# 3d display with lasers?

jk22
Is the following realizable :

We suppose two non visible lasers whose direction and phase could be changed very quickly.

The energy of just one laser would correspond to no possible transition in the atomic spectra of the molecules present in the atmosphere of the room.

However if both lasers meet at one point in space, their constructive interference would correspond to the atomic transition of let say oxygen. Then that point would emit let say visible light ?

Were that somehow a possible basis for a technology of 3d display ?

## Answers and Replies

Staff Emeritus
Are you saying that the constructive interference would cause the frequencies to add? Sorry, it doesn't work that way.

jk22
Gold Member
Sounds like 2-photon microscopy, which is a real thing. Not counting the part about exciting O2 to get visible emission and a 3-D image, that part needs more thought. 2-photon processes are hard to do, you need very high intensity.

jk22
In fact ##\cos(at)+\cos(bt)=2\cos(\frac{a+b}{2})\cos(\frac{a-b}{2})##

So it is the average frequency. One could maybe use one UV and one IR lasers ?

Maybe the frequencies appearing due to the pulse shape contains higher frequencies ?
Or if one computes the projection of ##\cos(x/2)## over an incommensurable frequency the result is not zero over ##[0,2\pi]##, since the integration is over a finite time interval of the pulse. Could this have any effect ?

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Staff Emeritus
I think you're expressing the question awkwardly. To make a 3D laser display in the sky, you depend on the reflections from dust or smoke particles. When two beams cross, they reflect more light from the crossing point. That has nothing to to with the two beams interfering with each other.

Remember, for the display to be seen by a person, the light has to reverse direction and come back to the eye of the beholder.

There are lots of outdoor laser light shows. Research them.

Mentor
The energy of just one laser would correspond to no possible transition in the atomic spectra of the molecules present in the atmosphere of the room.

However if both lasers meet at one point in space, their constructive interference would correspond to the atomic transition of let say oxygen. Then that point would emit let say visible light ?

You might look some at "Optical-optical double resonance" which is somewhat related to what you are asking. Many years ago when displays were one of my specialties, I looked into OODR to try to do what you are asking. At the time, it wasn't practical.

Gold Member
I think the interaction of the photons and atoms is better thought of as a quantum mechanical process, with individual photons instead of classical waves. It's more about the probability of an atom (or molecule) absorbing two photons at essentially the same time. The ability of an atom to absorb or emit photons is really determined by their wavelength, and normally happens one photon at a time. That's why you got the comment above about interference not increasing the light's frequency. Interference is more about how many photons are you likely to find at the different locations.

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Staff Emeritus
Are you saying that the constructive interference would cause the frequencies to add? Sorry, it doesn't work that way.

It still doesn't work this way (using QM and considering air as the medium).

However if both lasers meet at one point in space, their constructive interference would correspond to the atomic transition of let say oxygen. Then that point would emit let say visible light ?
Were that somehow a possible basis for a technology of 3d display ?
The fact that the Earth's normal sea level atmosphere is transparent, and does not glow, suggests that the broadband radiation from the Sun does not generate significant frequency mixing and that the cold atmospheric gasses are remarkably linear.

It would require a non-linear characteristic such as ionisation before you could get frequency mixing, up or down. Maybe take a look at the polar aurora mechanisms, or the Luxembourg effect.
https://en.wikipedia.org/wiki/Luxemburg–Gorky_effect

If you could condense water on command, then you could generate a cloud pattern like in a cloud chamber. Unfortunately, adding energy to air does not increase condensation, it reduces it.

If the product of a non-linear reaction could be seen at night, the radiation produced would need to be visible to us. The local oscillators (UV and IR lasers) would need to be absorbed by an optically non-linear gas to provide the frequency mixing. The range would be limited by that same absorption by the atmosphere. You might do some very local magic given a non-linear transparent smoke.

Maybe you would have better luck generating an image in a tank of solvent. You are looking for solution chemistry that supports; IR * n = visible, or UV – UV = visible.

Gold Member
Several aspects here, essentially a stream-of-consciousness memory dump.

It so happens that if you use a tank of liquid, water with some milk added works rather well. You only need one visible laser with fast beam positioning, gating, and focusing. The fat particles in the milk act as scattering centers.

For use in the atmosphere, I recall when my wife and I were motorcycle touring and we ended up spending a few nights on the North rim of the Grand Canyon. There were no towns anywhere near so the sky was really dark. We joined a night tour that was astronomy related. The instructor/group leader used a five-cell flashlight as a pointer to celestial objects. There was enough moisture in the air that the backscatter was easily visible as a beam of light.

If you have seen searchlights used for advertising, it is the same effect.

What you are proposing requires enough energy to excite atoms or molecules to a higher energy state, which then decay to ground state and emit the excess energy as visible light. This is the way Neon signs and lamps work, and also the lasers you are proposing to use.

If you consider how much power your laser needs to operate, that will give you a vague idea of the power needed to ionize the atmosphere... a rather difficult/expensive task, I'm afraid. You would have to start with a large diameter beam and have it focus at the position you want to illuminate, or use multiple beams that converge at the target region. You have to somehow get X amount of energy to a small volume; with X being the energy needed to excite Nitrogen or Oxygen to a higher energy state. I expect that in the U.S. both the Federal Aviation Administration(FAA) and the Occupational Safety and Health Administration (OSHA, https://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_6.html) would have something to say.

If I recall correctly, there was a military research project many years ago that built a large chemical laser and mounted it in a large transport airplane. A chemical laser because that was the only way to get high enough power. They were trying to develop it as a weapon. It was functional but the effort was abandoned as impractical, I don't recall the details.

Hope it helps... and Have Fun!

Cheers,
Tom

p.s. Just remembered that there are holographic displays that project images in space. Though they are probably not suitable for your present project because they are back projection systems where the laser shines through a hologram towards the observer. The effect is a 3D image appearing to be in front of the observer, i.e. effectively the retina is the screen.

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