对于紧聚焦高斯光束，如果考虑纵向成分，也就是计算倏逝场，在最大横波数中，设置为大于ewfd.k0的时候，目前对这种光束汇聚空间，或者说是相对传播方向的负空间没有很好的数值定义，所以仅能够对正空间部分，也就是发散的正空间进行仿真。所以焦点只能在边界。如果是不考虑倏逝成分，则可以焦点设置在场内，但此时有一定的误差。详细信息可以阅读该作者的其余相关文章。

你好，目前暂时没有模型可以提供。

可以尝试：1.背景场公式，例如平面波乘上虚部的幅值衰减因子；2.边界模式分析，选择那些具有合适虚部的结果，或者已知端口的分布以及相应的复传播系数，然后在用户定义端口中设定。

Hi, Dr. Yosuke
Thank you for your kindness help again. I see what you mean.
Best wishes
Sincerely,
Miao Peng

Hi Miao,
I think you have to consider if the background field feature is appropriate or not for your application. We’ve been discussing under an assumption that you can somehow use the background field formulation by doing some pre-calculations. But it may be more appropriate to use the Gaussian beam boundary condition, rather than the (domain) background. We introduced the Gaussian beam feature in the scattering boundary condition at version 5.5 but it’s only the paraxial formulation. For your high NA case, you can use the integral formula that I wrote in this blog.
Best regards,
Yosuke

Hi，Dr. Yosuke
I am sorry to bothering you again.
But I still have doubts about your solution last time.
1. In COMSOL 5.5, does the plane wave expansion formula of Gaussian beam is transmitted in vacuum by default? So when we think about transmission in other media, we need to consider the refractive index of the media, right?
2. You said, “Calculate a rough spot radius from the above result, r = 0.61*lambda/NA_vac.” If my focus is in a medium such as water or air, do I need to consider the effect of the refractive index of the medium on the spot size?
3. In “ewfd” setting, the default value of the “Focal plane along the axis” of the beam is zero, right? If so, when the focus position of the tightly focused Gaussian beam is in the water, is the calculated “focal position shift” should be directly enter in the text box of “Focal plane along the axis”?
I am very sorry to take up your precious time to answer for me again. I am looking forward to your reply.
Best wishes
Sincerely,
Miao Peng

You are welcome!
Best regards,
Yosuke