Wikipedia discusses this - Terahertz radiation is absorbed at the frequencies of the quantized rotation of h2o. If you can get these to emit/relax at once, you

Wikipedia discusses this - Terahertz radiation is absorbed at the frequencies of the quantized rotation of h2o. If you can get these to emit/relax at once, you get a laser analogue. Trying to use something that’s absorbed by water vapour is going to be difficult in that the frequencies will have to be chosen carefully or “dry” radiation pipes will be required. This is an experiment that was done in second semester p-chem lab in ancient times and might still be being done now. A good double beam spectrometer and a source that can emit both near IR (vibrational bands) and far IR (rotational bands).