Z. Gaburro

Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction

Light propagation can be controlled with plasmonic interfaces that introduce abrupt phase shifts along the optical path. 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.

Aberration-Free Ultrathin Flat Lenses and Axicons at Telecom Wavelengths Based on Plasmonic Metasurfaces

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 V-shaped nanoantennas that introduce a radial distribution of phase discontinuities, thereby generating respectively spherical wavefronts and nondiffracting 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.

A Broadband, Background-Free Quarter-Wave Plate Based on Plasmonic Metasurfaces

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.

Ultra-thin plasmonic optical vortex plate based on phase discontinuities

A flat optical device that generates optical vortices with a variety of topological charges is demonstrated. This device spatially modulates light beams over a distance much smaller than the wavelength in the direction of propagation by means of an array of V-shaped plasmonic antennas with sub-wavelength separation. Optical vortices are shown to develop after a sub-wavelength propagation distance from the array, a feature that has major potential implications for integrated optics.

Out-of-plane reflection and refraction of light by anisotropic optical antenna metasurfaces with phase discontinuities.

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.

Flat Optics: Controlling Wavefronts With Optical Antenna Metasurfaces

Conventional optical components rely on the propagation effect to control the phase and polarization of light beams. One can instead exploit abrupt phase and polarization changes associated with scattered light from optical resonators to control light propagation. In this paper, we discuss the optical responses of anisotropic plasmonic antennas and a new class of planar optical components (“metasurfaces”) based on arrays of these antennas. To demonstrate the versatility of metasurfaces, we show the design and experimental realization of a number of flat optical components: 1) metasurfaces with a constant interfacial phase gradient that deflect light into arbitrary directions; 2) metasurfaces with anisotropic optical responses that create light beams of arbitrary polarization over a wide wavelength range; 3) planar lenses and axicons that generate spherical wavefronts and nondiffracting Bessel beams, respectively; and 4) metasurfaces with spiral phase distributions that create optical vortex beams of well-defined orbital angular momentum.

Porous silicon-based rugate filters

We report an experimental study of porous silicon-based rugate filters. We performed filter apodization, following a half-apodization approach, which successfully attenuated the sidelobes at both sides of the photonic stop band. We achieved successful reduction of interference ripples through the insertion of index-matching layers on the first and last interfaces. An apodized dielectric mirror and a rugate filter are compared: Appreciable differences in the harmonic presence and stop-band performance were observed and are commented on. Bandwidth control when index contrast is modified is also demonstrated. Finally, the possibility of combining different rugate filter designs to attain more complex responses is demonstrated by the achievement of a multi-stop-band filter. Numerical calculations for design optimization and comparison with experimental data are reported too.

Nonlinear optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition

The real and imaginary parts of third-order nonlinear susceptibility χ(3) have been measured for silicon nanocrystals embedded in SiO2 matrix, formed by high temperature annealing of SiOx films prepared by plasma-enhanced chemical vapor deposition. Measurements have been performed using a femtosecond Ti–sapphire laser at 813 nm using the Z-scan technique with maximum peak intensities up to 2×1010 W/cm2. The real part of χ(3) shows positive nonlinearity for all samples. Intensity-dependent nonlinear absorption is observed and attributed to two-photon absorption processes. The absolute value of χ(3) is on the order of 10−9 esu and shows a systematic increase as the silicon nanocrystalline size decreases. This is due to quantum confinement effects.

Free-standing porous silicon single and multiple optical cavities

Porous silicon free-standing microcavity structures, with different layer designs, have been fabricated. Single microcavities show transmission resonances in the technologically relevant wavelength region of 1.55 μm with quality factors up to 3380. High-order cavities show sub-nm transmission peaks over the whole stop band. Coupled microcavity structures, where splitting of the degenerate cavity mode occurs, lead to multiple transmission peaks in a limited region of the stop band. We also report incident angle-dependent measurements, where transmission peak blueshift and splitting of transverse electric and transverse magnetic polarized modes due to porous silicon birefringence were observed.

Giant birefringence in optical antenna arrays with widely tailorable optical anisotropy

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.