Finding a retroaction with something: I think the best way is to try, the Cat Tongue, in first place. It is the easiest machine to build. After applying the gaz to the allied substance (once on the static object or before!), you will need to aim to something (don't try something bigger that the smallest thing you can build). The fact is that those rays goes on both direction! In the case you succed to apply it corectly to the base :)
Plastic: A transluency substance with carbon chains or resonant cycles, will do the trick (the principle is that an oscillation must be natural in those constituants: OLED, will send electron from one end to the other end of the chain, so it can be OLED; Acetate: look too short but cellulose acetate look better but a little bit cheap!). Use the substance that they built neutrino detector with (not the preferable way!). Place yourself in the dark with photonic detector (depending on the size of the transmiter and the substance to be targeted), maybe the eyes will be enough. I personally experimented to see those effects with my own eyes (I don't know what was the parameters!)
Sugar: Phenylalaline, will certainly do it, but this time it won't be ligth, but heat :() You may considering to use a little bit bigger Gun to produce detectable heat in a small piece of that sugar: Almost free to buy. But beware to not build a killing Gun! Yes Ether-electron could kill :( So, it may be oscillating transistor or Snap-on tool, to detect heat.
Trying to intercept more than qualitatively, those rays: Building of the Cat Ears. It becoming more complex to me to try, without having experimented it, but let's go...
Having built the best transmiter that could be built: The fact of retreiving the heat data, at such rates it's close to the limit of Physical sciences. But, believe me, it can. You can consider using, the plastic or the sugar, but it looks worst :() But capturing the emmited light of the OLED (an old trick of me :) ), may do that what is expected, but to acheive rate of the brain frequency...
The Brain: Haaaahaaa, what is the resonance frequency? I don't know :) But you will achieve that :() There is the density aspect of the situation: To listen to large distance. There is billion of neurone in a brain, let's say 1 Billion :) (100 Trillion in yours :) )
The density: 1 billion / every degree of 3D = 3-D polar system of coordinate: at 1 000 000 meters ---> few rays left per unit of time. Maybe someday I will calculate it, but considering being close to the target first.
Decoding: That is a crazy thing! But you can consider using brain to decode Brain :)
Digital data: The next step, because it permit to be close to synthetic voice. It could be a great asset, on inferior being, to make them believe they are listening to such person or an other... It won't do on experimented being or smart enough one, but, usely yes :)
Fifth and final Cat Ether-electron step
Building an Army, with poor being soldier, that will have the power of the final tough Police :)
This is the most difficult part: What is the good way to think?