We theoretically study propagation of light in a vertically coupled whispering gallery mode resonator (WGMR) waveguide consisting of a chain of disc WGMRs etched on the surface of a cylinder made of an optically transparent material. The waveguide is capable of reducing the group velocity of light by as much as a factor of a billion, is much more efficient than usual coupled resonator optical waveguides, and compete with slow light atomic systems. We discuss practical as well as fundamental advantages and disadvantages of the resonator and atomic delay lines.
We theoretically study propagation of light in a vertically coupled whispering gallery mode resonator (WGMR) waveguide consisting of a chain of disc WGMRs etched on the surface of a cylinder made of an optically transparent material. The waveguide is capable of reducing the group velocity of light by as much as a factor of a billion, is much more efficient than usual coupled resonator optical waveguides, and compete with slow light atomic systems. We discuss practical as well as fundamental advantages and disadvantages of the resonator and atomic delay lines.
Suitably configured whispering-gallery-mode (WGM) optical resonators have been proposed as delay lines for trains of short light pulses. Until now, it has been common practice to implement an optical delay line as a coiled long optical fiber, which is bulky and tends to be noisy. An alternative has been to implement an optical delay line as a coupled-resonator optical waveguide (a chain of coupled optical resonators), which is compact but limits the width of the pulse spectrum to the width of an optical resonance and thereby places a lower limit on the duration of a pulse. In contrast, a delay line according to the proposal could be implemented as a single WGM resonator, and the pulses delayed by the resonator could be so short that their spectral widths could greatly exceed the spectral width of any single resonance. The proposal emerged from theoretical and experimental studies of the propagation of a pulse train in a WGM resonator. An important element of the theoretical study was recognition that the traditional definition of group velocity in effect, the velocity of a single pulse comprising a packet of waves propagating in a medium, the responsivity of which is a monotonous function of frequency does not necessarily apply in the case of a WGM resonator or other medium having a spectrum consisting of discrete resonance peaks at different frequencies. A new definition of group velocity, applicable to a train of pulses propagating in such a medium, was introduced and found to lead to the discovery of previously unknown features of propagation. Notably, it was found that in a micro - sphere optical resonator that supports a suitable combination of WGM modes, the group velocity for a train of light pulses could be positive, zero, or negative. A positive group velocity could be so small that the delay could be much longer than the ring-down time of the resonator; a delay of such great length is impossible for a single pulse interacting with either a linearly responding lossless resonator or a coupled- resonator optical waveguide.
Quantum information is the next frontier in communications. to realize quantum communications, the quantum mechanical properties of today's best communication medium, light, must be harnessed in a scalable and efficient manner. Whispering-gallery mode resonators (WGMRs), a type of optical cavity, have advantages over traditional designs that can enhance processes used in the generation of nonclassical (quantum) states of light. In particular they reduce the power threshold for intensity-dependent nonlinear phenomena. One such process, second harmonic generation, can reduce the shot noise of light below the standard quantum limit. This dissertation explores the theoretical analysis and experimental tests of noise reduction through second harmonic generation in a crystalline whispering-gallery mode resonator. We also observe the generation of another nonlinear optical process, hyper-Raman scattering, at modest optical powers inside a crystalline WGMR. The change in optical properties of vanadium dioxide due to an optically-induced phase transition is also studied as a potential Q-switching material in a WGMR-type cavity.
Slow light and Whispering Gallery Mode Resonators are two interesting and useful fields with applications in optical sensing and laser stabilisation among others. Slow light can lead to long interaction times which are useful in light-matter interactions such as sensitive non-linear optics. Whispering Gallery Mode Resonators have small mode volumes and strong beam confinement leading to high Quality Factors and intense trapped light. Whispering Gallery Mode Resonators exhibit very narrow resonance linewidths which are beneficial in frequency sensitive measurements. This thesis uses a Praseodymium REI doped Yttrium orthosilicate crystal, formed into a disc shaped Whispering Gallery Mode Resonator. The aim is to design a setup which can be used to combine the two fields, slow light and Whispering Gallery Mode Resonators, in an attempt to achieve ultra-high Quality Factors and ultra-narrow resonance linewidths. The thesis details the experimental mode coupling procedure learnt from a group at Max Planck Institute for the Science of Light in Erlangen, Germany. The thesis provides a foundation for Whispering Gallery Mode Resonator experimentation in the Quantum Information group in Lund, where the thesis work was undertaken. The resulting knowledge gained is applied to design a setup holder for use in a cryostat, which provides the necessary temperatures to create slow light structures. Two beam alignment setups are constructed and used to couple into the Whispering Gallery Mode Resonator at room temperature. The first of the two setups (free beam) was difficult to operate and control over the beam alignment and focal spot shape was poor. The resulting mode coupling had a low efficiency. The second setup (fibre coupled) provided a more user friendly setup with good control over the beam alignment and focal spot shape. Results using this setup were taken for a variety of wavelengths between 593.1 nm and 610.4 nm to investigate the effect of the ions wavelength dependent absorbance, on the losses. Due to the low efficiency of the coupling in both regimes it was not possible to isolate the cause. A mismatch of focal spot size and shape is suggested as a potential area that introduces loss and therefore requires more extensive investigation. With improved beam alignment, in addition to the focal spot geometry, it should be possible to perform the absorbance measurements again in order to determine the contribution. Suggestions for further measurements including the surface roughness and mode simulations are made in an attempt to optimise the mode coupling regime to be able to accurately estimate the Quality Factor of the Whispering Gallery Mode Resonator. This is critical in order to observe changes in the Quality Factor value at cryogenic temperatures, which can be attributed to slow light effects alone.
We theoretically describe and study coupling between two whispering-gallery-mode microcavities through a side coupled fiber taper. The two microcavities can couple to each other bidirectionally with only one fiber taper waveguide due to the modal coupling between a pair of whispering-gallery modes traveling in opposite directions. By choosing appropriate physical parameters, some interesting spectra, such as electromagnetically-induced transparency-like coherent transmission and band-like reflection, can be obtained in such a system. Slow light is discussed in this paper as a direct application of the coupled microresonator structure. This structure also has potential application in coupled cavity quantum electrodynamics.
Optical interference and the related effects are key elements which could exhibit the properties of both classical and quantum optical systems. Usually, the interference between different optical paths may produce intriguing effects, such as the electromagnetically induced transparent (EIT) and so on. In this work, we study the interference effects in directly coupled whispering-gallery mode (WGM) micro-resonators, in which the emergence of EIT and electromagnetically induced absorption (EIA) could be observed. We find by tuning the coupling strength between the resonators, the modulation phase may control the appearance and disappearance of EIT (EIA). Furthermore, we prove the existence of fast and slow light, and propose a scheme for effective switching between them. This work provides a new strategy for obtaining slow light and quantum storage in the WGM micro-resonators.
Asymmetrical waveguide-whispering-gallery resonator (WGR) couplings are proposed to manipulate single-photon transport properties in a hybrid system composed of a linear waveguide, a WGR and two separated two-level atoms. Tri-frequency/quadri-frequency photon attenuators and all-pass filters (caused by decoupling effects) are realized via the symmetrical/asymmetrical waveguide-WGR couplings and the phase difference induced by two atoms. The study of group delay shows slow-light and fast-light effects in this hybrid system, and dissipation effects imposing on the transmitted single photon are also analyzed.