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Creating visible light using an antenna

  1. Jun 22, 2009 #1
    If you could get a circuit to oscillate quickly enough, could visible light be produced using a simple metallic antenna?
     
  2. jcsd
  3. Jun 23, 2009 #2
    The question is, could you get a circuit to oscillate fast enough?
     
  4. Jun 23, 2009 #3

    Born2bwire

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    Could we manufacture an antenna and system on scale to resonate at visible light?

    Generally the classical electromagnetic theory holds well until around the Terahertz range. From there, it is usually best to use quantum electrodynamics with a mix of classical electromagnetics when appropriate. So if you want to design a device that radiates visible light, classical EM would give you a device that is physically unrealizable. Quantum electrodynamics gives you options like black body radiators or emmission from excited states that can be used to design real devices that will emit visible light. This can vary from heating the tungsten wires in your incadescent lights to flourescent lights or even light emitting diodes or transistors (I am not sure if they have gotten the LET into visible light region yet, my understanding from the talks that I attended a few years ago is that the LET was only recently invented).
     
  5. Jun 23, 2009 #4
    Good question.... perhaps if we could create a sharp enough rising edge. Perhaps if two micromachined surfaces were brought together quickly.
     
  6. Jun 23, 2009 #5
    Are there any sort of reverse dipole geometries that resonant at a wavelength that is some fraction of it's characteristic lengths?

    Yeah, I know there are plenty of devices that can create light, it's this breakdown of classical electromagnetics that I find interesting.

    It got me wondering whether you could build a carbon dioxide scrubber with a circuit oscillating in the THz range. If you could excite the molecular vibration mode with enough energy in theory it could cause ionization, facilitating the construction of carbon dioxide scrubbers that do not require consumable materials.
     
  7. Jun 24, 2009 #6
    I am fairly conviced that ordinary vibratory dynamics or standard electromagnetic theory cannot do this, but is there some QED concept that can be used here, i am sorry i did not follow the post on the QED part can some one make it a little more verbose

    Thanks
     
  8. Jun 24, 2009 #7

    Born2bwire

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    Classical electromagnetics operates on the idea that electromagnetic waves are produced by currents. However, QED allows for electromagnetic waves to be created from other mechanisms. The black body radiator states that an object of a given temperature will radiate electromagnetic waves over all frequencies in a certain distribution. This has to do with the moving molecules/atoms of the object spontaneously or through stimulation converting some of their momentum into light. I believe this would be similar to how an incandescent bulb works. The excitation of the electrons in the orbitals of an atom will produce electromagnetic waves when the excited electron falls down to lower energy orbitals. This is how a low pressure sodium vapor (older street lights) lamps work. QED allows for more mechanisms for electromagnetic radiation than classical electromagnetics.

    The problem with classical electromagnetics at very small scales is that quantum effects start to take over. In addition, at higher frequencies objects behave less classically. For example, we treat a simple wire antenna at microwave frequencies and lower as a good or perfect conductor. However, a lot of metals, like silver, will behave like a plasma at the Terahertz range. The change in the properties of the metal, though they can be given a classical equivalent as in the case of treating the metal as having a surface plasma, makes them difficult or impossible to use regular microwave design techniques.

    That doesn't mean it is impossible. I have seen Terahertz bow tie antennas made using nanoscale lithography. These bow tie antennas are used to intensify the electric fields produced at a quantum dot. In this case, the theory is a mixture of classical and quantum electrodynamics. The quantum dot will produce fields in the Terahertz range and it is placed at the excitation point of the bow tie antenna. The bow tie antenna will have a surface plasma mode excited on it from the fields from the quantum dot. The antenna will resonate and help intensify the potential well of the quantum dot (I think that's the end result that was desired).

    So there are a number of problems, the first is that most classical antennas will generally not work at the length scales for them to emit visible light radiation. It is difficult to actually produce the fields needed to feed these antennas and it is difficult to create a feeding network to the desired antennas. I say difficult only because I am not one to say that something is impossible but I would say in this case that difficult means I highly doubt it would ever be feasible.

    So certainly failing to be less verbose but hopefully clearer.
     
  9. Jun 24, 2009 #8
    I've read in an article some time ago that this is possible (I've to do some searching to dig it up, though).

    If you look at an ordinary radio-antenna and look at how it works from a quantum mechanical perspective, what you see is that an antenna absorbs quanta from the radiation field, coherently. Suppose you have an antenna coupled to a superconducting LC circuit that resonates at some frequency omega. The energy levels of the circuit are (n+1/2) hbar omega. The antenna couples this circuit to the radiation field. The LC circuit can absorb a photon of energy hbar omega and make a transition to a higher energy level.

    In principle, all radio-circuit work like this, but in the limit of large currents and voltages we can ignore quantum effects and pretend that classical equations apply. In reality, just like like position and momentum, the current and voltage satisfy a similar commutation relation and you have an uncertainty relation between them.


    This can, in principle, also be done for light, using very small devices (presumably made of nanotubes). You could think using a very large array of these devices to detect light coherently (so that you keep the phase information). You can then operate a telescope just like we can operate radio telescopes. You can record the data from different telescopes on a computer and then combine them to do interferometry.

    With ordnary telescopes, this is not possible and you have to do interferometry with the light from the different telescopes itself. The moment you detect the light, the phase information is lost.
     
  10. Jun 24, 2009 #9
    Here is an article by the American Institue of Physics regarding lightwave antenna development using nanotubes by Boston College:
    http://www.aip.org/pnu/2004/split/701-1.html [Broken]
    I understand they have made progress since this publication.
     
    Last edited by a moderator: May 4, 2017
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