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.

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.

Field-effect active plasmonics for ultracompact electro-optic switching Appl. Phys. Lett. 101, 121113 (2012) Highly complex optical signal generation using electro-optical systems with non-linear, non-invertible transmission functions Appl. Phys. Lett. 101, 071115 (2012) Preparation of an exponentially rising optical pulse for efficient excitation of single atoms in free space Rev. Sci. Instrum. 83, 083104 (2012) High-speed waveguide-coupled graphene-on-graphene optical modulators

The propagation of light in nonperiodic quasicrystals is studied by ultrashort pulse interferometry. Samples consist of multilayer dielectric structures of the Fibonacci type and are realized from porous silicon. We observe mode beating and strong pulse stretching in the light transport through these systems, and a strongly suppressed group velocity for frequencies close to a Fibonacci band gap. A theoretical description based on transfer matrix theory allows us to interpret the results in terms of Fibonacci band-edge resonances.

We explore the relationship between the near-field enhancement, absorption, and scattering spectra of localized plasmonic elements. A simple oscillator model including both internal and radiative damping is developed, and is shown to accurately capture the near- and far-field spectral features of linear optical antennas, including their phase response. At wavelengths away from the interband transitions of the metal, we expect the absorption of a plasmonic element to be red-shifted relative to the scattering, and the near-field to be red-shifted relative to both.

The manipulation of light by conventional optical components such as lenses, prisms, and waveplates involves engineering of the wavefront as it propagates through an optically thick medium. A unique class of flat optical components with high functionality can be designed by introducing abrupt phase shifts into the optical path, utilizing the resonant response of arrays of scatterers with deeply subwavelength thickness. As an application of this concept, we report a theoretical and experimental study of birefringent arrays of two-dimensional (V-and Y-shaped) optical antennas which support two orthogonal charge-oscillation modes and serve as broadband, anisotropic optical elements that can be used to locally tailor the amplitude, phase, and polarization of light. The degree of optical anisotropy can be designed by controlling the interference between the waves scattered by the antenna modes; in particular, we observe a striking effect in which the anisotropy disappears as a result of destructive interference. These properties are captured by a simple, physical model in which the antenna modes are treated as independent, orthogonally oriented harmonic oscillators.plasmonics | resonance | metasurfaces | scattering T he general function of optical devices consists of the modification of the wavefront of light by altering its phase, amplitude, and polarization in a desired manner. The class of optical components with varying phase retardation includes lenses, waveplates, spiral phase plates (1), axicons (2), and more generally spatial light modulators, which are able to imitate many of these components by means of a dynamically tunable spatial response (3). All of these conventional optical components rely on gradual evolution of phase, amplitude, and polarization as the wave propagates through an optically thick medium. The introduction of abrupt phase changes into the optical path by using the resonant behavior of plasmonic nanostructures allows one to achieve control over the wavefront without relying on gradual phase accumulation (4). This approach is now enabling the design of various optical devices which are thin compared to the wavelength of light (5-7).Our previous work on phase discontinuities involved spatially inhomogeneous configurations of V-shaped optical antennas (4, 5, 7). Here, we report that homogeneous arrays optical antennas supporting two independent and orthogonally oriented current modes operate as highly birefringent meta-surfaces. We consider arrays of V-shaped antennas, creating a connection with our previous work, and Y-shaped antennas in which the anisotropy can be widely tailored or extinguished via interference between the scattered light from the two current modes. A simple, analytical two-oscillator model for such two-dimensional (2D) optical antennas is developed which captures the physics of these antennas and provides an intuitive way to understand how engineering of the amplitude and phase of the scattered light provides control over the optical anisotropy of the resulting meta-sur...