Evidence of Polarons and Their Ultrafast Optical Undressing in a Topological Semimetal

Polarons, formed by coupling electrons with lattice vibrations, are of intense interest in topological semimetals owing to their crucial role in dictating high-speed topological optoelectronics. However, polarons in topological semimetals have thus far been exclusively observed as light-induced transient quasiparticles, leaving equilibrium polarons and their ultrafast dynamics elusive. Here, using time-resolved magneto-terahertz spectroscopy, we demonstrate the ultrafast optical undressing of polarons in a Cd3As2 thin film. In equilibrium, a magnetic-field-induced spectral splitting near the 1.22-terahertz phonon mode reveals an 87% enhancement of the carrier effective mass due to the polaron effect. Upon photoexcitation, the enhanced carrier effective mass collapses within 5 ps, reflecting the transient undressing of this pre-existing polaronic state. Notably, this anomalous suppression contrasts sharply with prior studies where optical excitation exclusively triggers polaron formation in topological semimetals. This study provides deep insights into the ultrafast magneto-optical dynamics of pre-existing polarons, advancing the design of high-performance topological devices.