In this paper, a displacement sensor with an electrically extremely small size and high sensitivity is proposed based on an elaborately designed metamaterial element, i.e., coupled split-ring resonators (SRRs). The sensor consists of a feeding structure with a rectangular opening loop and a sensing structure with double-layer coupled SRRs. The movable double-layer structures can be used to measure the relative displacement. The size of microwave displacement sensors can be significantly reduced due to the compact feeding and sensing structures. By adjusting the position of the split gap within the resonator, the detection directions of the displacement sensing can be further expanded accordingly (along with the x- or y-axis) without increasing its physical size. Compared with previous works, the extremely compact size of 0.05λ₀ × 0.05λ₀ (λ₀ denotes the free-space wavelength), a high sensitivity, and a high quality factor (Q-factor) can be achieved by the proposed sensor. From the perspective of the advantages above, the proposed sensor holds promise for being applied in many high-precision industrial measurement scenarios.

A three-port plasmonic array is proposed to achieve a dual-field superfocusing using the forward-propagation method based on spatial Fourier transform (SFT). The algorithm of fast Fourier transform (FFT) is used to facilitate the design process. Both the electric and magnetic fields can be superfocused on the focal plane of z = 0.1 (is the free-space wavelength) based on an ef-fect of radiationless electromagnetic interference (REI). The full widths of the half-maximum (FWHMs) of both two fields are 0.060 and 0.072, respectively. Besides, the metrics of the du-al-field superfocusing are 1.67 and 1.39. The proposed theory can simplify the design process of superfocusing, and the plasmonic array with brief three ports can sufficiently facilitate the con-struction of the desired source. This work can be beneficial for the development of beam steering, sophisticated holography, su-per-resolution imaging, and other applications of near- or far-field devices, etc.