Near-Field Metasurfaces: Subdiffraction Focusing of Terahertz Waves

Using a gradient-index (GRIN) metasurface to control the electromagnetic near field, we demonstrate sub-diffraction focusing at ~ 1 THz. The near-field metasurface consists of three-cell layers of square-slot elements and focuses terahertz plane waves to a spot of size ≈ 0.17 λ0 (half-power beamwidth) at a distance of 0.2 λ0, where λ0 is the free-space wavelength. These metasurfaces will find applications in super-resolution imaging, microscopy, and lithography.