Vladimir P. Safonov

Fractals in Microcavities: Giant Coupled, Multiplicative Enhancement of Optical Responses

Lasing at extremely low pump intensities, below 1 mW, and dramatically enhanced Raman scattering were observed for a novel class of optical materials, microcavities doped with nano‐structured fractal aggregates. Nanostructured fractal aggregates lead to giant local‐fields concentrated in “hot spots” much smaller in size than the size of the fractal and the wavelength. Coupling the localized plasmon modes in fractal aggregates with microcavity resonances further increases the local fields. These microcomposites possess unique optical characteristics, including extremely large probabilities of radiative processes. An extremely pronounced time‐dependent lasing effect of dye molecules adsorbed on silver particles was also observed in our experiment This time dependence is likely due to photo‐stimulated aggregation of silver colloid particles and the gradient forces pulled fractal aggregates into the high‐field area associated with the morphology‐dependent resonance mode.

Surface-plasmon-enhanced radiation effects in confined photonic systems

Combined action of a resonant cavity and metal nanocomposites is shown to result in dramatic enhancement of spontaneous emisson rate in the optical spectral range. The cavity leads to modification of the optical density of states (Parcell effect), whereas the metal nanoparticles lead to giant local-field effects in spontaneous emission occurring because of surface-plasmon-enhanced vacuum field fluctuations. It is also shown that the surface-plasmon resonances may result in a full photonic gap in a metal-dielectric composite, even for a completely random structure of the composite. In our experiments, lasing at extremely low pump intensities, below 1 mW was observed for a novel class of optical materials, microcavities doped with fractal aggregates of metal nanoparticles.

OPTICAL NONLINEARITIES OF METAL-DIELECTRIC COMPOSITES

Nonlinear optical properties of metal-dielectric composites, such as fractal colloid aggregates and clusters created by ion implantation, are studied. Strong fluctuations of local fields result in huge enhancements of optical nonlinearities in fractal colloid aggregates. The real and imaginary parts of the cubic susceptibility of silver colloid aggregates are measured. It is found that the coefficient of nonlinear absorption strongly depends on the laser wavelength and intensity. Optical limiting effect in fractal silver colloids is observed. Nondegenerate forward four-wave mixing technique is used to investigate the third-order nonlinear susceptibility for nanocomposite material with Au nanocrystals formed inside a SiO2 glass matrix. The Au nanocrystals are formed by the ion implantation and annealing method that produces very high volume fraction of nanoparticles. The large value |χ(3)|=1.3×10-7 esu is measured. Two characteristic relaxation times, 5.3 ps and 0.66 ps, are estimated from the detuning curve of |χ(3)|, as the probe beam wavelength changes. A novel class of optical materials, microcavities doped with nanostructured fractal aggregates, is also studied. In our experiments, lasing at extremely low pump intensities, below 1 mW, and dramatically enhanced Raman scattering was observed in microcavity/fractal composites.

Nonlinear Optical Phenomena in Nanostructured Fractal Materials

The results of theoretical and experimental studies of nonlinear optical phenomena in fractal nanostructures are reviewed. Localization of optical excitations in fractal metal nanocomposites, such as colloidal aggregates, self-affine and semicontinuous films, results in strong enhancements of optical nonlinearities. The localized modes of fractals cover a broad spectral range, from the visible to the far-infrared. A number of optical processes are studied such as the Kerr-type nonlinearities, four-wave mixing, second and third harmonic generation and frequency- and polarization-selective photomodification.

Excitation localization and nonlinear optical processes in colloidal silver aggregates

We present the results on investigation of the photomodification and optical nonlinearities of colloidal aggregates, which are indicative of the localization of optical excitation in fractal clusters. It is shown experimentally that the enhanced optical nonlinearity of aggregated metal colloids is connected with an enhancement of the local field in resonant domains of large clusters. The enhancement factor of nonlinear absorption in aggregated silver colloids is about 700 as compared with nonaggregated samples.

