Nonlinear optical spectroscopy in solid state media is intriguing as it opens up additional experimental degrees of freedom which can help to disentangle congested linear spectra, while providing improved sensitivity. Most notably, this can be exploited to isolate different symmetry contributions to the measured signal and in consequence gain information about the crystal structure of the sample at hand. In particular, second-harmonic generation (SHG) as an even-order nonlinear process is (in leading electric-dipole order) only allowed in non-centrosymmetric media which makes it a very suitable tool for the investigation of ferroelectrics due to their naturally broken inversion symmetry [1]. However, these approaches have so far been restricted to the visible and near-infrared. Here, we demonstrate second-harmonic phonon spectroscopy [2] in the far-IR as a novel approach to study polar dielectrics with broken inversion symmetry and exemplify the technique for α-quartz. We observe a strong enhancement of the SHG yield at transversal optical phonon frequencies whereby the azimuthal behavior allows distinguishing the different symmetries of the respective modes. Temperature-dependent measurements demonstrate the sensitivity of our technique to symmetry changes upon the α-β phase transition selectively for the different phonon modes. Given the sensitivity to structure via phonon resonances and crystal symmetry via the nonlinear susceptibility tensor, our novel technique presents itself as a promising tool to study ferroic materials such as multiferroics and ferroelectrics.
References
[1] Denev et al., J. Am. Ceram. Soc. 94, 2699 (2011).
[2] Paarmann et al., Phys. Rev. B 94, 134312 (2016).