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Abstract

We demonstrate midinfrared second-harmonic generation as a highly sensitive phonon spectroscopy technique that we exemplify using α-quartz (SiO₂) as a model system. A midinfrared free-electron laser provides direct access to optical phonon resonances ranging from 350 to 1400cm⁻¹. While the extremely wide tunability and high peak fields of a 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 T_c=846K 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.