Terahertz Near-Field Metasurfaces: Amplitude-Phase Combined Steering and Electromagnetostatic Dual-Field Superfocusing

Subwavelength resolution imaging entails a sharpened superfocusing to overcome the diffraction limit. However, conventional high-numerical-aperture superfocusing lenses are bulky and expensive. Relying on the abrupt amplitude and phase discontinuities, metasurfaces have been recently explored as an ultrathin platform to generate arbitrary wavefronts over subwavelength thicknesses. In this paper, a dual-field superfocusing is demonstrated for terahertz wavelengths using a near-field metasurface by simultaneously tailoring the amplitude and phase of the evanescent field, which satisfies both electrostatic and magnetostatic limits. A combined analytic theory is introduced to describe the physical mechanism and the entire design process of the near-field metasurface. With three-layer minuscule elements characterized by extremely small electric sizes, the metasurface can force both incident electric and magnetic fields to converge to a sharpened focus spot (0.066 and 0.045 wavelength focus spots at a distance of 0.073 wavelength) through radiationless electromagnetic interference. Practical implementations of these metasurfaces hold promise for near-field terahertz beam steering, laser-based microscopy, noncontact sensing, imaging, and lithography applications.