Two Questions on Lasers

[calbiterol]

The first of my two questions regarding lasers and the like is relatively straightforward. Is there any way to make a system in which two lasers with frequencies in the non-visible range intersect to produce visible light at their point of intersection? The only way I would think this would be possible would be by having one laser in the infrared range and another in the ultraviolet, and somehow getting them to combine in a way that produces a point of visible light... I'm not that knowledgeable in physics, so I don't know how absurd that sounds (hence the question), but any answers, whether affirmative or not, would be helpful.

 

 

 

Along the same lines, given the following situation:

A contained liquid or gas which is excitable, and emits visible light when excited

What would be the best way to produce excitation using two lasers in a gid-like system? In other words, is there any way to use two lasers (or another means of excitation, like two electron beams) to excite the atoms at only the point of intersection, and not at any other point along the two laser (or other means of excitation) beams? My only ideas on this have involved something along the lines of using a specific frequency to excite the atom (perhaps its resonant frequency?) and tuning each laser to half of that frequency, so that at their intersection, the full frequency would be reached, and excitation will occur. I've thought of numerous problems with this method, though, and I am wondering if there is any other way to produce this effect. I have been pointed in the general direction of two-photon transitions, but I don't know exactly what these are (I have a very vague idea) or when/how they occur.

 

Any and all answers are helpful.

[The Rebel]

Without a third entity I think the lasers would simply pass through each other. The only reason red and green make yellow (for example), is because both red and green are visible to us.

 

I suppose a gas that is reactive to both UV and IR radiation would have to be fed into the air, what sort of gas to use though??

 

I know humiseal, detergents are good and reacting to UV.

[calbiterol]

I know humiseal, detergents are good and reacting to UV.

 

Can you explain these chemicals? Do you have any other ideas?

[swansont]

What you want to investigate is a field called "four wave mixing" If you shine two laser frequencies, w₁ and w₂, onto certain nonlinear materials, you can get the sum and difference frequencies out: w₃ = w₁+w₂ and w₄ = w₁-w₂ (assuming w₁>w₂) A special case, where w₁ = w₂ is called second-harmonic generation. You get twice the original frequency and there is no difference frequency.

 

If the materials were transparent to the frequencies involved, you would in principle have minimal absorption except at the overlap region. However, four wave mixing has certain alignment requirements; I'm not certain if the beams can be perpendicular, or have to be copropagating or have some other alignment/polarization relationships.

[timo]

I don´t want to formulate a text atm so I´ll just throw in a few thoughts of mine:

 

- As said by The Rebel photons (light beams) do not interact.

 

- The effect of creating reactions (like emmiting light) at a cetain point within a substance by pointing two lasers at this point should be possible in theory. I´d think about the two photons in the IR-range being absorbed to create an excited state. This excited state could decay back to the ground state by absorbing a single photon in the visible range. Conservation of momentum might be a problem here.

 

- I remember that some years ago some experimentalists at my former university showed me their labs. They were experimenting with holographic crystals which were supposed to be used as the new generation of memory chips (they allready were able to store and reread data back then -6 years ago- dunno why it didn´t come out as a new technology, yet). They used two lasers to create interference patterns in a crystal. The link to the group is here: http://www.physik.tu-darmstadt.de/lto/pro/index.html . Be aware that from my experience the pages of the TUD are not very informative, but you could read around in the "Glossary of frequently used terms" or maybe check their links.

 

- The idea to cast rays of something from different directions to create some effect at a certain point has at least one practical application I know of: The GSI ("Gesellschaft fuer Schwerionenforschung"="Corporation for heavy ion research", also, located in Darmstadt) in cooperation with the physicians of the university of Heidelberg are using neutron beams to destroy cancer tumors. Neutrons in matter have the very pleasant feature that they give away most of their energy within a certain range. If you now adjust your beam so that this range is within the tumor and shoot beams from different directions you have a very good ration of "energy deposited in the tumor / energy deposited in healthy cells". Or in other words: You can destroy the tumor with minimal damage to the patient.

[calbiterol]

Atheist, können Sie bitte mir helfen? Mein Deutsch ist nicht so gut, und so kann ich nicht alle von TUD verstehen. Danke sehr.

[swansont]

- The effect of creating reactions (like emmiting light) at a cetain point within a substance by pointing two lasers at this point should be possible in theory. I´d think about the two photons in the IR-range being absorbed to create an excited state. This excited state could decay back to the ground state by absorbing a single photon in the visible range. Conservation of momentum might be a problem here.

 

The decay can only occur if a single-photon transition is allowed. What momentum conservation concerns do you have?

[timo]

My momentum conservation concerns are the following:

Assume the two original photons have energy and momentum of and respectivey (c=1 for simplicity). The released photon has energy and momentum of when you assume that there´s no recoil. But as (note that the photons´ momentums are explicitely not colinear), the simple process "2 photons -> excited state -> 1 photon" is not possible in the simple form my post suggested. You have to store excess energy in the cystal or consider recoil, for example.

