Optical microcavities confine light at resonant frequencies for extended periods of time and fundamentally alter the interaction of light with matter. The wavelength-scale optical confinement and low optical loss of nanophotonic devices dramatically enhance the interaction between light and matter within these structures, making them the basis of numerous applied and fundamental studies, such as cavity QED, nonlinear photonics and sensing. Optical microcavities can be characterized by two key quantities: an effective mode volume Veff, which describes the per photon electric field strength within the cavity, and a quality factor Q, which describes the photon lifetime within the cavity. The quality factor Q is defined as Q ≡ ωo/δω = ωoτ/2, in which ω o is the resonant angular frequency, δω is the resonance linewidth, and τ is the photon lifetime. Cavities with a small Veff and a high Q offer an ideal building block for sensing applications, as any minute change in the vicinity of the cavity can be reflected in the resonance frequency shift or the quality factor change. Chip-based devices are particularly appealing, as planar fabrication technology can be used to make optical structures on a semiconductor chip that confine light to wavelength-scale dimensions, thereby creating strong enough electric fields that even a single photon can have an appreciable interaction with matter.
Despite their good performance in the infrared (IR) region, Si resonators suffer from significant material loss in the visible range of the optical spectrum, making them unsuitable for visible light applications. Unlike silicon, low-Si LPCVD Si3N4 offers a very low material loss throughout the optical range (wavelengths from 300 nm to several microns), and a moderately high refractive index (n ≈ 2). Si3N4, being a dielectric material, does not suffer from free carrier absorption, which is an important limiting factor in Si high-Q resonators. This makes the fabrication process more straightforward by eliminating the complicated surface treatment post processes necessary for high-Q silicon resonators [2]. On the other hand, having the absorption band edge located at approximately 300 nm wavelength, two photon absorption is not a limiting factor at high intensities at wavelengths larger than 600 nm. Thus, provided that the Si3N4 layer is optically isolated from the lossy Si substrate, it can guide the visible light without significant loss in this wavelength range. Nevertheless, most of the research on silicon nitride has been concentrated on infrared (IR) [9, 10, 11, 12, 13, 14] or near infrared (NIR) [15, 3] applications. The few reported works on Si3N4 photonic crystals [16, 17, 18, 19, 20, 21] and microring resonators [22] in the visible range were of considerably lower fabrication quality compared with their Si counterparts.
In this thesis, monolithic, high Q, compact Si 3N4/SiO2 resonators are demonstrated in the visible range, and critical coupling of the resonators to in-plane waveguides is also demonstrated (at λ=652–660 nm).
In chapter 2 the theoretical aspects are investigated. The mode structure of the microdisks is investigated, and the coupled mode theory is used to calculate the coupling between the resonators and the waveguides.
Details of fabrication of the devices are explained in section 3. The first demonstration of ultra high Q resonators is presented in 4.
The characterization setup and the experimental results are discussed in section 4.2. Coupling of high Q resonators to adjacent waveguides and its optimization are covered in section 4.3.
In chapter 5 a new coupling scheme for single mode coupling is presented. It will be shown that if the bus waveguide wraps around the microdisk, the coupling to only one of the microdisk modes will be enhanced. The structures presented in 4 and 5 will be integrated with micro/nano fluidic channels in 6.
In section 7 the high resolution superprism spectrometers are investigated. Additionally, AWG spectrometers will be compared with superprisms in 8. The AWGs will be cascaded with microring resonators. This leads to wide-band, high-resolution spectrometers operating in the visible range.
In part 9 the possibility of fabrication of high Q and small mode volume photonic crystal cavities is presented. And finally in 10 it is shown that the structures shown all through the thesis can work without thermal perturbations. (Abstract shortened by UMI.)
