# Interaction of THz radiation with matter

1. Jan 7, 2009

### jgrant333

hello.

I'm studying a project on Thz radiation and have to establish what material would make a good THz detector.

To this end I've tried to find equations linking the aborption of THz radiation in matter and how the absorption varies with wavelength (one of the things I'd like to eastablish is how thick I need to make my detector to get 100% absorption of the THz radiation).

Anyway up until this point I have been unable to come across any such equations.....seriously frustrating (perhaps I need to refine how I search for informartion).

Could anyone help me out?

Many thanks!

2. Jan 7, 2009

If no one else replies I'll give you my 2 cents. Absorption is fine and all, but not so useful, but first things first:
Absorption is a property of a material, just like color. It's a ***** to predict ab initio (i.e. from the theoretical ground work). For many material classes there are established theories. As THz radiation is basically normal non ionizing radio waves, you might want to look into radio wave absorption. For metals in bulk for example there should be plenty of theory, but metals also reflect a lot.
Now about the absorption: If you simply use an absorbing material it will only get warmer. You wouldn't know if you are really measuring THz waves or microwaves, or something else that is absorbed by your material. With radio waves you use antennas, and separated charges, with light you can use electron excitations in semiconductors, there are many more things imaginable where your material shows some effect A under THz radiation and you have a good detector for A. So I would look at the techniques already in use for measuring radiation, and for molecules on the length scale of the wavelength, that can resonate. Theory will not deliver what you are searching for.

3. Jan 8, 2009

### jgrant333

I take your point about differentiating between Thz waves and other electromagnetic radiation. However I plan on using a dedicated Thz radiation source along with suitable filters so that the majority of the EM radiation striking my detector is Thz.

I actually would like to fabricate a focal plane array microbolometer - i.e. just as you say in your post I want to measure how much the detector heats up when Thz radiation is impingent on it.

In many papers Vanadium Oxide is used as a detector material (it has a high temperature coefficient of resistance) however in no paper can I find an explanation giving % THz radiation absorbed with thickness of detector material.

Obviously I'd like 100% absorption of Thz radiation in my detector material to obtain maximum detector responsivity.

As a side note could anyone recommend any books on EM radiation interaction in matter?

Many thanks

4. Jan 8, 2009

Interesting, since I think finding filter materials is pretty much the same problem, as finding absorbers.

The way you phrase your question I am not sure if you know what you are searching for. Absorption is generally a statistical process, with an exponential behavior, so you are actually looking for 99.999% absorption or so, otherwise the detector must be unreasonably long to "catch the last photon". But not even really that, because it will probably ruin your response time if you need to heat up or cool down a thick block of material, in the end you'll probably want "most radiation absorbed" or "just enough radiation absorbed".

Yes but only for molecules and quantum dots. So that doesn't help. I hope you are familiar with Maxwells equations in media, and complex valued permitivity, then you could check out the links in http://en.wikipedia.org/wiki/Dielectric_spectroscopy
Took me about 5 minutes to find, but I don't really know what level of physics you're at.