Low-threshold lasing and broad-band multiphoton-excited light emission from Ag aggregate-adsorbate complexes in microcavity

A novel class of composites for optics, microcavities doped with metal fractal aggregates, is studied. Lasing and broad-band Stokes and anti-Stokes emission from (Ag colloidal aggregates)/(adsorbed molecules)/(micro-cavity) composite at low-intensity cw and pulse laser excitation has been found. At 633 nm cw excitation wavelength the emission spectrum contains many peaks, spanning a range from wavelength 200 nm to 800 nm. Experiments with pulse excitation of Ag/dye/microcavity composite show that the duration of the observed broad-band anti-Stokes emission significantly exceeds the pump pulse duration, dye molecule fluorescence time, and relaxation times in silver particles. It may be interpreted as a luminescence governed by long-living triplet states of dye molecules. These observations were made possible by use of a fractal-microcavity composite, where coupling the localized plasmon modes in fractal aggregates with microcavity resonances is provided. The important role of 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.

Polarization Effects in Nanoaggregates of Silver Caused by Local and Nonlocal Nonlinear-Optical Responses

A theoretical and experimental study is made into the combined manifestation of local and nonlocal optical responses in a cubic nonlinear isotropic medium such as an aggregated colloidal silver solution. The phenomenological treatment of polarization effects is performed for the general case with due regard for the frequency dispersion of both local and nonlocal nonlinearities and for the noncollinear propagation of pump and probe light waves. The inverse Faraday effect, the optical Kerr effect, and the self-rotation of the polarization ellipse in a fractal-disordered nonlinear medium are observed for the first time. The tensor components of the local and nonlocal cubic nonlinearities of colloidal silver solutions are measured for different degrees of aggregation. It is demonstrated that, as the size of silver aggregate increases, the nonlocal nonlinear response increases much more strongly than the local one. An inference is made that the mechanical motion of metal nanoparticles because of their dynamic interaction with the light wave field can contribute to the nonlinear polarization effects.

Giant nonlinear optical activity in an aggregated silver nanocomposite

Nonlinear optical activity due to spatial dispersion is observed in a colloidal solution of silver. It is shown experimentally that the effect is substantially enhanced (by a factor of ∼102) when the silver particles aggregate into fractal clusters. The self-rotation angle of the plane of polarization is 2 mrad at an intensity of 2 MW/cm2 for λ=0.532 μm and a pulse duration of 11 ns. A method of separating the contributions of the local and nonlocal effects to the rotation of the plane of polarization is proposed and implemented.

Nonlinear Optical Effects and Selective Photomodification of Colloidal Silver Aggregates

Colloidal silver aggregates of nanoparticles were studied experimentally using optical spectroscopy, electron microscopy, near-field optics, and nonlinear optics. Changes in absorption spectra, local structure, and near-field optical response after the irradiation of fractal colloidal aggregates with a laser pulse (selective photomodification) were studied. The diameters of the selectively photomodified domains decreased as the laser wavelength increased, in accordance with the theory of the optics of fractal clusters. Giant enhancements of nonlinear optical responses were found for aggregated nanocomposites compared with nonaggregated. The enhancements are due to excitation of the collective plasmon modes in the aggregates. The plasmon modes are anisotropic and chiral. Nonlinear effects governed by local and nonlocal responses (degenerate four-wave mixing, nonlinear absorption, refraction and gyrotropy, inverse Faraday effect, and ellipse self-rotation) were studied.

On the Theory of Optical Properties of Fractal Clusters

A theory of optical properties of clusters of spherical metal nanoparticles characterized by an arbitrary size distribution is developed in a quasi-static dipole approximation. The equations for coupled dipoles and general relations are formulated in terms of reduced dipole moments. It is shown that the dipole resonant frequencies and amplitudes, the absorbed power, and the acting-field magnitudes strongly depend on the ratios of particle radii in a cluster. Properties of linear, planar, and three-dimensional systems are examined.