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Abstract
The refractive index of commercial chalcogenide glasses (ChGs) available in the market is generally 2.4 to 2.7, which is relatively low and has huge room for improvement. In this paper, different ratios of Ag/Pb were doped into commercial glasses by the melt-quenching method to substantially increase their refractive index. The refractive index of the commercial Ge28Sb12Se60 glass was increased from 2.6 to 3.05 by external doping with 20 atomic percentage (at%) of Ag. And the refractive index of commercially available Ge33As12Se55 glass was increased from 2.45 to 2.88 by external doping with 9 at% of Pb. These improvements effectively reduce the thickness of commercial lenses at the same radius of curvature and focal length. The physical and optical properties of commercial glasses doped with Ag/Pb in different proportions were systematically characterized.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Infrared (IR) lenses are increasingly taking on an important role in military and civilian imaging systems, including night vision detector [1], missile tracking [2], temperature detection [3] and medical imaging [4]. Common commercial infrared imaging systems typically consist of a combination of infrared crystals and ChGs. Typical infrared crystals such as ZnSe/S and Ge are used as part of the imaging system components, and are difficult to modify in the imaging system because their optical properties are determined by a fixed crystal structure [5–7]. ChGs are non-oxide amorphous compounds based on S, Se and Te and combined with Ge, As, Sb, Ga, Ag, Pb, etc. They have the ability to substantially modulate the compositions and their properties, and maintain extremely good infrared optical properties. Common commercial ChGs include Ge28Sb12Se60, Ge33As12Se55, As2Se3, etc. These ChGs have high infrared transmittance, good mechanical and thermodynamic stability. With the increasing market demand for lighter and smaller lenses, increasing the refractive index of ChGs to reduce lens thickness is the simplest and most efficient solution. The refractive index of these commercial ChGs generally range from 2.4 to 2.7, which is relatively low and has huge room for improvement. Increasing the refractive index can effectively reduce the thickness of the lens and expand the field of view of the lens. Therefore, it is of great interest to study the improvement of refractive index of commercial ChGs [8–12].
Current methods for increasing the refractive index of ChGs include high-energy radiation, ultra-high pressure densification and external doping of elements [13]. High-energy radiation is generally used to locally crystallize thin films, and has a very limited increase in refractive index (about 10−3). This method can only change the refractive index of the local area of the glasses, and it is difficult to change the overall refractive index of the glasses. In addition, the crystallization of chalcogenide glass by high-energy radiation easily leads to a significant decrease in near infrared transmittance [14]. Ultra-high pressure densification is generally used to improve the density and refractive index of silica glass by high temperature and high pressure. Typical work such as, V. Keryvin et al. demonstrated that the density and refractive index of silica glass can be increased by about 20% and 14%, respectively [15,16]. But it is easy to destroy commercial ChGs and its properties. Compared with the above two methods, the refractive index of the whole glass can be directly and effectively enhanced by external doping of ChGs with specific elements, and parameters such as refractive index and density can be accurately controlled by the content of doping. Numerous research works have reported the enhancement of ChG properties by external doping [17–20].
Ag and Pb, as additives, have been widely used to dope ChGs to enhance properties such as electrical conductivity [21,22]. On the other hand, Ag and Pb are well suited to increase the density and refractive index of ChGs due to their heavy metal nature and their high solubility in ChGs [21,23]. Compared to the common chalcogenide elements, Ag and Pb have larger atomic numbers and polarization indices, which obviously have a more pronounced effect on the refractive index according to Lenz-Lorentz formula [13]. Ge28Sb12Se60 and Ge33As12Se55 glasses are widely used in commercial infrared imaging systems due to their superior mechanical, thermal, and optical properties. However, the refractive indices of these glasses are still low. Therefore, in this paper, these two glasses are chosen as the base glasses for Ag/Pb doping. The refractive index of the glasses is increased to the greatest extent possible while ensuring the stability of the glasses. The thermal, optical and mechanical properties of all glasses and the evolution of the glass network structure were analyzed and evaluated in detail. The effect of lens thickness lightening for a given model was calculated and analyzed by means of Fermat's principle.
