Thz Time Domain Spectroscopy Emission and Transmission Setup in a nutshell

Описание: In the case of terahertz transmission, we steer a terahertz pulse to a sample and detect the transmitted terahertz pulse as the output signal. To do this, we have a zinc-telluride crystal which emits terahertz when being pumped by a femtosecond optical pulse.

Whereas for the terahertz emission, the optical pulse hits the sample directly. The output signal is a terahertz pulse emitted by the sample itself.

For both transmission and emission measurements, the output terahertz signal lasts several picoseconds.

Because standard optical detectors do not have a picosecond resolution, a type of pump-probe detection technique is used to measure the output signal with a sub-picosecond resolution.

The technique is called electro-optic sampling, where an electro-optic crystal is hit by the output terahertz signal as the pump, and a weak optical pulse as the probe.

By measuring the change in the probe pulse transmitting through the crystal, we can construct the output terahertz signal in the time-domain.

Before running an actual measurement, the detection optical assembly needs to go through a calibration procedure without any terahertz signal.

A probe pulse exiting the crystal without appropriate phase would result in a non-zero detector output even without any signal.

The compensation procedure consists in adjusting the quarter-wave plate until the reading of the balanced photodiode is close to zero volts, or, equivalently, until the probe-pulse exiting the quarter-wave plate is circularly polarised.

Once this calibration has been carried out, the terahertz pulse can be measured.

The terahertz-induced birefringence modifies the electric field of the probe from circular to elliptical. This deviation, which is proportional to the terahertz field, is detected by the balanced photodiode.

By probing the photodiode output as a function of the delay between the terahertz-pulse and the probe-pulse, the terahertz electric field can be reconstructed in the time-domain.

A lock-in amplifier is used with the choppers shown earlier to improve the signal to noise ratio of the reconstructed signal.

Full video: https://scivpro.com/manuscript/10_323...

COPYRIGHT BULLAKI LTD