16 research outputs found
Fluorescence quenching near small metal nanoparticles
We develop a microscopic model for fluorescence of a molecule (or
semiconductor quantum dot) near a small metal nanoparticle. When a molecule is
situated close to metal surface, its fluorescence is quenched due to energy
transfer to the metal. We perform quantum-mechanical calculations of energy
transfer rates for nanometer-sized Au nanoparticles and find that non-local and
quantum-size effects significantly enhance dissipation in metal as compared to
those predicted by semiclassical electromagnetic models. However, the
dependence of transfer rates on molecule's distance to metal nanoparticle
surface, , is significantly weaker than the behavior for flat metal
surface with a sharp boundary predicted by previous calculations within random
phase approximation.Comment: 7 pages, 5 figure
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
Coulomb and quenching effects in small nanoparticle-based spasers
We study numerically the effect of mode mixing and direct dipole-dipole
interactions between gain molecules on spasing in a small composite
nanoparticles with a metallic core and a dye-doped dielectric shell. By
combining Maxwell-Bloch equations with Green's function formalism, we calculate
lasing frequency and threshold population inversion for various gain densities
in the shell. We find that gain coupling to nonresonant plasmon modes has a
negligible effect on spasing threshold. In contrast, the direct dipole-dipole
coupling, by causing random shifts of gain molecules' excitation frequencies,
hinders reaching the spasing threshold in small systems. We identify a region
of parameter space in which spasing can occur considering these effects.Comment: 7 pages, 6 figure
Cooperative emission of light by an ensemble of dipoles near a metal nanostucture: The plasmonic Dicke effect
We identify a new mechanism for cooperative emission of light by an ensemble
of N dipoles near a metal nanostructure supporting a surface plasmon.The
cross-talk between emitters due to virtual plasmon exchange leads to a
formation of three plasmonic super-radiant modes whose radiative decay rates
scales with N, while the total radiated energy is thrice that of a single
emitter. Our numerical simulations indicate that the plasmonic Dicke effect
survives non-radiative losses in the metal.Comment: 4 pages, 4 figure
Microscopic theory of surface-enhanced Raman scattering in noble-metal nanoparticles
We present a microscopic model for surface-enhanced Raman scattering (SERS)
from molecules adsorbed on small noble-metal nanoparticles. In the absence of
direct overlap of molecular orbitals and electronic states in the metal, the
main enhancement source is the strong electric field of the surface plasmon
resonance in a nanoparticle acting on a molecule near the surface. In small
particles, the electromagnetic enhancement is strongly modified by quantum-size
effects. We show that, in nanometer-sized particles, SERS magnitude is
determined by a competition between several quantum-size effects such as the
Landau damping of surface plasmon resonance and reduced screening near the
nanoparticle surface. Using time-dependent local density approximation, we
calculate spatial distribution of local fields near the surface and enhancement
factor for different nanoparticles sizes.Comment: 8 pages, 6 figures. Considerably extended final versio