8 research outputs found
Resonant Gain Singularities in 1D and 3D Metal/Dielectric Multilayered Nanostructures
We
present a detailed study on the resonant gain (RG) phenomena
occurring in two nanostructures, in which the presence of dielectric
singularities is used to reach a huge amplification of the emitted
photons resonantly interacting with the system. The presence of gain
molecules in the considered nanoresonator systems makes it possible
to obtain optical features that are able to unlock several applications.
Two noticeable cases have been investigated: a 1D nanoresonator based
on hyperbolic metamaterials and a 3D metal/dielectric spherical multishell.
The former has been designed in the framework of the effective medium
theory, in order to behave as an <i>epsilon-near-zero-and-pole</i> metamaterial, showing extraordinary light confinement and collimation.
Such a peculiarity represents the key to lead to a RG behavior, a
condition in which the system is demonstrated to behave as a self-amplifying
perfect lens. Very high enhancement and spectral sharpness of 1 nm
of the emitted light are demonstrated by means of a transfer matrix
method simulation. The latter system consists of a metal/doped-dielectric
multishell. A dedicated theoretical approach has been set up to finely
engineer its doubly tunable resonant nature. The RG condition has
been demonstrated also in this case. Finite element method-based simulations,
together with an analytical model, clarify the electric field distribution
inside the multishell and suggest the opportunity to use this device
as a <i>self-enhanced loss compensated</i> multishell, being
a favorable scenario for low-threshold SPASER action. Counterintuitively,
exceeding the resonant gain amount of molecules in both systems causes
a significant drop in the amplitude of the resonance
Thermoplasmonic Effects in Gain-Assisted Nanoparticle Solutions
We
report a detailed characterization of the photoinduced heating
observed in gain-assisted solutions of gold nanoparticles (AuNPs).
AuNPs, with sizes ranging from 14 to 48 nm and concentration of 2.5
Ă 10<sup>â10</sup> M, are exposed to different intensity
values of a resonant continuous laser (532 nm), used to excite their
localized surface plasmon resonance (LSPR), responsible for the photogeneration
process. In this way the optimal conditions to achieve the maximum
temperature variation with the least laser dosage are obtained. By
addition of an organic dye to the solutions, whose emission band overlaps
to the LSPR, we found that the contribution to the photothermal efficiency
is enhanced if the solutions are excited at 405 nm. This happens in
the case of smaller NPs, due to a strong coupling effect between the
two subunits, which causes an increase of the extinction cross section
of the whole gain-assisted system. On the other hand, for the larger
AuNPs, an opposite behavior is found: a loss compensation mechanism,
based on a resonant energy transfer process from gain units to plasmonic
nanoparticles, limits the increase of the absorption cross section
with a consequent lowering of the photothermal efficiency. The presented
quantitative analysis of a dispersion of AuNPs results as fundamental
for biomedical applications as well as for integrated plasmonic devices
based on loss compensation effects, where the impact of undesirable
thermal effects cannot be ignored
Loss-Mitigated Collective Resonances in Gain-Assisted Plasmonic Mesocapsules
Inherent optical losses of plasmonic
materials represent a crucial
issue for optoplasmonics, whereas the realization of hierarchical
plasmonic nanostructures implemented with gain functionalities is
a promising and valuable solution to the problem. Here we demonstrate
that porous silica capsules embedding gold nanoparticles (Au NPs)
and fabricated at a scale intermediate between the single plasmonic
nanostructure and bulk materials show remarkable formâfunction
relations. At this scale, in fact, the plasmonâgain interplay
is dominated by the location of the gain medium with respect to the
spatial distribution of the local field. In particular, the hollow
spherical cavities of these structures allow regions of uniform plasmonic
field where the energy transfer occurring between chromophoric donors
and the surrounding plasmonic acceptors gives rise to a broadband
attenuation of losses
Interface of Physics and Biology: Engineering Virus-Based Nanoparticles for Biophotonics
Virus-based nanoparticles (VNPs)
have been used for a wide range
of applications, spanning basic materials science and translational
medicine. Their propensity to self-assemble into precise structures
that offer a three-dimensional scaffold for functionalization has
led to their use as optical contrast agents and related biophotonics
applications. A number of fluorescently labeled platforms have been
developed and their utility in optical imaging demonstrated, yet their
optical properties have not been investigated in detail. In this study,
two VNPs of varying architectures were compared side-by-side to determine
the impact of dye density, dye localization, conjugation chemistry,
and microenvironment on the optical properties of the probes. Dyes
were attached to icosahedral cowpea mosaic virus (CPMV) and rod-shaped
tobacco mosaic virus (TMV) through a range of chemistries to target
particular side chains displayed at specific locations around the
virus. The fluorescence intensity and lifetime of the particles were
determined, first using photochemical experiments on the benchtop,
and second in imaging experiments using tissue culture experiments.
The virus-based optical probes were found to be extraordinarily robust
under ultrashort, pulsed laser light conditions with a significant
amount of excitation energy, maintaining structural and chemical stability.
