| Surface Plasmon Polariton(SPP)is a surface wave that propagates along the metal surface generated by the interaction of free electrons and photons on the metal surface.Optical devices based on SPP have the characteristics of break through diffraction limit,sub-wavelength local enhancement and ultra-features such as focusing effect,which are expected to realize sub-wavelength optical path integration.At present,the effective excitation and directional control of SPP has become a research focus in the field of optical modulators and optical switches.Due to the uncontrollability of the direction of SPP excited in general optical devices,the energy loss of SPP is caused.In order to improve the directional emission performance of SPP,this paper studies the directional emission performance of SPP with single slit,double slits,multi-slits and grating structure by using the finite difference time domain method,explores the influence of device structure and structural parameters on SPP directional emission,and obtains a device structure with excellent directional emission performance.In this paper,the directional emission performance of the device is evaluated by the unidirectional factor,the processing tolerance of the device is evaluated by the allowable structural parameters,and the conversion efficiency of the device is evaluated by the unilateral SPP intensity.Firstly,the directional emission performance of SPP based on the principle of multimode interference is studied.By optimizing the incident light angle,the width and height of the slit,the maximum unidirectional factor is 27.5d B,and the unilateral SPP intensity is 0.173W/m~2.Based on the single slit structure,the cascaded single slit structure(MIDS)and the single-slit-resonator structure are proposed.The influence of the width,height and spacing of the MIDS and single-slit-resonator on the orientation performance of SPP is studied respectively and the MIDS structure is obtained.The highest intensity of SPP has better orientation performance,its unidirectional factor is 13d B,and the maximum intensity of SPP is 0.31W/m~2,and the reason of its high orientation is analyzed by multi-mode interference.However,the width of the single-slit-resonator has a maximum allowable structural parameter of 400nm,and has the largest processing tolerance.Secondly,the directional emission performance of the SPP based on the interference of the double slits structure is investigated.When the light is normally incident,the amplitude and phase of SPP in the single slit structure are analyzed with the width and height,and the initial width and height of the double slits are determined.On this basis,the effect of double slits spacing and height on the directional performance of SPP is studied,and the unidirectional factor is finally obtained as 38d B.The reason for its high directional performance is explained by coherent superposition of SPP.Thirdly,the structure with single slit as the basic unit(ICSS)and the structure with double slits as the basic unit(ICDS)were constructed respectively,and the performance of the directional emission SPP was studied.For the ICDS structure,when the number of cascades is 5,the SPP strength of the ICDS structure is higher and the orientation performance is better.For the ICSS structure,the six-unit ICSS has higher SPP strength and slightly higher unidirectional factor than the ICDS structure.Finally,the SPP directional emission performance of the grating structure is studied.The effect of grating structures with different shapes on the orientation performance of SPP is studied.The unidirectional factor and optimal structural parameters of various gratings are calculated,and the wave vector calculated by the grating coupling formula is compared with the wave vector obtained by the final structure.Among them,the SPP intensity of the right-angle triangle grating is higher and has the best orientation performance.This study provides a theoretical basis for the design of SPP unidirectional excitation devices. |
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