Strongly nonlinear optical glass fibers from noncentrosymmetric phase-change chalcogenide materials

We report that the one-dimensional polar selenophosphate compounds APSe₆ (A = K, Rb), which show crystal-glass phase-change behavior, exhibit strong second harmonic generation (SHG) response in both crystal and glassy forms. The crystalline materials are type-I phase-matchable with SHG coefficients χ⁽²⁾ of 151.3 and 149.4 pm V^-1 for K ⁺ and Rb⁺ salts, respectively, which is the highest among phase-matchable nonlinear optical (NLO) materials with band gaps over 1.0 eV. The glass of APSe₆ exhibits comparable SHG intensities to the top infrared NLO material AgGaSe₂ without any poling treatments. APSe₆ exhibit excellent mid-IR transparency. We demonstrate that starting from noncentrosymmetric phase-change materials such as APSe₆ (A = K, Rb), we can obtain optical glass fibers with strong, intrinsic, and temporally stable second-order nonlinear optical (NLO) response. The as-prepared glass fibers exhibit SHG and difference frequency generation (DFG) responses over a wide range of wavelengths. Raman spectroscopy and pair distribution function (PDF) analyses provide further understanding of the local structure in amorphous state of KPSe₆ bulk glass and glass fiber. We propose that this approach can be widely applied to prepare permanent NLO glass from materials that undergo a phase-change process.