Simulation of gas based THG and its output profile

[LTChou01]

Hi Dr. Karpowicz,

Thank you very much for this amazing work, it really helps me on my experiments a lot! I am wondering if the LWE can be used to simulate THG generation in gas? Since I have checked the issue you have answered about the filamentation in air, I did not find the THG.
Or would it be possible that the implementation of THG coupling is needed as shown in the section 2.4.2 of this tutorial: "Couairon, Arnaud, et al. "Practitioner’s guide to laser pulse propagation models and simulation: Numerical implementation and practical usage of modern pulse propagation models." The European Physical Journal Special Topics 199.1 (2011): 5-76." ?

Also the phenomena I encountered is the interesting conical profile of the THG, and I found an article stating the involvement of fifth order nonlinearity (Vaičaitis, Virgilijus, Vygandas Jarutis, and Dionysios Pentaris. "Conical third-harmonic generation in normally dispersive media." Physical review letters 103.10 (2009): 103901.). Therefore, I am wondering if the simulation of THG is available in LWE, can it also simulate this phenomena?

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

Sincerely,
LuTing

[NickKarpowicz]

Hi there,
there will always be a Gouy shift when solving the wave equation (except in 1D) - it's an intrinsic part of the solution to the Helmholtz equation. So it's always going to be there in LWE. THG will also happen - when chi(3) is there, there is no selection of different mixing processes, they're all included. I think maybe you didn't see the THG in the case you were looking at because it was weak? It's not phase-matched, so it doesn't build up over long propagation like the optical Kerr effect will.

Here's a simulation with tighter focusing conditions, more like the paper you referenced. I don't have a chi(5) there - instead, there's the mechanism of plasma generation, a multiphoton resonance, which is likely to be what you'll see, or at least it's a good approximation until the intensity gets into the tunneling regime.

Here you can see the conical emission (I set the blue channel to be centered at 900 THz, with a gain of 80).

Maybe use this as a starting point? One note of caution, it can be very hard to converge simulations with plasma generation and a very long propagation length, so you'll need small step sizes and a lot of patience to get a good result.

thg.zip

[LTChou01]

Hi Dr. Karpowicz,

Thank you very much for the reply! This starting point is important for me! I will check more details with LWE, and maybe share some interesting findings if I figure out the problem I encountered.

Best regards,
LuTing

[LTChou01]

Hi Dr. Karpowicz,

I would like to confirm if the calculation for the materials' refractive index has included the oscillation strength part in Sellmeier equation? Or if I can add my own wavelength-n curve into LWE? Thank you

Best regards,
Lu-Ting

[NickKarpowicz]

Hi there,
yes there is an oscillator strength included in the refractive index equations - depending on which form, it is either associated with a Lorentzian or Gaussian lineshape. You can add your own materials to a custom database by following this tutorial here: https://youtu.be/qlcy_RBLGoU

I also have an online version of the worksheet for adding materials here: https://nickkarpowicz.github.io/widgets/crystalmaker/ where you can put in complex refractive index data and do the fitting to Lorentzians or Gaussians and make the material definition to copy/paste into the CrystalDatabase.txt file where the materials are stored

Best wishes,
Nick