基于复合光栅和梯度超表面的多功能器件设计

Multifunctional Device Design Based on Composite Gratings and Gradient Supersurfaces

设计了一种金属/介质/金属复合光栅与梯度超表面相结合的多功能器件。通过复合光栅的非对称结构实现光的非对称传输,当调制后的波矢量与表面等离极化激元的波矢量相匹配时,金属光栅激发的表面等离极化激元产生单波段光传输;通过梯度超表面实现光的异常反射。当x偏振态光波入射时,不同入射方向可在不同波段激发表面等离极化激元,由于上下光栅的不同周期和非对称的介质环境,光栅可分别在1128 nm和1550 nm处实现高透射率;另外在反向入射方向,该器件在1300~1400 nm整个波段表现为零吸收,反射率大于90%,可作为反射器。当y偏振态光波入射时,在反向入射方向器件通过相位梯度超表面实现异常反射。该结构可为各种偏振相关的多功能器件和集成光学元件提供参考。

Most of the current research can only realize the asymmetric transmission of light singly,in the face of the demand for multifunctional application scenarios,the design of multifunctional integrated devices has become a development trend.Therefore,the combination of grating and super-surface,which can realize different functions in different incidence and polarization states,is a multifunctional device combining a metal/medium/metal composite grating and a gradient super-surface structure.The asymmetric transmission of light is realized through the asymmetric structure of the composite grating,and when the modulated wave vector matches the wave vector of the Surface Plasmon Polaritons(SPPs),the SPPs excited by the metal grating produce single-band light transmission,and the anomalous reflection of light is realized through the gradient hypersurface.When an x-polarized light wave is incident,the SPPs are unidirectionally excited at 1550 nm in the forward-incidence direction,and the forward transmittance at this wavelength is up to 0.9.In the reverse-incidence direction,the SPPs are unidirectionally excited at 1128 nm,and the backward transmittance at this wavelength is up to 0.86.Due to the different periods of the upper and lower gratings and the difference in permittivity at the incident interfaces,the surface-iso-polarized excitations can be excited in a single-band light transmission.The upper and lower gratings cannot excite SPPs in the same band at different incidence directions,and the reverse transmission is suppressed in the forward excitation band and vice versa.In order to obtain the best performance of the asymmetric transmission characteristics,the parameters of the device are optimized to enhance the interaction between the beam and the grating.The effect of the grating transverse position on the transmission spectrum is investigated.Changes in the grating transverse position alter the strength of the coupling effect,leading to the splitting of the transmission peaks,the emergence of double peaks or even multiple peaks,and the broadening of the transmission spectrum,and the effective refractive index of the double-layered grating and the relative phases of the light passing through the sub-wavelength grooves of the two gratings are also varied,leading to changes in the resonance wavelengths and the transmission spectra.The changes in the transmission spectra of the upper and lower metal gratings and the intermediate SiO2 film under different parameters are also investigated.In addition,in the direction of reverse incidence,the device exhibits zero absorption in the entire band from 1300 to 1400 nm,and the reflectivity is greater than 0.9,which can be used as a reflector.In order to verify that the asymmetric transmission phenomenon occurs only when x-polarized light waves are incident,the transmission spectra of y-polarized light waves incident on the device in different incident directions are also simulated,and it is found that the device does not exhibit asymmetric transmission when y-polarized light waves are incident,due to the fact that the SPPs field component reaches its maximum at the metal/dielectric boundary,and decays exponentially in the dielectrics at the two ends of the metal.In visible and infrared light,the real part of the permittivity of most metals is negative,so that the permittivity of the metal is different from that of the surrounding permittivity,and only light waves in the x-polarized state can efficiently excite the SPPs.Given that the asymmetric transmission properties and reflection properties of gratings at the incidence of light waves in the x-polarized state have been extensively studied,the anomalous reflections are achieved by irradiating the underlying phase-gradient hypersurfaces with light from the other wavelength band in order to realize the versatility of the device.When the y-polarized state light wave is incident,in the reverse incidence direction,the light directly irradiates the phase gradient super-surface,and according to the Generalized Snell′s Law,in order to form a phase discontinuous super-surface to make the length of the underlying Ag nanostructures of each unit structure in the super-unit is different,which forms the reflective phase delay,and produces the phenomenon of anomalous reflections when the light is irradiated.To solve the problem of asymmetric transmission and single function of anomalous reflection devices,unidirectional excitation of SPPs and the Generalized Snell′s Law are combined to realize multifunctionality,which provides a reference for a variety of polarization-related multifunctional devices and integrated optical components.