When it comes to designing man-made structures with unusual or desirable optical properties, one popular goal is the realization of three-dimensional bulk metamaterials. However, thin surfaces with custom-designed optical properties are potentially useful too. Now, Shulin Sun and colleagues from Taiwan and China have fabricated a gradient metasurface from an array of nanoantennas whose properties vary across the sample. The surface is an efficient reflector of near-infrared light whose angle of reflection can be tailored by adjusting the gradient of the metallic structures. In addition, the researchers shifted operation to shorter wavelengths than previous experiments by reducing the size of the features that make up the surface. The metasurface is made from a 130-nm-thick gold reflecting plane coupled with gold rod antennas on an upper layer with a 50-nm-thick MgF spacer. The phase of the reflected or scattered field is engineered by varying the lengths of the antenna rods across the sample. The rod length determines the resonant frequency and hence the phase imparted to the scattered field from each point on the reflector. The structure operates across a wide wavelength range of 750–900 nm, owing to the broad resonance of lossy low-Q-factor resonators and to the gradient of the antennas' resonant frequencies across the sample. The researchers hope this approach will be useful for applications such as optical absorbers and optical mode couplers.
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Pile, D. Tailoring reflection. Nature Photon 7, 84 (2013). https://doi.org/10.1038/nphoton.2013.14