Phase-matched second harmonic generation at the Dirac point of a 2-D photonic crystal.

We study second harmonic generation in a 2-D photonic crystal with the pump field tuned at the Dirac point of the structure. The simultaneous generation of both forward and backward phase-matched second harmonic is achieved by exploiting a peculiar regime in which the interacting waves have zero phase velocity in the lattice. This regime can be attained even when strong material dispersion is present and therefore lends itself well to be implemented in semiconductor-based frequency conversion devices.

Impedance matched thin metamaterials make metals absorbing.

Metals are generally considered good reflectors over the entire electromagnetic spectrum up to their plasma frequency. Here we demonstrate an approach to tailor their absorbing characteristics based on the effective metamaterial properties of thin, periodic metallo-dielectric multilayers by exploiting a broadband, inherently non-resonant, surface impedance matching mechanism. Based on this mechanism, we design, fabricate and test omnidirectional, thin (<1 micron), polarization independent, extremely efficient absorbers (in principle being capable to reach A > 99%) over a frequency range spanning from the UV to the IR.

All-optical bistability and switching near the Dirac point of a 2-D photonic crystal.

We investigate all-optical switching at the guided mode resonances originating near the Dirac point of a finite, 2-D photonic crystal consisting of a square lattice of dielectric columns possessing a cubic nonlinearity. The peculiar field localization properties of these Dirac-point guided mode resonances conspire to yield extremely low switching threshold at near-to-normal incidence for remarkably low filling factors of the nonlinear material.

Long range plasmon assisted all-optical switching at telecommunication wavelengths.

We exploit the properties of ultranarrow, Fano-like resonances generated by the coupling of long range surface plasmons with ultrathin (~10 nm), metallic, subwavelength gratings embedded in a nonlinear, cubic material to obtain all-optical switching at telecommunication wavelengths for extremely low input power. We provide an example of a silver metallic grating embedded in a chalcogenide glass (As2S3), and we show the concrete possibility to achieve all-optical switching at local field intensities compatible with the photo-darkening threshold of the material.

Thermal emission from a metamaterial wire medium slab.

We investigate thermal emission from a metamaterial wire medium embedded in a dielectric host and highlight two different regimes for efficient emission, respectively characterized by broadband emission near the effective plasma frequency of the metamaterial, and by narrow-band resonant emission at the band-edge in the Bragg scattering regime. We discuss how to control the spectral position and relative strength of these two emission mechanisms by varying the geometrical parameters of the proposed metamaterial and its temperature.

Broadband metamaterial for nonresonant matching of acoustic waves.

Unity transmittance at an interface between bulk media is quite common for polarized electromagnetic waves incident at the Brewster angle, but it is rarely observed for sound waves at any angle of incidence. In the following, we theoretically and experimentally demonstrate an acoustic metamaterial possessing a Brewster-like angle that is completely transparent to sound waves over an ultra-broadband frequency range with >100% bandwidth. The metamaterial, consisting of a hard metal with subwavelength apertures, provides a surface impedance matching mechanism that can be arbitrarily tailored to specific media.

Ultraviolet surface-enhanced Raman scattering at the plasmonic band edge of a metallic grating.

Surface-enhanced Raman Scattering (SERS) is studied in sub-wavelength metallic gratings on a substrate using a rigorous electromagnetic approach. In the ultraviolet SERS is limited by the metallic dampening, yet enhancements as large as 10(5) are predicted. It is shown that these enhancements are directly linked to the spectral position of the plasmonic band edge of the metal/substrate surface plasmon.

All-optical switching at the Fano resonances in subwavelength gratings with very narrow slits.

We theoretically discuss all-optical switching at the Fano resonances of subwavelength gratings made of a chalcogenide glass (As(2)S(3)). Particular attention is devoted to the case in which the grating possesses extremely narrow slits (channels ranging from a∼10 nm to a∼40 nm). The remarkable local field enhancement available in these situations conspires to yield low-threshold switching intensities (~50 MW/cm(2)) at telecommunication wavelengths for extremely thin (d∼200 nm) gratings when a realistic value of the As(2)S(3) cubic nonlinearity is used.

Plasmonic Brewster angle: broadband extraordinary transmission through optical gratings.

