Question about Crystal φ of BBO

[AniviaBai]

Hi Dr. Karpowicz,

Thank you again for this amazing work, it really helps me on my experiments a lot.

I have noticed that when performing the SHG calculation using BBO, the polarization of the fundamental frequency light should always be in the y-direction, and the φ angle should be set to 30°. Only in this way can an effective result be obtained. However, when the polarization of the fundamental frequency light is set to x, no matter how the φ angle is changed, the result is always very low. I have always been curious about why this happens and whether there is any solution to it.

In the frequency conversion of femtosecond light, in practical operations, it is sometimes necessary to slightly rotate the angle of BBO to adjust the wavelength of the frequency doubling. However, in the software, the crystal seems unable to rotate. Can this operation be achieved by changing the NC angle of the incident light? Considering that only the polarization light can be set in the y direction, I attempted to fix the cutting angle θ and scan the NC angle and φ angle. However, the results of the parameter scanning are usually not convincing. If the polarization light can be set in the x direction, I think the results might be easier to understand (because the adjustment of the NC angle is limited to the xz plane only).

These are some of my confusions and I am looking forward to your reply.

[NickKarpowicz]

Hi, you rotate the crystal effectively by rotating the polarization of the input field - just set the polarization to 90 instead of 0 to put the input field on the slow axis. The theta and phi angles are independent of this, think of them as the direction of the beam (or surface normal) - that doesn't determine the polarization. If you're making a sequence of crystals, there is the rotate(angle) function which will have a similar effect to rotating the crystal...