Research On Terahertz Filter Based On Metal Subwavelength Structure

Terahertz,as the electromagnetic spectrum which has not been widely used so far,has great potential in various fields,such as communication,detection,medical treatment and so on.As an indispensable functional device in terahertz system,terahertz filter has always attracted the attention of researchers around the world.Filters with high performance and simple structure in terahertz band have always been the goal of researchers in various countries.Metallic subwavelength structures arranged according to a certain periodicity can exhibit novel electromagnetic properties in the terahertz band.Based on fractal geometry and split-ring structure,THz filters with three different frequency selection functions,single-band,dual-pass band and multi-band,have been designed in this paper to provide a reference for the design of new THz filters.The main research contents are as follows:1.A terahertz bandpass filter based on the koch curve fractal structure is designed.The filter is etched out of the koch curve fractal structure on the metal film.The narrow band filter in the terahertz band is realized when the terahertz wave is incident vertically to the filter.In the process of filter design,the theory and experiment are combined.Firstly,the filter model is established in electromagnetic simulation software to optimize the parameters.The results show that the resonant frequency of the filter is 0.715 THz,the transmission coefficient can reach 0.92 and-3 d B bandwidth is 21.9 GHz.The electromagnetic parameters of terahertz filter are obtained by S parameter inversion of the simulated scattering parameters,which theoretically explains the transmission enhancement of terahertz wave at the resonant point.The femtosecond laser micromachining system is used to process the samples,and the experimental tests are carried out in the terahertz time domain spectrum system.The experimental data obtained are in good agreement with the simulation results.2.The dual-pass band terahertz filter is designed by introducing Fibonacci fractal structure.The filter consists of three metal layers and two dielectric layers.The upper and lower metal layers are metal patches with the same size and hollowed-out fibonacci fractal structure.The middle metal layer is a square coil complementary to the metal patch and the two dielectric layers separate the three metal layers.The whole filter is metal-medium-metal-medium-metal structure.Firstly,the influence of different order fractal structures on the filter performance is analyzed,and the second order Fibonacci fractal structure with the best performance is selected to design the terahertz filter.Then the equivalent circuit is used to analyze the causes of the two passbands,and the full wave simulation of the filter is carried out,the results of the two are consistent.The first passband center frequency of the filter is 0.32 THz,the-3d B bandwidth is 164 GHz,the second passband center frequency is 0.74 THz,and the-3d B bandwidth is 176 GHz.3.The multi-band terahertz band stop filter is designed based on the metal double defect ring structure.The filter is to place the metal double defect ring structure on the polyimide dielectric layer.The metal double defect ring structure is two concentric metal ring structures with average four splitting and the two rings are connected with four metal bars.The whole structure is about x axis and y axis symmetry,respectively.Firstly,the influence of the size change of the metal double-defect ring structure on the transmission performance of the filter is studied.After determine the optimized size,the filter exhibits better three-band band-stop filter characteristics.Center frequency of the first stopband f₁=0.502 THz,the transmission coefficient is 0.058;the center frequency of the second stopband f₂=0.693 and the transmission coefficient is 0.064;the center frequency of the third stopband f₃=0.815 THz,and the transmission coefficient is 0.105.It is found that the multi-band filter performance can be generated by changing the structure,which makes it possible to design multi-band terahertz filter.