High-harmonic spectroscopy of quantum materials

We report the evidence of generation of high-order harmonics from the surface states of a three-dimensional topological insulator, Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, subject to strong mid-infrared laser fields. The response from the surface is identified by analyzing the polarization of high-harmonics and by exploiting the fact that inversion symmetry is broken at the surface. In experiments, high-order harmonics up to 17$^{\mathrm{th}}$ order are observed without sample damage at the peak intensity of approx. 3x10$^{\mathrm{10}}$ W/cm$^{\mathrm{2}}$. The non-trivial response of the topological material is seen to manifest itself through a significantly enhanced harmonic yield for circularly polarized driving laser fields.