Terahertz and multi-terahertz spectroscopy of light-driven 3D Dirac semimetal Cd3As2

We developed a switchable terahertz and multi-terahertz spectroscopy system to investigate the optical and electrical properties of a light-driven three-dimensional Dirac semimetal, Cd$_{3}As_{2}$. The femtosecond multi-terahertz probe pulse enabled us to track the ultrafast change of the broadband response functions in the 12-45 THz frequency region. We observed a large reduction of the refractive index after pulsed mid-infrared excitation, which is attributed to a combination of a light-induced shift in the plasma frequency and a long scattering time specific to Dirac semimetals. Upon changing the pump light to a narrowband multi-terahertz pulse which lies inside the probed frequency band, we observed strong stimulated Rayleigh scattering characterized by an absorption peak and an emission peak sandwiching the driving frequency. Microscopic calculations showed that this phenomenon originates from optical transitions among the light-dressed energy bands, called the Floquet-Bloch bands, which are enhanced by the collective plasma mode responsible for the large reduction of the refractive index. We propose a possible route to slow light generation based on the large stimulated Rayleigh scattering in a metallic material at room temperature. We also transformed the driving multi-terahertz pulses into circularly polarized ones, to study the light-induced anomalous Hall conductivity through the terahertz polarization rotation. Comparison of experiment and theory showed that the observed anomalous Hall conductivity mainly originates from the circular photogalvanic effect allowed by the terahertz electric field, and that the theoretically anticipated Floquet-Weyl semimetal plays a minor role under the present experimental conditions. We also revealed that the isospin degree of freedom in Dirac semimetals can be injected by circularly polarized light and detected through the anomalous Hall conductivity after photoexcitation. These studies elucidate both the interband and intraband physics in the massless Dirac fermions.