Excitation Mechanisms of Whispering Gallery Modes with Direct Light Scattering

Year: 2021

Authors: Zambrana-Puyalto X., D’Ambrosio D., Gagliardi G.

Autors Affiliation: Ist Italiano Tecnol, Via Morego 30, I-16163 Genoa, Italy; CNR, Ist Nazl Ott INO, Via Campi Flegrei 34 Comprensorio A Olivetti, I-80078 Pozzuoli, Na, Italy

Abstract: In optics, whispering gallery modes (WGMs) are modes of light that arise in cylindrically symmetric structures. Their intensity profile is strongly confined around the interface of the structure, and their Q-factors are some of the highest ever measured with light. Here, the physical mechanisms governing the coupling of a tangential beam into a WGM of a microsphere are analytically demonstrated. For that, Mie theory and the symmetries of light are made use of. It is demonstrated that the coupling mechanism is not related to any evanescent tunnelling effect. Rather, it is shown that it has to do with the angular momentum matching between the available inner WGMs of the sphere and the angular momentum content of the incident beam. The model is valid for any homogeneous sphere, for any wavelength, and for any incident cylindrically symmetric beam, focused or not. It quantitatively predicts the optical coupling effciency to the resonator for any tangential position of the incident beam. And it sketches four different regimes of interaction depending on the beam position with respect to the sphere, properly matching experimental resonance spectra observed with free-space laser scattering.


Volume: 15 (5)      Pages from: 2000528-1  to: 2000538-10

More Information: The authors are grateful to Marco Conti, Andrea Passarella, and Claudio Cicconetti for the availability of the CNR-IIT high-performance computer facility. X.Z.-P. acknowledges funding from the European Union´s Horizon 2020 research and innovation programme under the Marie SklodowskaCurie grant agreement No 795838. D.D. acknowledges funding from Italian Ministry for University & Research (MUR) in the framework of PRIN2017 PELM project (20177PSCKT grant).
KeyWords: angular momentum, Mie theory, whispering gallery mode
DOI: 10.1002/lpor.202000528

Citations: 18
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