|MadSci Network: Physics|
Thanks for your interesting question. When I first read the question, I thought it was about the subject of my PhD, of which one of the applications is ultra-powerful lasers in the infrared, and another is terahertz radiation.
You, however, are looking at 100 Hz radiation. From the top of my head, that's a wavelength of about 3000 km. Hence, I don't really understand what you mean with "coherent" . This, in laser physics, means that the individual light "particles" move at the same pace, in phase, and all oscillate, well, coherently, like soldiers marching. However, if your wavelength is 3000 km, you need to have things awfully far out of phase to make it seem noncoherent, on "human" length scales. If you use a chopper, and just emit radiation in a certain short timeframe, with a well-defined phase at the beginning of the device, you have close to coherent radiation. In other words, I don't think this is a big issue.
Directionality, on the other hand, can be obtained with a proper antenna. However, such antennas typically need to be of the same size as the wavelength of the radiation produced, or 3000 km. Your suggestion to use a material with a high index of refraction is clever, but not workable, as there are no with optical properties coming even close to what you want, as far as I know. Again, if efficiency is not an issue, you could perhaps just block all radiation out of a certain radiation angle.
In other words, what you could do is use a common wire (radiation is 50 (Europe) or 60 (US) Hz and multiples of that frequency). Then, you chop it, say in microsecond pulses. This will give you coherence. Next, discard all radiation not in the direction you are interested in.
As for your own suggestions, they are clever and creative, but unfortunately not workable. No remotely normal material that I can think of has transitions in the order of 100 Hz. Even if these transitions were to exist, the energy would be so tiny, in the order of fractions of a millionth of an electronvolt, with an electronvolt the typical energy of an optical photon, that for any realistic temperature, the natural radiation of the object, called blackbody radiation , would drown the radiation from this transition.
Your suggestion of using a free-electron laser is again clever, but I'm pretty sure the laser has to be as least as large as several wavelengths, and thus not feasible.
If you are interested, and would like to divulge more about your idea, I'd be more than happy to talk to you about this. I'm convinced you can easily find my email, and if not, I bet the site administrators will want to help you. Of course, I promise I won't "steal" your idea or anything.
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