What is THz?

Terahertz radiation is electromagnetic radiation of frequencies between 10 GHz and 30 Thz (in the lab). THz = 10¹² Hz This puts it in the far-infrared part of the spectrum with wavelengths (in case you can’t do the math) on the order of 100 microns.

This part of the spectrum is interesting because it lies in what’s known as the “THz gap” between traditional photonics and microwave technologies. Traditional electronics cannot switch fast enough to generate these pulses, and commercial lasers cannot generate that low of frequency of waves.

So how do we generate these waves? This is where ultrafast optics comes in. Using a technique called “modelocking” a standing wave is setup in a laser cavity and waves constructively interfere to create pulses with lengths on the order of femtoseconds (10^-15 s). These pulses can then be amplified in a multi-pass system and set into a specific repetition rate (1080 Hz in my lab). Each ~100 fs pulse contains approximately 1/2 mJ. These pulses are then of high enough intensity to cause nonlinear effects in most materials. We now enter nonlinear optics land. When this light is incident on specific nonlinear crystals (in our case Zinc Telluride), a second order nonlinear effect called optical rectification occurs. This effect essentially creates a DC bias inside the crystal and current essentially flows through the crystal itself. This accelerating currents creates electromagnetic radiation in the THz regime.

THz pulses are interesting to study due to their absorption properties. Water is the primary absorber of these wavelengths, while plastics, papers, and fabrics have much lower absorption amounts. This allows for the application of techniques for THz imaging. For instance a THz wave can scan through clothes but gets absorbed by water or reflected by metal. This allows for the potential use of security scanning where police can essentially ‘see’ through a person’s clothes and determine if there are any weapons on them. The resolution is better than 1 mm, which is good enough to see most weapons. The following image shows a THz imaging scan:

There are several other ways to generate THz radiation (which I am presently working on a few), as well as many more applications (inlcuding THz-TDS). It is an exciting field that I am enjoying as I learn more about it.