High sensitivity photonic time-stretch electro-optic sampling of terahertz pulses

Single-shot recording of terahertz electric signals has recently become possible at high repetition rates, by using the photonic time-stretch electro-optic sampling (EOS) technique. However the moderate sensitivity of time-stretch EOS is still a strong limit for a range of applications. Here we present a variant enabling to increase the sensitivity of photonic time-stretch for free-propagating THz signals. A key point is to integrate the idea presented in Ref. [Ahmed et al., Rev. Sci. Instrum. 85, 013114 (2014)], for upgrading classical time-stretch systems. The method is tested using the high repetition rate terahertz coherent synchrotron radiation source (CSR) of the SOLEIL synchrotron radiation facility. The signal-to-noise ratio of our terahertz digitizer could thus be straightforwardly improved by a factor $\approx 6.5$, leading to a noise-equivalent input electric field below $1.25$~V/cm inside the electro-optic crystal, over the 0-300~GHz band (i.e, 2.3~$\mu$V/cm/$\sqrt{\text{Hz}}$). The sensitivity is scalable with respect to the available laser power, potentially enabling further sensitivity improvements when needed.