amorphous nature of the glass was confirmed by X-ray diffraction (XRD, Bruker D2 Phaser, λ=0.15406 nm, 30 kV, 10 mA, CuKα) using a conventional X-ray diffractometer in a 2θ scan mode. Thermal analysis was performed by using differential scanning calorimetry (DSC, TA Q20 Thermal Analysis) with a heating rate of 10 °C/min from room temperature to 350 °C under the protection of flowing N2 atmosphere. The linear refractive indices (n) were measured for single side-polished bulk glass samples with 2 mm thickness using an infrared ellipsometer (J.A. Woollam, IR-Vase II, USA). IR transmission spectra (2.5–25 µm) were measured using a Fourier transform infrared (FTIR) spectrophotometer. The density of the samples was obtained by the Archimedes method with alcohol as the immersion liquid. All measurements were carried out at room temperature. Samples from each glass composition were weighed five times and the average density was recorded.
2.2 SPS Sintering
The glass rods with specific components were mechanically ground for more than 4 hours at the speed of 1200 rpm and protected with high-purity argon. Micron-sized powders could be obtained after mechanical grinding. The thickness and weight of each layer of powder were set and calculated according to the mass density volume relationship. The glass powder of S0, S4, S8, and S16 was selected to study the SPS sintering process. The glass layer thickness after densification was set as 600 microns, and the weight of the powder was 0.476 g, 0.466 g, 0.457 g, and 0. 439 g respectively. The glass powders were stacked in the graphite mold (15 mm diameter) in the specified direction. To prevent carbon pollution in the sintering process, molybdenum sheets were used to isolate the upper, lower and surrounding parts of the powder in the mold.
The schematic diagram of the preparation process is shown in Fig. 1. Separate homogeneous glass powders with different element components were stacked in the graphite mold in the specified direction. The powder was sintered by spark plasma sintering (LABOX-1575F spark plasma sintering system) under vacuum at a temperature about 100 °C higher than the glass transition temperature (Tg). Maximum pressure of 40 MPa was applied. The heating rate was 80 °C/min, and the cooling rate was about 50 °C/min. Then, the sample was heat-treated. The resulting samples were optically polished for the required optical measurements. Finally, the resulting samples were embedded in epoxy resin, cut through the center, polished, and tested. The composition profile was measured and characterized by the EPMA line scan.
3. Result and discussion
3.1 Basic glasses
Figure 2(a) presents the linear refractive index of the Ge11.5As24Se(64.5-x)Sx glasses as a function of sulfur content. With the increase of sulfur content, the refractive index of glass decreases gradually. Therefore, several groups of candidate materials were prepared by replacing selenium with sulfur. The maximum refractive index difference is Δn≈0.18. Figure 2(b) intuitively shows the relationship between refractive index and S content. The upper axis refers to the characteristic X-ray intensity generated by the excitation of the S element in the glass system in the unit time of the EPMA test. As shown in the figure, the intensity is positively correlated with the concentration. Therefore, the change in refractive index can be revealed by changes in intensity.
Table 1 shows the density and glass transition temperature (Tg) of the Ge11.5As24Se(64.5-x)Sx glasses as a function of sulfur content. With increasing sulfur content, the density changes from 4.495 g/cm3 at x = 0 to 3.783 g/cm3 at x = 32. While it is reasonably expected that the Ge11.5As24Se(64.5-x)Sx glasses have a similar structure, the decreasing density of the glass by S-substitution is due to the lighter atomic weight of S (32.06) compared with that of Se (78.96).
The refractive index of chalcogenide glass is closely related to its composition, and the relationship between its density and refractive index can be expressed as :
The Tg of these seven glasses were measured and found that Tg changed from 201 °C to 205 °C without any obvious compositional dependence, indicating that the substitution of S for Se has no significant modification of the network structure. Temperature is an important parameter in the SPS experiment, and the selection of temperature parameters often refers to the temperature range of (Tg+100 °C). Therefore, the glass has similar Tg, which can reduce the influence of temperature parameters on powder sintering and ensure the good transmittance of the resulting samples.
