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Ultra-high- silica microtoroids are fabricated through a series of three steps. (a) Oxide is lithographically defined and etched using buffered oxide etchant, forming circular oxide pads of controllable diameter; the silicon is then isotropically etched using xenon difluoride gas, creating high- silica microdisks. (b) The oxide disks are reflowed using a laser, forming the UHQ microtoroids. Power is coupled into and out of the microtoroid resonator using tapered optical fibers.
The ultra-high- silica microtoroid is coupled to the fiber taper waveguide. After being immersed in either or , a coverslip is placed on top.
Transmission spectra of (a) a highly undercoupled microtoroid resonator in at . The inset shows a single high- resonance (black line) with lorentzian fit (red line). (b) The transmission spectra of a microtoroid resonator in at bands. In this spectra, the resonator is also undercoupled but closer to being critically coupled.
(a) Quality factors measured and predicted in the band plotted versus toroid major diameter. increases with major diameter over the range of diameters wherein radiation loss is the dominant loss mechanism. It then plateaus at values set by absorption of the aqueous environment. Above data taking is unreliable due to laser-linewidth stability limitations. The maximum quality factor achieved in was and in was . (b) Quality factors measured and predicted in the wavelength band. Both the radiation-loss-limited (small toroid diameter) and aqueous-absorption-loss limited regimes ( plateau) are apparent. The measured absorptive-loss limits are (in ) and (in ). (c) Quality factors measured and predicted in the band. In , the maximum quality factor achieved is . By changing to , the maximum quality factors increased to .
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