Ultra-high Q Microtoroid 2.1 Microtoroid Resonator Overview

The silica microtoroid is an ultra-high Q (UHQ) and ultra-small mode volume microcavity fabricated on silicon using standard microelectronics techniques. Although silica microspheres have shown higher quality factors than microtoroids (8×10 compared to 5×10), their geometry and fabrication method present practical limitations [32]. The physical dimensions of a microsphere are difficult to control during melting, since there is no physical stop for the surface-tension induced reflow. Secondly, the microsphere’s mode spectrum is more dense and complicated than the microtoroid, because the optical mode is not restricted in azimuthal and vertical degrees of freedom as the microtoroid is [33]. These challenges, and lack of planar integration of the microsphere in a compact package, led to the invention of the microtoroid [10]. The microtoroid is the first microcavity that offers ultra-high quality factor on silicon. The highest Q factor recorded in a microtoroid to date is 4 × 10, which corresponds to a cavity finesse (F = λQ πnD ) of 1 × 10 . Also, the microtoroid’s cavity dimensions can be accurately controlled during fabrication to produce the desired resonator. For instance, small diameter toroids are needed for cQED experiments, where as larger toroids are important for laser operation in water. A SEM image of a typical silica microtoroid is shown in Figure 2.1, with 60 μm major diameter (D) and 5 μm minor diameter (d). After fabrication, the microtoroid can be described as a glass ring cavity with a dumbbell-like cross section, suspended over a silicon pillar by a silica membrane. In microtoroids, like optical fiber and microspheres, the silica is amorphous. Alternatively, crystalline quartz rods have been carefully polished into WGM resonators with ultra-high Q (5 × 10) [34]. The advantage of amorphous silica is that it can easily be melted or drawn into the desire shape, though care should be taken not to freeze significant refractive index variations in the glass that cause scattering loss. Input light, for example from a fiber taper, orbits the microtoroid confined by TIR until it is absorbed, scattered out, or coupled out by another waveguide.