Abstract
Refractive index modification in glass or crystalline materials typically involves conversion of state (amorphous to crystalline or crystalline to amorphous) through a homogeneous, external stimulus such as laser- or current-induced heating, melting, or localized (resonant) bond modification. With the exception of traditional phase change materials that exploit reversibility, usually at high speeds and over multiple cycles, localized patterning of the refractive index is most frequently employed to induce a complete change of phase to enable the creation of embedded or surface optical structures. The present effort employs a novel, laser-induced vitrification (LIV) process developed to spatially modify the refractive index in a fully homogeneous glass ceramic material. Such processing leads to a local re-vitrification of the pre-existing nanocrystalline microstructure within the material to realize spatially-defined, refractive index profiles. Post-processing refractive index modification on the order of ∆n ~-0.062 was realized in a partially crystallized, multi-component chalcogenide glass ceramic nanocomposite, subjected to bandgap laser exposure. Spatially-varied phase modification in the lateral and axial directions within a bulk glass ceramic is quantified and the optical function of the resulting structure is demonstrated in the formation of an infrared grating. The underlying mechanism associated with the resulting local refractive index modification is explained through quantification of the multi-phase material attributes including parent glass properties, crystal phase identity and phase fraction as determined through micro-XRD and electron microscopic analysis. This correlation validates the proposed mechanism associated with the modification. A threshold power density for LIV in the starting glass ceramic has been determined based on exposure conditions and material attributes.
Highlights
Infrared (IR) optical systems have increased in their applications across a wide range of commercial and defense platforms [1,2,3,4]
The Raman spectra of the glass ceramic exhibits two bands centered at wavenumbers of 205 cm−1 and 246 cm−1 where the peak located at a wavenumber of 205 cm−1 corresponds to Ge-Se bonds within GeSe2 units, while the peak located at a wavenumber of 246 cm−1 consists of two major peaks located at wavelengths of 225 cm−1 and 240 cm−1 which can be attributed to As-Se vibrations found .within As2Se3
The laser-modified microstructure is not entirely identical to the parent glass droplet/matrix morphology but similar enough to that shown in the dark field transmission electron microscopy (TEM) image of a glass material (Fig. 3e top) and the corresponding selected area electron diffraction (SAED) pattern [10]
Summary
Infrared (IR) optical systems have increased in their applications across a wide range of commercial and defense platforms [1,2,3,4]. Efforts to further tailor a glass’ local index in a spatially varying way has been demonstrated through the partial crystallization of the glass, forming an optical nanocomposite [10,14,16] or more recently, through a micro-poling process [17] In the former efforts, a glass ceramic is formed where the nanoscale crystallites are dispersed in the glassy phase, yielding an effective refractive index, neff, which differs from that of the parent starting material. We present the findings of our investigation of an approach converse to the laser-assisted crystallization technique This laser-induced vitrification (LIV) method starts with a glass ceramic and selectively re-amorphizes it through the controlled conversion of the crystalline phase(s) present. The resulting reamorphization creates a laser-modified region possessing a higher volume fraction of glassy phase with a lower index than the surrounding (starting) glass ceramic material, opposite to the previously reported index gradient imparted through the growth of the higher index crystalline phases within a starting (lower index) glass material [14,16,21]
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