Heat

Don't know of any examples...

https://en.wikipedia.org/wiki/Extremely_low_frequency

Well, that is what we hams call CW and light people call coherent light, from laser or from a marching coherence device. Radiation of one single frequency.

When I see the term "Heat" I think of thermal energy. When I see the term Infra Red I think of electromagnetic energy emitted by objects "heated" to a thermal state where they begin to emit electromagnetic radiation at a frequency just below the visible spectrum. The thermal state is directly proportional to Plank's Constant and the speed of light but inversely proportional to wave length?

If an inductor of fixed length c/f where f is the whole wave or one of the harmonic resonant frequencies of that fixed length then heat could be created as the induced energy is dissipated through the damping of the circuit resistance.

The challenge for thermal conversion at those very very long wave lengths (y) would appear to be achieving those long inductive lengths or compensative capacitance where the whole or harmonic c/y would approximate 1/2pi(sqrt(LC)) ?

As far as I know, the closest you could get would be a Free Electron Laser (FEL). These lasers are tunable over an extremely wide range with a narrow bandwidth. I believe that a 10 to 100 hertz bandwidth is beyond what is achievable.

http://www.sciencedirect.com/science/article/pii/0168900291908114

Development and Applications of Free Electron Lasers

My electric fire runs at 50Hz.

you're so consistent with your punch line crabtree, quite exceptional.

Maybe, but why? We already know that RF energy in the 2.4 GHz band can easily be converted to "heat" in objects containing water; this is how microwave ovens and "area denial" weapons work.

I thought about heat in a material is governed by molecular resonance, and it's probably in the range of the size of molecules or may be-grain structure at certain size from micro to nanoscale for most materials.

I thought about it as a dance in tuned with frequencies when molecules are most agitated in their corresponding wavelengths.

Say for example, Silicon and Germanium in PV cell has unique band responses.

Above is, assuming that I was right. May be someone could provide a clearer insights here.

TWTs and Magnetrons are out, because the dimensions would be too small to guarantee any accuracy or efficiency. At 80THz, I suspect the Boltzmann variations in an electron stream would preclude the bandwidth you are seeking, unless you were able to do it all at near absolute zero temperatures, with multiple filters. How good are dichroic mirrors or equivalent at selecting a NARROW band of frequencies? Can you reproduce filters all having exactly the same frequency range? If the filters are adjustable, how do you tune all of them to the same frequency? Reminds me of the problem of selecting a narrow-band signal with a TRF receiver and regeneration, easy by comparison.