2. Experimental Methods
The (Ge28Sb12Se60)100-xAgx (x = 0, 5, 10, 15, 20) and (Ge33As12Se55)100-xPbx (x = 1, 3, 5, 7, 9) glasses were prepared by the standard melt-quenching method. Ge (5N), Sb (5N), As (5N), Se (5N), Ag (4N), and Pb (3N) raw materials were accurately weighed and vacuumed in a quartz tube at 100°C for 2 h, and then sealed in vacuum (10−4 Pa). And these mixtures were subsequently melted in a rocking furnace at 950 °C for at least 15 h. The (Ge28Sb12Se60)100-xAgx glass and (Ge33As12Se55)100-xPbx glass were cooled down to 600 °C and 550 °C, respectively, followed by rapid quenching in water and annealing at a glass transition temperature below 10 °C. The diameter of all glasses is 15 mm, and the final weight of all glasses is 50 g. About 2 mm thick slices were cut from each cylindrical glasses rod for polishing.
IR transmission spectra of the glasses were measured with a Fourier infrared spectrometer (Thermo Nicolet 380) in the range of 2.5-20 µm. The Tg, crystallization temperature (Tx) and the difference between Tg and Tx (ΔT) were measured by a differential scanning calorimeter (Naichi 204F1) with a heating rate of 10 K/min. The hardness of the glasses is obtained by averaging 5 stable measurements with a microindenter. The densities of the glasses were obtained by the Archimedes method with alcohol as the immersion liquid. Each density was averaged from 5 stable measurements. The X-ray diffraction patterns of glasses were measured by X-ray diffractometer (Bruker, D2 PHASER, GER). The refractive index was measured using an IR ellipsometer (J.A. Woollam, IR-Vase II, USA) on a single-sided polished slice. The Raman spectrum was excited by a 785 nm laser diode and recorded on a micro-Raman spectrometer (Renishaw, In Via, GBR).
3. Results and discussion
The XRD patterns of the two groups of glasses are shown in Fig. 1. It can be seen from Fig. 1(a) that there are no sharp crystal peaks in the XRD diffraction patterns as the Ag content increases. The glasses of the (Ge28Sb12Se60)100-xAgx (x = 0, 5, 10, 15, 20) series are in the amorphous state. When the Ag content reaches 25 at%, distinct diffraction peaks appear in the XRD pattern. When the content of Pb reaches 11 at%, the glass precipitates crystals. There are two factors that affect the doping density. The proportion of Ag/Pb doping is mainly limited by the volume of doped atoms. The atomic volume of Ag atoms is 10.03 cm3/mol, while the atomic volume of Pb is 18.17 cm3/mol. The smaller the atomic volume, the higher the doping ratio. On the other hand, the ratio of Ag/Pb doping is also affected by the stability of matrix glass.
The refractive index of (Ge28Sb12Se60)100-xAgx and (Ge33As12Se55)100-xPbx series glasses are shown in Fig. 2(a) and Fig. 2(b). With the increase of Ag and Pb content, the refractive index of both groups of glasses increased obviously. When the Ag doping of commercial Ge28Sb12Se60 glasses reached 20 at%, the refractive index of this glasses at 10 µm increased from 2.6 to 3.05 with a refractive index difference (Δn) of 0.45. On the other hand, when the Pb doping of commercial Ge33As12Se55 glasses reached 9 at%, the refractive index of this glasses at 10 µm increased from 2.45 to 2.88 with a Δn of 0.43. It is evident that Ag and Pb have a significant effect on the refractive index enhancement of commercial ChGs, which stems from the effect of Ag and Pb on its glass network structure.
Fig. 2. Refractive index of (Ge28Sb12Se60)100-xAgx (a) and (Ge33As12Se55)100-xPbx (b) series glasses. (c) Density of (Ge28Sb12Se60)100-xAgx and (Ge33As12Se55)100-xPbx series glasses. (d) Relationship between refractive index and density of (Ge28Sb12Se60)100-xAgx and (Ge33As12Se55)100-xPbx series glasses. The error range for density measurement is (±0.01 g/cm3), and the error range for refractive index measurement is (± 0.01).
