Surface plasmons mediated energy transfer from a semiconductor quantum well to an organic overlayer

We consider the resonant energy transfer from a two-dimensional Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer (acceptor) when the active media are separated by a metallic layer, possibly an electrode. We characterize the effect of the surface plasmon on this process. Using realistic values of material parameters, we show that it is possible to increase the transfer rate within typically a factor of 5, with a 1.25 efficiency enhancement (up to a factor of 44 with ten times larger efficiency according to geometrical configuration). Foreseeing applications to light emitting diode, we then take into account the quenching of the organic luminescence due to the proximity to the metal. The latter is significant and affect negatively the total internal efficiency that we discuss for different geometries.     

Strong coupling between surface plasmons and excitons in an organic semiconductor

We report on the observation of a strong coupling between a surface plasmon and an exciton. Reflectometry experiments are performed on an organic semiconductor, namely, cyanide dye J aggregates, deposited on a silver film. The dispersion lines present an anticrossing that is the signature of a strong plasmon-exciton coupling. Mixed states are formed in a similar way as microcavities polaritons. The Rabi splitting characteristic of this coupling reaches 180 meV at room temperature. The emission of the low energy plasmon-exciton mixed state has been observed and is largely shifted from the uncoupled emission.     

Energy transfer between excitons and plasmons in semiconductor-metal hybrid nanostructures

We summarized our recent optical studies on semiconductor nanoparticle (NP) based hybrid nanostructures: isolated CdSe NPs on Au substrates, close-packed CdSe NP monolayers on Au substrates, and close-packed monolayers of mixed CdSe/Au NPs. Luminescence properties of semiconductor-metal hybrid nanostructures were studied by space-resolved optical imaging spectroscopy and time-resolved luminescence spectroscopy. The luminescence spectra and dynamics of isolated and assembled NPs depend on the local environments. We discuss exciton-plasmon interactions in semiconductor-metal hybrid nanostructures.     

Strong coupling between localized plasmons and organic excitons in metal nanovoids

Hybrid emitting exciton-plasmonic composites are constructed by coating arrays of spherical nanovoids embedded in a gold film with organic semiconducting molecular J-aggregate films. In such plasmonic crystals, localized plasmons confined inside the voids can be excited. We report the first observation of polaritonic spectral narrowing and strong coupling between localized plasmons and J-aggregate excitons with Rabi splittings of 230 meV at room temperature.     

Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.     

Electron energy bands in semiconductor films

The possible nature of the band structure in crystal films is examined on the basis of a consideration of the symmetry possessed by their structure.For germanium and silicon of the p- and n-types with various different orientations of the film the connection is found between the band structures of the film and of the bulk crystal. The result is obtained that with the orientation [111] an n-germanium film, like an n-silicon film with the same orientation must have a single-trough minimum.     

Nearly perfect triplet-triplet energy transfer from Wannier excitons to naphthalene in organic-inorganic hybrid quantum-well materials

We report the observation of extremely efficient energy transfer (greater than 99%) in an organic-inorganic hybrid quantum-well structure consisting of perovskite-type lead bromide well layers and naphthalene-linked ammonium barrier layers. Time-resolved photoluminescence measurements confirm that the transfer is triplet-triplet Dexter-type energy transfer from Wannier excitons in the inorganic well to the triplet state of naphthalene molecules in the organic barrier. Using measurements in the 10-300 K temperature range, we also investigated the temperature dependence of the energy transfer.     

Direct investigation of superparamagnetism in Co nanoparticle films

A direct probe of superparamagnetism was used to determine the complete anisotropy energy distribution of Co nanoparticle films. The films were composed of self-assembled lattices of uniform Co nanoparticles of 3 or 5 nm in diameter, and a variable temperature scanning-SQUID microscope was used to measure temperature-induced spontaneous magnetic noise in the samples. Accurate measurements of anisotropy energy distributions of small volume samples will be critical to magnetic optimization of nanoparticle devices and media.     

