Dear Cheng Wei Hsu,

There are some reasons possible that might need checking:

1) Do you have data on the pulse quality of your laser? Looking at the wavelength this seems like a Yb based laser. The duration seems very short for this material, so I am guessing that you have a laser system which works in a nonlinear regime. This might indicate either some pre- or postpulses are present or there is a (temporally long) pedestal beneath the short feature of the laser - the energy within these features is not useable for the SHG process and thus will reduce conversion efficiency when efficiency is in the pump depletion regime.

2) Does the laser have a high average power (several watts)? That might be - depending on the BBO residual absorption - leading to heating of the crystal. Heating will be more in the center of the beam....that leads to change of the local Phase-Matching conditions and the efficiency will drop. Try to decrease average power without changing all other parameters to gain insight here (pulse picker of the laser?).

In my opinion - and if the highest possible bandwidth is not the goal - using a longer crystal at a lower peak intensity is the better approach to reach high conversion efficiency when dealing with high average power.

Hope this helps and good luck!

Marcus

Hi Cheng,

I suppose you would not only like to reach a high conversion efficiency, but also keep a good pulse shape in space and time. This is not so easy. I am afraid, at 1.6TW/cm2 you are ovepumping quite strongly, and the pulse shape is considerably distorted. If you really want to understand what is going on in your crystal and get an impression of an optimum pumping, I would recommend you to dounload the free SNLO software by Dr. Arlee Smith (http://www.as-photonics.com/snlo). It is quite good and easy to understand and work with. You can even make a movie of your pulse and the second harmonic propagating through the crystal.

Best regards,

Alexander

Dear Cheng Wei Hsu,

According to: D. N. Nikogosi︠a︡n, Nonlinear Optical Crystals: A Complete Survey (Springer-Science, 2005) the bulk damage threshold for 800 nm 25 fs is higher than > 3.4 TW/cm^2.

I have simulated SHG of 50 fs (pulse FWHM), 130 uJ, w0 = 2.4 mm (27 GW/cm^2) with hussar software (http://ufs.edu.pl/index.php?article=hussar&lang=en , https://www.nature.com/articles/srep42889) and I’m getting 11.2 % with ideal Gaussian time/space profiles. When going down to w0 = 0.32 mm (1.62 TW/cm^2) I see some back conversion to 1030 nm so the 515 nm beam becomes a flat top (possibly to tight focusing) but anyway I’m getting > 65 %. This optimistic result comes from the fact that I’m taking ideal Gaussain profiles (with Hussar you can take your measured beam profile and spider/frog or at least experimental spectrum). I can also see some self/cross-phase modulation already present but maybe still acceptable.

I agree with @Marcus Beutler, you could check for thermal effects by observing the power of 515 nm just after crystal illumination with 1030 nm beam. The thermal effects take several seconds/minute to stabilize. Also I agree that a longer crystal in this case, however, from 0.55 mm up your SH pulse will become longer than fundamental (50 fs) due to GVM.