Thermo-optical Effect in Phase-Change Nanophotonics

Phase-change materials (PCMs) integrated with nanophotonic components enable tunable integrated optical devices and hold promise for emerging applications in long-term photonic data storage by exploiting the large contrast in optical properties when switched between amorphous and crystalline phase states. The phase transitions rely on short heat pulses, which can be conveniently supplied by optical means. Therefore, a proper understanding of the temperature-induced refractive index fluctuations, the thermo-optical effect, is essential for controlled and optimized device operation. Here we analyze the thermo-optical effect of Ge2Sb2Te5 nanojunctions integrated atop nanophotonic waveguides at telecommunication wavelengths. We extract the thermo-optical coefficients for the crystalline and amorphous phase state, both the real and imaginary part. We observe sign reversal of the thermo-optical coefficient upon phase change, which we trace back to different chromatic dispersion of the two phase states within the studied spectral regime. Our findings will allow for designing PCM-based modulators, nonvolatile memory systems, and reconfigurable photonic devices.