Intramolecular energy transfer by singlet and triplet excitons in macromolecules

Absorption, luminescence and photo-oxidation of polyvinylcarbazole (PVCa) and polyvinylbenzocarbazole (PVBCa) in weak solutions have been studied. It is found that fluorescence and phosphorescence spectra, observed at 293, 77 and 4.2 K, depend on the molecular mass M of the polymer and on the experimental conditions. Due to efficient intramolecular energy transfer the fluorescence spectrum includes the emission of excimers, emission of end groups and that of internal structural defects of a macrochain. The relative role of each emission markedly depends on M. Phosphorescence is largely determined by the annihilation effects between triplet excitations, these effects being pronounced in long macrochains. The data show the excitonic nature of the singlet and triplet excitation energy transfer in the polymers studied. The migration of one-dimensional singlet and triplet excitons strongly influences the luminescent properties of PVCa and PVBCa macromolecules, as well as their photochemical reactions. The range of singlet excitons in PVCa and PVBCa macromolecules is determined both at room temperature and at low temperatures.     

Resonant-Raman scattering from quasi-localized excitons in semiconductor quantum wells

Large polarization-dependent resonance enhancement is observed in first and second order Raman scattering in multiple quantum well $\text{GaAs}\text{(Al}{}_{\mathrm{x}}\text{Ga}{}_{\mathrm{1-x}}\text{)As}$ and $\text{GaAs}\text{AlAs}$ semiconductor heterostructures. Resonance enhancement is much stronger in the forbidden polarization than in the allowed and shows double resonance behaviour at incoming and outgoing photon energies. The resonance is stronger at incoming photon energies for one-phonon scattering and at outgoing photon energies for two-phonon scattering.     

Coherent exciton-plasmon interaction in the hybrid semiconductor quantum dot and metal nanoparticle complex

We studied theoretically the exciton coherent dynamics in the hybrid complex composed of CdTe quantum dot (QDs) and rodlike Au nanoparticles (NPs) by the self-consistent approach. Through adjusting the aspect ratio of the rodlike Au NPs, the radiative rate of the exciton and the nonradiative energy transfer rate from the QD to the Au NP are tunable in the wide range 0.05-4 ns(-1) and 4.4 x 10(-4) to 2.6 ns(-1), respectively; consequently, the period of Rabi oscillations of exciton population is tunable in the range 0.6 pi-9 pi.     

Observation of surface and bulk plasmons in semiconductor superlattices

We report far infrared measurements of oblique incidence power reflectivity and attenuated total reflection (ATR) on multiply Si δ-doped GaAs samples. The reflectivity spectrum in s-polarisation probes the in-plane component of the superlattice dielectric function, and is sensitive to the overall 3D electron density. The p-polarisation spectrum, however, also probes the out-of-plane component, resulting in an extra mode which is sensitive to the distribution of free electrons between wells and barriers. The p-polarisation ATR spectrum shows surface modes which are also sensitive to this distribution. The results are compared with a bulk slab model of the dielectric function.     

Theory of excitons, charged excitons, exciton fine-structure and entangled excitons in self-assembled semiconductor quantum dots

We show how the atomistic pseudopotential many-body theory of InGaAs/GaAs addresses some important effects, including (i) the fine-structure splittings (originating from interband spin exchange), (ii) the optical spectra of charged quantum dots and (iii) the degree of entanglement in a quantum dot molecule.     

New features of excitonic emission in metal nanoparticle/semiconductor quantum dot nanosystem

We study theoretically the excitonic emission properties of a hybrid nanosystem composed of a spherical metal nanoparticle (NP) and a spherical quantum dot (QD). We show that electromagnetic field (EMF) emitted by a single QD has only dipole, quadrupole, and octupole components, i.e., QD cannot in principle be regarded as an oscillating point dipole, which emits infinite series of multipoles. This leads to a substantial deviation of the characteristics of QD excitonic emission from the emission characteristics of point dipole (molecular fluorophore) located in a vicinity of metal NP at small interparticle distances. The observed fluorescence spectra of the CdTe QD/Ag NP nanostructure are found to be in good agreement with the calculated ones.