Research On Mode Excitation And Modulation In Whispering Gallery Mode Optical Microcavity

Due to the high quality(Q)factor and small mode volume,whispering-gallery mode(WGM)optical microcavity shows significant applications in numerous areas,such as optical interconnection,microscale laser,biosensing,nonlinear optics,quantum mechanics and so on.Among these applications,efficient mode coupling and modulation are the key factors.Usually,the cavity modes are excited through evanescent wave coupling in the traditional research.Although this coupling mechanism supports high coupling efficiency,the distance between waveguide and microcavity should be controlled precisely in nanometer scale,resulting in high fabrication cost.It makes the microcavity devices hard for commercialization.To address this problem,we derive,design and demonstrate an innovated waveguidemicrocavity coupling mechanism experimentally,which origins from the Maxwell’s functions.This device could help address the mentioned difficulty by bringing down the fabrication cost and promote the microcavity’s commercialization.The followings are the main work of this thesis:(1)In theory,the waveguide is connected with the edge of the microdisk directly.Considering the time-reversal process of the directional output along the connected waveguide,light could be injected into the microcavity through the same waveguide directly.This process is termed as end-fire injection.Combining with the eigenfrequency and transmission spectra analysis,this coupling mechanism is verified with the two-dimensional numerical simulation using finite-element method.Then,the coupling property is further analyzed using the three-dimensional finitedifference time-domain model.(2)In experiment,the waveguide-microcavity structure is fabricated on a silicon-on-isolator wafer.Then,the end-fire injection coupling mechanism is demonstrated experimentally by measuring the reflectance spectra.The experiment results show that light could be injected into the microcavity and excite the resonant modes efficiently,which agrees well with the numerical simulation.And the high coupling efficiency is robust to fabrication deviation,such as waveguide width and waveguide position.At the same time,the microcavity-waveguide device also supports high Q factors.By measuring the resonant wavelength shift of the high-Q modes,the device could be used to sense environment temperature variation,single nanoparticle and the nanoparticles number,where the sensing accuracy is compatible with traditional silicon optical microcavity.(3)Depending on the experiment and simulation verification of the end-fire injection,the waveguide-microcavity structure is further applied in deformed microcavity.To enhance the number of high-Q modes in waveguide-circular microcavity structure,the Quadruple microcavity is introduced and connected with the waveguide.Benefiting from the special field distributions of the high-order 4-bounce modes,a set of modes are excited both with high coupling-efficiency and high-Q factors,simultaneously.What’s more,the high-order 4-bounce modes show nonuniform field distributions along the cavity boundary.By connecting a channel waveguide at special location of low field distributions,the low-Q modes could be restrained effectively without damaging the high coupling efficiency and high-Q factor.(4)The abundant phase space structures of deformed microcavity are applied to study the physical phenomena around exceptional point,including mode coupling,the chaotic-to regular quantum tunneling and the quantum tunneling induced optical chirality.Benefiting from the huge difference in mode distributions of the chaotic microcavity,the resonant wavelengths of the two modes could be controlled by thermal-optic effect,individually.As a result,the strong mode coupling in a single microcavity has been demonstrated experimentally.What’s more,the waveguide is connected at the chaotic region of the defomed microcavity.By comparing the farfield laser emissions with and without connected waveguide,the chaotic to regular quantum tunneling has been demonstrated experimentally.Then,the resonant mode chirality is modulated by changing the location of the connected waveguide.This experiment shows that,besides the optical sensing,the waveguide-microcavity structure could become an ideal platform to study the fundamental quantum physics.In summary,this thesis takes optical microcavity as a research platform.Based on time-reversal process in optical microcavity,a high-efficiency and workable coupling mechanism is introduced and demonstrated.This device supports high performance,low fabrication cost as well as the high fabrication deviation tolerance,simultaneously.It may expedite the development of microcavity’s commercialization,especially in the low-cost biosensors and the large-scale integrated optical chips.In addition,the waveguide-microcavity structure could also act as an ideal platform for fundamental quantum mechanics research.