| Metasurface is a new artificial planar electromagnetic(EM)metamaterial with powerful manipulation ability of EM waves,whose array configuration is the periodic or aperiodic arrangement of subwavelength meta-particles.In 2014,prof.Tie Jun Cui proposed the concept of coding metasurface to control EM waves in digital way,which provides a bridge between the physical world and information science by loading digital information.The introduction of active devices realizes the leap-forward development of information metasurfaces,which are transformed from static control to dynamic control,even real-time control.The main focus of this research object is reflection-transmission metasurface,especially in manipulation of full EM waves,mainly studies the following contents:1)An ultra-thin Huygens metasurface with high transmission amplitude and 360° phase shift is proposed by using of antisymmetric structure design.The Huygens metasurface is composed of only two layers of metal structure without vias,and has a thickness of 0.033 wavelengths.In the case of normal incidence,the metasurface can cover the 360° transmission phase shift under the transmission amplitude of no less than-2.5dB criteria.Two practical applications based on the above the Huygens metasurface:First,a high-gain and ultra-thin transmitarray antenna is designed.The measured aperture efficiency of the antenna is 61.04%,which is far superior to that of the same type of transmitarray antenna.This design is expected to contribute to the design of ultra-thin and high-performance transmitarray antenna.Second,based on the Huygens metasurface and combined with holographic focusing imaging algorithm,an ultra-thin 3D imaging metasurface is designed.The imaging metasurface can simultaneously display different images at different imaging focal distance.2)A single polarization full-space metasurface design is realized by combining passive structure and active element.Then,a 1-bit switchable reflection-transmission integrated digital coding metasurface loaded with PIN diode is designed.By changing the states of the PIN diodes,it can be represented as a transmitarray or a reflectarray.3)A full-space reconfigurable anisotropic metasurface is developed and designed by loading PIN diodes in the horizontal and vertical directions respectively.The metasurface can independently control not only the near/far-field modes of EM waves,but also the transmission and reflection modes of EM waves under different polarization incidences.As an application demonstration,near-field holography imaging is choosed in this chapter.When x-polarized and y-polarized EM waves are incident,indepent images can be achieved without affecting each other.At the same time,by adjusting the control voltages of the diodes in the horizontal and vertical directions respectively,the switching of the reflection-transmission working mode under the incident x-polarized and y-polarized EM waves can be achieved independently.4)A novel reflection-transmission reconfigurable digital coding metasurface is further proposed.The metasurface can not only realize the independent control of x-and y-polarized EM waves,but also the reflection mode and transmission mode of x-and y-polarized EM waves can be independently controlled.As example verification,a(far-field)EM diffusion and(near-field)holography encryption design is carried out based on the proposed digital coding metasurface.When the metasurface exhibits a reflection mode,the effective RCS(Radar Cross Section)reduction for x-polarized and y-polarized EM waves can be achieved.When the metasurface exhibits a transmission mode,it can realize effective holography encryption based on OAM(Orbital Angular Momentum)for both x-and y-polarized transmission EM waves,ensuring the safe of transmission information.5)A multitasking shared bidirectional digital coding metasurface is proposed,which can achieve independent dual-band control of reflected and transmitted waves regardless of whether EM waves are incident from the front or back of the metasurface.Further,reflected and transmitted waves can also work at completely different frequencies to achieve multi-band(six bands)control of full space EM waves.As two proof-of-concepts,both the simulated and fabricated two-band full-space metasurfaces and six-band full-space metasurfaces have good performance. |
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