Broadband Terahertz Absorbers Based On Subwavelength Structures

By artificially designing the structure size and array configuration,subwavelength structure can freely regulate electromagnetic waves and obtain superior performance that cannot be realized by natural materials.Terahertz wave is widely utilized in communication,detection,imaging,stealth and other fields due to its many advantages such as low energy,short pulse,small damage to human body and so on.The terahertz absorber based on subwavelength structure is one of the important devices in the field of electromagnetic detecting and absorption.At present,the terahertz absorbers generally have problems such as narrow absorption bandwidth,complexed fabrication procedure,and sensitivity to incident angle.Aiming at these shortages,this thesis carries out the following research work:(1)For narrow absorption bandwidth of the existing THz absorbers,this thesis designs an ultra-broadband terahertz absorber based on subwavelength structure and verifies the simulation results through experiments.Different unit sizes of the double-layer 2D gratings were optimized to make the equivalent indices(EIs)form interval values between free space and the substrate.The impedance is matched at the interface,so that a broadband absorption is obtained in the range of 0.77-5 THz.The photoresist dielectric material SU-8 was used to cover the grating structure,so that the EI has smaller interval.The absorption bandwidth of the terahertz absorber is further extended to 0.44-10 THz,and the relative bandwidth is 183%.This thesis designs a simple processing flow based on conventional photolithography and completes preparation of high quality terahertz absorber sample.The absorption spectra of the sample show that the absorption bandwidth is 0.3-10 THz with absorption above 87%,which verifies the ultra-broadband terahertz absorption performance of the absorber.The simulation result displays that the absorber still has 90% absorption when the incident angle is up to 30° for TE polarization and 70° for TM polarization.(2)Three terahertz absorbers are designed based on three different structure profiles including linear,parabolic and catenary,and their EIs are continuously distributed.Compared with the absorber on the basis of interval-EI,the three absorbers achieve superior relative bandwidth and higher absorption under normal incidence conditions,which indicates that absorbers based on the continuous EI can realize better impedance matching.In addition,aiming at the sensitivity of incident angle,a large-angle broadband terahertz absorber is designed based on the polynomial continuous EI optimized by genetic algorithm.The simulation results show that the absorber obtains superior absorption performance at 70°.For TE polarization,the absorber presents high absorption above 90% from 0.54 to 5 THz,and the maximum absorption reaches up to 99.86%.Compared with the absorber based on the interval-EI,the absorption is improved at least 30%.For TM polarization,the bandwidth is 0.61-5 THz with the absorption larger than 80%.