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Evidence of spatially selective refractive index modification in 15GeSe2-45As2Se3-40PbSe glass ceramic through correlation of structure and optical property measurements for GRIN applications

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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.

© 2017 Optical Society of America

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Figures (9)

Fig. 1
Fig. 1 (A) The glass forming region (to the left of the red line shown) as described by Wang and Yang et al. [42,43], in the As2Se3-GeSe2-PbSe ternary system. The 15GeSe2-45As2Se3-40PbSe [154540 GAP-Se] composition is shown by the blue star. (B) A picture of a 30 mm polished slice of this glass in the visible.
Fig. 2
Fig. 2 (Left) Sample sectioning protocol: a rod was sliced into disks, which were then quartered. All of the quarters of one slice were then HT at a single nucleation temperature. Each of the quarters were then grown at a different growth HT temperatures. (Right) Nucleation-like (I) and growth-like (U) curve for the base 154540 GAP-Se glass [27]. When the sample goes through a nucleation step before growth, the growth curve shifts to lower values.
Fig. 3
Fig. 3 Raman spectra were measured at 0.25 mm increments in the z-direction, and 1 mm in the x-direction (depicted by red dots, and is not to scale) on a cross-sectioned disk to quantify the extent of the modifications across the 2D sample volume. The background coloration (not to scale) is representative of the expected temperature profile while the sample is irradiated, while held at 190°C. The green, Gaussian shaped “beam” incident on the top surface is representative of the beam size with respect to the sample diameter, with a Gaussian intensity profile depicted on the sample surface.
Fig. 4
Fig. 4 (A) XRD spectrum for the base glass confirming the glass’ as-melted x-ray amorphous nature. (B) 2-D within-slice refractive index homogeneity map (λ = 4.515μm) measured at 21 locations across the 30 mm diameter slice. The dots show measurement locations and the spot size of the measurement was ~2 mm. The color variation shows the range of index variation (C) Transmission spectrum for a 2 mm thick sample, not corrected for Fresnel loss. (D) MicroRaman spectra of the base glass (λexc = 785nm).
Fig. 5
Fig. 5 (A) Representative XRD spectra for the 154540 GAP-Se glass following nucleation at 190°C, with furnace HT at temperatures shown for 30 min. (B) Measured refractive index (λ = 4.515 μm) versus growth temperature for various nucleation temperatures. (C) Measured (experimental) refractive index values obtained as a function of HT temperature, as compared to calculated refractive index based on volume fraction of crystal phase formed from XRD spectra.
Fig. 6
Fig. 6 (A) Raman spectra at λexc = 785nm for base glass prior to and following nucleation (190°C) and growth treatments. Spectra have been normalized to the 200 cm−1 peak. (B) Raman ratio versus heat treatment temperature for various nucleation protocols. (C) Direct correlation between Raman ratio and refractive index indicative of fractional conversion of glass to glass ceramic.
Fig. 7
Fig. 7 (Top) Simulated temperature profile induced for the λ = 532 nm laser irradiation conditions used, incorporating thermal properties measured on the bulk glass. (Bottom) The experimentally predicted temperature profile as determined by the change in the Raman ratio with temperature.
Fig. 8
Fig. 8 The mapped refractive index values predicted by simulation (top) and prediction from the experimentally measured Raman (middle). The bottom plot is the difference between the two spectra in refractive index units.
Fig. 9
Fig. 9 Focal spot as measured in a laser-written 3-D GRIN structure. (Left) focal spot of measurement beam (collimated 2µm laser) without and with sample inserted, where the color scale shows the center beam intensity (yellow high, purple low). (Right) beam waist measurement as a function of position, and fitted (solid line) to extrapolate the measured focal length of the GRIN lens 84.3 ± 2.7mm. The blue line represents the plane where the images on the left were taken.

Tables (2)

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Table 1 Thermal properties for 15As2Se3-45GeSe2-40PbSe that were used in simulations.

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Table 2 The base glass optical properties of the melt prior to irradiation and/or HT. The index value is the average of the measurements used in Fig. 4(b), and the error is the standard deviation of these measurements.

Equations (1)

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Q in =P( 1R )( 2 w 0 2 ) e ( 2 r 2 w 0 2 )
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