Historically, the methods used to describe the electromagnetic response of random, three-dimensional (3D), metal-dielectric composites (MDCs) have been limited to approximations such as effective-medium theories that employ easily-obtained, macroscopic parameters. Full-wave numerical simulations such as finite-difference time domain (FDTD) calculations are difficult for random MDCs due to the fact that the nanoscale geometry of a random composite is generally difficult to ascertain after fabrication. We have developed a fabrication method for creating semicontinuous metal films with arbitrary thicknesses and a modeling technique for such films using realistic geometries. We extended our two-dimensional simulation method to obtain realistic geometries of 3D MDC samples, and we obtained the detailed near- and far-field electromagnetic responses of such composites using FDTD calculations. Our simulation results agree quantitatively well with the experimentally measured far-field spectra of the real samples.
In this work, we report the substantial compensation of loss of propagating SPPs at the interface between silver film and
optically pumped polymer with dye. The large magnitude of the effect, nearly threefold change of the reflectivity,
enables a variety of applications of "active" nanoplasmonics. In order to quantify the observed phenomenon, we have
extended the theoretical formalism relating the reflectivity in ATR experiment and the SPP propagation length to the
case of active dielectric media.
The light-induced forces in the aggregates of metal nanoparticles are studied with the Newton's and coupled
dipole equations. The simulations imply a linear intrinsic optical response of the particles and host medium.
Large and relatively fast third-order optical nonlinearity of such a nanocomposite is shown to originate from the
light-induced motion of the particles. The nonlinear absorption coefficients of an artificial medium composed of
5-particles aggregates are simulated at different intensities and frequencies of the incident light. The calculated
nonlinear absorption appears to be of the same order of magnitude as typically measured for the silver colloids.
Adaptive silver films (ASFs) have been studied as a substrate for protein microarrays. Vacuum evaporated silver films fabricated at certain range of evaporation parameters allow fine rearrangement of the silver nanostructure under protein depositions in buffer solution. Proteins restructure and stabilize the ASF to increase the surface-enhanced Raman scattering (SERS) signal from a monolayer of molecules. Preliminary evidence indicates that the adaptive property of the substrates make them appropriate for protein microarray assays. Head-to-head comparisons with two commercial substrates have been performed. Protein binding was quantified on the microarray using the streptavidinCy3/biotinylated goat IgG protein pair. With fluorescence detection, the performance of ASF substrates was comparable with SuperAldehyde and SuperEpoxy substrates. Additionally, the ASF is also a SERS substrate and this provides an additional tool for analysis. It is found that the SERS spectra of the streptavidinCy5 fluorescence reporter bound to true and bound to false sites show distinct difference.
Adaptive surface-enhanced Raman scattering (SERS) substrates exhibit unique properties which make them well suited for SERS studies of proteins on surfaces. Specifically, adaptive silver films (ASFs) allow nanoscale restructuring of metal particles during protein deposition which yields a three-fold benefit of soft protein adsorption, protein-metal complex stabilization, and increased SERS signal. In this work ASF fabrication and characterization methods are introduced, with special attention paid to characterization methods that provide insight into the adaptive nature of the substrates, such as UV-vis spectrophotometry, field-emission scanning electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. These ASF substrates show SERS enhancement factors in the range of 10<sup>6</sup> for an area-averaged signal, and have been successfully used for sub-monolayer protein detection. The addition of a thick metal layer in the ASF fabrication structure typically increases the SERS signal by a factor of four or five. Finally, several examples of current SERS protein studies using ASF substrates are provided.
High SERS sensitivity for protein detection has been accomplished with semicontinuous silver films. Specifically, an insulin surface density as low as 80 fmol/mm<sup>2</sup> and 25 amol in a probed area has been readily detected.
Optical properties of metal nanowires and nanowire composite materials are studied experimentally and theoretically. We suggest that a nanowire composite, constructed from parallel pairs of nanowires has both effective magnetic permeability and dielectric permittivity negative in the visible and near-infrared spectral ranges due to resonant excitation of surface plasmon polaritons.
Experimental results confirm excitation of surface plasmons polaritons in periodical array of nanowires.
This report presents the discovery of greatly enhanced, broad-range, multiphoton excited emission from Ag aggregate- adsorbate complexes seeded into a cylindrical microcavity. The emission spectrum contains descrete peaks spanning the wavelength range from the 632 nm HeNe laser exciting wavelength down to 200 nm. Observation of multiphoton processes at the low exciting light intensity (20 W/cm<SUP>2</SUP>) became possible due to using a fractal-microcavity composite, where coupling the localized plasmon modes in fractal aggregates with microcavity resonances is provided. The important role of the multiphoton resonant transitions between discrete states of a finite-size metal particle in enhanced local fields is shown. Analysis, based on the model of a spherical potential well, shows that the observed spectra contain fingerprints of the quantum size effect.
We describe the characteristics of a novel optical material developed in our laboratory, a fractal/microcavity composite. Both components of this composite exhibit resonance behavior whereby the amplitudes of spectral emissions, generated by molecules either adsorbed onto the composite or located remotely from it, are significantly enhanced. In the composite, the individual enhancement factors combine multiplicatively with the result that spectral emissions are enhanced by an extremely large factor. In particular, the extremely large enhancement factors facilitate the generation of nonlinear optical processes in the composite, which may be exploited in the fabrication of ultra-sensitive detectors.
Reported is the observation of nonlinear gyrotropy of the aggregated colloidal silver solution, caused by the spatial dispersion of nonlinear response. In experiments, we measured the angle of self-induced rotation of the polarization plane of the pulsed laser source at (lambda) equals 0.532 micrometers and (tau) equals 11 ns. The technique developed allowed to separate the spatial dispersion effect from the nonlinear rotation occurring due to local response of a medium. The latter was measured in the pump-probe configuration via inverse Faraday effect.
The applications of the new interferometric techniques to the measurement of nonlinear refractive index are summarized in this paper. Developed methods are based on dispersion and shearing interferometry and allow to perform the direct phase measurements in the near field. The majority of the limitations of traditional interferometers have been eliminated in these schemes. At the same time, the benefit of using these methods comparable to currently used is shown for the cases of complicated dependence of the refractive index on the intensity and when the response time of nonlinearity is comparable with the laser pulse duration.
In the present paper we report about experimental investigation of acousto-optical mode locking in various types of lasers: gas, solid-state and dye. The theoretical description which takes into account the amplifying process of short pulses in resonant active medium had been carried out. Experimentally the pulse location inside the temporal mode-locker `window' and its shape as a function of both active medium gain and the resonator length detuning have been done. The pulse shape comprises satellite(s) following the main pulse. The number of the satellites with their relative amplitude and spacing from the main pulse was also one of the subjects of investigation.