Extraordinary optical transmission through metallic gratings is a well established effect based on the collective resonance of corrugated screens. Being based on plasmonic resonances, its bandwidth is inherently narrow, in particular, for thick screens and narrow apertures. We introduce here a different mechanism to achieve total transmission through an otherwise opaque screen, based on an ultrabroadband tunneling that can span from dc to the visible range at a given incidence angle.

Enhanced second-harmonic generation from resonant GaAs gratings.

We theoretically study second harmonic generation in nonlinear, GaAs gratings. We find large enhancement of conversion efficiency when the pump field excites the guided mode resonances of the grating. Under these circumstances the spectrum near the pump wavelength displays sharp resonances characterized by dramatic enhancements of local fields and favorable conditions for second-harmonic generation, even in regimes of strong linear absorption at the harmonic wavelength.

Second harmonic generation from metallo-dielectric multilayered structures in the plasmonic regime.

We present a theoretical study on second harmonic generation from metallo-dielectric multilayered structures in the plasmonic regime. In particular we analyze the behavior of structures made of Ag (silver) and MgF2 (magnesium-fluoride) due to the straightforward procedure to grow these materials with standard sputtering or thermal evaporation techniques. A systematic study is performed which analyzes four different kinds of elementary cells--namely (Ag/MgF2)N, (MgF2/Ag)N, (Ag/MgF2/Ag)N and (MgF2/Ag/MgF2)N--as function of the number of periods (N) and the thickness of the layers.

Transmission function properties for multi-layered structures: application to super-resolution.

We discuss the properties of the transmission function in the k-space for a generic multi-layered structure. In particular we analytically demonstrate that a transmission greater than one in the evanescent spectrum (amplification of the evanescent modes) can be directly linked to the guided modes supported by the structure. Moreover we show that the slope of the phase of the transmission function in the propagating spectrum is inversely proportional to the ability of the structure to compensate the diffraction of the propagating modes.

Energy considerations for a superlens based on metal/dielectric multilayers.

We investigate the resolution and absorption losses of a Ag/GaP multilayer superlens. For a fixed source to image distance the resolution is independent of the position of the lens but the losses depend strongly on the lens placement. The absorption losses associated with the evanescent waves can be significantly larger than losses associated with the propagating waves especially when the superlens is close to the source.

Gap solitons in a nonlinear quadratic negative-index cavity.

We predict the existence of gap solitons in a nonlinear, quadratic Fabry-Pérot negative index cavity. A peculiarity of a single negative index layer is that if magnetic and electric plasma frequencies are different it forms a photonic band structure similar to that of a multilayer stack composed of ordinary, positive index materials. This similarity also results in comparable field localization and enhancement properties that under appropriate conditions may be used to either dynamically shift the band edge, or for efficient energy conversion.

Negative refraction and sub-wavelength focusing in the visible range using transparent metallo-dielectric stacks.

We numerically demonstrate negative refraction of the Poynting vector and sub-wavelength focusing in the visible part of the spectrum using a transparent multilayer, metallo-dielectric photonic band gap structure. Our results reveal that in the wavelength regime of interest evanescent waves are not transmitted by the structure, and that the main underlying physical mechanisms for sub-wavelength focusing are resonance tunneling, field localization, and propagation effects. These structures offer several advantages: tunability and high transmittance (50% or better) across the visible and near IR ranges; large object-image distances, with image planes located beyond the range where the evanescent waves have decayed.

Accessing quadratic nonlinearities of metals through metallodielectric photonic-band-gap structures.

We study second harmonic generation in a metallodielectric photonic-band-gap structure made of alternating layers of silver and a generic, dispersive, linear, dielectric material. We find that under ideal conditions the conversion efficiency can be more than two orders of magnitude greater than the maximum conversion efficiency achievable in a single layer of silver. We interpret this enhancement in terms of the simultaneous availability of phase matching conditions over the structure and good field penetration into the metal layers.

Second-harmonic generation at angular incidence in a negative-positive index photonic band-gap structure.

