Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat's principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.

The concept of optical phase discontinuities is applied to the design and demonstration of aberration--free planar lenses and axicons, comprising a phased array of ultrathin subwavelength spaced optical antennas. The lenses and axicons consist of radial distributions of V--shaped nanoantennas that generate respectively spherical wavefronts and non--diffracting Bessel beams at telecom wavelengths.Simulations are also presented to show that our aberration--free designs are applicable to high numerical aperture lenses such as flat microscope objectives.The fabrication of lenses with aberration correction is challenging in particular in the mid--and near--infrared wavelength range where the choice of transparent materials is limited.Usually it requires complex optimization techniques such as aspheric shapes or multi--lens designs [1,2], which are expensive and bulky.Focusing diffracting plates offer the possibility of designing low weight and small volume lenses. For example the Fresnel Zone Plate focuses light by diffracting from a binary mask that blocks part of the radiation [1]. A more advanced solution is represented by the Fresnel lens, which introduces a gradual phase retardation in the radial direction to focus light

We demonstrate optically thin quarter-wave plates built with metasurfaces that generate high-quality circularly polarized light over a broad wavelength range for arbitrary orientation of the incident linear polarization. The metasurface consists of an array of plasmonic antennas with spatially varying phase and polarization responses. Experimentally demonstrated quarter-wave plates generate light with a high degree of circular polarization (>0.97) from λ = 5 to 12 μm, representing a major advance in performance compared to previously reported plasmonics-based wave plates.

The replacement of bulk refractive optical elements with diffractive planar components enables the miniaturization of optical systems. However, diffractive optics suffers from large chromatic aberrations due to the dispersion of the phase accumulated by light during propagation. We show that this limitation can be overcome with an engineered wavelength-dependent phase shift imparted by a metasurface and demonstrate a design that deflects three wavelengths without dispersion. A planar lens without chromatic aberrations at three wavelengths is also presented. Our design is based on low-loss dielectric resonators which introduce a dense spectrum of optical modes to enable dispersive phase compensation. The suppression of chromatic aberrations in metasurface-based planar photonics will find applications in lightweight collimators for displays, and chromatically-corrected imaging systems.Refractive and diffractive optical components share many similarities when they are used to manipulate monochromatic light but their response to broadband light is very different. For a material with normal dispersion, refractive lenses have larger focal distances for red light than for blue light and prisms deflect longer wavelengths by a smaller angle; the contrary occurs for diffractive lenses and gratings (1, 2). This contrasting behavior arises because two different principles are used to shape the light: refractive optics rely on the phase gradually accumulated through propagation, while diffractive optics operate by means of interference of light transmitted through an amplitude or phase mask. In most transparent materials in the visible the refractive index n(λ) decreases with increasing wavelength ("normal dispersion"). Since the deflection angle θ of a prism

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Experiments on ultrathin anisotropic arrays of subwavelength optical antennas display out-of-plane refraction. A powerful three-dimensional (3D) extension of the recently demonstrated generalized laws of refraction and reflection shows that the interface imparts a tangential wavevector to the incident light leading to anomalous beams, which in general are noncoplanar with the incident beam. The refracted beam direction can be controlled by varying the angle between the plane of incidence and the antenna array.

Nanoscale optical resonators enable a new class of flat optical components called metasurfaces. This approach has been used to demonstrate functionalities such as focusing free of monochromatic aberrations (i.e., spherical and coma), anomalous reflection, and large circular dichroism. Recently, dielectric metasurfaces that compensate the phase dispersion responsible for chromatic aberrations have been demonstrated. Here, we utilize an aperiodic array of coupled dielectric nanoresonators to demonstrate a multiwavelength achromatic lens. The focal length remains unchanged for three wavelengths in the near-infrared region (1300, 1550, and 1800 nm). Experimental results are in agreement with full-wave simulations. Our findings are an essential step toward a realization of broadband flat optical elements.

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