158 research outputs found
Origin of second-harmonic generation enhancement in optical split-ring resonators
We present a study of the second-order nonlinear optical properties of
metal-based metamaterials. A hydrodynamic model for electronic response is
used, in which nonlinear surface contributions are expressed in terms of the
bulk polarization. The model is in good agreement with published experimental
results, and clarifies the mechanisms contributing to the nonlinear response.
In particular, we show that the reported enhancement of second-harmonic in
split-ring resonator based media is driven by the electric rather than the
magnetic properties of the structure
One-dimensional chirality: strong optical activity in epsilon-near-zero metamaterials
We suggest that electromagnetic chirality, generally displayed by 3D or 2D
complex chiral structures, can occur in 1D patterned composites whose
components are achiral. This feature is highly unexpected in a 1D system which
is geometrically achiral since its mirror image can always be superposed onto
it by a 180 deg rotation. We analytically evaluate from first principles the
bi-anisotropic response of multilayered metamaterials and we show that the
chiral tensor is not vanishing if the system is geometrically one-dimensional
chiral, i.e. its mirror image can not be superposed onto it by using
translations without resorting to rotations. As a signature of 1D chirality, we
show that 1D chiral metamaterials support optical activity and we prove that
this phenomenon undergoes a dramatic non-resonant enhancement in the
epsilon-near-zero regime where the magneto-electric coupling can become
dominant in the constitutive relations.Comment: 5 pages, 4 figures. Accepted for publication on Physical Review
Letter
Polariton excitation in epsilon-near-zero slabs: transient trapping of slow light
We numerically investigate the propagation of a spatially localized and
quasi-monochromatic electromagnetic pulse through a slab with Lorentz
dielectric response in the epsilon-near-zero regime, where the real part of the
permittivity vanishes at the pulse carrier frequency. We show that the pulse is
able to excite a set of virtual polariton modes supported by the slab, the
excitation undergoing a generally slow damping due to absorption and radiation
leakage. Our numerical and analytical approaches indicate that in its transient
dynamics the electromagnetic field displays the very same enhancement of the
field component perpendicular to the slab, as in the monochromatic regime. The
transient trapping is inherently accompanied by a significantly reduced group
velocity ensuing from the small dielectric permittivity, thus providing a novel
platform for achieving control and manipulation of slow light
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
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
Low-damping epsilon-near-zero slabs: nonlinear and nonlocal optical properties
We investigate second harmonic generation, low-threshold multistability,
all-optical switching, and inherently nonlocal effects due to the free-electron
gas pressure in an epsilon-near-zero (ENZ) metamaterial slab made of
cylindrical, plasmonic nanoshells illuminated by TM-polarized light. Damping
compensation in the ENZ frequency region, achieved by using gain medium inside
the shells' dielectric cores, enhances the nonlinear properties. Reflection is
inhibited and the electric field component normal to the slab interface is
enhanced near the effective pseudo-Brewster angle, where the effective
\epsilon-near-zero condition triggers a non-resonant, impedance-matching
phenomenon. We show that the slab displays a strong effective, spatial
nonlocality associated with leaky modes that are mediated by the compensation
of damping. The presence of these leaky modes then induces further spectral and
angular conditions where the local fields are enhanced, thus opening new
windows of opportunity for the enhancement of nonlinear optical processes
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
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