The radiative and nonradiative decay rates of lissamine dye molecules, chemically attached to differently sized gold nanoparticles, are investigated by means of time-resolved fluorescence experiments. A pronounced fluorescence quenching is observed already for the smallest nanoparticles of 1 nm radius. The quenching is caused not only by an increased nonradiative rate but, equally important, by a drastic decrease in the dye's radiative rate. Assuming resonant energy transfer to be responsible for the nonradiative decay channel, we compare our experimental findings with theoretical results derived from the Gersten-Nitzan model. DOI: 10.1103/PhysRevLett.89.203002 PACS numbers: 33.50.-j, 81.07.Pr Resonant energy transfer (RET) systems consisting of organic dye molecules and noble metal nanoparticles have recently gained considerable interest in biophotonics [1][2][3][4] as well as in materials science [5,6]. Closely related are donor-acceptor pairs of organic dye molecules forming Förster resonant energy transfer (FRET) systems. They have been theoretically modeled [7] and applied in biophysics extensively during the past decade (see, e.g., [8]). Yet these classical purely dye-based systems show disadvantages regarding quenching efficiency [4] and photostability [9].If the donor molecule is placed in the vicinity of a metal surface instead of an organic acceptor, not only resonant energy transfer takes place but also the radiative lifetime of the donor molecule changes. For metal films this has been investigated extensively [10 -13]. Much less is known about donor molecules in the vicinity of metal nanoparticles. Theoretical treatments of the moleculenanoparticle problem [14 -17] predict energy transfer rates and radiative decay rates that deviate substantially from what has been found for dye molecules in front of a metal film. Both radiative and nonradiative rates are expected to depend critically on size and shape of the nanoparticle, the distance between the dye molecule and the nanoparticle, the orientation of the molecular dipole with respect to the dye-nanoparticle axis, and the overlap of the molecule's emission with the nanoparticle's absorption spectrum. Recent experimental investigations deal with metal island films or rough surfaces only (see [18,19] and references in [20]), where the above mentioned parameters are undefined.Here we report results of time-resolved fluorescence experiments on a donor-acceptor system composed of lissamine molecules (donor) chemically attached to a gold nanoparticle (acceptor). The distance between the lissamine molecule and the surface of the nanoparticle is kept constant at 1 nm, whereas the nanoparticle radius is varied between 1 and 30 nm. We find time constants for the energy transfer on a picosecond time scale which turn out to decrease with increasing nanoparticle size. In addition, the dye's radiative rate is reduced by more than an order of magnitude. Both effects are responsible for the drastic quenching of the fluorescence yield as predicted by the so-called...

The fluorescence quantum yield of Cy5 molecules attached to gold nanoparticles via ssDNA spacers is measured for Cy5-nanoparticle distances between 2 and 16 nm. Different numbers of ssDNA per nanoparticle allow to fine-tune the distance. The change of the radiative and nonradiative molecular decay rates with distance is determined using time-resolved photoluminescence spectroscopy. Remarkably, the distance dependent quantum efficiency is almost exclusively governed by the radiative rate.

Light emission at the particle plasmon frequency is observed in optically excited spherical gold nanoparticles. We find a photoluminescence efficiency of 10 −6 , which is essentially independent of particle size and four orders of magnitude higher than the efficiencies determined from metal films. Our experimental findings are explained with a process in which excited d-band holes recombine nonradiatively with sp electrons, emitting particle plasmons. These plasmons subsequently radiate, giving rise to the photoluminescence observed in the experiment. We determine the quantum efficiencies involved in this process.

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We determine group index and group velocity dispersion (GVD) of SOI single-mode strip waveguides (photonic wires) with 525x226nm cross-section over the entire telecommunication bandwidth by employing an integrated Mach-Zehnder interferometer. The measured GVD yields 4400 ps/(nm*km) at 1550 nm and exceeds that of standard single-mode fibers by almost three orders of magnitude. In the photonic wires the GVD is mainly determined by strong light confinement rather than by material dispersion. Our results indicate that despite this high GVD, dispersion-induced signal impairment is negligible in photonic circuits for data rates up to 100-Gb/s and total waveguide lengths as long as about 1 meter. The measured group index and GVD are used as benchmarks to compare model calculations originating from four different theoretical methods.

We measure the transmission of ps-pulses through silicon-on-insulator submicron waveguides for excitation wavelengths between 1400 and 1650 nm and peak powers covering four orders of magnitude. Self-phase-modulation induced spectral broadening is found to be significant at coupled peak powers of even a few tens of mW. The nonlinear-index coefficient, extracted from the experimental data, is estimated as n(2) ~ 5*10(-18) m(2)/W at 1500 nm. The experimental results show good agreement with model calculations that take into account nonlinear phase shift, first- and second order dispersion, mode confinement, frequency dispersion of n(2), and dynamics of two-photon-absorption-generated free carriers. Comparison with theory indicates that an observed twofold increase of spectral broadening between 1400 and 1650 nm can be assigned to the dispersion of n(2) as well as first order- rather than second-order dispersion effects. The analysis of pulse broadening, spectral shift and transmission saturation allows estimating a power threshold for nonlinearity-induced signal impairment in nanophotonic devices.

The electric field directed layer-by-layer assembly (EFDLA) method is used to fabricate a pattern of two different types of CdTe nanocrystals on a structured indium-tin-oxide (ITO) substrate. Such a pattern can be used to fabricate pixel arrays of CdTe light-emitting devices that emit electroluminescence of different colors. The highly fluorescent CdTe nanocrystals are negatively charged and alternated by a polycation, poly(diallyldimethylammonium chloride) (PDDA), in the fabrication of multilayer PDDA/CdTe films on ITO and Au surfaces by the EFDLA method. The depositions of the charged species on different electrodes are controlled by the polarity of an applied bias voltage. We define the deposition that becomes easier in the electric fields as favorable deposition and that becomes more difficult as unfavorable deposition. Experimental results obtained by quartz crystal microbalance reveal that a series of unfavorable depositions of CdTe and PDDA performed in an alternate way can at most result in one bilayer of PDDA/ CdTe on the Au surface at 0.6 V, whereas favorable depositions show a net growth of deposited mass with the increased number of deposition cycles. Similar results can also be obtained by using ITO electrodes, but a relatively high voltage of 1.4 V is required to achieve a contrast of 99% between favorable and unfavorable depositions. The large contrast enables the fabrication of dual-color photoluminescence and electroluminescence patterns of PDDA/CdTe films by successively depositing two differently sized CdTe nanocrystals on different ITO electrodes on one common substrate by the EFDLA method.

The strong dispersion and large third-order nonlinearity in Si photonic wires are intimately linked in the optical physics needed for the optical control of phase. By carefully choosing the waveguide dimensions, both linear and nonlinear optical properties of Si wires can be engineered. In this paper we provide a review of the control of phase using nonlinear-optical effects such as self-phase and cross-phase modulation in dispersion-engineered Si wires. The low threshold powers for phase-changing effects in Si-wires make them potential candidates for functional nonlinear optical devices of just a few millimeters in length.