93 research outputs found
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
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
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
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
Gain assisted harmonic generation in near-zero permittivity metamaterials made of plasmonic nanoshells
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
Role of Antenna Modes and Field Enhancement in Second Harmonic Generation from Dipole Nanoantennas
We study optical second harmonic generation from metallic dipole antennas with narrow gaps. Enhancement of the fundamental-frequency field in the gap region plays a marginal role on conversion efficiency. In the symmetric configuration, i.e., with the gap located at the center of the antenna axis, reducing gap size induces a significant red-shift of the maximum conversion efficiency peak. Either enhancement or inhibition of second-harmonic emission may be observed as gap size is decreased, depending on the antenna mode excited at the harmonic frequency. The second-harmonic signal is extremely sensitive to the asymmetry introduced by gap’s displacements with respect to the antenna center. In this situation, second-harmonic light can couple to all the available antenna modes. We perform a multipolar analysis that allows engineering the far-field SH emission and find that the interaction with quasi-odd-symmetry modes generates radiation patterns with a strong dipolar component
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