132 research outputs found
Theory of a Directive Optical Leaky Wave Antenna Integrated into a Resonator and Enhancement of Radiation Control
We provide for the first time the detailed study of the radiation performance
of an optical leaky wave antenna (OLWA) integrated into a Fabry-P\'erot
resonator. We show that the radiation pattern can be expressed as the one
generated by the interference of two leaky waves counter-propagating in the
resonator leading to a design procedure for achieving optimized broadside
radiation, i.e., normal to the waveguide axis. We thus report a realizable
implementation of the OLWA made of semiconductor and dielectric regions. The
theoretical modeling is supported by full-wave simulation results, which are
found to be in good agreement. We aim to control the radiation intensity in the
broadside direction via excess carrier generation in the semiconductor regions.
We show that the presence of the resonator can provide an effective way of
enhancing the radiation level modulation, which reaches values as high as 13.5
dB, paving the way for novel promising control capabilities that might allow
the generation of very fast optical switches, as an example.Comment: 10 pages, 14 figure
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
Array of dipoles near a hyperbolic metamaterial: Evanescent-to-propagating Floquet wave transformation
We investigate the capabilities of hyperbolic metamaterials (HMs) to couple
near-fields (i.e., evanescent waves) emitted by a two-dimensional periodic
array of electric dipoles to propagating waves. In particular, large order
Floquet harmonics with transverse magnetic (TM) polarization, that would be
evanescent in free space and therefore confined near the array surface, are
transformed into propagating spectrum inside the HM, and thus carry power away.
Because of this property, independent of the finite or infinite extent of the
HM, the power generated by an array of elementary electric dipoles is strongly
enhanced when the array is located near a HM surface and is mostly directed
into the HM. In particular, the power coupled to the HM exhibits narrow
frequency features that can be employed in detection applications. The results
shown in this paper provide a clear signature on wave dynamics in HMs. A link
between the results pertaining to the case of an isolated dipole on top of HM
and the planar array is found convenient to explain both wave dynamics and
spectral power distribution. The narrow frequency emission features appear in
the array case only; they depend on its spatial periodicity and remarkable on
the HM thickness.Comment: 13 pages, 12 figure
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
Electrodynamic modeling of strong coupling between a metasurface and intersubband transitions in quantum wells
Strong light-matter coupling has recently been demonstrated in sub-wavelength
volumes by coupling engineered optical transitions in semiconductor
heterostructures (e.g., quantum wells) to metasurface resonances via near
fields. It has also been shown that different resonator shapes may lead to
different Rabi splittings, though this has not yet been well explained. In this
paper, our aim is to understand the correlation between resonator shape and
Rabi splitting, and in particular determine and quantify the physical
parameters that affect strong coupling by developing an equivalent circuit
network model whose elements describe energy and dissipation. Because of the
subwavelength dimension of each metasurface element, we resort to the
quasi-static (electrostatic) description of the near-field and hence define an
equivalent capacitance associated to each dipolar element of a flat
metasurface, and we show that this is also able to accurately model the
phenomenology involved in strong coupling between the metasurface and the
intersubband transitions in quantum wells. We show that the spectral properties
and stored energy of a metasurface/quantum-well system obtained using our model
are in good agreement with both full-wave simulation and experimental results.
We then analyze metasurfaces made of three different resonator geometries and
observe that the magnitude of the Rabi splitting increases with the resonator
capacitance in agreement with our theory, providing a phenomenological
explanation for the resonator shape dependence of the strong coupling process.Comment: 10 pages, 10 figure
Microclimate Management for the Preservation of Cultural Heritage
This paper presents a line of research aimed at studying in detail the interaction between air conditioning/heating systems and thermo hygrometric conditions inside museums. An experimental test was carried out on the microclimatic conditions inside a building envelope, using as case studies the interiors of the Castle of Chiaramonte in Favara (Agrigento), and the Church of Saint Matthew (Palermo). In this phase of the work the “real” measured values and the recommended “optimal” values were compared for the various museum exhibits displayed. The analysis of the complex dynamic interaction of climate conditions, the need for the museum pieces to be preserved in a situation of maximum stability, the problems regarding thermal comfort in relation to users’ enjoyment of the exhibition space, the construction model of the building envelope – the synergy between all these variables led to establishing the final design project. This consists of an air conditioning system with radiant panels, floor heating, cooling system and primary air, in which the benefits seen from an environmental and energy point of view, are fully explained. The conservation of cultural heritage collections in museums, on the one hand implies that the attentionof the specialists must turn to the prevention of the natural degradation caused through time and on the other to improve the quality of the environment. This means environmental parameters must be checked and monitoring techniques used. Since the connective tissue between the museum and its container is the environment, the building-plant system must be held responsible for the “degradation suffered by the museum itself” or “be largely responsible for its optimal conservation with the passing of time”
Fano collective resonance as complex mode in a two dimensional planar metasurface of plasmonic nanoparticles
Fano resonances are features in transmissivity/reflectivity/absorption that
owe their origin to the interaction between a bright resonance and a dark
(i.e., sub-radiant) narrower resonance, and may emerge in the optical
properties of planar two-dimensional (2D) periodic arrays (metasurfaces) of
plasmonic nanoparticles. In this Letter, we provide a thorough assessment of
their nature for the general case of normal and oblique plane wave incidence,
highlighting when a Fano resonance is affected by the mutual coupling in an
array and its capability to support free modal solutions. We analyze the
representative case of a metasurface of plasmonic nanoshells at ultraviolet
frequencies and compute its absorption under TE- and TM-polarized, oblique
plane-wave incidence. In particular, we find that plasmonic metasurfaces
display two distinct types of resonances observable as absorption peaks: one is
related to the Mie, dipolar resonance of each nanoparticle; the other is due to
the forced excitation of free modes with small attenuation constant, usually
found at oblique incidence. The latter is thus an array-induced collective Fano
resonance. This realization opens up to manifold flexible designs at optical
frequencies mixing individual and collective resonances. We explain the
physical origin of such Fano resonances using the modal analysis, which allows
to calculate the free modes with complex wavenumber supported by the
metasurface. We define equivalent array dipolar polarizabilities that are
directly related to the absorption physics at oblique incidence and show a
direct dependence between array modal phase and attenuation constant and Fano
resonances. We thus provide a more complete picture of Fano resonances that may
lead to the design of filters, energy-harvesting devices, photodetectors, and
sensors at ultraviolet frequencies.Comment: 6 pages, 5 figure
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