Broadband and ultrafast terahertz modulation with GeTe thin films

Chalcogenide phase change materials (PCMs) are gaining much interest for optoelectronic applications because of their capability to perform nonvolatile phase transition between the amorphous and crystalline states under Joule heat generated by applied electrical bias or optical excitation. Utilizing the high dielectric tunability of PCMs, we experimentally demonstrated the terahertz (THz) modulation with germanium telluride (GeTe) thin films. With THz time-domain spectroscopy (THz-TDS), we showed that the transmission of the 250-nm GeTe film can be tuned from 95% (amorphous) to 35% (crystallized) in the range of 0.1–1.2 THz. The high optical contrast of the GeTe film was also confirmed by Fourier transform infrared spectrometer and scattering-type scanning near-field optical microscope at higher frequencies. Ultrafast time-resolved optical pump–THz probe experiments were performed with 400-nm laser pulses. The amorphous and crystalline phases of GeTe exhibit different volatile THz modulation capabilities on the picosecond time scale and their decay times are 1.39 ps and 0.73 ps, respectively. •Terahertz transmission tuned by thin films of GeTe, a phase change material.•High optical contrast of GeTe also evident with near-field nanoimaging.•Volatile modulation of free charge carriers under optical pumping on picosecond scale.•Combination with laser switching to tune the local terahertz resonance anticipated.