Inverse Optical Contrast in Metastable GeTe 2 : An Anomaly Enabling Low‐Loss Phase Change Photonics

ABSTRACT Phase‐change materials (PCMs) typically exhibit a significant increase in refractive index and extinction coefficient upon crystallization. This work reports an unconventional optical phase inversion in during its amorphous‐to‐metastable crystalline transition, which challenges the established paradigm for chalcogenide PCMs. Spectroscopic measurements reveal that the transformation from the amorphous state to a metastable phase results in a negative refractive index change of = −0.23 at the telecommunication wavelength of 1550 nm, while remarkably maintaining zero absorption in both phases. Optical bandgap widening explains the physical origin of this anomaly. The metastable phase possesses an indirect bandgap of 0.98 eV, which significantly exceeds the 0.86 eV observed in the amorphous state. This behavior is unique to the metastable phase, as subsequent thermal decomposition into GeTe and Te results in a dramatic decrease in the bandgap and a return to high‐index, lossy behavior. These findings demonstrate as a promising candidate for low‐loss non‐volatile integrated photonics.