Questions about cascading process of SHG and SFG

[AniviaBai]

Hi Dr. Karpowicz,

Thank you very much for this amazing work, it really helps me on my experiments a lot! However, I encountered a difficulty while trying to simulate the following sequence: "SHG first in one BBO crystal, SFG later in another BBO.".

I simulated the SHG process 800.0 nm (o)+ 800.0 nm (o) = 400.0 nm(e). The next step I want to do is the conversion of the 400 nm (e) light into 400 nm (o) and then the SFG process 800.0 nm (o) + 400.0 nm (o) = 266.7. nm (e).

I found that "Rotate (90)" cannot change the polarization of the SH while maintaining the polarization of the fundamental light unchanged. The solution I can think of at present is to use a spectra filter to separately obtain the remaining FH and the generated SH, and then introduce them as two new pulses. However, this method seems overly cumbersome. Is there a better approach?

By the way, in the experiment, I can achieve a better SFG efficiency by changing the time delay between the two beams using a delay line. I wonder if this be achieved in simulation.

Many thanks for this impressive work and looking forward for your reply.

Sincerely,
Anivia Bai

[NickKarpowicz]

Hi Anivia,

the most straightforward way to do that is how we actually did it in the lab when we wanted to change the relative polarization of the second harmonic and fundamental: use a thin waveplate that is full-wave at the second harmonic, and half-wave at the fundamental. You could for example rotate by 45, introduce a thin quartz piece, and then rotate 45 again before the second crystal.

We've actually used similar systems like this in two-color high harmonic generation in an attosecond beamline. The time-delay (which you also noticed matters!) there was controlled with yet another thin quartz piece, where the second harmonic went purely on the fast axis, and the fundamental on the slow axis. This crystal should come before the waveplate.

Would this kind of physical optic do what you want, or do you need to introduce a pure polarization change?

Another option would be this:
polarizer(45)
rotate(-45)
filter(600, 2000, 4, 1.4142, 0)

Which will project the two polarizations onto each other and correct the amplitude with a very wide filter with a sqrt(2) multiplier.

I don't have something in there at the moment to shift the timing by hand, unfortunately, but maybe the trick of using a thin quartz plate will also work for you?

Best wishes,
Nick