Impact of nanoparticle-induced scattering of an azimuthally propagating mode on the resonance of whispering gallery microcavities

Abstract

Optical whispering gallery microcavities with high-quality factors have shown great potential toward achieveing ultrahigh-sensitivity sensing up to a single molecule or nanoparticle, which raises a huge demand on a deep theoretical insight into the crucial phenomena such as the mode shift, mode splitting, and mode broadening in sensing experiments. Here we propose an intuitive model to analyze these phenomena from the viewpoint of the nanoparticle-induced multiple scattering of the azimuthally propagating mode (APM). The model unveils explicit relations between these phenomena and the phase change and energy loss of the APM when scattered at the nanoparticle; the model also explains the observed polarization-dependent preservation of one resonance and the particle-dependent redshift or blueshift. The model indicates that the particle-induced coupling between the pair of unperturbed degenerate whispering gallery modes (WGMs) and the coupling between the WGMs and the free-space radiation modes, which are widely adopted in current theoretical formalisms, are realized via the reflection and scattering-induced free-space radiation of the APM, respectively, and additionally exhibits the contribution of cross coupling between the unperturbed WGMs and other different WGMs to forming the splitting resonant modes, especially for large particles.

Advances of Optofluidic Microcavities for Microlasers and Biosensors

“…Even if a single bio-particle of nanoscale is locally attached to the cavities’ surface, it would also rearrange the electric field distribution because of highly enhanced light-particle interaction. Recently, much research attention has been put on the WGM-based detection of single particles, such as virus, DNA and single proteins [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 ]. Some reviews had made detailed descriptions of the theories and the recent biosensing applications of optical microresonators [ 27 , 28 , 67 , 68 ].…”

“…An external light source (white source or tunable laser) is used to couple photons into the cavities by the taper fiber or the waveguide. By monitoring the shift or splitting the resonant peaks of transmission spectra, analyte concentration or molecule attaching can be detected [ 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 ]. As the optical confinement elements, the cavities with high Q factor would greatly enhance the light-matter interaction and would result in high sensitivity.…”

Three-Dimensional Simulation of Particle-Induced Mode Splitting in Large Toroidal Microresonators

“…This method is less sensitive to the environment, but the optical properties and size of the perturbing particle cannot be uniquely determined by the spectral measurement alone. Finally, sensing based on mode-splitting makes use of both resonance frequency and linewidth changes of pairs of degenerate WGMs, i.e., the clockwise (CW) and the counter-clockwise (CCW) modes that share the same resonance frequency and field distribution in the unperturbed resonator but travel in opposite directions [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. Unlike the other modalities, mode-splitting can size particles based on spectral features alone and is robust to environmental temperature and pressure variations, which do not induce mode-splitting.…”

Whispering gallery mode microlaser based on a single polymer fiber fabricated by electrospinning

“…Among all of these resonators, WGM confines the light near a circular ring boundary by total internal reflection. In addition, WGM reasonable microcavity has been extensively studied due to the advantages of inherently high quality (Q) factor, low mode volume, enhanced optical field, relatively simple fabrication, very narrow spectral linewidth, and a large optical density [3][4][5][6][7][8]. In 1961, WGM microspherical laser resonator was firstly observed by C. G. Garrett in the spherical sample of CaF2: Sm ++ [9].…”