Metamaterials are artificial materials made of subwavelength elementary cells that give rise to unexpected wave properties that do not exist naturally. However, these properties are generally achieved due to 3D patterning, which is hardly feasible at short wavelengths in the visible and near-infrared regions targeted by most photonic applications. To overcome this limitation, metasurfaces, which are the 2D counterparts of metamaterials, have emerged as promising platforms that are compatible with planar nanotechnologies and thus mass production, which platforms the properties of a metamaterial into a 2D sheet. In the linear regime, wavefront manipulation for lensing, holography, and polarization control has been achieved recently. Interest in metasurfaces operating in the nonlinear regime has also increased due to the ability of metasurfaces to efficiently convert incident light into harmonic frequencies with unusual polarization properties. However, to date, the nonlinear absorption of metasurfaces has been mostly ignored. Here, we demonstrate that plasmonic metasurfaces behave as saturable absorbers with modulation performances superior to the modulation performance of other 2D materials and exhibit unusual polarimetric nonlinear transfer functions. We quantify the link between saturable absorption, the plasmonic resonances of the unit cell and their distribution in a 2D metasurface, and finally provide a practical implementation by integrating the metasurfaces into a fiber laser cavity operating in pulsed regimes driven by the metasurface properties. As such, this work provides new perspectives on ultrathin nonlinear saturable absorbers for applications where tunable nonlinear transfer functions are needed, such as in ultrafast lasers or neuromorphic circuits.

We present a design for a highly efficient and omnidirectional color-selective filter for the visible spectrum, based on a Fabry-Perot metal-dielectric-metal nanoresonator. The filter can have the same color transmitted in a range of incident angles from 0° up to 60° for TM polarization. The dielectrics used for each color filter are carefully chosen so that the angle-insensitive resonance conditions are satisfied while transmission values from 44.3% to 78.36% are achieved. We calculated the dielectric thickness for each filter and analyzed the optimal Ag thickness for maximum transmission. The proposed filters have a simple multilayer structure and do not require complex lithographic fabrication processes.

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Fig. I. Schematics of the lens composed by concentrical metallic rings.Abstract-We present the design and analysis of subwalength lenses based on air cylindrical waveguides surrounded by metal for the focusing of light in very small areas. The lenses are composed by several concentrically disposed air-metal waveguides with different widths in order to control the phase shift ing necessary to focus the light. The lenses we designed to operate at visible frequencies then, the propagation mechanism of light along the air-metal waveguides is obtained by the surface plasmon polaritons (SPPs). The structures have been analyzed by an efficient finite element formulation in cylindrical coordinates with perfectly matched layer (PMLs). Incident ligth650 nm lens r z I. INTRO DUCTIO NThe transmission of light through sub wavelength metallic apertures with low losses brought up a new era for the design of new types of nanophotonic circuits [1][2][3][4][5]. It makes possible to design several devices such as bends, nanowaveguides, and lenses.In this paper we analyze and design lenses by using an array of variant width nano-slits formed by air cylindrical waveguides surrounded by metal. We follow [I], where the designed lenses were used for focusing light in two dimensions (20). Here, we analyze the tridimensional (3D) subwavelength focusing and confirm the possibility of this light confining. The 3D focusing mechanism has been obtained by considering the cylindrical counterpart of 20 lenses. It was obtained by rotating the designed lenses in rectangular coordinates creating a new structure, which is compo sed by concentrically disposed metallic ring s as can be seen in Fig. I. The lenses are then a sequence of air-metal-air ring with different width and same depth. The variant width let introduce a phase shift in the output wave in order to obtain constructive interference at the desired focal distance. The metal here considered was silver for their fabrication feasibilities. The designed structures have been analyzed by an efficient finite element formulation in cylindrical coordinates with perfectly match ed layer (PMLs) [6].978-1 -4244-5357-3/09/$26.00©20091 EEE II. M ETAL AIR META L WAV EGUID ESWe analyzed first the propagated mod es in a metal air metal waveguide by solving the transcendental equation to obtain their propagation constants. We consider the complex nature of the refractive index of metals operating at visible frequencies. The propagation constants were obtained by solving [I] tanh(~(32-k~ed w)=_ ed~(32-k~em (1) 2 e 1(32 -ee m" 0 d where k o is the free space wavenumber, Cd and C m are the relative permittivity of air (dielectric) and the metal and w is the waveguide width.We use a multiple step refinement process in conjunction with an iterative bisection scheme to solve efficiently (1) and obtain the values offJ.The real and imaginary parts of fJ shown in Fig. 2, determine the phase velocity and the propagation loss of SPPs inside the metallic slit, respectively. The waveguides widths have been consider...

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