Dear Mohamed,

One way to achieve a cavity with CP (helicity) preserving modes is to exploit the properties of grazing reflections. As you write in your question, perpendicular reflection at an interface flips helicity completely. But this is not true as your angle of incidence changes from the normal. It turns out that, in the limit of grazing incidence, helicity is perfectly preserved due to the degeneracy of the reflection coefficients of TE and TM components. This effect is quite independent of the actual planar interfaces.

Then, the modes that bounce back and forth between two "walls" with very large angles of reflection are largely helicity preserving. Coincidentally, there is a recent publication that proposes exploiting this effect for enhancing the circular dichroism signal of chiral molecules.

Article Helicity-Preserving Optical Cavity Modes for Enhanced Sensin...

Best regards

Ivan

Ivan Fernandez-Corbaton Thanks Ivan for your answer, I really like the work in the paper you shared. I have also found another interesting solution: using a chiral mirror that " selectively reflects only one spin state of light while preserving its handedness, with a near-unity level of circular dichroism ".

https://www.nature.com/articles/s41377-020-0256-5

Hi Mohamed Ismail Abdelrahman ,

this is indeed an interesting way to realize a proper helical cavity mode by incorporating these mirrors that preserve handedness (or chirality, if you will) upon reflection at normal incidence. I have been playing with these structures and got some preliminary results kind of showing the formation of a helical mode, although there is A LOT of caveats related to polarization mixing happening inside.

Are you trying to do the same?

Hi Denis G. Baranov ,

thanks for your answer. I am exploring this idea and I agree with you that it seems challenging, especially that the 'chiral' mirror imposes many restrictions to the cavity performance.

Interesting question and good answers. Let me add to the discussion that some authors have considered "circular polarization handedness inverters" or "co-circular polarization reflectors" to address your question. Some examples:

• Article Perfect Co-Circular Polarization Reflector: A Class of Recip...

• Article Helicity-Preserving Omnidirectional Plasmonic Mirror

• Article Nanostructured optical thin films fabricated by oblique angl...

• Article Circular Dichroism Metamirrors with Near-Perfect Extinction

• Article Spin-preserving chiral photonic crystal mirror

Telecommunications and Radio Engineering, 77(12):1029-1039 (2018)