Effect of metal/dielectric substrates on photopolymerization of BITh thin films.

We have studied effects of metal-dielectric substrates on photopolymerization of [2,2'-Bi-1H-indene]-1,1'-dione-3,3'-diyl diheptanoate (BITh) monomer. We synthetized BITh and spin-coated it onto a variety of dielectric, metallic, and metal-dielectric substrates. The films were exposed to radiation of a UV-Visible Xe lamp, causing photo-polymerization of monomer molecules. The magnitude and the rate of the photo-polymerization were monitored by measuring the strength of the ~ 480 nm absorption band, which existed in the monomer but not in the polymer. Expectedly, the rate of photo-polymerization changed nearly linearly with the change of the pumping intensity. In contrast with our early study of photo-degradation of semiconducting polymer P3HT, the rate of photo-polymerization of BITh is getting modestly higher if the monomer film is deposited on top of silver separated from the monomer by a thin insulating MgF2 layer preventing a charge transfer. This effect is partly due to a constructive interference of the incident and reflected light waves, as well as known in the literature effects of metal/dielectric substrates on a variety of spectroscopic and energy transfer parameters. At the same time, the rate of photopolymerization is getting threefold larger if monomer is deposited on Ag film directly and charge transfer is allowed. Finally, Au substrates cause modest (~ 50%) enhancement of both monomer film absorption and the rate of photo-polymerization.

Ultra-sensitive plasmonic sensing based on gold nanostrip arrays.

The extra-narrow reflection feature in angular dependence of reflectivity of periodic gold nanostrip arrays presents interest for sensing application. We explore its behavior in a modified dielectric environment and characterize the capability for sensing small changes in the dielectric permittivity of the environment.

Directional emission of rhodamine 6G on top of a silver grating.

We have observed directional spontaneous emission of rhodamine 6G dye deposited on top of a silver grating and found that its angular distribution patterns were very different in TE and TM polarizations. The latter was related to the dispersion curves determined based on the polarized reflection spectra measured at multiple incidence angles. The most intriguing finding of this Letter was a resonance, which was coupled with TE-polarized light and determined the characteristic double-crescent patterns in the TE-polarized spontaneous emission. This observation, as well as nearly similar resonance observed in TM polarization, was tentatively explained in terms of leaky waveguide modes supported by a film of dye-doped polymer.

Collective plasmonic oscillations in gold nanostrips arrays.

Excitation of collective plasmonic modes and their effect on optical behavior are experimentally and theoretically studied in 1D arrays of gold nanostrips in comparison with continuous gold films with periodically modulated profile. In strips, the angular dependence of the reflectivity demonstrates a peak at the resonance condition as opposed to a dip observed in continuous sine wave gratings. In addition, an extremely narrow feature in the reflection is observed in strips and tentatively ascribed to the bright Wood-Rayleigh anomaly. Theoretical calculations based on the combined transfer-matrix coupled-wave analysis and coordinate transformation method are shown to fit the experimental angular and spectral behavior of the plasmonic resonances. The effects are also discussed in terms of a simple equivalent circuit model.

Spontaneous emission of electric and magnetic dipoles in the vicinity of thin and thick metal.

Strong modification of spontaneous emission of Eu(3+) ions placed in close vicinity to thin and thick gold and silver films was clearly demonstrated in a microscope setup separately for electric and magnetic dipole transitions. We have shown that the magnetic transition was very sensitive to the thickness of the gold substrate and behaved distinctly different from the electric transition. The observations were described theoretically based on the dyadic Green's function approach for layered media and explained through modified image models for the near and far-field emissions. We established that there exists a "near-field event horizon", which demarcates the distance from the metal at which the dipole emission is taken up exclusively in the near field.

Modification of electric and magnetic dipole emission in anisotropic plasmonic systems.

In order to investigate the effects of plasmonic environments on spontaneous emission of magnetic and electric dipoles, we have studied luminescence of Eu³âº ions in close vicinity to gold nanostrip arrays. Significant changes in the emission kinetics, emission polarization, and radiation patterns have been observed in the wavelength range corresponding to the plasmonic resonance. The effect of the plasmonic resonance on the magnetic dipole transition 5D0-->7F1 is found to be very different from its effect on the electric dipole transitions. This makes Eu³âºâ‚‹ containing complexes promising for mapping local distributions of magnetic and electric fields in metamaterials and plasmonic systems.

Effect of metallic surface on electric dipole and magnetic dipole emission transitions in Eu3+ doped polymeric film.

