Measuring the Electro-optic Response of Quartz for Accurate Sampling of Intense THz Fields

The advancement of THz sources with peak fields exceeding 1 MV/cm poses the challenge to accurately measure these electric field transients without distortions in the electro-optic detection. Z-cut $\alpha$-quartz has been recently found as a suitable electro-optic sampling (EOS) crystal, but its accurate response function, which allows the THz electric field to be exactly determined from the measured EOS signal, was still elusive. Here, we employ intense THz fields (0.5-4 THz) generated via optical rectification in LiNbO
3
to measure EOS in quartz. By comparison with a traditional EOS crystal, we determine the quartz response function in both experiment and theory, and apply it to extract the exact THz field waveform. Our work therefore establishes a new standard for accurate measurement of intense THz electric fields using quartz, wherever conventional EOS materials face saturation effects or nonlinear distortions.