Giant surface second harmonic generation from layered topological type-II Dirac semimetal PdTe₂

PdTe₂ belongs to the ATe₂ family (A = Pd, Pt, Ni) of layered, centrosymmetric, type-II Dirac semimetals (DSMs). With broken inversion symmetry at the surface, it hosts non-trivial topological surface states leading to peculiar light-matter interactions that are excellent for future optoelectronic and spintronic applications. In this work, we studied the surface-specific nonlinear optical property of PdTe₂ via rotational-anisotropic second harmonic generation (SHG). To our best knowledge, this is the first experimental study of the surface-specific nonlinear optical property of type-II DSMs. Our measurements verify that SHG originates from the surface of PdTe₂ by following its C_3v surface symmetry via the electric-dipole contribution. Moreover, fresh crystal surfaces prepared by in situ cleavage exhibit a profound time-dependent evolution of the surface SHG by following surface oxidation kinetics. Quantitatively, we characterized all independent non-zero complex second-order surface nonlinear optical susceptibilities, χ(2)_S,ijk, in the steady state as well as the transient state right after cleavage. The results indicate a giant magnitude of the out-of-plane χ(2)_S. Detailed discussions about the electronic origin for giant χ(2)_S and mechanisms causing the transient evolution of SHG will be given in the talk.