I have gone through some of the articles on whispering gallery modes which talks about exciting the modes through prism coupling, taper fiber and others.. But i could not find any article on coupling through plane wave although in some places it's mentioned that it's difficult to do.
So, my question is, "Is it theoretically not possible to observe it through plane wave excitation" also is there some dispersion relation [as in case of Surface plasmon polaritons(see Stefan A. Maier for ref.) ]which prohibits it's excitation by plane wave?
This may be the simple answer you are looking for. A whispering gallery mode is confined to the resonator through total internal reflection (TIR). Think of a plane wave incident on a planar interface at greater than the critical angle. It is not possible to excite this wave from the outside. It would refract to an angle less than the critical angle. So illuminating a whispering gallery resonator from the outside with a plane wave is not possible. It would couple into a wave inside the resonator that is not a whispering gallery mode.
@Raymond Rumpf
I think i got your point. But what about it's dispersion relations?
It would be very helpful if you could suggest some Article on it.
The dispersion relation is a concept behind what I said earlier. The resonator has a higher refractive index and the dispersion relation shows that a rapidly varying field is allowed. Outside of the resonator is a low refractive index and the dispersion relation does not allow the field to vary as rapidly. This mismatch makes it so that a plane wave cannot excite the guided mode without the help of a prism, grating, etc.
If by plane wave you mean a beam-like field -- then I think, there is nothing prohibiting excitation fundamentally -- however, all coupling schemes you mentioned share one thing in common: evanescent, highly localized and therefore intense fields,
I think this is just efficiency argument
@Roman Actually you are right i have now figured it out it's not that plane wave do not excite wgm It's just that they are not so efficient. Because we get WGM in scattering profiles(pg 300 "bohren and huffman") of spherical resonators calculated using Mie theory which considers a plane wave incidence.
Anyways thank you for adding your answer.
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