Abstract
Thermally-induced nucleation and growth of secondary crystalline phases in a parent glass matrix results in the formation of a glass ceramic. Localized, spatial control of the number density and size of the crystal phases formed can yield ‘effective’ properties defined approximately by the local volume fraction of each phase present. With spatial control of crystal phase formation, the resulting optical nanocomposite exhibits gradients in physical properties including gradient refractive index (GRIN) profiles. Micro-structural changes quantified via Raman spectroscopy and X-ray diffraction have been correlated to calculated and measured refractive index modification verifying formation of an effective refractive index, neff, with the formation of nanocrystal phases created through thermal heat treatment in a multi-component chalcogenide glass. These findings have been used to define experimental laser irradiation conditions required to induce the conversion from glass to glass ceramic, verified using simulations to model the thermal profiles needed to substantiate the gradient in nanocrystal formation. Pre-nucleated glass underwent spatially varying nanocrystal growth using bandgap laser heating, where the laser beam’s thermal profile yielded a gradient in both resulting crystal phase formation and refractive index. The changes in the nanocomposite’s micro-Raman signature have been quantified and correlated to crystal phases formed, the material’s index change and the resulting GRIN profile. A flat, three-dimensional (3D) GRIN nanocomposite focusing element created through use of this approach, is demonstrated.
Highlights
Gradient refractive index (GRIN) optics have been increasingly sought after to enhance optical system performance while reducing the number, size, and/or weight of the optical components needed in a system, while maintaining optical performance
This paper examines the structural evolution of a multicomponent infrared glass as it is converted to an optical nanocomposite through both two-step furnace heat treatment (HT), a furnace nucleation plus laser growth HT, and for the first time demonstrates a technique that predicts the structural changes associated with formation of the crystalline phase(s) responsible for an effective refractive index change which yields a GRIN optical element
A calculated effective index from X-ray diffraction (XRD) data was shown to agree with experimentally measured refractive index of the samples
Summary
Gradient refractive index (GRIN) optics have been increasingly sought after to enhance optical system performance while reducing the number, size, and/or weight of the optical components needed in a system, while maintaining optical performance (size, weight, and power – SWaP). It employs micro-Raman spectroscopy which can give structural selectivity combined with the spatial control of a microscopic measurement This technique has the potential to be well suited for glass-ceramic systems since it can probe the fine structural changes that develop upon the early stages of conversion of glass networks when traditional tools cannot detect nucleation, through the steps of crystal growth inside the glassy matrix. The parent glass composition, 15GeSe2-45As2Se340PbSe (154540 GAP-Se) has been evaluated in its base form, following a homogeneous (furnace) nucleation HT, and following growth with (i.) a homogeneous furnace growth HT or (ii.) through use of a Gaussian laser heating HT Through this comparison, samples were examined to determine if analysis of a localized micro-Raman signature would prove viable for indirectly measuring the phase and refractive index evolution of a two-step cerammed material in a spatially selective way. The resulting sample was evaluated for optical functionality as a secondary confirmation that the gradient HT profile resulted in an index change in the material
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