Impedance-Matching Technique of Metasurfaces Generating Evanescent Fields for Subwavelength Focusing

We propose an impedance-matching technique for metasurfaces generating evanescent fields with complex free-space impedance for subwavelength focusing. The proposed technique is mandatory for the subwavelength focusing far beyond the diffraction limit. First, we theoretically show the fact that the wave impedance becomes a complex number due to the generated evanescent fields and give an explicit form of the wave impedance as a ratio of local electric and magnetic fields. Then, we discuss the impedance-matching technique for the distributed complex wave impedance by a metasurface with a position-dependent impedance matrix. To demonstrate the proposed technique, we design a subwavelength focusing 1-D metasurface with the null-to-null beamwidth of one-twentieth of the free-space wavelength at 2.45 GHz, and its performance is verified by wave propagation simulations based on the equivalent circuit model. In order to further verify the validity of the proposed technique, a metasurface is implemented with the trilayer configuration, and the subwavelength focusing is experimentally demonstrated.