218 research outputs found
Surface Plasmon Excitation of Second Harmonic light: Emission and Absorption
We aim to clarify the role that absorption plays in nonlinear optical
processes in a variety of metallic nanostructures and show how it relates to
emission and conversion efficiency. We define a figure of merit that
establishes the structure's ability to either favor or impede second harmonic
generation. Our findings suggest that, despite the best efforts embarked upon
to enhance local fields and light coupling via plasmon excitation, nearly
always the absorbed harmonic energy far surpasses the harmonic energy emitted
in the far field. Qualitative and quantitative understanding of absorption
processes is crucial in the evaluation of practical designs of plasmonic
nanostructures for the purpose of frequency mixing
Quantum Conductivity for Metal-Insulator-Metal Nanostructures
We present a methodology based on quantum mechanics for assigning quantum
conductivity when an ac field is applied across a variable gap between two
plasmonic nanoparticles with an insulator sandwiched between them. The quantum
tunneling effect is portrayed by a set of quantum conductivity coefficients
describing the linear ac conductivity responding at the frequency of the
applied field and nonlinear coefficients that modulate the field amplitude at
the fundamental frequency and its harmonics. The quantum conductivity,
determined with no fit parameters, has both frequency and gap dependence that
can be applied to determine the nonlinear quantum effects of strong applied
electromagnetic fields even when the system is composed of dissimilar metal
nanostructures. Our methodology compares well to results on quantum tunneling
effects reported in the literature and it is simple to extend it to a number of
systems with different metals and different insulators between them
Gain assisted harmonic generation in near-zero permittivity metamaterials made of plasmonic nanoshells
We investigate enhanced harmonic generation processes in gain-assisted,
near-zero permittivity metamaterials composed of spherical plasmonic
nanoshells. We report the presence of narrow-band features in transmission,
reflection and absorption induced by the presence of an active material inside
the core of the nanoshells. The damping-compensation mechanism used to achieve
the near-zero effective permittivity condition also induces a significant
increase in field localization and strength and, consequently, enhancement of
linear absorption. When only metal nonlinearities are considered, second and
third harmonic generation efficiencies obtained by probing the structure in the
vicinity of the near-zero permittivity condition approach values as high as for
irradiance value as low as . These results clearly demonstrate that a
relatively straightforward path now exists to the development of exotic and
extreme nonlinear optical phenomena in the KW/cm2 rang
Nonlinear dynamics in low permittivity media: the impact of losses.
Slabs of materials with near-zero permittivity display enhanced nonlinear processes. We show that field enhancement due to the continuity of the longitudinal component of the displacement field drastically enhances harmonic generation. We investigate the impact of losses with and without bulk nonlinearities and demonstrate that in the latter scenario surface, magnetic and quadrupolar nonlinear sources cannot always be ignored
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. The interpretation of
transmittance values greater than unity for evanescent waves is clarified with
respect to the associated absorption losses.Comment: to be published in Optics Expres
Second harmonic double resonance cones in dispersive hyperbolic metamaterials
We study the formation of second harmonic double-resonance cones in
hyperbolic metamaterials. An electric dipole on the surface of the structure
induces second harmonic light to propagate into two distinct volume
plasmon-polariton channels: A signal that propagates within its own peculiar
resonance cone; and a phase-locked signal that is trapped under the pump's
resonance cone. Metamaterial dispersion and birefringence induce a large
angular divergence between the two volume plasmon-polaritons, making these
structures ideal for subwavelength second and higher harmonic imaging
microscopy
Electrodynamics of Conductive Oxides: Intensity-dependent anisotropy, reconstruction of the effective dielectric constant, and harmonic generation
We study electromagnetic pulse propagation in an indium tin oxide nanolayer
in the linear and nonlinear regimes. We use the constitutive relations to
reconstruct the effective dielectric constant of the medium, and show that
nonlocal effects induce additional absorption resonances and anisotropic
dielectric response: longitudinal and transverse effective dielectric functions
are modulated differently along the propagation direction, and display
different epsilon-near-zero crossing points with a discrepancy that increases
with increasing intensity. We predict that hot carriers induce a dynamic
redshift of the plasma frequency and a corresponding translation of the
effective nonlinear dispersion curves that can be used to predict and quantify
nonlinear refractive index changes as a function of incident laser peak power
density. Our results suggest that large, nonlinear refractive index changes can
occur without the need for epsilon-near-zero modes to couple with plasmonic
resonators. At sufficiently large laser pulse intensities, we predict the onset
of optical bistability, while the presence of additional pump absorption
resonances that arise from longitudinal oscillations of the free electron gas
give way to corresponding resonances in the second and third harmonic spectra.
A realistic propagation model is key to unraveling the basic physical
mechanisms that play a fundamental role in the dynamics
Nonlocal and Quantum Tunneling Contributions to Harmonic Generation in Nanostructures: Electron Cloud Screening Effects
Our theoretical examination of second and third harmonic generation from
metal-based nanostructures predicts that nonlocal and quantum tunneling
phenomena can significantly exceed expectations based solely on local,
classical electromagnetism. Mindful that the diameter of typical transition
metal atoms is approximately 3{\AA}, we adopt a theoretical model that treats
nanometer-size features and/or sub-nanometer size gaps or spacers by taking
into account: (i) the limits imposed by atomic size to fulfill the requirements
of continuum electrodynamics; (ii) spillage of the nearly-free electron cloud
into the surrounding vacuum; and (iii) the increased probability of quantum
tunneling as objects are placed in close proximity. Our approach also includes
the treatment of bound charges, which add crucial, dynamical components to the
dielectric constant that are neglected in the conventional hydrodynamic model,
especially in the visible and UV ranges, where interband transitions are
important. The model attempts to inject into the classical electrodynamic
picture a simple, perhaps more realistic description of the metal surface by
incorporating a thin patina of free-electrons that screens an internal,
polarizable medium.Comment: Submitted to PR
Analog image processing with nonlinear nonlocal flat optics
Digital signal processing has revolutionized many fields of science and engineering, but it still shows critical limits, mainly related to the complexity, power consumption, and limited speed of analogue-to-digital converters. A long-sought solution to overcome these hurdles is optical analog computing. In this regard, flat optics has been recently unveiled as a powerful platform to perform data processing in real-time, with low power consumption and a small footprint. So far, these explorations have been mainly limited to linear optics. Arguably, significantly more impact may be garnered from pushing this operation towards nonlinear processing of the incoming signals. In this context, we demonstrate here that nonlinear phenomena combined with engineered nonlocality in flat optics devices can be leveraged to synthesize Volterra kernels able to outperform linear optical analog image processing
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