Research On The Subwavelength Grating Device Based On Guided-mode Resonance

Subwavelength optical elements have special optical properties that can not be achieved by conventional optical elements.The special diffraction characteristics of subwavelength gratings with only zero order diffraction are widely concerned.The guided mode resonance effect of subwavelength grating has high diffraction efficiency,narrow bandwidth,simple structure and so on.It is used to design a lot of optical devices,such as filter,beam splitter,and optical modulator.However,the period of the gratings is usually the same or less than the operating wavelength,scalar analysis method can not accurately calculate the diffraction characteristics.The rigorous vector analysis is needed.In this paper,three numerical methods are analyzed for the subwavelength gratings.The equivalent dielectric theory is used to approximate the grating diffraction.Using the rigorous coupled wave theory and time domain finite difference algorithm,the spectral response is solved accurately.this paper proposes and studies two optical device based on the mode resonance effect of the subwavelength gratings,a tunable narrowband filter and a low threshold bistable grating.The main contents of this paper are summarized as follows:1.According to the equivalent waveguide model,the subwavelength grating layer is regarded as the plane uniform waveguide medium with the average refractive index of the grating.The position of the mode resonance is obtained by calculating the intrinsic equation.The influence of different grating parameters on the spectral response is calculated by the rigorous coupled wave theory and the finite difference time domain algorithm.2.A separate grating filter structure is proposed in which the waveguide layer is separated by an air thin layer.The spectral response of the filter is calculated when the thickness of the slot is different.The results show that the thickness of the air layer has a control effect on the resonance position.Through the optimization of the grating structure,the 1515?1558nm wavelength range can be tuned.Due to the shallow modulation grating design,line width is less than 0.6nm.3.A low threshold bistable grating structure is proposed.The structure of the waveguide layer is composed of a high refractive index layer filled with a thin layer of low refractive index.The grating layer adopts strong modulation of shallow etching Grating,non-linear material selection of common optical material silicon as a grating layer and waveguide layer.In the case of the TM polarization,the electric field is strongly confined and enhanced in the low refractive index layer due to the boundary condition,so as to improve the nonlinear performance of the structure and realize a low threshold.The results show that the switching threshold of our structure is about an order of magnitude lower than that of the traditional bistable grating.In this paper,the thickness of the low refractive index layer is analyzed,that is,the trend of different grating thickness and exchange threshold,and explain the trend from the point of view of its light field distribution.In this paper,the tunable narrowband filter with excellent characteristics can play an important role in the field of optical communications;not only that,the structure also has the potential for detection and sensing of pressure;The low threshold bistable structure has some practical value for realizing the optical communication network or other optical fields.