Tunable Coupling Between An Ultra-High-Q Microtoroid Cavity And Graphene

In recent years,whispering gallery mode optical microcavities have drawn massive atttentions in the field of photonics micro/nano photonics due to their brilliant optical characteristics,like ultra-high quality factor and small mode volume.Silica optical microcavities have been widely exploited in fundamental physics such as cavity opto-mechanics,cavity quantum electrodynamics,nonlinear optics,and applied sciences such as microlasers,biosensors.They can also be integrated on a silicon chip with other photonic components like waveguides,electrodes,and can play the role of optical filters and laser sources.These optical microcavities may become more and more important in the field of optical communications and information processing,which need to manipulate the optical characteristics of the microcavities in a tunable manner.In this thesis,by introducing graphene as an absorber,we realized manipulation of the optical characteristics of a chip-based ultra-high Q silica microcavity.Graphene is only one atom thick,but it has excellent electrical and optical properties.It has extremely high charge carrier mobilities,low sheet resistance,broad absorption band and high transmissivity,so it was widely' utilized in optoelectronic devices like displays and detectors.In this work,we have investigated the interactions between graphene and ultra-high Q microtoroid cavity and have studied how the graphene changes the optical characteristics of the microcavity.In our experiment,the silica microtoroid cavity was fabricated on the corner of a silicon chip.To support the graphene,we also fabricated a freestanding silica microdisk substrate with a thickness of 800 nm.This kind of the substrate was simulated and experimentally confirmed that can avoid the energy leakage when coupling it to the microtoroid cavity.In order to realize an effective coupling between the microcavity and the graphene,the microtoroid cavity was invertedly mounted on a nano-positioner,while the graphene was placed beneath the microtoroid.We vertically coupled the graphene and the ultra-high Q microvavity by controlling the gap between them.When the gap between them decreased to a certain range,we can obviously see the changes of the cavity mode.During the experiment,the optical Q-factor of the microtoroid cavity was finely tuned from 1.59×107 to 1.20×105 by controlling the positons of the graphene.At the same time,the resonant wavelength of the cavity mode shifted from 1587.0636 nm to 1587.0838 nm.To show the application of this hybrid system,we also demonstrated a bandwidth tunable filter by controlling the fiber taper coupling during changing the gap between the graphene and the microcavity.This controllable hybrid structure may serve as a promising platform for applications such as optical modulators,low-threshold microlasers and quantum information processing.