Sum-frequency phonon spectroscopy on α-quartz

We demonstrate here, using alpha-quartz as an example, that IR-visible sum-frequency (SF) spectroscopy can be employed to probe zone-center optical phonons of crystals without inversion symmetry. Because only modes both IR and Raman active are allowed in the SF process, the observed phonon spectrum is greatly simplified. The intrinsic flexibility of SF spectroscopy also permits more accurate determination of relative magnitudes and signs of relevant Raman polarizabilities.