3,932 research outputs found
Plasmon-mediated superradiance near metal nanostructures
We develop a theory of cooperative emission of light by an ensemble of
emitters, such as fluorescing molecules or semiconductor quantum dots, located
near a metal nanostructure supporting surface plasmon. The primary mechanism of
cooperative emission in such systems is resonant energy transfer between
emitters and plasmons rather than the Dicke radiative coupling between
emitters. We identify two types of plasmonic coupling between the emitters, (i)
plasmon-enhanced radiative coupling and (ii) plasmon-assisted nonradiative
energy transfer, the competition between them governing the structure of system
eigenstates. Specifically, when emitters are removed by more than several nm
from the metal surface, the emission is dominated by three superradiant states
with the same quantum yield as a single emitter, resulting in a drastic
reduction of ensemble radiated energy, while at smaller distances cooperative
behavior is destroyed by nonradiative transitions. The crossover between two
regimes can be observed in distance dependence of ensemble quantum efficiency.
Our numerical calculations incorporating direct and plasmon-assisted
interactions between the emitters indicate that they do not destroy the
plasmonic Dicke effect.Comment: 12 pages, 10 figure
Purcell effect in wire metamaterials
We study theoretically the enhancement of spontaneous emission in wire
metamaterials. We analyze the dependence of the Purcell factor dependence on
wire dielectric constant for both electric and magnetic dipole sources, and
find an optimal value of the dielectric constant for maximizing the Purcell
factor for the electric dipole. We obtain analytical expressions for the
Purcell factor and also provide estimates for the Purcell factor in realistic
structures operating in both microwave and optical spectral range.Comment: 15 pages, 7 figure
Microscopic model of Purcell enhancement in hyperbolic metamaterials
We study theoretically a dramatic enhancement of spontaneous emission in
metamaterials with the hyperbolic dispersion modeled as a cubic lattice of
anisotropic resonant dipoles. We analyze the dependence of the Purcell factor
on the source position in the lattice unit cell and demonstrate that the
optimal emitter position to achieve large Purcell factors and Lamb shifts are
in the local field maxima. We show that the calculated Green function has a
characteristic cross-like shape, spatially modulated due to structure
discreteness. Our basic microscopic theory provides fundamental insights into
the rapidly developing field of hyperbolic metamaterials.Comment: 9 pages, 11 figure
Changes in metabolic profiling of sugarcane leaves induced by endophytic diazotrophic bacteria and humic acids.
Plant growth-promoting bacteria (PGPB) and humic acids (HA) have been used as biostimulants in field conditions. The complete genomic and proteomic transcription of Herbaspirillum seropedicae and Gluconacetobacter diazotrophicus is available but interpreting and utilizing this information in the field to increase crop performance is challenging. The identification and characterization of metabolites that are induced by genomic changes may be used to improve plant responses to inoculation. The objective of this study was to describe changes in sugarcane metabolic profile that occur when HA and PGPB are used as biostimulants. Inoculum was applied to soil containing 45-day old sugarcane stalks. One week after inoculation, the methanolic extracts from leaves were obtained and analyzed by gas chromatography coupled to time-of-flight mass spectrometry; a total of 1,880 compounds were observed and 280 were identified in all samples. The application of HA significantly decreased the concentration of 15 metabolites, which generally included amino acids. HA increased the levels of 40 compounds, and these included metabolites linked to the stress response (shikimic, caffeic, hydroxycinnamic acids, putrescine, behenic acid, quinoline xylulose, galactose, lactose proline, oxyproline and valeric acid) and cellular growth (adenine and adenosine derivatives, ribose, ribonic acid and citric acid). Similarly, PGPB enhanced the level of metabolites identified in HA-treated soils; e.g., 48 metabolites were elevated and included amino acids, nucleic acids, organic acids, and lipids. Co-inoculation (HACPGPB) boosted the level of 110 metabolites with respect to non-inoculated controls; these included amino acids, lipids and nitrogenous compounds. Changes in the metabolic profile induced by HA+PGPB influenced both glucose and pentose pathways and resulted in the accumulation of heptuloses and riboses, which are substrates in the nucleoside biosynthesis and shikimic acid pathways. The mevalonate pathway was also activated, thus increasing phytosterol synthesis. The improvement in cellular metabolism observed with PGPB+HA was compatible with high levels of vitamins. Glucuronate and amino sugars were stimulated in addition to the products and intermediary compounds of tricarboxylic acid metabolism. Lipids and amino acids were the main compounds induced by co-inoculation in addition to antioxidants, stress-related metabolites, and compounds involved in cellular redox. The primary compounds observed in each treatment were identified, and the effect of co-inoculation (HACPGPB) on metabolite levels was discussed
Completeness of evanescent modes in layered dielectrics
In the presence of a dielectric slab, the modes of the free electromagnetic field comprise traveling modes, consisting of incoming, reflected, and transmitted parts, as well as trapped modes that are subject to repeated total internal reflection and emerge as evanescent field outside the slab. Traveling modes have a continuous range of frequencies, but trapped modes occur only at certain discrete frequencies. We solve the problem of which relative weight to use when summing over all modes, as commonly required in perturbative calculations. We demonstrate the correctness of our method by showing the completeness of electromagnetic field modes in the presence of a dielectric slab. We derive a convenient method of summing over all modes by means of a single contour integral, which is very useful in standard quantum electrodynamic calculations
Extrapolation-CAM Theory for Critical Exponents
By intentionally underestimating the rate of convergence of
exact-diagonalization values for the mass or energy gaps of finite systems, we
form families of sequences of gap estimates. The gap estimates cross zero with
generically nonzero linear terms in their Taylor expansions, so that
for each member of these sequences of estimates. Thus, the Coherent Anomaly
Method can be used to determine . Our freedom in deciding exactly how to
underestimate the convergence allows us to choose the sequence that displays
the clearest coherent anomaly. We demonstrate this approach on the
two-dimensional ferromagnetic Ising model, for which . We also use it
on the three-dimensional ferromagnetic Ising model, finding , in good agreement with other estimates.Comment: 21 pages, Submitted to Journal of Physics A; new section added
discussing rate of convergence and relation to Finite-Size Scalin
Remote optical addressing of single nano-objects
We present a scheme for remotely addressing single nano-objects by means of
near-field optical microscopy that makes only use of one of the most
fundamental properties of electromagnetic radiation: its polarization. A medium
containing optically active nano-objects is covered with a thin metallic film
presenting sub-wavelength holes. When the optical tip is positioned some
distance away from a hole, surface plasmons in the metal coating are generated
which, by turning the polarization plane of the excitation light, transfer the
excitation towards a chosen hole and induce emission from the underlying
nano-objects. The method, easily applicable to other systems, is demonstrated
for single quantum dots (QDs) at low temperature. It may become a valuable tool
for future optical applications in the nanoworld
Rigorous Theory of Optical Trapping by an Optical Vortex Beam
We propose a rigorous theory for the optical trapping by optical vortices,
which is emerging as an important tool to trap mesoscopic particles. The common
perception is that the trapping is solely due to the gradient force, and may be
characterized by three real force constants. However, we show that the optical
vortex trap can exhibit complex force constants, implying that the trapping
must be stabilized by ambient damping. At different damping levels, particle
shows remarkably different dynamics, such as stable trapping, periodic and
aperiodic orbital motions
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