Ultrahigh–Q Microtoroid On-Chip Resonators for Low Threshold Microlasers

Abstract

Recently demonstrated silica toroidal microcavities, as on-chip resonant cavities, become one of the most promising laser resonators due to their exceptional ability to confine optical energy temporarily and spatially (high Q-factor and small mode volume) while being integrated on a silicon substrate. In the first part of this thesis, semianalytic theory is presented for an in-depth understanding of the high-Q toroidal microcavity coupled to a tapered fiber waveguide. Basic properties of toroidal microcavities such as cavity mode field, resonance wavelength, cavity mode volume, radiative Q-factor, and phase-matching condition are described within the limit of an iterative perturbation expansion method. With this theoretical background, various laser systems with different gain media, utilizing the high-Q toroidal microcavity as a laser resonator, are demonstrated in the latter parts. As a first example, II-VI semiconductor nanocrystal, CdSe/ZnS (core/shell), quantum dots are coated on the surface of ultrahigh-Q toroidal microcavities. By pulsed excitation of quantum dots on the surface, either through tapered fiber waveguides or free-space, lasing is observed at both room and liquid nitrogen temperature. Use of a tapered fiber coupling substantially lowered the threshold energy when compared to the case of free-space excitation. Further threshold reduction down to 9.9 fJ was made possible by quantum dot density control. Lasing from an erbium-implanted high-Q silica toroidal microcavity is demonstrated and analyzed in the next chapter. A minimum threshold power as low as 4.5 uW and a maximum output lasing power as high as 39.4 uW are obtained. Control of lasing wavelength is demonstrated by changing the cavity loading. Analytic formulas predicting threshold power, differential slope efficiency are derived and their dependence on cavity loading, erbium ion concentration and Q-factor is found and compared with the experimental results. The nonlinear oscillation in an ultrahigh-Q silica toroidal microcavity is investigated in the last chapter. A controllable and reversible transition between parametric and Raman oscillation is experimentally demonstrated and theoretically analyzed. By direct change of cavity loading and indirect adjustment of frequency detuning, parametric and/or Raman oscillation can be accessed selectively without modification of cavity geometry in a toroidal microcavity with large enough aspect ratio. Based on an effective cavity gain theory, this transition is analyzed in terms of cavity loading and frequency detuning leading to a better understanding of the combined effects of parametric and Raman processes in silica microcavities.