Ultrabroadband Dispersion of Third-Order Terahertz Nonlinearities

Terahertz (THz) radiation sources based on two-color femtosecond plasmas in air is becoming a mature technology for coherent spectroscopy and strong-field physics across the extended THz range to several tens of THz, or photon energies >100 meV. Here we highlight the performance of THz time-domain spectroscopy using such sources with examples of reflection spectroscopy of single-crystal gallium arsenide (GaAs) and transmission spectroscopy of thin-film fused silica (amorphous SiO2) using air-biased coherent detection (ABCD). We present our implementation of solid-state biased coherent detection (SSBCD) with >30 THz bandwidth and minimum probe energy requirements in the sub-nanojoule range. We use a back-to-back measurement with ABCD and SSBCD as a novel method to measure the absolute value and dispersion of the relevant third-order nonlinearity χ (3) (-(2ω±Ω); ω,ω, Ω) for fused silica (SiO2) and silicon nitride (SiN) across the Ω/ 2π =1-40 THz frequency range. We demonstrate that vibrational modes in SiO2 in the THz range leads to strong, resonant enhancement of the nonlinearity. The method presented here for measurement of higher-order nonlinearities induced by ultrabroadband THz radiation will be advantageous for advanced pulse detection concepts and promising for static, pump-probe and nonlinear multidimensional spectroscopy.