Volatile and Nonvolatile Switching of Phase Change Material Ge 2 Sb 2 Te 5 Revealed by Time-Resolved Terahertz Spectroscopy.

Phase change materials exhibit unique advantages in reconfigurable photonic devices due to drastic tunability of photoelectric properties. Here, we systematically investigate the thermal equilibrium process and the ultrafast dynamics of Ge 2 Sb 2 Te 5 (GST) driven by femtosecond (fs) pulses, using time-resolved terahertz spectroscopy. Both fs-pulse-driven crystallization and amorphization are demonstrated, and the threshold of photoinduced crystallization (amorphization) is determined to be 8.4 mJ/cm 2 (10.1 mJ/cm 2 ). The ultrafast carrier dynamics reveal that the cumulative photothermal effect plays a crucial role in the ultrafast crystallization, and modulation depth of volatile (nonvolatile) THz has switching limits up to 30% (15%). A distinctive phonon absorption at 1.1 THz is observed, providing fingerprint spectrum evidence of crystalline lattice formation driven by intense fs pulses. Finally, multistate volatile (nonvolatile) THz switching is implemented by tuning optical pump fluence. These results provide insight into the photoinduced phase change of GST and offer benefits for all optical THz functional devices.