[timo]

Atheist, können Sie bitte mir helfen? Mein Deutsch ist nicht so gut, und so kann ich nicht alle von TUD verstehen. Danke sehr.

 

The pages are not too informative anyways. It was just a suggested start to browse. The "glossary of frequently used terms" are in english (though, they are quite short) and the links go out to english pages. I´m also not completely sure if you actually will find usefull information on the linked pages because I didn´t visit them to check it out.

[calbiterol]

Okay, thanks.

 

[Edit: Can you explain this?

 

My momentum conservation concerns are the following:

Assume the two original photons have energy and momentum of and respectivey (c=1 for simplicity). The released photon has energy and momentum of when you assume that there´s no recoil. But as (note that the photons´ momentums are explicitely not colinear), the simple process "2 photons -> excited state -> 1 photon" is not possible in the simple form my post suggested. You have to store excess energy in the cystal or consider recoil, for example.

 

I'm not very knowledgeable about physics, but I pick up on things fast. I get the general gist of that, but not the full meaning, and I'd like some help figuring out what it means.]

[swansont]

My momentum conservation concerns are the following:

Assume the two original photons have energy and momentum of and respectivey (c=1 for simplicity). The released photon has energy and momentum of when you assume that there´s no recoil. But as (note that the photons´ momentums are explicitely not colinear), the simple process "2 photons -> excited state -> 1 photon" is not possible in the simple form my post suggested. You have to store excess energy in the cystal or consider recoil, for example.

 

Yes, there's recoil. That's why laser cooling works. But h/^{[math]\lambda[/math]} is small. Recoil for a free atom is a few mm/sec to a few cm/sec with the photon in the visible spectrum. (for hydrogen it's like 3 m/sec, and that's a 10.2 eV photon and the smallest atomic mass you can have) Thermal speeds are hundreds of m/sec

[timo]

@calbiterol:

I´m not really sure what part you didn´t understand. Perhaps it´s best if you say what you understood (or at least what you think you understood) and what parts were problematic for you.

As a sidenote: Originally my post included some variables, equations and one inequality. They seem to have disappeared somehow (at least they don´t show up for me right now). Is the missing math the problem?

 

 

@swansont:

EDIT: I´ve even edited my post before posting it. Atm I also think that conservation of momentum is not going to be a problem. However, if you´re interested in knowing were my original concerns lied, then go ahead reading...

 

Yes, of course there´s recoil. And indeed for free atoms it seems quite easy to find parameters that make conservation of energy and momentum (and thus the whole process) possible. What I had in mind, however, was the process to take place in a crystal. For crystals, I think things become a bit more complicated:

 

- You need to find a a phonon state (or a combination of those, perhaps) with a fitting pair of (E, p) to ensure conservation of energy and momentum.

 

- Atm I cannot see if the accoustical branch of the phonons is going to be much of a help, because for small momentums E(k) it is also linear as for the photons. Intuitively Id think this might be a problem.

EDIT: On 2nd though this should be no problem since you can simply make the recoil happen in opposite direction as the released photon´s momentum. If you then increase the ammount of recoil beginning from zero you reduce the emiited photon´s energy and also increase it´s momentum. You can do that until conservation of energy and momentum are true. Only problem might be the quantization of the phonon spectrum but for a macroscopic crystal this should be small enough.

 

- The optical branches can easily absorb energy with no or little momentum being absorbed. This would make them suited for compensating the excess of energy. However, if the optical branch is narrow and does not include the energy required then it doesn´t really help.

 

- Combinations with more than one phonon involved are going to reduce the probability for the process to happen.

 

- A last possibility that might actually work would be giving the recoil to the crystal as a whole. Since E=P²/2m is pobably neglectible you can simply chose a recoil momentum in a way that energy and momentum are conserved. However, I remember something like "the possible momentums a crystal can take over are quantized to hbar*G", where G is a vector in the reciprocal lattice. I don´t really know why this should be because I can hardly imagine the momentums for a free particle -regardless of it´s size- to be quantized but it seems to be the case (this quantzation leads to the Moessbauer effect if I remember that correctly). So if the quanta are large enough this might also be a problem for the conservation laws.

 

 

Huh, didn´t want to write that much on this, originally. Well, once you start with it ... .

However, I´d like to emphasize again that I said "conservation of momentum might be a problem" not that the process is impossible due to conservation of momentum. It was also just a sidenote that came to my mind when I wrote my post and I didn´t bother (as I also didn´t do now) to do any calculations on it.

[swansont]

Yes' date=' of course there´s recoil. And indeed for free atoms it seems quite easy to find parameters that make conservation of energy and momentum (and thus the whole process) possible. What I had in mind, however, was the process to take place in a crystal. For crystals, I think things become a bit more complicated:

 

- You need to find a a phonon state (or a combination of those, perhaps) with a fitting pair of (E, p) to ensure conservation of energy and momentum.

[/quote']

 

Yes, there are direct and indirect bandgap materials. Some photon interactions will have lower probability in crystals.