Optical Microresonators Based on Tapered Optical Fibers

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We aim to realize high Q microresonators in structured tapered optical fibers. The resonators have a highly prolate shape. They sustain whispering gallery modes which exhibit, in contrast to the equatorial modes commonly used in silica microsphere resonators, two spatially well separated caustics with a resonantly enhanced field strength. The position of the caustics strongly depends on the number of revolutions the closed optical path corresponding to the resonator mode carries out around the resonator axis. Different modes can therefore be selectively excited by coupling light in and out of the resonator at the respective caustic. This will be accomplished by means of auxiliary tapered optical fibers using frustrated total internal reflection. Moreover, the free spectral range of our so-called "pendulum modes" is significantly smaller than for equatorial whispering gallery modes. Therefore, a tuning of the pendulum modes over one free spectral range can easily be achieved. The advantageous mode geometry of the resonators, in combination with their tunability, opens interesting perspectives for confining and controlling light. As a first application we aim to couple organic molecules to the resonator modes and observe a modification of their fluorescence.