Research On Unidirectional Emission From PT-symmetric Annular Microcavity

Optical microcavity based on the whispering gallery mode has become a research focus in recent years.Its high quality factor and low threshold value have good application prospects in the fields of integrated optics and information optics.At present,the shapes of optical microcavities are also various,including microsphere cavities,microring cavities,and microdisk cavities.This article mainly studies the various phenomena of the exit end after introducing circular holes and scattering particles into the microdisk cavity.And by introducing the PT symmetry(Parity-time symmetry,PT symmetry for short)structure to further influent the internal ray dynamics of the microcavity,the mode selection of the microcavity and the directionality of the laser at the exit end of the microcavity are modulated.For annular cavities,this paper analyzes the coupling between low-order modes and high-order modes when introducing non-concentric circular holes into circular microdisks,and it is concluded that the low-order modes with poor directivity can pass through the high-order modes with good directivity.Through the mode analysis method,the reason why the annular cavity can achieve one-way emission is found.For a point scattering cavity,when the scattering particles are located at the focus of the disk,a specific pattern of light in the cavity can be effectively scattered and propagated along the inverse focusing path,thereby achieving a one-way exit of the laser.COMSOL Multiphysics software is used to perform eigenmode analysis,and uses the MATLAB program to calculate the angular momentum distribution in the model.Through the analysis of the annular cavity and the point-scattering microcavity,the limitations of the unidirectional laser emission are obtained:modes with higher quality quality factors tend to have worse directivity,while those with better directivity have has a relatively low quality factor and is therefore difficult to excite.This paper introduces PT symmetrical structure to solve this problem.When the system satisfies PT symmetry,its eigenvalue can be real,and its overall gain and loss are balanced to be neutral.Under certain conditions,the PT symmetry of the system is broken.At this time,the eigenvalue of the system is no longer a real number,and usually only one mode satisfies the gain condition,thereby achieving single-mode emission.By applying the simple concept of PT symmetry,we show that well-designed directional resonances can be selectively excited in optical microcavities.In addition,this paper also found that the PT symmetrical modulation on the imaginary part of the refractive index in the circular microdisk cavity will affect the internal ray dynamics,so that for the first time,a highly directional laser emission can be generated without changing the microdisk structure.Based on the previous ring cavity design theory,the inner circle is plated with a PT symmetrical structure that is exactly opposite to the outer circle modulation.By breaking the PT symmetry,unidirectional laser emission and single-mode operation are achieved.This research can promote basic research and practical applications of PT symmetrical systems.