The most effective fluorescence output was achieved through dye placement
at optimized densities coupled to the exterior surface avoiding conjugated
ring systems. Lifetime measurements indicate that fluorescence output
depends not only on spacing the fluorophores, but also on dimer stacking
and configurational changes leading to radiationless relaxationî¸and
these processes are related to the conjugation chemistry and nanoparticle
shape. For biological applications, the particles were also examined
in tissue culture, from which it was found that the optical properties
differed from those found on the benchtop due to effects from cellular
processes and uptake kinetics. Data indicate that fluorescent cargos
are released in the endolysosomal compartment of the cell targeted
by the virus-based optical probes. These studies provide insight into
the optical properties and fates of fluorescent proteinaceous imaging
probes. The cellular release of cargo has implications not only for
virus-based optical probes, but also for drug delivery and release
systems
Extraordinary Effects in Quasi-Periodic Gold Nanocavities: Enhanced Transmission and Polarization Control of Cavity Modes
Plasmonic
quasi-periodic structures are well-known to exhibit several
surprising phenomena with respect to their periodic counterparts,
due to their long-range order and higher rotational symmetry. Thanks
to their specific geometrical arrangement, plasmonic quasi-crystals
offer unique possibilities in tailoring the coupling and propagation
of surface plasmons through their lattice, a scenario in which a plethora
of fascinating phenomena can take place. In this paper we investigate
the extraordinary transmission phenomenon occurring in specifically
patterned ThueâMorse nanocavities, demonstrating noticeable
enhanced transmission, directly revealed by near-field optical experiments,
performed by means of a scanning near-field optical microscope (SNOM).
SNOM further provides an intuitive picture of confined plasmon modes
inside the nanocavities and confirms that localization of plasmon
modes is based on size and depth of nanocavities, while cross talk
between close cavities <i>via</i> propagating plasmons holds
the polarization response of patterned quasi-crystals. Our performed
numerical simulations are in good agreement with the experimental
results. Thus, the control on cavity size and incident polarization
can be used to alter the intensity and spatial properties of confined
cavity modes in such structures, which can be exploited in order to
design a plasmonic device with customized optical properties and desired
functionalities, to be used for several applications in quantum plasmonics
Analysis of Different European Hazelnut (<i>Corylus avellana</i> L.) Cultivars: Authentication, Phenotypic Features, and Phenolic Profiles
Hazelnuts
exhibit functional properties due to their content in
fatty acids and phenolic compounds that could positively affect human
health. The food industry requires precise traits for morphological,
chemical, and physical kernel features so that some cultivars could
be more suitable for specific industrial processing. In this study,
agronomical and morphological features of 29 hazelnut cultivars were
evaluated and a detailed structural characterization of kernel polyphenols
was performed, confirming the presence of protocatechuic acid, flavan-3-ols
such as catechin, procyanidin B2, six procyanidin oligomers, flavonols,
and one dihydrochalcone in all the analyzed cultivars. In addition,
an innovative methodology based on the MALDI-TOF mass spectrometric
analysis of peptide/protein components extracted from kernels was
developed for the authentication of the most valuable cultivars. The
proposed method is rapid, simple, and reliable and holds the potential
to be applied in quality control processes. These results could be
useful in hazelnut cultivar evaluation and choice for growers, breeders,
and food industry
Mid-Infrared Plasmonic Excitation in Indium Tin Oxide Microhole Arrays
Transparent
conducting oxides (TCOs) are emerging as possible alternative
constituent materials to replace noble metals such as silver and gold
for low-loss plasmonic applications in the near-infrared (NIR) and
mid-infrared (MIR) regimes. In particular, TCO-based nanostructures
are extensively investigated for biospectroscopy exploiting their
surface-enhanced infrared absorption (SEIRA). The latter enhances
the absorption from vibrational and rotational modes of nearby biomolecules,
making TCO nanostructures a promising candidate for IR sensing applications.
Nevertheless, in order to produce inexpensive devices for lab-on-a-chip
diagnostics, it would be favorable to achieve surface-enhanced infrared
absorption with very simple microstructures not requiring nanosize
control. In this work, we attempt to demonstrate a SEIRA effect with
the least challenging fabrication, Îźm-scale instead of nm-scale,
by tailoring both device design and charge density of the indium tin
oxide (ITO) film. We show that microperiodic hole arrays in a ITO
film are able to produce SEIRA via grating coupling. Such a study
opens the way for innovative and disrupting biosensing devices
Tailoring Electromagnetic Hot Spots toward Visible Frequencies in Ultra-Narrow Gap Al/Al<sub>2</sub>O<sub>3</sub> Bowtie Nanoantennas
Plasmonic
bowtie nanoantennas are intriguing nanostructures, capable
to achieve very high local electromagnetic (EM) field confinement
and enhancement in the hot spots. This effect is strongly dependent
on the gap size, which in turn is related to technological limitations.
Ultranarrow gap bowtie nanoantennas, operating at visible frequencies,
can be of great impact in biosensing applications and in the study
of strong lightâmatter interactions with organic molecules.
Here, we present a comprehensive study on the structural and optical
properties of aluminum bowties, realized with ultranarrow gap by He<sup>+</sup>-ion milling lithography, and operating from the near-infrared
to the red part of the visible range. Most importantly, this analysis
demonstrates that large EM near-field enhancement and different hot
spot spatial positions, as a function of nanometer-sized gaps, are
constrained by the native aluminum oxide, thus, working as hot spot
ruler