| Non-reciprocal transmission of light is fundamental in all-optical information processing.Similar to an electronic diode with p-n junctions,an all-optical diode?AOD?allows light to transmit only in one direction,blocking it in the opposite direction.and thus it has broad application prospects in the fields of all-optical calculation,laser technology,and all-optical information processing.The ideal all-optical diode?AOD?is not only compatible with semiconductor CMOS processes,but should also have high-contrast nonreciprocal transmission?defined as the ratio of the transmission intensities for forward and backward incident signal under the same conditions?,broad-bandwidth and high transmittance.Currently,Fano resonances are widely used to achieve ultrahigh transmission contrast rate,however,the resonant enhanced nonlinear diode has a small bandwidth?typically less than0.005 nm?,which limits the practical application.On the other hand,in all-optical information processing,sometimes we want the conduction direction of the AOD to be controlled to reverse,in order to achieve more flexible light control.However,very little physical mechanism to complete it,and its controllable inversion can only target signal light of different frequency,it cannot achieve controlled inversion of the same signal light.The main contents of this paper and the research results are summarized as follows:Chapter one,briefly introducing the basic concept of photonic crystals and two important characteristics and related applications,comprehensively introducing the main methods and research status of realizing non-reciprocal transmission of light.Chapter two,combining the theory of electromagnetic of photonic crystals and Maxwell's equations,we introduce the plane wave expansion method and the finite difference time domain method?FDTD?and focuses on FDTD in 2D.Chapter three,we construct a microcavity-waveguide asymmetric structure all-optical diode?the photonic crystal waveguide is formed by a row of circular Si material micro-media columns with reduced diameter?.In the linear case,we study the effect of the dimensional change of the micro-media column on the transmission spectrum of the waveguide and the coupling coefficient between the front and rear waveguides and the microcavity.As result,we can optimize the structure,which prepare for the later study of AOD under nonlinear conditions.Chapter fourth,useing the nonlinear microcavity-waveguide asymmetric coupling structure to realize the all-photo diode.In this paper,we focus on nonlinear time-domain coupled mode theory,the Kerr effect is introduced into the photonic crystal microcavit,due to the local characteristics,the nonlinear effect becomes more and more obvious in the cavity.The refractive index n in the resonant cavity increases as the light intensity I increases,which causes the resonant frequency of the microcavity to red-shift and continuously move toward the long wavelength direction.Based on the optimization design in the third chapter,we studied the transmission spectra in the case where??16?is equal to??17?and??16?is not equal to??16?,and the reversible AOD that works at the same wavelength is proposed for the first time.This is very important for signal processing.After simulation,we obtained AOD that can achieve high transmittance,high contrast and wide bandwidth at the same time.Chapter five,using a similar mechanism principle,we designed another microcavity-waveguide asymmetrical structure.By constantly adjusting the distance s between the cavity wall and the microcavity,the coupling coefficient between the front and rear waveguides and the microcavity is significantly different,and all-optical diodes with higher transmission?average transmission over 60%?,higher contrast?close to 20 dB?and wider operating bandwidth?over 5 nm?are also achieved. |
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