Second-harmonic phonon spectroscopy of α-quartz

We demonstrate mid-infrared second-harmonic generation as a highly sensitive phonon spectroscopy technique that we exemplify using α-quartz (SiO2) as a model system. A mid-infrared free-electron laser provides direct access to optical phonon resonances ranging from 350 cm−1 to 1400 cm−1. While the extremely wide tunability and high peak fields of an free-electron laser promote nonlinear spectroscopic studies – complemented by simultaneous linear reflectivity measurements – azimuthal scans reveal crystallographic symmetry information of the sample. Additionally, temperature-dependent measurements show how damping rates increase, phonon modes shift spectrally and in certain cases disappear completely when approaching Tc = 846 K where quartz undergoes a structural phase transition from trigonal α-quartz to hexagonal β-quartz, demonstrating the technique’s potential for studies of phase transitions.