Polarization-Controlled Multifrequency Coherent Perfect Absorption In Metamaterials

Metamaterials are artificial electromagnetic media with spatially well-designed meta-molecules on the sub-wavelength scale.They exhibit attractive electromagnetic properties.As a promising application,metamaterial absorbers have been widely studied due to incomparable advantages to the naturally existing materials.Coherent perfect absorption(CPA)is regarded as the time-reversed counterpart of laser emission and can be achieved via interferometric control of two or even more input waves incident upon two opposite sides of a sample.In this work,we have experimentally demonstrated polarization-controlled multifrequency CPAs in stereometamaterials with stacked twisted asymmetrically split rings(ASR).The subtle coupling effects in bilayered ASRs cause abundant electromagnetic responses that strongly depend on incident polarization states.The main works include as follows:Firstly,the interaction of coherent incident light with metamaterials was studied by using the transfer matrix and scattering matrix in the four-port device and the near-field coupling and far-field coupling.The difference of electromagnetic modes excited by incident single beam or the standing wave field formed by coherent light was analyzed.The coherent control technique is briefly introduced to separate and recognize the hidden electromagnetic modes in the absorption spectrum of metamaterials when a single beam is incident.Secondly,the coherent perfect absorption properties of two kinds of asymmetric split-ring metamaterials are mainly studied theoretically and experimentally in this work.When the single-beam x-polarized light is incident,the two metamaterials have three absorption peaks and the absorption value are nearly 0.5,where the two peaks are superimposed together,whereas the incident the single y-polarization,the metamaterial with θ = 0° has two absorption peaks near 0.5 and the other one with θ = 180° has an absorption peak near 0.7.On the basis of this,the absorption properties of two kinds of metamaterials when they are incident on coherent light are studied.When the two incident lights are x-polarized,three absorption peaks appear in the two kinds of metamaterials and there are two electromagnetic modes of the three absorption peaks.When the two incident lights are y-polarized,two kinds of metamaterials have two absorption peaks.For the metamaterial with θ = 0°,the two absorption peaks have the same frequency as the single beam incident and the electromagnetic mode is also the same.For the metamaterial with θ = 180°,the two absorption peaks have very close and there is different electromagnetic modes.Therefore,it is confirmed that one absorption peak of a single beam incident contains two electromagnetic modes.Thirdly coherent absorption at all resonant frequencies of two metamaterials at coherent light incidence may periodically change between 0 and 1 as the phase difference changing between the two incident light.At different polarization light incident,the two metamaterials produce different coherent perfect absorption bands.When x-polarized light is incident,the band with coherent perfect absorption occurs in the low frequency band of 4-6 GHz.When the y-polarized light is incident,the band of perfect coherent absorption occurs at 9-11 GHz.By different polarization state of the incident,excited different bands of multi-band coherent perfect absorption,the switch of coherent perfect absorption band could be controlled.