Perfect Absorption And Dynamically Switchable Functional Devices Based On Electromagnetic Metamaterials

The absorption of electromagnetic energy absorption underlies many significant applications,including optoelectronic detection,biosensing,electromagnetic stealth,spectral imaging,and so on.The proposal of metamaterial has broken the bottleneck of research in the field of materials.Significantly,metamaterial absorber has the advantages of light weight,thin thickness and strong absorption compared with traditional absorbing materials.With the rapid advance of communication technology,single-band and narrow-band devices cannot meet the needs of engineering applications.Furthermore,absorption performance relying on the original structure is fixed for these traditional absorbing materials or metamaterial absorbers,consequently,in order to adapt increasingly complex environment for equipments,reconfigurability and multi-function have become research hotspots in the field of electromagnetic metamaterials.Owing to the deficiencies of traditional metamaterials,consisting of single function,narrow and fixed operation frequency,the aim of this thesis is to propose approaches to realize reconfigurability and multi-functional metamaterials.Additionally,the absorption of electromagnetic energy can be made much more efficient by coherent perfect absorption.Absorption in this system can be manipulated coherently by the variation of the relative phase of two beams.Therefore,the dependence of coherent perfect absorption on incident light offer a novel platform for the control of optical scattering and absorption.Nevertheless,like the majority of coherent-enhancement schemes,the fulfillment of coherent perfect absorption is confined to the discrete resonant frequency or narrow band,which leads to the limitation of development of coherent perfect absorber in many applications.Thus,it is necessary for coherent perfect absorber to explore some approaches to enhance absorption bandwidth and realize the reconfiguration of frequency and bandwidth.In Part I of this thesis,a thermally tunable absorption metasurface based on phase change material vanadium dioxide（VO₂）and metal pattern is designed in the terahertz（THz）regime.VO₂is a kind of room temperature phase-change materials,which can show metal property at high temperature（VO₂in metal phase（MP））,and display dielectric feature at low temperature（VO₂in insulating phase（IP））.Thus,the THz tunable absorber can be realized by changing the external temperature.When the temperature is larger than phase change temperature,the absorptivity of the absorber is over 90%in the frequency range of 4.72-9.88 THz,and its relative bandwidth is 70.68%.At low temperature,there is a high absorption peak at 7.91 THz with an absorptivity of 94.2%.Furthermore,the proposed metasurface is polarization-insensitive.The demonstrated tunable metasurface has potential applications in the thermal detection,THz modulator and so on.In Part II of this thesis,we propose an ultra-broadband tunable absorber based on resistive film,which is innovatively tailored by the gravity field with changing the position of the liquid metal（eutectic gallium-indium）in theory.It is noted that the liquid metal is poured into the different parts of the glass cavities via the rotation under the action of the gravity field to obtain the tunable absorption.When such an absorber is in the operation state-1,for the transverse electric（TE）wave,the absorption rate exceeds 90%at 1.8 GHz to 57.5 GHz with relative absorption bandwidth of187.8%.However,for the transverse magnetic（TM）wave,the absorptivity goes beyond 90%at5.6-56.4 GHz（the relative bandwidth is 158.7%）.The presented absorber exhibits an ultra-broadband absorption in the frequency regime of 1.6-45.3 GHz（the absorption rate is above90%）in the operation state-2.While the absorption rate is near to zero at 1.6-60 GHz,the proposed absorber can operate as a reflector in the operation state-3.Therefore,the operation state of this abasorber can be dynamically manipulated according to practical requirement.Besides,the designed absorber can realize the high absorption under a wider incident angle.Compared with the traditional tunable devices,the gravity field regulation,which can change the operating frequency of absorber in a non-contact way,has the advantages of easy implementation and resource-saving.Taken together,this study suggests a role for such an absorber in promoting a novel application of tunable devices.In Part III of this thesis,we devise a multifunctional tailored metasurface based on based on dynamic control of light and heat.By controlling the multitasking metasurface,its room temperature,or pump illumination,the conductivities of VO₂and photoconductive semiconductor（PS）will be altered.As a consequence,we switch the function of absorption or polarization conversion（PC）on and off for this metasurface,and realize the tuning of absorptivity and polarization conversion rate（PCR）.Meanwhile,the construction of cylindrical air column（CAC）in the dielectric provides an effective channel to further promote the absorption performance.For the devised metasurface to behave as a polarization converter,the polarization conversion rate（PCR）over 90%at 0.82-1.6 THz.And by exciting the PS via an optical pump beam,the PCR at 0.82-1.6THz can be modulated continuously from over 90%to perfectly near zero.When the PS conductivity is fixed at 3×10⁴S/m and VO₂is in MP simultaneously,the presented metasurface switched to absorption mode exhibits ultra-broadband absorption with the absorptivity above 90%at 0.68 THz to 1.6 THz.By varying the optical pump power and thermally controlling the room temperature,at 0.68-1.6 THz,the absorptivity of this metasurface can be successively tailored from over 90%to near null.Constructing CAC in the dielectric substrate provide a viable method to strengthen absorption bandwidth.The designed metasurface can effectively promote the electromagnetic reconfigurable functionalities of the present multifunctional devices,which may be applied in more complicated operating context.In Part IV of this thesis,we present a coherent absorption metasurface（CAMS）based on resistive thin films and metal-medium composite structure to force the implementation of ultra-broadband coherent perfect absorption in THz regime.To realize ultra-broadband coherent absorption,we first characterize a metasurface integrated with two ring-shaped resistive films.By incorporating internal and external resistive films with attached phase-delay lines,the desired ultra-broadband coherent absorption with the coherent absorptivity higher than 90%can be obtained at 7.6-26.51 THz（the relative bandwidth is 110.87%）.Simultaneously,by building a metal-medium composite structure superseding the dielectric substrate,additional promotion of the coherent absorptivity over the operation frequencies is realized.The coherent absorptivity is above 95%at 8.34-25.07 THz（the relative bandwidth is 100.1%）.More importantly,manipulating the phase difference of two coherent beams,the coherent absorptivity at 8.34-25.07 THz can be tailored successively from over 95.7%to as low as 38.1%.The scattering matrix method and electric field are employed to illuminate the absorption mechanism of the designed CAMS.Moreover,the proposed CAMS possesses good omnidirectional coherent absorption response.Our finding offers an interesting approach to designing ultra-broadband coherent absorption devices and may serve applications in THz modulators,all-optical switches,and signal processors.In Part V of this thesis,we present a tunable metasurface based on toroidal coupling,combined PS to force fulfillment of tuned broadband coherent perfect absorption in the THz region.Utilizing the variation of phase difference between two coherent beams,the coherent absorptivity from 2.73THz to 4.04 THz can be modulated successively from over 90%to as low as 11.7%.By exciting the PS through a pump power to change its conductivity,the realization of quadruple-band,dual-band,and broadband coherent perfect absorption switching is demonstrated.Our findings provide a novel channel to devise tunable broadband coherent perfect absorption and broaden the application domain with electromagnetic response for toroidal dipole.The CAMS may serve applications in THz modulators and all-optical switches.