Highly Efficient Ultrafast Terahertz Modulation Enabled by Unconventional Carrier Dynamics in HgTe Semimetals

Low‐power and high‐speed terahertz (THz) photonic devices are of significant importance for advancing THz technologies, demanding exceptional functional materials to load. In this work, we systematically investigate the transient charge transport dynamics in semimetallic mercury telluride (HgTe) films utilizing optical pump‐THz probe (OPTP) spectroscopy. Remarkably, photoexcited bare HgTe exhibits an ultrasensitive transmission response in THz frequencies at room temperature–reaching a modulation depth of 28% at a mere pump fluence of 1 µJ/cm 2 and up to 73% at 21 µJ/cm 2 –surpassing all reported materials under comparable conditions. Moreover, the photocarrier relaxation remains ultrafast and nearly constant (∼14.3 ps) across a wide range of pump fluences. Through detailed analysis of THz photoconductivity spectra, Drude‐Smith model fitting, and temperature‐dependent transient dynamics, we reveal that HgTe's superior performance stems from its semimetallic character, featuring ultrahigh electron mobility enabled by long momentum scattering times and a small effective mass, as well as its gapless electronic structure. Our findings provide deep insights into the photocarrier relaxation mechanism in HgTe and highlight its immense potential for developing energy‐efficient, broadband, all‐optically controlled, ultrafast THz modulators and other high‐performance THz optoelectronic devices.