3.2 Glasses after SPS
The glass powder of S0, S4, S8, and S16 (Δn≈0.1) are selected to study the SPS sintering process. The infrared transmittance of the well-polished glasses sintered by SPS is shown in Fig. 3(a). The results show that the infrared transmittance is good at a long wavelength and decreases obviously at a short wavelength, which is mainly due to the Rayleigh scattering caused by impurities in the glass (impurities come from dust impurities introduced during glass grinding). The inset is an infrared photo of the sample.
The samples are tested by the EPMA, and the results are shown in Fig. 3(b). It can be seen from Fig. 2 that the change of glass refractive index has a good corresponding relationship with the change of S content. The change of refractive index of glass can be characterized by the change of S content. Therefore, the changes of the S element are detected here. The results show that the diffusion depth between layers is small, and the diffusion depth for layers with a large concentration difference is 12 µm (the test step is 4 µm). The inset of Fig. 3(b) is the sample high-resolution EPMA image of the cross-section. And we can see that the thickness of each layer is about 600 microns, which is the same as the thickness value we set. Figures 3(c) and 3(d) are schematic diagrams of the refractive index gradient of the 5-layer and 7-layer samples. As shown in the EPMA image, the number and thickness of powder layers can easily adjust by the powder stacking method, and the thickness of the powder layer is reduced to 300 microns.
Then, the two-layer (S0-S16) samples are prepared to explore the thermal diffusion depth under different temperature and time conditions. The experimental parameters are shown in Table 2. The transmittance of samples 6 and 8 is shown in Fig. 4(a). It is proved that the selected experimental parameters have little effect on the transmittance. To measure the relative concentration of S elements in the glass, EPMA line scanning is carried out in the center of the sample along the central axis of the joint surface. The results are shown in Table 2. The experimental results (1, 2, 4, 7, 8) show that a higher heat treatment temperature can obtain a larger diffusion depth. Experiments (3, 4, 5, 6) show that the extension of heat treatment time increases the diffusion depth. The diffusion distance of sample 8 exceeds 100 microns, as shown in Fig. 4(b). The blue curve in Fig. 4(b) shows the distribution of S elements in GRIN glass, and the content of specific S corresponds to the known glass components (S0-S16). The red triangle points in the figure represent the refractive index of the corresponding component glass (n@8 µm). Fitting the red triangle points with the same Boltzmann function as the element distribution line shows a good correlation.
The diffusion profile for a specific element can be predicted using Fick’s laws of diffusion :
And3) and (4) can be derived as :
From the calculated diffusion coefficient results, the diffusion coefficient decreases first and then increases with the decrease of concentration. It can be seen from Figs. 5(a) and 5(b) that the diffusion coefficient increases with the increase of heat treatment temperature. The diffusion coefficient of the sample at 400 °C is about one order of magnitude higher than that at 300 °C. It can be seen from Figs. 5(c) and 5(d) that the diffusion coefficient increases with the increase of heat treatment time. The diffusion coefficient of the sample at 144 h is about an order of magnitude higher than that at 24 h. It can also be found that the diffusion coefficient of the sample at 400 °C-48 h is greater than that at 330 °C-144 h. It can be concluded that compared with the time factor, the temperature is the main factor affecting the diffusion ability in the heat treatment process.
In summary, multilayer refractive index graded glass was fabricated through the SPS, which was feasible for the design of IR GRIN. The diffusion depth was explored using EPMA. A diffusion depth of 100 microns was achieved by heat treatment at 400 °C for 48 hours. The effects of heat treatment temperature and time on diffusion depth were discussed in detail. The concentration-dependent diffusivity of the system was calculated and analyzed by the Boltzmann-Matano method. These results indicate that this technique offers a new way to fabricate IR GRIN lenses.
Joint Funds of the National Natural Science Foundation of China (U21A2056); National Natural Science Foundation of China (61775111, 61975086); Key R&D program of Zhejiang Province, China (2021C01025); Natural Science Foundation of Ningbo (202003N4180); K. C. Wong Magna Fund in Ningbo University; Zhejiang Provincial Natural Science Foundation of China (LY19F050004).
The authors declare no conflicts of interest.
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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