Dont too much about terahertz waves but i know that the are very hard to detect because they are between Microwaves and Radiowaves, in that frequency range. They could be very useful in applications such as medicine because they are almost completely safe and can pass through objects such as flesh but not bone...(just like x-rays but safe) and some more things...
"There are also security applications. They can be used, at short ranges, to detect and even image metal items concealed under clothing.
Detection of terrahertz signals could be problematic. There are atmospheric absorption problems in that band. IR is usually detected with semiconductor bandgaps, microwaves are detected with antennae. For terrahertz, you could go either way, but it requires extaordinary measures. You could use lithographically deposited arrays of tiny metal antennae, or engineer a semiconductor material using superlattices or quantum dots to get the right band structure. You would probably need a quantum cascade effect to detect terrahertz in semiconductors."
this is what Njorl told me about them so i cant take credit for it
i suppose u could look on google like i have been told to hehe