| With the development of information technology,information war now has great influences on construction of national defense.Under this background,metasurfaces,with great power of manipulating electromagnetic(EM)waves,have been seen as a pivotal technology to realize EM information safety.In this paper,several new types of metasurfaces are designed for radar cross section(RCS)reduction and Doppler stealth.The main works of this thesis are summarized as follows:1.A mechanism is proposed to optimize tunable metasurfaces based on multi-objective genetic algorithm and a tunable metasurface is designed which can realize RCS reduction arbitrarily in a wide frequency band accordingly.The field and circuit co-simulation method is used to optimize the structural parameters of the metasurface with multi-objective genetic algorithm.Then,by applying an external voltage source to control the working state of PIN diodes,the reflectivity of the metasurface is dynamically controlled.Therefore,the RCS of metasurface can be effectively modified in an ultra-wide frequency band.This efficient optimization method accelerates the pace of structural design and the designed metasurface has great ability of reflectivity manipulation.2.A new method is proposed to verify the linear Doppler effect with time-domain coding metasurface.A metasurface whose reflection phase is controlled by the voltage of varactor diodes is designed and fabricated.The metasurface can generate a frequency shift to incident EM waves by adding a specific voltage-time coding sequence,so that the linear Doppler effect can be observed without any real movements.In the experiment,the moving target in the real scene is simulated by transferring the energy from the center frequency to the harmonic frequency.3.An amplitude reconfigurable metasurface is proposed to generate the rotational Doppler effect.The propeller blade rotation process is simulated by dynamically manipulating the reflection amplitude of the metasurface in different regions and the actual Doppler frequency shift is calculated theoretically.A new method is also presented to verify the rotational Doppler effect by using the orbital angular momentum(OAM)metasurface.The OAM EM waves are generated by quantifying the phase gradients of different regions.Finally,the rotational Doppler effect is experimentally demonstrated by an OAM metasurface,in which the spatiotemporal reflection phase of each region is modulated.This type of metasurface provides a new method for the generation of orbital angular momentum beam. |
