Measurement of the Dispersion of χ ( 3 ) $\chi ^{(3)}$ of SiO 2 ${\rm SiO}_2$ and SiN Across the THz and Far-Infrared Frequency Bands

Abstract

Terahertz (THz) radiation sources based on two-color femtosecond plasmas in air are becoming a mature technology for coherent spectroscopy and strong-field physics across the extended THz range to several tens of THz. The field-resolved detection of such THz transients relies on the third-order nonlinearity of the detection medium. Here, a comparative measurement is demonstrated with air-biased coherent detection (ABCD) and solid-state biased detection (SSBCD) as a novel method to measure the dispersion of the magnitude and phase of the relevant third-order nonlinearity ${\chi }^{\left(3\right)}(2\omega \pm \Omega ;\omega ,\omega ,\pm \Omega )$ for fused silica (${\mathrm{SiO}}_2$) and silicon nitride (SiN). Based on the development of the ultrabroadband SSBCD device with a detection bandwidth exceeding 30 THz, ${\chi }^{\left(3\right)}$ measurements are obtained across the 1–28 THz frequency range, hence covering the THz and far-infrared. It is shown that the vibrational modes in ${\mathrm{SiO}}_2$ and SiN in the THz range lead to strong resonant enhancement and dispersion of the nonlinearity. The SSBCD devices operate down to nanojoule (nJ) probe energy, and their is demonstrated by measuring the dielectric function of the Lorentzian line profile of transverse-optical (TO) phonon mode at 9 THz in single-crystal gallium arsenide (GaAs) and observing the weak phonon combination bands near the TO phonon.