Mounted THz Detector Crystals for Electro-Optic Sampling

Overview

These are electro-optic sampling crystals supplied ready-mounted in a carrier, so the fragile thin element is protected and can be dropped straight into a standard optic holder. Very thin electro-optic crystals are difficult to handle unmounted; pre-mounting removes the most common cause of breakage during a first build and makes the detection arm repeatable between users.

How electro-optic sampling works

The terahertz electric field induces a transient birefringence in the crystal. A co-propagating femtosecond probe pulse therefore picks up a small polarisation rotation proportional to the instantaneous terahertz field. A quarter-wave plate biases the probe to circular polarisation, a Wollaston prism splits it into two orthogonal components, and a balanced photodetector reads the difference. Scanning the probe delay reconstructs the field waveform in time, and the Fourier transform gives amplitude and phase together.

Thickness sets the trade-off

A thicker crystal gives a longer interaction length and a larger signal, but the velocity match between the optical group velocity and the terahertz phase velocity holds over a narrower frequency range, so high-frequency components wash out. Thin crystals do the opposite. Choose thickness against the bandwidth the experiment actually needs rather than defaulting to the largest signal.

Alignment notes

Overlap the terahertz focus and the probe focus on the same spot, keeping the terahertz spot larger than the probe so the probe samples a uniform field. Electro-optic crystals are not rotationally symmetric: rotate the crystal about the beam axis to maximise the differential signal before optimising anything else. Keep the crystal inside the purged volume with the rest of the terahertz path.