Infrared transmission and refractive index dispersion of mixed-chalcogen Ge-Sb-S-Se glasses for use in molded lens applications
Introduction
Demands on infrared cameras capable of visualizing thermal images in the long-wavelength infrared (LWIR) region, i.e., 8–12 μm, are growing fast in various civilian sectors along with their already existing military applications [1]. For a successful employment in the civilian applications, a competent infrared camera needs to resolve, in addition to concerns related to cost effectiveness, technical issues associated with miniaturization that requires it to be slim enough for integration into mobile electronic devices like smartphone, while keeping its performances to be further enhanced. In order to improve its practicality, together with technologically advanced uncooled microbolometer-type image sensors, many different chemical compositions for optical-grade LWIR-transmitting glasses need to be secured. Specifically, diversifying optical parameters of the LWIR glasses such as refractive index and its dispersion over wavelength, i.e., broadening the LWIR Abbe diagram, elevates degrees of freedom in configuring a lens assembly with minimized optical aberrations [2]. Being judged based on these considerations, chalcogenide glasses (ChGs) are most likely to undertake the material of choice for LWIR-transmitting lens applications in terms of property controllability, cost effectiveness and mass productivity. These merits stem from the inherent compositional flexibility and molding-process compatibility of ChGs, which cannot be featured by their crystalline counterparts such as Ge and ZnSe [3]. Glass formed out of ternary Ge-Sb-Se system, i.e., a prototypical ChG, is one of the well-known ChGs already commercialized for use as lens materials [4]. Here, it should be mentioned that all of the commercially available ChGs for the LWIR lens applications, including ternary Ge-Sb-Se glasses as well, are categorized basically as a low-dispersion glass, i.e., their LWIR Abbe number (ν10) exceeding 100 [4, 5]. Here, we have where n8, n10 and n12 are refractive indexes measured at 8 μm, 10 μm and 12 μm, respectively. This Ge-Sb-Se system exhibits a quite broad glass-forming area centered at the Se-sufficient compositions against the GeSe2-Sb2Se3 stoichiometric tie-line. However, it has been experimentally verified that a Se-deficient but Ge-sufficient compositional region is more appropriate for a precision glass molding (PGM) process to produce LWIR lenses [6], [7], [8], [9].
In an effort to assure highly dispersive ChGs, in addition to the already existing low-dispersion ChGs, we proposed recently that quaternary Ge-Ga-Sb-S glasses compositionally optimized in consideration of thermal and mechanical properties as well as LWIR transmittance are characterized by high dispersion, i.e., ν10 lower than ∼40 [10]; nevertheless, we notice that there is still a gap between the LWIR Abbe numbers of these ternary selenide and quaternary sulfide glasses, further demanding a compositional engineering for exploiting ChGs of which refractive index dispersion falls into a moderate-dispersion category in the LWIR Abbe diagram. This is then supposed to facilitate designing all-ChG-based lens assemblies well equipped for diverse functionalities required in the LWIR range.
On the other hand, it is verified that infrared transmission edge (denoted here as ωc and defined as a wavenumber value corresponding to half-maximum infrared transmittance of a 2-mm-thick chalcogenide glass disk, if not mentioned otherwise) of Ge-based sulfide or selenide glasses can be simply correlated with their chemical compositions via average bond energy (Eave) and molar mass (M) of any given ChGs in accordance with a postulation that a ChG behaves as the single average harmonic oscillator (SAHO) to reveal its ωc via multiphonon absorption process [11]. It is worthwhile to note that refractive index dispersion over the LWIR range can be estimated in a quantitative manner for a ChG composition in the framework of the SAHO model, and in this regard it is curious whether or not ChGs containing both S and Se as major constituents also follow the SAHO model. With these in mind, in this study, we have investigated quaternary mixed-chalcogen Ge-Sb-S-Se glasses: Thermal, mechanical and optical properties are delineated as a function of S content substituting Se in Ge27.5Sb12.5Se60-xSx glasses with aims to screen candidate compositions suitable for the PGM process as well as moderate refractive index dispersions. Universality of the SAHO model is then confirmed through analyzing their composition-dependent ωc values, and then their refractive index and ν10 values are elucidated for some representative compositions. In particular, it is pointed out that these mixed-chalcogen Ge-Sb-S-Se glasses behave like typical sulfide glasses in their composition dependence of ωc values, implying that the S-related vibrational modes dominate in the infrared transmittance of ChGs.
Section snippets
Experimental
Quaternary mixed-chalcogen glasses with nominal composition of Ge27.5Sb12.5SxSe60-x (0 ≤ x ≤ 45 at%) were prepared via conventional melt-quenching technique for synthesis of ChGs [12]. It should be noted that the ternary selenide composition Ge27.5Sb12.5Se60 was selected for addition of sulfur to form the present sulfur-selenide glasses on the basis of previous studies concerning compositional screening performed upon the Ge-Sb-Se glass-forming system, which reveals that only a relatively
Results and discussion
Presented in Fig. 1(a) are changes of VH values upon increase of S content: A monotonous enhancement of VH is noticed, and this tendency also appears in the case of Tg and Ts as shown in Fig. 1(b). This behavior of VH and Tg in Ge27.5Sb12.5SxSe60-x glasses is in agreement with previous reports [14], [15], [16]. On the other hand, as displayed in Fig. 1(c), an insignificant change is found among α values for the substitution of sulfur. It is worth mentioning that the present Ge27.5Sb12.5SxSe60-x
Summary
Mixed chalcogen Ge-Sb-S-Se glass is compositionally engineered in this study for the purpose of securing LWIR-transmitting glasses for the molded lens applications. Substitution of Se with S exerts some conspicuous influences: Infrared transmission edge is blueshifted; VH as well as Tg and Ts tends to be enhanced upon the introduction of sulfur, whereas no nontrivial changes in α are observed. The linear correlation between ωc and values is reconfirmed also in the present
Declaration of Competing Interest
None
Acknowledgements
This work has been supported by the Industry Core Technology Development Project funded by the Ministry of Trade, Industry and Energy of Korea (Grant No. 20002803), and also supported by Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technologyof Korea (2018R1D1A1B07040938).
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