Research On Frequency-controllable Multi-band Metamaterial Absorbers

Metamaterial(MM)absorbers are a kind of artificial sub-wavelength structures whose unit cells are periodically arranged,and usually composed of metal-dielectric-metal three layers.MM absorbers can dissipate the incident electromagnetic(EM)waves almost completely at a specific frequency band through the ohmic loss of metal and the dielectric loss of dielectric substrate,Since MM absorbers were proposed,researchers have proposed a variety of traditional MM absorbers,such as single-band,dual-band,multi-band and broadband absorbers over operating frequency from microwave to optical band.With the advancement of technology and research,flexible,frequency-tunable and switchable MM absorbers have attracted much concern due to the important application prospects in many fields such as reflectors,antennas,sensors,smart radars and filters.Frequency-controllable MM absorbers include tunable and switchable metarmaterial absorbers.This paper studies three controllable MM absorbers including a frequency-tunable MM absorber with three bands,a dual-band and a three-band switchable MM absorber/reflector.The absorption characteristics and mechanism of the proposed controllable absorbers are studied by simulation and experiment.The details are as follows:In Chapter 1,the research background,significance and progress of MM absorbers at home and abroad are introduced.In Chapter 2,based on the realization methods of frequency-tunable MM absorbers,a frequency-tunable three-band MM absorber with loaded varactors is designed.By adjusting the reverse bias voltage of the varactors in the unit cells,the three-band absorption characteristics can be realized,in which the two absorption peak frequencies can be dynamically tuned.When the reverse bias voltage on the varactor diode changes from 0 V to-15 V,the frequencies of the first and second absorption peaks can be dynamically tuned from 2.77 GHz-4.36 GHz and 8.11 GHz-8.41 GHz,respectively,while the third absorption peak frequency is hardly shifted.By analyzing the surface current distributions at each resonant frequency when the bias voltage is-10 V,the multi-band tunable absorption mechanism is explained.The measurement results of the fabricated absorber sample confirm the frequency-tunable absorption characteristics of the multi-band MM absorber.In Chapter 3,based on the realization methods of switchable MM absorbers,a dual-band switchable MM absorber/reflector in S-C band is designed.By controlling the switching state of the switch diodes embedded in the splits of the inner and outer rings,the arbitrary switching among two kinds of single-band perfect absorption,one kind of dual-band perfect absorption and the perfect reflection is realized.Meanwhile,the switchable absorption mechanism of the structure is analyzed by analyzing the surface current distributions at each resonant frequency when the structure works in three absorption states.Finally,the influences of polarization angle,incident angle and main structure parameters on the absorption characteristics of the structure are studied.In Chapter 4,on the base of the third chapter,a three-band switchable MM absorber/reflector with eight working modes is designed.By welding three sets of identical semiconductor switch diodes in the gaps of three different sub-units resonators on the top layer and skillfully designing the stereoscopic feeding network,which can control the three sets of silicon diodes independently,so that the designed structure can realize arbitrary switching between triple-band absorption mode(111),dual-band absorption modes(011,101 and 110),single-band absorption modes(001,010 and 100),and reflection mode(000).In addition,the absorption mechanism of the structure is studied by analyzing the current distributions at each resonant frequency in eight working modes.Meanwhile,we investigate the absorption characteristics of the proposed absorber under different polarization and incident angles.By measuring the fabricated sample,the measurement results are in good agreement with the simulation results,which verifies the switching characteristics of the proposed absorber.In Chapter 5,we summarize the full contents and look forward to the future work on MM absorbers.