In addition, the densities of the glasses increase monotonically with increasing Ag and Pb doping content as shown in Fig. 2(c). And it is noteworthy that the patterns of variation in refractive index and density are very similar. Figure 2(d) demonstrates the relationship between the refractive index and the density of the (Ge28Sb12Se60)100-xAgx and (Ge33As12Se55)100-xPbx series glasses. It is quite evident that the refractive index of these two groups of Ag/Pb doped commercial ChGs is highly correlated with density. And a mathematical model for refractive index (n) and density (ρ) is given by the Lenz-Lorentz relation [13,24]:
Where Rm is the molecule polarizability and NA is the Avogadro number. It can be seen from the Lenz-Lorentz relation that the refractive index is mainly related to the atomic molecule polarizability and the density of the glasses. Therefore, the molecular polarizability and density of the glass are critical issues to evaluate in terms of improving the refractive index of the glass. The left side of the equation is a monotonically increasing function of the refractive index, and the right side of the equation is a product function of the polarizability and density. There is also a relationship between polarizability and density, with the higher the polarizability, the higher the density. Therefore, the right side of the equation can also be seen as a monotonically increasing function of density, and the refractive index and density are similar to a linear relationship. Ag and Pb possess higher polarizabilities compared to sulfur group elements, and they cause an increase in the polarizability of the glasses [25]. As for the evolution of Ag and Pb doping on the glass density, it can be explained by the following equation [26]: Where M is the molecular weight, and Nf is the number of atoms per unit, and Vmean is the average volume of atoms. Ag and Pb are generally attached at the end of structures such as GeSe4/2 or interspersed in such structures [21,27], and the addition of these two heavy elements causes a substantial increase in M, while Nf remains constant and Vmean does not change much, so the density increases monotonically. Based on the evolution of the two factors, polarizability and density, the doping of Ag and Pb leads to a monotonic increase of Rm and ρ which makes the refractive index to be monotonically increasing.The thickness of the lens depends mainly on its own refractive index. For single-lens imaging, there is a close relationship between focal length, radius of curvature, lens thickness and refractive index of materials, which can be expressed as follows:
Where f is the focal length, r is the radius of curvature, t is the thickness of the lens, and n is the refractive index. At the same focal length condition, and the original lens thickness is set to 1, the following equation can be obtained: Where n1 is the refractive index of the original glasses, n2 is the refractive index of the high refractive index glasses, and Δn is the difference between n1 and n2. It is easy to obtain that at the same focal length and radius of curvature, the thickness of the lens decreases as the refractive index difference increases. On the other hand, for the same lens thickness, the focal length decreases as the refractive index increases and thus the field of view of the lens expands. The schematic diagram of single lens imaging is shown in Fig. 3. The optimized lens thickness of commercial Ge28Sb12Se60 glasses and Ge33As12Se55 glasses are also shown in Fig. 3(a) and Fig. 3(b) respectively.Fig. 3. Single lens imaging schematic of (Ge28Sb12Se60)100-xAgx (a) and (Ge33As12Se55)100-xPbx (b) series glasses.
Dispersion is one of the important indexes to evaluate the performance of imaging lens. The dispersion of homogeneous infrared materials is characterized by Abbe number (V) [28]:
where nc, ns, and nl represent the refractive index at the center, short and long wavelengths, respectively. The Abbe numbers of these glasses at long wave IR (LWIR, 8∼12 µm) and medium wave IR (MWIR, 3∼5 µm) bands were calculated, as shown in Fig. 4. The V values for all glasses are large positive values in both LWIR and MWIR bands, indicating their low dispersion. In particular, the Abbe numbers of (Ge28Sb12Se60)100-xAgx series glasses with more than 10 at% Ag doping still show no decreasing trend. Only the Abbe numbers of (Ge33As12Se55)100-xPbx series glasses have a slight decreasing trend in the LWIR band. These high-refractive-index, low-dispersion sulfur-based glasses certainly offer more design freedom for high-definition imaging.IR transmission is also an important indicator to measure the optical performance of the lens. Figure 5 shows the excellent IR transmission of (Ge28Sb12Se60)100-xAgx and (Ge33As12Se55)100-xPbx series glasses. With the gradual increase of Ag/Pb content, the IR transmission of the glass gradually decreases due to the increase of refractive index. The relationship between the theoretical transmission (T) and refractive index (n) of the glasses can be expressed as:
And the measured transmission of these glasses corresponds well to the theoretical value. By the way, The absorption peaks near 2.8, 4.5, 6.3, 9.3, and 12.8 µm are vibrational absorptions from H-O, Se-H. H2O, As-O, and Ge-O bonds. The Ge-O absorption peak at 12.8 µm is mainly caused by the oxidation of raw materials and the entry of oxygen elements in the air into the quartz ampoule. And these absorptions need to be eliminated by purification techniques [29–31].
Fig. 5. IR transmission spectra of (Ge28Sb12Se60)100-xAgx (a) and (Ge33As12Se55)100-xPbx (b) series glasses. The thickness of the two series of glass samples is about 2 mm.
