Non-traditional whispering gallery modes inside microspheres visualized with Fourier analysis

Abstract

Non-traditional whispering gallery modes are studied in a glass microsphere. Geometrical ray tracing is used to explain and calculate these modes. Thermal emission and Raman scattering are used as an internal light source to excite these modes inside the glass microsphere. The thermal and Raman emission spectra are modified due to the existence of these modes. Fourier analysis is then used to distinguish the individual modes. The understanding of these non-traditional WGM may lead to alternative design strategies for sensor applications or laser cavity configurations.

Fractional Optical Angular Momentum and Multi-Defect-Mediated Mode Renormalization and Orientation Control in Photonic Crystal Microring Resonators

“…Perturbations that disturb the geometry from the circular symmetry change this picture. For example, in deformed microcavities, geometric ray optics predicts more generalized WGMs with fractional angular momentum numbers of m/n 4,5 , that is, the optical field exhibits m × 2π phase oscillations across n round trips. Recently, such fractional angular momentum has been demonstrated in a quadrupolar microdisk 6 , in contrast to numerous free-space approaches that have been developed because of its potential applications in optical trapping, micromanipulation, and providing extra channels for multiplexing [7][8][9][10][11][12][13][14] .…”

Fractional optical angular momentum and multi-defect-mediated mode re-normalization and orientation control in photonic crystal microring resonators

“…The circular geometry dictates the behavior of the electromagnetic field in the azimuthal (φ) direction, as matching the field phase across one round trip (i.e, a φ=2π rotation) results in integer azimuthal mode numbers. By disturbing the geometry from the circular shape, geometric ray optics predicts more generalized whispering gallery modes with fractional angular momentum numbers of m/n 17,18 , that is, the optical field exhibits m × 2π phase oscillations across n round trips. Recently, such unconventional WGMs have been demonstrated in a quadrupolar microdisk whose Q was largely deteriorated due to the deformed geometry 19 .…”

“…There are many interesting questions that rise from this work on half-integer angular momentum modes in the WGM platform. An immediate question is whether this work can be extended to arbitrary fractional angular momentum (m/n, with both m and n as integers) as has been demonstrated in deformed microcavities that subscribe to a ray-optics description [17][18][19] . One possible path towards this end is to use a photonic crystal ring with multiple coherent mode couplings simultaneously 35 .…”