Study On Characteristics Of Multi-band Coherent Perfect Absorption Of Patterned MIM Structure

Electromagnetic metamaterials can adjust the amplitude,phase,and polarization of electromagnetic waves to achieve beam polarization,electromagnetic absorption,nonlinear optics,and hyperspectral imaging.In fact,electromagnetic absorption plays an important role in many applications such as solar cells,biosensing,electromagnetic stealth and thermal emissions.Metasurface which can completely absorb electromagnetic waves have been extensively studied and designed,almost covering all wavelength ranges.Among them,the patterned metal-medium-metal structure(MIM structure)can significantly enhance the absorptivity by exciting electromagnetic resonance.Moreover,by changing the geometry of the patterned MIM structure or the material of the dielectric layer,the resonance absorption frequency can be flexibly adjusted.This type of structure is widely used in the design of electromagnetic absorbers and has great freedom.However,once the structure of the conventional electromagnetic absorber is fixed,its absorptivity is also fixed and difficult to control actively,which greatly hinders their applications in optical switches and modulators.In recent years,the emergence of coherent perfect absorption provides a new method for achieving tunable absorption.Coherent perfect absorption refers to the complete electromagnetic absorption in the lossy material by the interference of two back-propagating coherent waves,which is also interpreted as the time reversal process of the laser.However,as a time-reversed laser,most of the coherent perfect absorbers only work at single frequency and the absorption bandwidth is very narrow,which is limited in broadband applications such as solar photovoltaic.Therefore,it is important to study the multi-band coherent perfect absorption.Based on the effects of metal wafer size and dielectric layer material on the absorption frequency in patterned MIM structure,this thesis study the coherent absorption properties of multi-size metal wafer-dielectric layer-metal wafer metasurface,multi-size MIM wafer-dielectriclayer-MIM wafer metasurface and graphene elliptical patch-dielectric layer-elliptical patch metasurface,the detailed research work includes:1.The influence of the wafer radius on the coherent absorption peak frequency in the Metal wafer-Insulator-Metal wafer(M(w)-I-M(w))metasurface.A multi-size M(w)-I-M(w)structure is designed to numerically simulate the absorption performance with anti-symmetric inputs(the phase difference between the two coherent incident lights is 180°).It is found that for a single M(w)-I-M(w)structure,when the radius of the wafer increases,the absorption peak frequency is red-shifted;otherwise,the blue shift will occur.Multi-size M(w)-I-M(w)structure combined by four different radius metal wafers can achieve four peak absorption.At the same time,when the phase difference between the two incident lights is adjusted,the absorption at 106.5 THz and 112.8 THz varies between 99.56%-2.58% and 99.90%-3.03%,respectively,and the modulation depth is 38.49 and 32.89.Study the absorption performance under oblique incidence and adjust the structural parameters to optimize the absorption of TE waves at an incident angle of 45°,in order to be effectively applied to the beam splitter.The results obtained in this paper provide a basis theoretically for the expansion of single-frequency to multi-frequency coherent absorption to achieve broadband coherence perfect absorption.2.The influence of dielectric layer material on the coherent absorption peak frequency in M(w)-I-M(w)metasurface.A multi-size MIM(w)-I-MIM(w)(MIM wafer-Insulator-MIM wafer)structure is designed to numerically simulate the absorption performance with anti-symmetric inputs(the phase difference between the two coherent incident lights is 180°)and symmetrical inputs(phase difference between two beams of coherent incident light is 0°).It is found that for the M(w)-I-M(w)structure,when the dielectric constant of the dielectric layer increases,the absorption peak frequency will be red-shifted,and vice versa.MIM(w)-I-MIM(w)structure can achieve dual-band coherent perfect absorption.And then explore the phase adjustment capability about the absorption rate in MIM(w)-I-MIM(w)structure,studies have shown that at symmetric inputs,only the magnetic resonance in the upper and lower dielectric layers can be excited,and the number of absorption peaks will be reduced by half compared to the antisymmetric inputs.Change the upper and lower dielectric layer materials,the absorption peak frequency can be flexibly adjusted between low frequency and high frequency.Combining the influence of wafer size and dielectric material on the absorption peak frequency,the absorptionbandwidth is nearly nine times larger than that of the single-frequency absorption in the designed multi-layer multi-size MIM metasurface.The results obtained in this paper can theoretically achieve the broadband coherence perfect absorption and greatly expands the bandwidth capacity of optical calculation.In addition,it provides a theoretical basis for the design of frequency selective coherence perfect absorber.3.The absorption properties of graphene elliptical patch-Insulator-elliptical patch(M(e)-I-M(e))metasurface for TE and TM polarization and the application of graphene's electrically tunable properties in coherent absorption.The effect of long and short axis length of elliptical patch and graphene Fermi level on coherent absorption was studied by numerical simulation.Studies have shown that due to the size,the elliptical M(e)-I-M(e)structure will absorb electromagnetic waves of different frequencies when the TE and TM polarized waves are incident.Keep the long axis of the elliptical patch unchanged while change the size of the short axis,the absorption under TM polarization does not change significantly and the absorption peak under TE polarization will move,vice versa.Arranging multiple elliptical patches in a symmetrical manner,the absorption will be polarization independent.The study of graphene shows that the adjustment of the Fermi level can make the absorption peak frequency move within a certain range while the absorption rate can be tuned by the phase modulation at the specific frequency.Thus this design can realize the double adjustment of absorption frequency and absorption rate.The results obtained in this paper provide a theoretical basis for the design of frequencyadjustable or polarization-dependent coherent perfect absorbers.