| Due to its unusual electromagnetic response characteristics,artificial electromagnetic meta-surfaces have been widely used in electromagnetic absorption,terahertz devices,nonlinear optics,spectral imaging and microwave communication.In fact,electromagnetic absorption plays an important role in military and civil aspects such as stealth technology,electronic information countermeasure,electromagnetic compatibility,radiation protection,biosensor and new energy technology.Therefore,electromagnetic super-surface absorbers with sub-wavelength size and perfect absorbing effect have been widely studied and designed.At present,the commonly used electromagnetic absorbers are single channel absorbers with electromagnetic wave incident in one direction,which pursue the characteristics of high absorptivity,wide absorption band,polarization insensitive and wide incidence angle,etc.In recent years,the coherent perfect absorber is a dual-channel absorber that works under the reverse propagation of two electromagnetic waves.It uses the interference cancellation of coherent waves to achieve complete electromagnetic absorption in lossy materials.The potential application value of its powerful absorption rate tunability in optical switches and modulators has attracted people’s attention.Water has good dielectric loss and frequency dispersion characteristics in the microwave frequency band,and is a low-cost material suitable for designing broadband absorbers.At the same time,its dielectric constant and temperature-related characteristics provide the possibility to realize the tunability of the absorber.In this paper,based on water materials,the typical single-channel absorber and the dual-channel coherent perfect absorber with the ability to dynamically adjust the absorption rate are studied and designed.The specific content is as follows:1.A "pyramid" type of wideband water-based all-medium absorber is designed for typical single-channel absorber.The absorptivity is maintained above 90% in the two frequency bands of 7.51-9.84 GHz and 11.49-90 GHz,and the bandwidth of the working band is80.84 GHz,in which the relative bandwidth of the main working band is 154.7%.In addition to the characteristics of insensitivity to polarization,wide angle of incidence,and insensitivity to temperature in the operating frequency band,it also has the characteristics of dynamic adjustment of the absorption peak by manipulating the water temperature in the non-operating frequency band to achieve a wider operating band.2.An ultra-thin structure with a "sandwich" configuration composed of water and metal SRR rings is proposed as a design prototype of a dual-channel coherent perfect absorber,which can achieve modulation of the coherent absorption peak intensity by changing the phase difference,So as to achieve the switch between completely "absorbed" and completely "transparent".By adjusting the relative opening orientations of SRR rings on both sides of the intermediate layer,the TE polarized and TM polarized electromagnetic wave coherent absorption channels can be overlaps and separated,and the maximum frequency splitting interval is 500 MHz.By changing the water temperature,the interaction between the coherent meta-surface and the electromagnetic wave can be controlled to realize the dynamic tuning of the absorbing performance.Then,the package design of the prototype of the coherent perfect absorbing meta-surface was carried out,and by comparing the performance difference before and after the package,it was proved that the prototype has a good tolerance to the package method.Finally,the possibility of adjusting the polarization sensitivity of the coherent perfect absorber by active regulation is discussed,which is equivalent to the change of the relative opening orientation of the SRR ring periodically embedded before and after the water layer by controlling the PIN switch diode on and off.3.In order to break the limitation that most coherent perfect absorbers can only work at a single band frequency,a multi-band coherent perfect absorber is designed by combining different resonant units.Under antisymmetric coherent wave incidence(the phase difference between the two coherent incident waves is 180°),three-band coherent perfect absorption at frequencies 7.54 GHz,11.04 GHz and 15.91 GHz is achieved,and the reflection coefficients at the three absorption peaks are summed.The transmission coefficient relationship satisfies the condition of perfect interference cancellation of coherent waves.The field analysis is used to explain that the formation of multi-band absorption is caused by the resonant operation of different size elements.Combined with the influence analysis of structural parameters,the working mechanism of the multi-band coherent perfect absorber is further proved.By changing the water temperature,the frequencies of the three absorption peaks can be shifted,which shows greater application value than the coherent perfect absorber in single-frequency operation. |
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