Research On Unidirectional Propagation Based On Artificial Metamaterial And Its Application To Photonic Devices

In ordinary photonic devices,backscattering is a major source of loss that degrades the performance of photonic devices.The electromagnetic one-way edge states analogous to quantum Hall edge states in metamaterials may provide a good solution to the key problem in ordinary photonic devices due to its unique unidirectional transportation and complete backscattering suppression characteristics.This ideal transmission property is unprecedented in photonic systems and has shown a broad prospect in constructing high performance photonic devices that are immune to fabrication imperfections or disorders.In this dissertation,we focus on the study of unidirectional transmission based on artificial metamaterials and its applications in photonic devices.On the one hand,the applications of one-way edge modes based on magneto-optical photonic crystal in resonator,slow light waveguide and other photonic devices are studied.On the other hand,new metamaterial structures are designed to realize the unidirectional transmission effect in the high frequencies,and its applications in topological resonators and a continuously tunable power splitter are studied.Based on theoretical analyses and numerical simulations,the main research results are summarized as follows:(1)To solve the problem that the performance of optical devices is susceptible to backscattering losses caused by manufacturing defects or environmental changes,a planar photonic crystal ring resonator based on the magneto-optical photonic crystal is studied.Due to the unique unidirectional transportation and complete backscattering suppression characteristics of the edge modes in magneto-optical photonic crystal,such a resonator only supports one-way rotating waves and the rotating direction can be controlled by the direction of an externally applied magnetic field normal to the rotation plane.Moreover,the rotation direction of the cavity modes in the second band gap and third band gap is totally opposite.The quality factor of the cavity is 3.2?10~6,more than one order of magnitude higher than previously reported cavities with the same ring size.Numerical simulations show that the unique unidirectional rotation characteristic makes its corresponding quality factors almost unaffected by the on-purposely introduced obstacles in the resonator.This feature ensures that the design is tolerant of fabrication imperfections.(2)To solve the problem that the performance of conventional channel-drop filter is susceptible to waveguide defects,a unidirectional channel-drop filter based on the magneto-optical photonic crystal is studied.The filter is composed of a one-way rotating photonic crystal ring resonator coupled with two unidirectional waveguides.In the traditional channel drop filter,the output port is selected by controlling the symmetry of the excited modes and the relative coupling between the resonator and the waveguide.However,the unidirectional channel-drop filter can route the signal to any preselected port by controlling the direction of the externally applied magnetic field.Numerical simulations demonstrate that a 96.7%backward dropping efficiency can be achieved at the resonant frequency in the filter,which is not substantially affected by the defects in the filter.The design is immune to defects or disorders,allowing implementation of multi-channel filter by cascading the four-port designs.The results may find applications in wavelength-division multiplexing and related integrated photonic techniques.(3)To solve the problem that the photonic crystal slow light system is particularly sensitive to backscattering,a unidirectional slow light waveguide design based on the magneto-optical photonic crystal is studied.By optimizing the structural parameters and adjusting the magnitude of the externally applied magnetic field,slow light with large normalized delay bandwidth product and low group velocity dispersion is realized.Based on both finite-element frequency and time domain numerical simulations of the propagation of electromagnetic waves in the presence of different types of interfacial defects or obstacles in the waveguide,it is demonstrated that the edge modes in the waveguide are immune to backscattering.Compared with the traditional slow light waveguide,the proposed structure not only has higher normalized delay bandwidth product and lower group velocity dispersion value,but also is immune to backscattering.(4)To extend the study of unidirectional transmission to the visible light range,a photonic crystal with a layered plasmo-semiconductor-gyroelectric metamaterial as the background medium is proposed.By applying an external magnetic field normal to the plane,a band gap of 24%relative size is obtained.The existence of the edge modes in the band gap is verified by the numerical calculation of the projected band diagram.These edge states are shown to possess unidirectional propagation characteristics that are robust against various types of defects and obstacles.Furthermore,Since the edge modes lie outside the light cone,the electromagnetic wave travels along the interface of the photonic crystal without the need of an ancillary cladding and the transmission direction depends on the orientation of an externally applied magnetic field.As an application,a geometry-independent topology cavity is proposed.Such a cavity only supports either clockwise or anticlockwise,but not both,rotating waves and can tolerate sharp corners in different angles.(5)To extend the study of unidirectional transmission to the THz regime by using homogeneous continuous media,the hyperbolic gyroelectric metamaterial based on alternating dielectric and magnetized semiconductor layers is proposed.By applying an external magnetic field,a spatial gap appears between two bulk modes of the metamaterial.The existence of the surface modes in the spatial gap is verified by theoretical calculation.The unidirectionality and the immunity to backscattering is further demonstrated by numerical simulation of electromagnetic waves propagation around sharp corners.By connecting two identical metamaterial structures and applying reversed magnetic fields,the number of one-way surface mode doubles.As a possible application,a continuously tunable power splitter is studied.