Ultrafast terahertz and multiterahertz spectroscopy for a nonequilibrium Dirac semimetal Cd3As2 with periodic driving by light

Optical properties of topological Dirac semimetals are attracting growing interest because 3D massless electrons show unique electromagnetic responses with a large interaction volume with light, providing a fascinating platform for studying ultrafast control of matter and promising application in optoelectronics and nonlinear optics [1]. Their unique infrared responses originate from the gapless band structure, where interband and intraband transitions occur in a near energy scale and their balance can drastically change in nonequilibrium. To thoroughly derive their novel functionalities, in-depth understanding of nonequilibrium broadband complex response functions is indispensable. We have developed a phase-stable time-domain spectroscopy system in the multiterahertz range (10-50 THz in frequency, 40-200 meV in energy, or 6-30 μm in wavelength) [2], and studied ultrafast dynamics of a photoexcited Cd3As2 thin film with 30-fs time resolution. We found that photoexcitation largely suppresses the infrared refractive index by a factor of 5 due to the elevated plasma frequency [3]. We also investigated how the response function changes during coherent light-matter interaction with periodic driving, i.e., under the formation of Floquet-Bloch state. Under a 30-THz periodic lightwave, we observed that stimulated Rayleigh scattering resonating between the Floquet subbands dominates the conductivity spectrum with the assistance of the collective plasmonic response [4]. The result may pave a way for slow light generation in conductive materials at room temperature. We also introduce observation of anomalous Hall effect in the 3D Dirac semimetal induced by circularly-polarized light, and discuss it by considering the formation of the Floquet-Weyl state.

[1] B. Cheng*, N. Kanda*, RM et al., Phys. Rev. Lett. 124, 117402 (2020).
[2] N. Kanda, RM et al., Opt.Express 29, 3479 (2021).
[3] N. Kanda, RM et al., arXiv:2110.09689.
[4] Y. Murotani*, N. Kanda*, RM et al., arXiv:2112.13113.