www.ceramics.org | American Ceramic Society Bulletin, Vol. 100, No. 8 32 Bioinspired optics: Chalcogenide glass-ceramic nanocomposites mark a . . . Celebrating 100 years phologically homogeneous media across a wide range of compositions.11,12 This difference is directly associated with their typical deposition technique. In efforts by our team, chalcogenide films are depos- ited predominantly using thermal evapo- ration where evaporated source materials experience excessively fast cooling as they are deposited on a substrate maintained at either room or a cold temperature. As a result of the process, the source materials are instantaneously frozen into a highly metastable amorphous phase. Because there is no time for constituent atoms/molecules to rearrange during the fast condensation process, their resulting morphology exhibits that of the “parent” vapor phases. As the medium is homoge- neous, this situation necessitates a differ- ent strategy to functionalize the material toward a GRIN medium. The highly metastable, homogeneous films have a strong tendency to separate into lead-rich and lead-deficient phases at an activation energy lower than that required for as-quenched bulk glasses. Meanwhile, the facile phase transforma- tion comes with a caveat. Relying only on heat treatment would induce the spontaneous formation of crystals with a large size distribution. Crystallites with sizes in the upper part of the size distribution, outside the effective media approximation’s regime, are likely to scatter incident IR light, leading to opti- cal loss. Also, their random spatial and size distributions leave none to very little controllability over the formation of GRIN profiles. To overcome the challenge, direct laser writing was introduced and sequen- tially combined with heat treatment.11,12 The rationale behind an addition of the laser process includes 1) It activates the process whereby a starting homogeneous film can be sepa- rated into lead-rich and lead-deficient glassy phases 2) Tunability of the magnitude and location of phase separation by control- ling fluence and laser exposure 3) Post heat treatment of the laser- induced nanocomposite allows ener- getically unstable lead-rich phases to be exclusively converted into high index crystals while keeping the lead-deficient low index phases glassy at a temperature lower than that required for a heat treat- ment-only condition and 4) The size of the high index crystal- line phases within the resulting glass- ceramic film is small and narrowly dis- tributed, allowing the nanocomposite to remain transparent. This approach enables GRIN layers to be applied to bulk spherical optics, thereby creating aspheric optical functions. Such an approach can result in considerable cost savings in the manufacturing process. The three representative cases dis- cussed above indicate that a specific choice of post stimulation process suit- able for the realization of GRIN media is largely determined by the starting medium’s morphology (phase separated or homogeneous) and microstructure (glassy or glass-ceramic). The starting mor- phology-stimulation method correlation is summarized in Fig. 2d. Here, the top row illustrates morphological/microstructural transitions that the material system can undergo: from i) a homogeneous glass to ii) a glass nanocomposite to iii) a glass- ceramic nanocomposite to iv) a partially vitrified glass-ceramic nanocomposite. No matter where the starting point is, a key requisite for the realization of GRIN media is to convert a starting morphology into a glass-ceramic nano- composite consisting of small (sub-100 nm) monosized high index crystallites within a low index glassy phase. The transmission electron microscope images in Fig. 2d summarize how three post- stimulation processes including heat treatment-only, laser-only, and a hybrid of these two processes (i.e., photo- thermal process) are coupled with the specific starting stages of target materials and induce them to evolve into glass- ceramic media.9–12,19,20 Figure 2e shows a TEM image and corresponding X-ray energy dispersive spectroscope maps collected from a nanocomposite with 20 mol% PbSe as a representative composition within the material system’s immiscibility dome.10 The presence of phase separation into lead-rich particles and a lead-deficient matrix is clearly indicated by the red map corresponding to lead. To quantify the extent of phase separation, the atomic percentages of four constituent elements were extracted along the black line in the TEM image. The spatial profiles in Fig. 2e show how each element’s quantity var- ies over the distance across particle and matrix regions, highlighting a large differ- ence in local atomic percentages of lead in particle and matrix regions. Refractive index modifications How the microstructural/morpho- logical evolution leads to changes in the composite’s refractive index is presented for heat treatment-only (Figs. 3a–c), photo-thermal (Figs. 3d–f), and laser-only processes (Figs. 3g–i). The heat treatment of bulk glasses converts them into glass- ceramics whereby the type and volume fraction of each crystalline phase is dic- tated by the composition of the parent glasses, as shown in Fig. 3a.10 It is important to note that the vol- ume fraction of lead-containing crystal- line phases (either Ge 0.1 Pb 0.9 Se or PbSe) increases with lead content of starting glasses. Because the refractive indices of these phases in the mid-wave IR for Ge 0.1 Pb 0.9 Se (4.81) and PbSe (4.90) crys- tals are far greater than those of As 2 Se 3 (2.41), selenium (2.65) crystals as well as the parent glass matrices (nominally around 2.48) at a wavelength of 4.515 µm as an example, the lead-containing crystals are clearly responsible for an increase in effective refractive index upon conversa- tion of glasses into glass-ceramics.10 The impact of starting composition and heat treatment on an increase in effective refractive index of nanocomposites is summarized in Fig. 3b, highlighting an ability to target a specific index through a specific choice of “knobs” of the com- position and thermal processing.10 Also, Fig. 3c shows that while there is an optical loss in the short-wave IR originated from scattering of incident light at interfaces between high index crystals and a low index matrix, the nanocomposite media still retain their mid-wave IR transparency upon glass to glass-ceramic conversion. Figures 3d and 3e show an example of such index tailorability via a photo- thermal process carried out on films.9 Specifically, an increase in laser exposure fluence followed by a fixed heat treat- ment protocol leads to a greater volume
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