Time domain thz spectroscopy

Who's doing what and when?

To see who is measuring what and when click here to see the official terahertz system calendar for The Bilbro and the Lubashevsky.

Time Domain Terahertz Spectroscopy:

TDTS works by the excitation of a source and activation of a detector by ultrafast femtosecond laser pulses. The laser has, via a beam splitter, its radiation split to fall on source and detector Auston switches. After the fast illumination by the femtosecond laser pulse, the source switch's resistance falls to a few hundred ohms and with a bias by a few tens of volts, charge carriers are accelerated across the gap on a time scale of a few picoseconds. Their acceleration produces a pulse of almost single-cycle radiation, which then propagates through free space and, collimated by mirrors and lenses, interacts with the sample. After passing through the sample, THz radiation falls on the second Auston switch, which is also activated by the femtosecond pulse. Current can flow only across the second switch with a direction and magnitude proportional to the transient electric field at the instant the short femtosecond pulse impinges on it. A delay line is then advanced and the THz electric field at different relative times can be measured. In this way, the entire electric field profile as a function of time can be mapped out.

There are a number of unique aspects to TDTS that allow it to work exceptionally. Since the detected signal is proportional to the instantaneous electric field, and not the power, the measured transmission function is the complex transmission coefficient for the electric field, which has both amplitude and phase. This allows one to invert the data directly to get the real and imaginary optical constants (e.g. the complex conductivity) of the material.

In the Armitage lab, we have an older THz system built and improved upon by Luke Bilbro in the past 4 years. This system, "The Bilbro" is able to measure as low as 1.42K in temperature. We also have a newer THz system, built in 2012. This new system, deemed "The Lubashevsky", is fully automated and currently able to go as low as ~3K in temperature. The laser that illuminates The Lubashevsky is also sent to another THz system, which includes a magnet. The arrival of the magnet in the summer of 2012 really expanded the opportunities for research in the Armitage lab!

Time Domain Terahertz Ellipsometry:

We have another spectroscopic instrument: the terahertz time-domain ellipsometer (TDE) in our lab. Mohammed Neshat is heading this project, which includes developing novel THz photoconductive antenna detectors. This makes the terhartz TDE capable of simultaneously measuring two electric field polarizations. Standard THz time domain spectroscopy requires transmission of the beam through the test sample and will not work for opaque materials. Ellipsometry measurements with the TDE will overcome this limitation by directly detecting the polarization change of the incident beam after reflection due to the material properties of the target surface.

Interactive View of the THz Lab: Click on any image for a view as if you were in our THz lab!

A view of the whole THz lab, especially the older system:

A view of the older system and the ellipsometry set-up:

A view of the new THz system (still in production):