Research On Dynamically Tunable Coherent Perfect Absorption Properties Based On Metasurfaces

Perfect absorption is important in a variety of applications such as energy harvesting,biosensing,modulation and detection.Perfect absorption based on metasurfaces can flexibly manipulate electromagnetic waves,which is particularly important in the nano and electromagnetic fields.Traditional absorbers are mostly single-port devices,that is,light is irradiated into the system from one side.Recently,studies have shown that perfect absorption can also be achieved when two coherent beams that meet certain conditions are injected into the system from opposite directions,and this phenomenon is called Coherent Perfect Absorption（CPA）.CPA provides a flexible way to control light absorption,becoming an emerging research hotspot.However,the absorption efficiency and operating frequency of most coherent absorbers are determined by their own structures,and it is difficult to achieve active tuning.In addition,most coherent absorbers can only work at a single frequency,which is challenging to extend to multiple frequency bands.Thence,it is essential to look for a simple and convenient method to realize multi-band tunable CPA.In this thesis,based on the two characteristics of tunable and multi-band metasurface coherent absorbers,the main work and research results are as follows:In this thesis,a metasurface composed of intersecting vertical Bulk Dirac Semimetal（BDS）strips and Si O₂is designed to realize CPA in the terahertz band.Combining numerical theoretical analysis and simulation calculations,the coherent absorptivity can be consecutively modulated from nearly 0(about 6.2×10^-5)to 99.97%by changing the phase difference of the two counter-propagating coherent beams,and an high modulation depth of about 1.6×10⁴（42.04 d B）can be obtained.The proposed structure can also achieve tuning of the absorption by changing the relative light intensity,reflecting the superior flexibility of the coherent absorber.Most importantly,the absorber can achieve dynamic regulation of absorption without reconfiguration by controlling the Fermi energy of BDS while maintaining high absorption efficiency.In addition,the effect of various geometric parameters of the metasurface on the absorption is also investigated.Further research found that the structure has broadband angle sensitivity,the CPA frequency is divided into two frequency bands of TE and TM polarization under oblique incidence,and the overall variation range of CPA frequency is from 1.563 to 1.604 THz.This provides an efficient method for manipulating the interaction between light and BDS,potentially offering applications such as optical switches and light modulators.A multi-band tunable CPA in the terahertz band is realized by integrating indium antimonide strips of different sizes in the same unit cell.The study demonstrates that the indium antimonide strips of different sizes support independent plasmonic resonances at different frequencies,and the total photoresponse on the metasurface is a linear superposition of the photoresponses of each subunit.Combining numerical theory and simulation calculations,independent and continuous control of absorption can be achieved at each resonance frequency by changing the relative phase or relative intensity of the two coherent beams,with a modulation depth of up to1.332×10⁴（41.25 d B）.In addition,the influence of various geometric parameters on absorption is also investigated,which is also the basis for structural design and optimization.It is further found that the structure has certain polarization selectivity and angle sensitivity,and can achieve longitudinal adjustment of absorption under the premise of fixed operating frequency,which may play a role in some occasions requiring specific polarization and incident angle.Interestingly,the device is extremely thermally tunable,and since the permittivity of indium antimonide changes with temperature,controlling the temperature can flexibly tune the coherent absorption performance.Simulation results show that this not only avoids structural reconstruction,but also maintains a high absorption efficiency.Based on the existing laws,the design can be extended to more bands of coherent absorbers,which may potentially be applied in the field of multi-band tunable coherent absorption and control.