Study On Multi-band/Broadband Absorbers And Polarization Converters Based On Electromagnetic Metamaterials

As a new synthetic materials,metamaterials(MMs)are composed of periodically arranged electromagnetic(EM)resonance unit cells,which has some singular physical properties and phenomena that aren't found in the natural materials.Among many proposed MM devices,the MM absorbers can dissipate the incident EM wave;while the MM polarization converters can control the polarization state of the EM wave.The MM absorbers and polarization converters have the advantages of thin thickness,light weight,low cost and easy integration with electronic devices,they have become research hotspots in radar stealth,electromagnetic compatibility(EMC),wireless communication and remote sensing with high application value.At present,the research of MM absorbers and polarization converters has been extended from the initial microwave band to terahertz(THz)and optical band,among which multi-band and broadband devices have more application prospects.In this dissertation,we study the multi-band/broadband MM absorbers and polarization converters by combining numerical simulation and experimental measurement.In addition,we explore the multifunctional MM absorbers and polarization converters.The main contents are summarized as follows:(1)Firstly,three polarization-sensitive petal-shaped structure absorbers are investigated.Based on this,a hollow quad-petal-shaped structure absorber(QPA4)with rotational symmetry is designed to achieve perfect absorption at 16.6 GHz and 24.4 GHz.This structure is insensitive to the polarization and incident angles of the incident EM wave.Finally,the sample is measured in free space,and the measured results are in good agreement with the simulated results.(2)A polarization controllable single-/penta-band MM absorber is designed,it can be switched between single-band and penta-band perfect absorption by controlling different polarization directions of the EM wave.The proposed absorber exhibits a single-band absorption with absorptivity of 99.8%for TM polarization wave at 6.64 GHz,and penta-band absorption with absorptivity of more than 99%for TE polarization wave at11.68 GHz,13.58 GHz,15.48 GHz,17.38 GHz and 19.28 GHz,respectively.The absorber possesses good flexibility with good absorption performance when it is bent to a certain extent due to the ultrathin thickness that 1/48 and 1/140 of the minimum and maximum operating wavelength.In addition,the sample is measured for different polarizations and radius of curvatures in free space.The measured results are in good agreement with the simulated ones.(3)By combining the polarization converter and the perfect absorber into one unit cell and embedding the PIN switch diodes in the appropriate positions,a reconfigurable multifunctional broadband MM polarization converter/absorber is designed.By controlling the state of the PIN diodes,the structure can realize arbitrary switching between broadband polarization conversion and perfect absorption functions.The results show that when the PIN diodes are in OFF-state,the MM device operates in polarization conversion mode with polarization conversion ratio(PCR)of above 90%in the range of2.97 to 6.03 GHz;while the PIN diodes are in ON-state,the operating mode is switched to perfect absorption with absorptivity of above 90%from 2.56 to 7.62 GHz.Finally,the sample is measured to verify the performances of the designed MM polarization converter/absorber.(4)Based on hybridized MM with photosensitive silicon(Si)and vanadium dioxide(VO₂),we design two multifunctional asymmetric MM polarization converters operating in the THz band.The converter structure I can achieve two different broadband linear polarization conversions for forward and backward incident EM waves,and when the external excitation conditions are different,the operating frequency bands of forward and backward incident can be interchanged.However,the converter structure II can operate in forward reflection linear polarization conversion,forward transmission circular polarization conversion and backward transmission linear polarization conversion modes under different excitation conditions.