The Research On Directional Emission Of Inner Boundary Deformed Annular Cavity And Its Application

Whispering gallery(WG) based optical microcavities trap light via total internal reflection along the cavity boundary for a long time and form ultrahigh quality(Q) factors, showing bright future in optical sensing and microsized coherent light sources. The key parameters for such applications are the high Q factor, unidirectional emission, and large chirality. The typical design to achieve unidirectional output is tailoring the cavity boundary and modifying the intrinsic localization and emission properties. However, as the deformed cavity boundaries can not conserve the angular momentums of localized resonances, the Q factors of such cavities are usually dramatically spoiled. Moreover, the previous reported chiral resonances either show low Q modes or require very special cavity designs. All these shortages will limit the application of microcavity. Based on the above discussion, this thesis demonstrates a new, general, and robust design to obtain the combination of ultrahigh Q factors and unidirectional output. Then, by placing a circular cavity and our new design in proximity to compose a photonic molecule, besides the ultra-high Q factor and unidirectional output, it can also achieve large chirality simultaneously. Finally, the detection properties of single molecule based on the photonic molecule is numerically investigated.The main works in this thesis is shown as below:First of all, this thesis formed a novel inner boundary deformed annular cavity to obtain the combination of ultrahigh Q factors and unidirectional output. By tailoring the position, size, and shape of inner boundary of annular cavity, it showed that unidirectional emission could be formed in long-lived resonances in a wide range of resonance frequencies, and the frequency range for the high Q and unidirectional emission was very broad, making the new design to be suitable for practical applications. The above mechanism has been proved by COMSOL Multiphysics based on finite-element method(FEM).Second, the research extended the mechanism in photonic molecule. By placing a circular cavity and an annular cavity with spiral inner boundary in proximity, the numerical calculations showed that ultra-high Q factor, unidirectional output, and chirality could be obtained simultaneously. And the co-propagating directions of the non-orthogonal mode pairs can be reversed by counting the mode coupling in photonic molecule. The advantages of the photonic molecule can further improve the detection sensitivity, and will be very important for the optical sensor and detection.Third, the research applied the photonic molecule to the single molecule detection. By attaching a molecule to a photonic molecule around an exceptional point(EP), the main resonant behaviors were strongly disturbed. In addition to typical mode splitting, the far-field pattern of the photonic molecule was significantly changed. Taking polystyrene as an example, this thesis demonstrated that a single molecule, with radius as small as 1 nm to 7 nm can be simply monitored through the variation of the far-field pattern. Compared with conventional methods, this approach is much easier and does not rely on high-cost equipment, showing more potential in free-label single molecule detection.In this thesis, the novel deformed inner boundary annular cavity will illuminate new advances in the fundamental research, such as EP and mode coupling. Moreover, the research also shows that it will provide a new platform for the practical applications. The research will shed light on the advances of the applications of the optical cavities.