Why do I get an elliptical polarization after a 45-degree reflection in a mirror?

  • 5 July 2019
  • 2 replies

Userlevel 4

Hello, I have set up a 45-degree mirror in sequential mode, and I run into a result that I want to make sure is a physical phenomenon and not a bug in the program.

In Sequential mode, I set up collimated light hitting a the MIRROR surface in a 45-degree angle. 

When I open the Polarization Pupil Map analysis window and set the input polarization axis to 45 degrees, I expect to see 45 degree polarization.

However, as you can see, the Polarization Pupil Map shows slightly elliptical polarization.

I would have thought that metal mirrors caused a phase shift of pi both for S- and P-polarized light. I wonder if what I see in the OpticStudio results corresponds to a real physics phenomena or a bug in the program.


Best answer by Katsumoto Ikeda 7 August 2019, 04:19

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2 replies

Userlevel 4

Thank you very much for the inquiry. Short answer is, YES this is a real physical phenomenon, and it is NOT a bug. This is a reflective property of metals.

Userlevel 4

Light reflected by a metal at non-normal incidence will generally have its ellipticity changed. Our case is a 45 degree angle, so it is non-normal.

When we set the surface property to “MIRROR”, it is set as some metal surface, and retains the properties of that metal.

For an ideal reflecting surface, I can set the coating of the MIRROR surface to “I.0”, which is a non-absorbing perfect reflecting surface. I get the result your intuition tells you, that the reflected polarization is not elliptical. This ideal reflection does not have any metallic or physical properties.

If I set the coating to “GOLD”, I get the following, much worse. 

The physics behind it is a little involved, but essentially, the S and P waves for the polarization are different with respect to the incident angle for most metals.

Below for the S-wave,

Below for the P-wave.

If we plot this out for a random metal, it looks something like this:

The incident angle is Theta_1, so you can see that the two reflections are different for non-zero indicent angles. For 45 degrees, we are talking roughly where the arrow for “P” points, so the difference between S and P are small in this case. 45-degrees is usually the tipping point that this happens, it's usually much worse at larger angles. We can also see that the reflection for Theta_1 = 0, or normal incidence, is Pi as expected.

I hope this helps.