Spontaneous emission of Eu(3+) ions is studied in thin organic films deposited onto several different substrates. It has been demonstrated that the presence of a metallic surface in close vicinity to emitting Eu(3+) ions causes modifications of their spontaneous emission spectra, in particular, the change in the relative strengths of magnetic-dipole and electric-dipole transitions. The character and the magnitude of the effect depend on the polarization and the observation angle. The experimental data are discussed in terms of modification of transition probabilities and account for the interference between directly emitted and reflected light waves.

NMR and spin relaxation in systems with magnetic nanoparticles: effects of size and molecular motion.

To better understand the specifics of nuclear magnetic resonance and spin relaxation in systems with magnetic nanoparticles and test the limits of the outer sphere model for the diffusion-related relaxation, iron oxide nanoparticle suspensions are studied in dependence on the particle concentration and size (5-40 nm). The model is modified to account for aggregation of the particles into clusters with an enlarged effective radius. For liquid suspensions containing small particles or clusters, both the longitudinal and transverse spin relaxation rates, T(1)(-1) and T(2)(-1), correspond well to the theory, which predicts passing of T(1)(-1) through a maximum and monotonic increase in T(2)(-1) with increasing particle size. For the largest particle sizes, as well as in the case of strong aggregation, the relaxation rates are significantly lower than theoretical predictions. An abrupt change in both the relaxation rates is observed in a narrow temperature range around the melting point of paraffin wax doped with magnetic nanoparticles. The applicability of fast-motion and fast-diffusion approximations is discussed for large effective sizes and limiting molecular motion cases.

Stimulated emission of surface plasmon polaritons.

We have observed laserlike emission of surface plasmon polaritons (SPPs) decoupled to the glass prism in an attenuated total reflection setup. SPPs were excited by optically pumped molecules in a polymeric film deposited on the top of a silver film. Stimulated emission was characterized by a distinct threshold in the input-output dependence and narrowing of the emission spectrum. The observed stimulated emission and corresponding compensation of the metallic absorption loss by gain enables many applications of metamaterials and nanoplasmonic devices.

Magnetic resonance in nanoparticles: between ferro- and paramagnetism.

Magnetic nanoparticles of γ-Fe(2)O(3) coated with organic molecules and suspended in liquid and solid matrices, as well as non-diluted magnetic fluid, have been studied by electron magnetic resonance (EMR) at 77-380 K. Slightly asymmetric spectra observed at room temperature become much broader and symmetric, and shift to lower fields upon cooling. An additional narrow spectral component (with a line-width of 30 G) is found in diluted samples; its magnitude obeys the Arrhenius law with an activation temperature of about 850 K. The longitudinal spin-relaxation time, T(1)≈10 ns, is determined by a specially developed modulation method. The angular dependence of the EMR signal position in field-freezing samples points to substantial alignment, suggesting the formation of dipolar-coupled aggregates. The shift and broadening of the spectrum upon cooling are assigned to the effect of the surface-related anisotropy. To describe the overall spectral shape, the 'quantization' model is used which includes summation of resonance transitions over the whole energy spectrum of a nanoparticle considered as a giant exchange cluster. This approach, supplemented with some phenomenological assumptions, provides satisfactory agreement with the experimental data.

NMR and spin relaxation in systems with magnetic nanoparticles.

The ¹H NMR spectra and spin dynamics of the host systems have been studied in liquid and solid suspensions of γ-Fe2O3 nanoparticles. Significant broadening of ¹H NMR spectra and growing relaxation rates were observed with increased concentration of nanoparticles in the liquid systems, with the relation T1/T2 depending on the particular host. Solid systems demonstrate inhomogeneous broadening of the spectra and practically no dependence of T1 upon the nanoparticle concentration. We explain the experimental results taking into account the predomination of self-diffusion as a source of the relaxation in liquid suspensions, and estimate effective parameters of relaxation in the systems under study.

Optical Studies of Nd-doped benzil, a potential luminescent and laser material.

Neodymium-doped benzil crystals have been synthesized and characterized for their absorption, emission, and kinetics properties. From Judd-Ofelt analysis, the radiative decay time of Nd emission (peaking at 1055 nm) is estimated to be equal to 441 mus. The experimental Nd lifetime (under Ar+ laser excitation) is equal to 19 mus. The broad emission band centered at approximately 700 nm (tau(decay) approximately 15 ns) and the Raman scattering with characteristic frequency shift of 1600 cm(-1) have been observed at excitation of benzil with 532-nm Q-switched laser pulses. We show that rare-earth-doped benzil can be considered as a potential candidate for luminescent and solid-state laser material.