Shashanka P. Ashili

Optical coupling and transport phenomena in chains of spherical dielectric microresonators with size disorder

The optical transmission properties of chains or circuits of touching polystyrene microspheres with sizes in the 3–20μm range and a size dispersion of ∼1% are studied. The dye-doped spheres with fluorescent peaks due to whispering gallery modes were attached to one end of the chains. The effects of optical transport were detected using spatially resolved scattering spectroscopy. The attenuation was shown to be ∼3 to 4 dB per sphere for the modes with the best transport properties. A mechanism for the observed transport is suggested based on the formation of strongly coupled photonic modes in the systems of randomly detuned resonators with size disorder. It is shown that such circuits possess broad bandpass waveguiding characteristics essential for applications in integrated all-optical network devices.

Optical Transport Phenomena in Coupled Spherical Cavities

The efficiency of optical transport is studied in one-dimensional (ID) chains and in 3D lattices of coupled microspheres with ~1-3% size disorder. To couple light into such structures we used sources of light formed by dye-doped fluorescent microspheres. Using techniques of spatially resolved scattering spectroscopy we observed large propagation losses (~ 3 dB per sphere) along the chain at the frequencies of whispering gallery modes (WGMs) in the source sphere. Away from the resonance with WGMs we observed much smaller losses (< 1 dB per sphere) due to formation of nanojet-induced modes. The propagation of light in 3D lattices of disordered coupled cavities with WGM resonances is interpreted in terms of percolation theory. In transmission spectra of such 3D structures we observed spectral signatures of strong coupling between multiple spheres with nearly resonant WGMs. The results indicate that the transmission properties can be significantly improved in 3D structures formed by more uniform spheres due to achieving an optical percolation threshold for WGM transport.

Integrated Circuits of Coupled Mircospheres for Optoelectronics Applications

The paper is devoted to theoretical modeling, fabrication, and advanced spectroscopic characterization of circuits of microspheres with high quality whispering gallery modes (WGMs). The discussion of results is based on three mechanisms of optical coupling between spherical cavities: (i) resonant coupling between WGMs in identical cavities, (ii) Fano resonance between a discrete energy state (true WGM excited in the source sphere) and a continuum of quasi-WGM states with distorted noncircular shape which can be induced in the receiving cavity, and (iii) photonic nanojet mechanism. For the case of identical cavities we present numerical modeling results demonstrating a good impedance matching, and a tunable nanosecond delay for propagating pulses. We experimentally studied optical coupling in size-mismatched bispheres with controllable inter-cavity separations, and showed the validity of mechanism based on Fano resonance. Finally, we observed photonic nanojets in extremely long chains of fluorescent microspheres with propagation losses less than 1 dB per sphere

Percolation of light in 3D lattices of coupled microspheres

The propagation of light in systems of disordered coupled cavities with whispering gallery resonances is interpreted in terms of percolation theory. The existence of well connected clusters is demonstrated in scattering spectra of such lattices.

Optical coupling between spherical dielectric atoms

It is shown that dielectric microspheres with strongly detuned whispering gallery modes can be effectively coupled. The transition from strong to weak coupling is demonstrated as a function of separation between nonidentical microspheres.

Optical coupling and transport phenomena in long chains of slightly disordered spherical microresonators

It is shown that weak optical coupling between polystyrene spherical microresonators with size disorders allows creating optical waveguides with attenuation ~2-3 dB per sphere, which can be evanescently coupled to resonant sources of light and used in chip-scale photonic circuits.