Controlling THz emission in topological materials

In thin polycrystalline films of the centrosymmetric Dirac semimetal PtSe₂, we observe a giant and highly tunable THz emission that is rapidly turned on at oblique incidence. Strikingly, we find the THz emission to be locked to both the in-plane photon momentum and polarization state of the incident pump beam, where the THz sign and amplitude are fully controlled by the incident pump polarization, helicity and photon momentum. Moreover, the emitted THz efficiency is two orders of magnitude larger than that of the standard THz-generating nonlinear crystal ZnTe, and approaches that of the record-setting topological material TaAs. Our work demonstrates how photon drag activates a rich and pronounced directional optical linearity that are available even in centrosymmetric and polycrystalline Dirac materials.