Second harmonic generation observed by dielectric relaxation at high electric fields: SHG without optics

We have measured second harmonic generation via the nonlinear dielectric permittivity of a polar glass-forming liquid, propylene glycol, with the inversion symmetry of the liquid broken by applying a dc bias field. For a given combined peak field, EB + E0, the highest second harmonic signals are obtained when the dc bias field and peak ac-field have the same amplitude, i.e., EB = E0. Second harmonic results measured in the static limit agree well with the theoretical prediction based upon the third-order nonlinear susceptibility term that connects polarization P with the static field E. Second harmonic signals are detectable at all frequencies at which dipole orientation contributes to permittivity, possibly occurring the instant a dc bias field is applied. The results imply that second harmonic generation as a signature of anisotropy, e.g., at interfaces, can be assessed by impedance spectroscopy at twice the fundamental frequency, not only by optical techniques.