Fig. 1: Electro-optic measurement of intra-cavity fields.

a The cavity THz electric field (red) induces a local, transient birefringence as it travels inside an α-quartz electro-optic cavity (EOC), which is read out from the transmitted polarization state of an ultrashort co-propagating probing pulse (green), as a function of the relative time delay \(t\) with respect to the incident single-cycle THz pulse (blue). b Incident ultra-broadband single-cycle THz field, measured by free-space EO sampling in z-cut α-quartz. c Intra-cavity EO signal generated from the input THz pulse in panel (b), for a quartz cavity length of 44 µm, and a nominal deposited gold mirror thickness of 14 nm. The standard error is represented here, and in panels (d, e), as a lighter area. d Quantitative, intra-cavity electric field derived from the inverse Fourier transform of the complex cavity spectrum in (e). e Normalized cavity spectrum derived from the measured EOC signal in (c) after applying the cavity correction function. Amplitude (red) and frequency-resolved phase (gray) are shown in comparison to the normalized spectrum of the incident broadband pulse in panel (b)