Abstract
Diffraction gratings disperse light in a rainbow of colors with the opposite order than refractive prisms, a phenomenon known as negative dispersion. While refractive dispersion can be controlled via material refractive index, diffractive dispersion is fundamentally an interference effect dictated by geometry. Here we show that this fundamental property can be altered using dielectric metasurfaces, and we experimentally demonstrate diffractive gratings and focusing mirrors with positive, zero, and hyper negative dispersion. These optical elements are implemented using a reflective metasurface composed of dielectric nano-posts that provide simultaneous control over phase and its wavelength derivative. In addition, as a first practical application, we demonstrate a focusing mirror that exhibits a five fold reduction in chromatic dispersion, and thus an almost three times increase in operation bandwidth compared to a regular diffractive element. This concept challenges the generally accepted dispersive properties of diffractive optical devices and extends their applications and functionalities.
Highlights
Most optical materials have positive dispersion, which means that the refractive index decreases at longer wavelengths
The reason diffraction gratings are said to have negative dispersion is that they disperse light similar to hypothetical refractive prisms made of a material with negative dispersion [Fig. 1(b)]
Dispersion is not related to material properties, and it refers to the derivative of a certain device parameter with respect to wavelength
Summary
Most optical materials have positive (normal) dispersion, which means that the refractive index decreases at longer wavelengths. Metasurfaces have attracted great interest in the recent years [3,4,5,6,7,8,9,10,11,12] because they enable precise control of optical wavefronts and are easy to fabricate with conventional microfabrication technology in a flat, thin, and lightweight form factor Various conventional devices, such as gratings, lenses, holograms, and planar filter arrays [7,8,9,13,14,15,16,17,18,19,20,21,22,23,24,25,26], as well as novel devices [27,28] have been demonstrated using metasurfaces. At will control of chromatic dispersion, adds a new functionality to metasurfaces not available in conventional diffractive or refractive devices
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