Whispering-gallery waves have been seen in the form of matter waves for neutrons,[38] and electrons,[39] and they have been proposed as an explanation for vibrations of a single nucleus.[40] Analogies of whispering-gallery waves also exist for gravitational waves at the event horizon of black holes.[1] A hybrid of waves of light and electrons known as surface plasmons has been demonstrated in the form of whispering-gallery waves,[41] and likewise for exciton-polaritons in semiconductors.[42] Galleries simultaneously containing both acoustic and optical whispering-gallery waves have also been made,[43] exhibiting very strong mode coupling and coherent effects.[44] Hybrid solid-fluid-optical whispering-gallery structures have been observed as well.