In the spectral region where the refractive index of the negative index material is approximately zero, at oblique incidence, the linear transmission of a finite structure composed of alternating layers of negative and positive index materials manifests the formation of a new type of band gap with exceptionally narrow band-edge resonances. In particular, for TM-polarized (transverse magnetic) incident waves, field values that can be achieved at the band edge may be much higher compared to field values achievable in standard photonic band-gap structures. We exploit the unique properties of these band-edge resonances for applications to nonlinear frequency conversion, second-harmonic generation, in particular.

Radiation pressure of light pulses and conservation of linear momentum in dispersive media.

We derive an expression for the Minkowski momentum under conditions of dispersive susceptibility and permeability, and compare it to the Abraham momentum in order to test the principle of conservation of linear momentum when matter is present. We investigate cases when an incident pulse interacts with a variety of structures, including thick substrates, resonant, free-standing, micron-sized multilayer stacks, and negative index materials. In general, we find that for media only a few wavelengths thick the Minkowski and Abraham momentum densities yield similar results.

Dynamics of short pulses and phase matched second harmonic generation in negative index materials.

We study pulsed second harmonic generation in metamaterials under conditions of significant absorption. Tuning the pump in the negative index range, a second harmonic signal is generated in the positive index region, such that the respective indices of refraction have the same magnitudes but opposite signs. This insures that a forward-propagating pump is exactly phase matched to the backward-propagating second harmonic signal.

Large enhancement of interface second-harmonic generation near the zero-n(-) gap of a negative-index Bragg grating.

We predict a large enhancement of interface second-harmonic generation near the zero-n(-) gap of a Bragg grating made of alternating layers of negative- and positive-index materials. Field localization and coherent oscillations of the nonlinear dipoles located at the structure's interfaces conspire to yield conversion efficiencies at least an order of magnitude greater than those achievable in the same length of nonlinear, phase-matched bulk material. These findings thus point to a new class of second-harmonic-generation devices made of standard centrosymmetric materials.

Nonlinear pulse propagation in one-dimensional metal-dielectric multilayer stacks: ultrawide bandwidth optical limiting.

We numerically study the nonlinear optical properties of metal-dielectric photonic band gap structures in the pulsed regime. We exploit the high chi3 of copper metal to induce nonlinear effects such as broadband optical limiting, self-phase modulation, and unusual spectral narrowing of high intensity pulses. We show that in a single pass through a typical, chirped multilayer stack nonlinear transmittance and peak powers can be reduced by nearly two orders of magnitude compared to low light intensity levels across the entire visible range.

Second-harmonic generation from a positive-negative index material heterostructure.

Resonant cavities have been widely used in the past to enhance material, nonlinear response. Traditional mirrors include metallic films and distributed Bragg reflectors. In this paper we propose negative index material mirrors as a third alternative.

Pulse propagation, dispersion, and energy in magnetic materials.

We discuss pulse propagation effects in generic, electrically and magnetically dispersive media that may display large material discontinuities, such as a surface boundary. Using the known basic constitutive relations between the fields, and an explicit Taylor expansion to describe the dielectric susceptibility and magnetic permeability, we derive expressions for energy density and energy dissipation rates, and equations of motion for the coupled electric and magnetic fields. We then solve the equations of motion in the presence of a single interface, and find that in addition to the now-established negative refraction process an energy exchange occurs between the electric and magnetic fields as the pulse traverses the boundary.

Dispersion-free pulse propagation in a negative-index material.

The possibility of controlling the spectral position of the zero group-velocity dispersion point of a negative-index material can be exploited by varying the ratio between the electric and the magnetic plasma frequency to obtain dispersion-free propagation in spectral regions otherwise inaccessible using conventional positive-index materials. Our predictions are confirmed by pulse propagation simulations where all the orders of the complex dispersion of the material are taken into account.

Generalized nonlinear Schrödinger equation for dispersive susceptibility and permeability: application to negative index materials.

A new generalized nonlinear Schrödinger equation describing the propagation of ultrashort pulses in bulk media exhibiting frequency dependent dielectric susceptibility and magnetic permeability is derived and used to characterize wave propagation in a negative index material. The equation has new features that are distinct from ordinary materials (mu=1): the linear and nonlinear coefficients can be tailored through the linear properties of the medium to attain any combination of signs unachievable in ordinary matter, with significant potential to realize a wide class of solitary waves.