Observation of selection rules for circularly polarized fields in high-harmonic generation from a crystalline solid

Abstract

Recently, circularly polarized high harmonics (HHs) have attracted great attention from the viewpoint of their application to ultrafast spectroscopy of magnetic materials and chiral molecules. However, circularly polarized HHs from gaseous media require a complicated setup due to strong constraints upon the high-harmonic generation (HHG) mechanism. HHG from solids is a new approach to this problem. Because the mechanism of solid HHG is intrinsically different from that of HHG from gaseous media, HHs can be generated under single-color circularly polarized excitation. Also, their polarization state has been predicted to obey simple selection rules. In this letter, we report on an experimental demonstration of circular HHG from solids which is fully consistent with the predicted selection rules. We irradiate circularly polarized mid-infrared (MIR) pulses to a thin gallium selenide crystal and measure the spectrum and polarization of the emitted HHs. The threefold rotational symmetry of the crystal causes extinction of every third-harmonic order, as well as generation of counter-rotating circularly polarized HHs. This result paves the way toward compact circularly polarized sources covering the spectra from the MIR to extreme ultraviolet.