Welcome to Laser Pointer Forums - discuss green laser pointers, blue laser pointers, and all types of lasers

How to Register on LPF | LPF Donations

Little help to understand SHG

L

louis54000

0
Joined
Jul 8, 2012
Messages
45
Points
0
Hi guys!
I need help to understand how SHG occurs in KTP crystals.
So far this is what I got :
In the crystal : the pump wave (light) creates a polarization wave twice it's frequency (due to non linearity). This wave then creates an electromagnetic field. Is this electromagnetic field considered as light (with double frequency)? I'm not quite sure I understand what's going on there.
And I read that it was essentially two photons "merging" into one with twice the energy but I don't know where that happens in the explanation I just gave.
If you guys could explain me how this works that'd be awesome !

I need this to explain how DPSS lasers work :)

Thanks !
 
Last edited:





yes I read these pages several times already (Wikipedia has a lot of complicated math in it)...
I wrote what I understood but I'm not sure it's correct. That's why I'm asking for help.
 
well once again, I already know all these links...
I already did a lot of research. All of this is not in my native language, and I only kindly asked for someone to explain me simply, so I have an overview of how it works.
Sams Laser faq helped me a lot for the global understanding of dpss laser, but I didn't find an easy explanation of the physics behind SHG in these crystals.
So I'd would really appreciate if someone could help me out, without bringing me links.. I really did a lot of research before asking here :)
Thank you!
 
Last edited:
apparently you did not read : Second-harmonic generation - Wikipedia, the free encyclopedia

The first paragraph clearly says :

"Second harmonic generation (also called frequency doubling or abbreviated SHG) is a nonlinear optical process, in which photons with the same frequency interacting with a nonlinear material are effectively "combined" to generate new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons."

That is as simple and as clear and overview as is possible to have.
 
Okay, I know that. This is not the physical mechanism described here (other link you gave):
The Physical Mechanism
The physical mechanism behind frequency doubling can be understood as follows. Due to the χ(2) nonlinearity, the fundamental (pump) wave generates a nonlinear polarization wave which oscillates with twice the fundamental frequency. According to Maxwell's equations, this nonlinear polarization wave radiates an electromagnetic field with this doubled frequency. Due to phase-matching issues (see below), the generated second-harmonic field propagates dominantly in the direction of the nonlinear polarization wave. The latter also interacts with the fundamental wave, so that the pump wave can be attenuated (pump depletion) when the second-harmonic intensity develops: energy is transferred from the pump wave to the second-harmonic wave.

It's the end I underlined that I don't fully understand.

I need to go more in depth than simply saying that 2 photons combine.
 
Last edited:
I think the tidbit you're missing is that photons are perturbations in an electromagnetic field. Remember the dual nature of light; particle and a wave. That twice frequency field that develops is the new shorter wavelength photons. Energy is conserved by transference from pump to harmonic.
 
Yes! You're right! I think (hope!) I got it now. That's what I didnt fully get! So it's the frequency of the new electromagnetic field that "dictates" the wavelength of the photons right? And as energy is conserved and if a photon as twice the frequency it has twice the energy, then it needs two "pump photons" to get one "harmonic photon" hence the "two photons combined in one" short explanation?
Thanks for helping! Much appreciated :)
 
louis54000 said:
Yes! You're right! I think (hope!) I got it now. That's what I didnt fully get! So it's the frequency of the new electromagnetic field that "dictates" the wavelength of the photons right? And as energy is conserved and if a photon as twice the frequency it has twice the energy, then it needs two "pump photons" to get one "harmonic photon" hence the "two photons combined in one" short explanation?
Thanks for helping! Much appreciated :)
Click to expand...

MAybe this will help you, also.

"When an optical wave, e.g. a laser beam, propagates in a medium, the associated electromagnetic field generates patterns of electric and magnetic polarization of the medium which propagate together with the generating optical wave. In most cases of propagation in transparent media (e.g. laser crystals or glasses), the electric polarization wave has a much stronger effect than the corresponding magnetic wave, because the electric susceptibility is much higher than the magnetic one. The interaction of the electric polarization wave with the electromagnetic wave usually reduces the phase velocity of the combined phenomenon below the vacuum velocity of light; the refractive index is the factor by which the phase velocity is reduced. Even though the wave propagation can still be described with Maxwell's equations, amended with some electric susceptibility, what propagates in the medium is actually more than only an electromagnetic wave.

For small electric fields, such as usually occur when lasers are not involved, the electric polarization is precisely proportional to the electric field strength. At high field strength, such as occur particularly in intense laser pulses, a significant nonlinearity of the polarization can occur, which can be described as additional polarization components proportional to higher integer powers of the electric field strength. Although the even orders of nonlinearity do not occur in most media due to certain symmetries, there are nonlinear crystal materials where the electric polarization also has a component proportional to the square of the electric field. In that case, a single laser beam also generates a nonlinear polarization wave with twice the optical input frequency, which propagates with the phase velocity of the input beam. That nonlinear polarization wave then radiates another optical field at that frequency. This is the phenomenon of frequency doubling, which usually becomes strong only when the radiated second-harmonic wave has the same phase velocity as the nonlinear polarization wave (→ phase matching)."

from: Encyclopedia of Laser Physics and Technology - polarization waves, nonlinear polarization, frequency conversion
 
Last edited:
Alright, thanks for that.
In this case,
electric polarization also has a component proportional to the square of the electric field.
Click to expand...
we just need to use this trigonometry identity : sin²(wt) = 1/2-cos(2wt)/2 and there we got it then.
Thanks again for your input.
 
louis54000 said:
Alright, thanks for that.
In this case, we just need to use this trigonometry identity : sin²(wt) = 1/2-cos(2wt)/2 and there we got it then.
Thanks again for your input.
Click to expand...

No idealized symbolic representation either in word symbols or math symbols is somthing that actually happens in the real world. How detailed a representation needs to be depends upon what you want or need to do with it and why.

This paper will give you a better understanding of SHG, I think: http://users.uj.edu.pl/~ufdzierz/PracFot/SHG_Dood.pdf

Maybe study/see: http://www.coqus.at/fileadmin/quantum/coqus/documents/Bahaa_Saleh/CH21-Nonlinear-Optics.pdf
 
Last edited:
Thanks, I'll have a look. Well I need to explain for 10 minutes green DPSS lasers. Therefore I need an overview of how SHG works, I don't need to go in depth (it's not a 1hr lecture, I wouldn't have time).
So I think with everything on this topic it should be ok.
Now I should put some calculations in my presentation but I really don't see what I could calculate that's not too complicated... and doesn't take half my presentation...
 
I liked this article:

http://photos.imageevent.com/qdf_fi...vecommunications/irledandlaserproject/nlo.pdf

I'm searching for information regarding what happens if you use 810nm, or to throw a number out there, 815nm instead of 808nm, how far from 808nm can the pump be off wavelength/frequency into the vanadate and then KTP before the 532nm output either doesn't work, or begins to drop off by 10 percent? Which crystal suffers the worse conversion efficiency when off freq, the YVO4 vanadate or the KTP, or do they both have similar off-wavelength inefficiencies?

Maybe I'm looking at this wrong, for example, if you shoot 1070nm into a LBO or KTP crystal you just end up with half the wavelength out or 535nm? Are all "532nm" DPSS laser pointers really 532nm?
 
Last edited:
You must log in or register to reply here.


Back
Top