Research On Photo-excited Switchable Multi-state Terahertz Metamaterial Absorbers

Metamaterial(MM)absorbers have attracted much attention in the microwave and optical band,especially in terahertz(THz)band due to their advantages of high absorptivity,wide-band,thin thickness,light weight,simple design,and easy processing,etc.It has been demonstrated that nearly perfect absorption of electromagnetic(EM)waves can be realized by reasonably designing and optimizing the shape,size and arrangement of the unit cell of the absorber to make its equivalent impedance match the impedance of free space.In recent years,research on MM absorbers has been gradually deepened,whose operating frequency has expanded from the microwave to optical frequency,and from single-band to multi-band or even broadband.In addition to high absorptivity,the polarization-insensitive and wide-angle incidence are also taken into account.In order to meet the needs of practical applications,and further expand the operating frequency and range of absorbers,most recently,tunable or switchable MM absorbers have become a hot spot.Based on the photosensitive properties of semiconductor materials,the conductivity of photosensitive semiconductor embedded in the designed structure can be effectively adjusted through changing the intensity of the external pump light,and then realize tunable frequency and absorptivity.In this paper,we focus on THz absorbers and propose two kinds of photo-excited switchable THz absorbers based on semiconductor materials,and the absorption characteristics and mechanism are studied by simulation.The main contents of this paper are as follows:1)Based on the photosensitive characteristics of semiconductor materials,a photo-excited switchable single-/dual-band THz absorber is designed by embedding semiconductor silicon(Si)blocks in the top resonator.For the different embedded position of the Si blocks,two kinds of single-/dual-band switchable absorption characteristics from 0.4 THz to 1.4 THz can beachieved.The switchable absorption mechanism is explained by the equivalent circuit of the structure and the electric field and current distribution at resonant frequencies.Also,simulation results show that the absorber has excellent absorption characteristics of polarization-insensitive and wide-angle incidence.2)Exploiting the different photosensitive characteristics of semiconductor Si and germanium(Ge),a photo-excited switchable tristate absorber operating in the range of 0.6 THz to 1.8 THz is designed by skillfully incorporating Si and Ge in appropriate position of MM unit cell.By applying pump light of different wavelengths on the absorber to control the excitation states of the Si and Ge blocks,the arbitrary switching among single-/single-/dual-band absorption states can be realized.Meanwhile,the absorption mechanism is demonstrated by analyzing the electric fields and surface current distribution at the corresponding peak frequencies for the three absorption states.Finally,the influence of the polarization and incident angle of the EM waves on the absorption characteristics are investigated.