Vickers hardness (Hv), Tg, Tx, and ΔT values of all glasses are listed in Table 1. With the increase of Ag and Pb content, the Vickers hardness of these glasses can be considered to be consistent essentially the same as that of commercial ChGs. The Vickers hardness of 163.4 kg/mm2 and 179.2 kg/mm2 for (Ge28Sb12Se60)100-xAgx and (Ge33As12Se55)100-xPbx glasses, respectively, were taken from the average of the measured values of their five different composition samples. The measurement error of Vickers hardness is about ±5 kg/mm2. Noted that the thermal properties of these glasses changed greatly with the change of Ag/Pb doping content. The Tg and Tx values of these glasses decrease progressively with Ag and Pb doping, which is attributed to the alteration of the glass network structure by these heavy metals. Increase of Ag/Pb content leads to decrease of the average coordination number, which leads to decrease of the transition temperature. This phenomenon has been documented in a number of literatures [32–34]. The ΔT values remain almost identical to that of the commercial base glasses with values greater than 110 °C. The larger ΔT indicates that the Ag/Pb-doped glasses inherits the good performance of the commercial base glass [35–37].
Table 1. Thermal and Mechanical Properties of Ag/Pb-Doped commercial ChGs
The variation of the refractive index of these glasses derives from the evolution of their glass network structure. As shown in Fig. 6(a), (Ge28Sb12Se60)100-xAgx glasses contain four obvious characteristic peaks. The broad peaks at 110∼130 cm−1 and 250 cm−1 correspond to the Se chain or Se ring, and the peaks near 163 cm−1 correspond to the symmetric structured [SbSe3/2] tetrahedral unit, The peaks near 198cm−1 correspond to the Se-free asymmetric structural unit of [Gex/4-Ge-Se(4-x)/2], and the broad peaks at 235 ∼330 cm−1 correspond to the stretching of Ge-Ge bond, similar to the bonding mode of amorphous Ge. It is obvious that with the increase of Ag content, the intensity of the broad peaks at 110∼130 cm−1 and 235∼330 cm−1 increases regularly, indicating that the increase of Ag causes a large number of unstable [Gex/4-Ge-Se (4-x)/2] unit bonds to break, and the excess Se forms Se chain or Se ring by itself. Others can form bonds with Ag with large polarity differences. Excess Ge forms a state similar to amorphous Ge. However, the peak intensity at 163 cm−1 remains unchanged and the peak intensity at 198 cm−1 gradually decreases, indicating that Ag doping only breaks and rearranges Ge-Ge bond, a weak polar bond, and has no effect on stable [GeSe4/2] in the glass network [38,39]. As shown in Fig. 6(b), with the increase of Pb content, the peak intensity corresponding to the stretching of Se-Se bond and the stretching of Ge-Ge bond gradually increases. The difference between Ge33As12Se55 and Ge28Sb12Se60 is that [Gex/4-Ge-Se(4-x)/2] units of Ge33As12Se55 remain unchanged, and [Asx/3-As-Se(3- x)/2] break off. The same for both is that the doping of Ag and Pb rearranges the weakly polar units, forming bonds with greater polarity differences [40].
4. Conclusion
In this paper, Ag and Pb with higher polarizabilities and atomic masses were doped into the commercial IR ChGs of Ge28Sb12Se60 and Ge33As12Se55. These glasses were characterized using optical, thermal and mechanical characterization techniques. The Vickers hardness of 163.4 kg/mm2 and 179.2 kg/mm2 for (Ge28Sb12Se60)100-xAgx and (Ge33As12Se55)100-xPbx glasses, respectively, were taken from the average of the measured values of their five different composition samples. The measurement error of Vickers hardness is about ±5 kg/mm2. And the stable ΔT value demonstrates its thermal stability. The refractive index of the Ge28Sb12Se60 is greatly increased by 0.45 due to the doping of Ag, and the refractive index of Ge33As12Se55 is increased by 0.43 due to the doping of Pb. The change of network structure of the glasses was analyzed by Raman spectroscopy. With the increase of heavy metal content, weak polar bonds were broken and rearranged to form bonds with large polarity difference. For a simple lens, we give a model of lens thickness versus refractive index for the same focal length and radius of curvature. In summary, this work provides a good case study for the doping modification of commercial glasses.
Funding
National Natural Science Foundation of China (U21A2056, 61975086, 62075110); the Key Research and Development Program of Zhejiang Province (2021C01025); the Major Program of Zhejiang Provincial Natural Science Foundation of China (LDT23F05012F05); the Fundamental Research Funds for the Provincial Universities of Zhejiang (SJLY2022004); Natural Science Foundation of Ningbo (202003N4180); Analysis and Measurement Foundation of Zhejiang Province (LGC19F050002); the K. C. Wong Magna Fund at 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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