# Laser Antenna

• B
Say you have a red laser and pointed it at a ~175 nm antenna, then amplified the signal into a speaker. Would you then theoretically get a very high frequency audio signal (in the terahertz range, i realize this isn't practical)?

My question is really on the nature of the light emitted from the laser. My intuition would tell me the light would only cause an emf in ONE direction on the antenna, so how would you get an oscillating AC signal to create a sound. Is the EM radiation that can induce a sound signal arranged in a particular way that would give you emf in both directions on the receiving antenna?

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berkeman
Mentor
Would you then theoretically get a very high frequency audio signal (in the terahertz range
What frequencies make up the typical audio range?

What frequencies make up the typical audio range?
Ya I realize its way out of reasonable range. Lets make it 10 khz laser light.

Your nano antenna would convert the light EM wave to an electric signal just like a radio antenna. The laser light electric field alternates as with any real photon, so the EMF on the antenna will alternate the same way. Problem is this electrical signal would travel a very short distance in your "wire" so it's difficult to process in any way other than rectification, called detection in traditional radio.

davenn
Gold Member
2019 Award
Ya I realize its way out of reasonable range. Lets make it 10 khz laser light.
you don't get 10kHz laser light, as in that isnt a freq of a laser, do you have any understanding of that ?

pointed it at a ~175 nm antenna,
and what is this antenna you refer to, do you have a reference ?

berkeman
Mentor
Ya I realize its way out of reasonable range. Lets make it 10 khz laser light.
That would be the frequency of modulation of the light beam. Which type of modulation do you have in mind? AM would be the simplest to discuss, probably. Are you familiar with how rectification ("envelope detection") is used in an AM receiver? That is what @drummin is referring to in his post.

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

f95toli
Gold Member
Say you have a red laser and pointed it at a ~175 nm antenna, then amplified the signal into a speaker. Would you then theoretically get a very high frequency audio signal (in the terahertz range, i realize this isn't practical)?

My question is really on the nature of the light emitted from the laser. My intuition would tell me the light would only cause an emf in ONE direction on the antenna, so how would you get an oscillating AC signal to create a sound. Is the EM radiation that can induce a sound signal arranged in a particular way that would give you emf in both directions on the receiving antenna?
The light emitted from a laser is essentially a coherent and monochromatic signal. Hence, from an EM point of view there is no difference between this and the signal
generated by any AC signal source.

In fact, the experiment you are describing can sort-of be done (although you would of course not be able to generate sound). Very far infrared radiation is at about the frequency range when antennas become practical (THz) so when working this field you end up using a mixture of optical component (e.g. lenses) and elementsalso used at lower frequencies (log-periodic antennas). The problem is of course that signals at ~1THz range can only travel short distances on a chip so most of the time you can't really do much more than say get the average power out or use mixers to down-mix to a lower frequency (which is how radio astronomy at these frequencies are done).

Interesting.

So when a photon is created, the system that emitted it is conserving both energy (E=hf) and angular momentum (the photon's spin), is this correct?

Now lets say the antenna absorbs the photons emitted by the laser. Both the photon's energy and angular momentum must be conserved by the antenna. Now it seems like an actual EM wave would need to be made of photons arriving with alternating spins to induce both directions of angular momentum in the antenna. (i.e. some of the photons are conserving the angular momentum for the positive swing of the AC signal, the others are conserving momentum for the negative swing).

In a laser, all the photons have the same spin don't they?

berkeman
Mentor
Interesting.

So when a photon is created, the system that emitted it is conserving both energy (E=hf) and angular momentum (the photon's spin), is this correct?

Now lets say the antenna absorbs the photons emitted by the laser. Both the photon's energy and angular momentum must be conserved by the antenna. Now it seems like an actual EM wave would need to be made of photons arriving with alternating spins to induce both directions of angular momentum in the antenna. (i.e. some of the photons are conserving the angular momentum for the positive swing of the AC signal, the others are conserving momentum for the negative swing).

In a laser, all the photons have the same spin don't they?
Sorry, but this is gibberish. I realize that this is a B=Basic thread, but misinformation is still not a good thing.

Please start a Conversation with me (click on my avatar to start the conversation) to provide some of the links to what you have been reading. I'd prefer not to burn the next 10 posts in this thread trying to figure that part out.

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