The Study On Mode Coupling And Its Effect On Enhancing Second Harmonic Generation From Metal Metasurfaces

Due to the rapid development of nanofabrication technology,metamaterials and metasurfaces have broad application prospects in many fields such as materials science,biomedicine,mechanics,and heat,and have become one of the current research hotspots.In the field of optics,optical metasurfaces whose thickness is much smaller than the wavelength of incident light can fully control the wave front of light,including phase,amplitude,and polarization.Among them,the metasurfaces composed of metal materials can support surface plasmon resonances(SPR),which can break the diffraction limit and increase the local field strength.It has great potential in optical imaging,sensing,quantum information processing,and light collection.As the significantly important optical phenomenon,nonlinear optical effect has extremely meaningful applications in many aspects,such as frequency conversion,optical switching,and optical modulation.Well-designed metal nanostructures can obtain efficient nonlinear optical process at sub-wavelength scale,effectively tune the working wavelength of nonlinear optical process,obtain a new phase matching mechanism,and realize the control and manipulation of local effective nonlinear polarization phase,etc.This is the huge advantage of nonlinear plasmon optics over traditional nonlinear optics.At present,the research of nonlinear plasmon optics has become a new hot spot,which is of great significance for both basic and potential application research.In this paper,we study the interaction between magnetic plasmon resonance provided by split ring resonators and surface lattice resonance,optical waveguide mode based on the metasurfaces composed of metal split ring resonator arrays,and its influence on the nonlinear efficiency of second harmonic generation.The specific content of the article includes:1.Diffractive coupling effect in metal split ring resonator arrays.We theoretically study the diffraction pattern of the SRR arrays in a homogeneous medium and the interaction with magnetic resonance of the SRR to achieve strong coupling.By changing the period of the structure in x and y direction,respectively,we realize that the diffraction pattern appear in only one direction within the wavelength range we study.According to the phenomenon that the position of the transmission valley in the linear transmission spectrum varies with the period,we found that when the period of arrays in the y direction is changed,the diffraction mode and the magnetic resonance mode could achieve strong coupling.2.Mode coupling characteristics in the composite system formed by metal split ring resonators-optical waveguide.We show the interaction between the magnetic resonance mode provided by SRR and the waveguide mode excited by the waveguide structure.Firstly,according to the relationship between the cut-off wavelength of the waveguide mode and the thickness of the waveguide layer,we choose the appropriate thickness of the waveguide layer to ensure that there is waveguide mode in the range we study.Then we arrange the SRR arrays on the waveguide layer to meet momentum conservation,which is enable to excite waveguide mode.Finally,we change the periods in x direction and y direction to realize strong coupling of TE waveguide mode and TM waveguide mode with magnetic resonance mode.3.The effect of mode coupling on the second harmonic conversion efficiency of metal metasurfaces.We study the second harmonic conversion efficiency of the metasurfaces designed by SRR arrays in the homogeneous medium and the waveguidebased systems.In the first system,considering the square structure and the rectangular structure separately.We found that,for the square period,the second harmonic intensity shows a downward trend with the increase of the period.For the rectangular period,due to the strong coupling between diffraction mode and magnetic resonance when the period is changed in the y direction,the local field is enhanced.Therefore,we still achieve the second harmonic intensity enhancement in the case of increasing the period;in the second system,both TE and TM waveguide modes could be strongly coupled with the magnetic resonance.Therefore,when the SRR number density decreases,we can obtain the second harmonic enhancement regardless of changing the period in the x direction or the y direction.Our research on mode coupling effects provides a new way to enhance the nonlinear conversion efficiency based on metal metasurfaces.