Small-particle composites. I. Linear optical properties

Abstract

Absorption and extinction spectra of fractal and nonfractal small-particle composites are studied. General solutions of the coupled-dipole equations with the exact operator for the dipole interaction ͑including the near-, intermediate-, and far-zone terms͒ are found and compared with those in the quasistatic approximation. Broadscale numerical simulations of optical spectra for clusters containing a large number of particles ͑up to 10 000͒ are performed. A significant fraction of dipolar eigenmodes in fractal aggregates is shown to be strongly localized. The eigenmodes cover a wide spectral region providing resonant enhancement in the visible and infrared parts of the spectrum. In contrast to previous predictions, the absorption spectrum is shown to be significantly different from the spectral distribution of the density of dipole eigenmodes. It clearly indicates the importance of symmetry properties of the modes and corresponding selection rules for the absorption by different modes in random fractal composites. Our experimental data obtained for extinction spectra of silver colloid fractal aggregates are in good agreement with the results of numerical simulations.

Direct observation of localized dipolar excitations on rough nanostructured surfaces

“…[6][7][8] Resonant dipolar excitations in fractal structures also can be localized in subwavelengthsized regions but, being the result of multiple scattering, exhibit strong frequency and polarization dependence of their spatial location. 9,10 This also means that the spatial location of light-induced dipole excitations is determined not only by the local topography, but also by the large-scale geometrical structure. Note that these features of light localization in fractals are similar to those observed for localization of surface plasmon polaritons ͑SPP's͒, 11,12 which is also an interference phenomenon related to multiple scattering of SPP's ͑in the surface plane͒ caused by the surface roughness.…”

“…13 The resonant optics of fractals has progressed immensely over the last decade, and many issues related to optical excitations in fractals have been theoretically clarified. [6][7][8][9][10]14 It was established that dipolar eigenmodes which resonate at very close frequencies and, therefore, can be excited simultaneously by a monochromatic source ͑as long as they lie within a single resonance width͒ can have very different localization lengths, from the maximum size of a sample to the minimum roughness scale. 9,[15][16][17][18] This phenomenon has been referred to as inhomogeneous localization.…”

“…[6][7][8][9][10]14 It was established that dipolar eigenmodes which resonate at very close frequencies and, therefore, can be excited simultaneously by a monochromatic source ͑as long as they lie within a single resonance width͒ can have very different localization lengths, from the maximum size of a sample to the minimum roughness scale. 9,[15][16][17][18] This phenomenon has been referred to as inhomogeneous localization. [15][16][17] However, in metal fractal nanocomposites the localization tends to increase, on average, when the wavelength changes from the plasmon resonance of an isolated metal spherule (ϳ400 nm) to the infrared, as was confirmed both theoretically 9,[19][20][21] and experimentally.…”

O efeito SERS na análise de traços: o papel das superfícies nanoestruturadas

“…Estruturas fractais são invariantes em função da escala 37 e podem ser associadas à rugosidade superficial das superfícies SERS-ativas como eletrodos metálicos, colóides e filmes de ilhas metálicas. Shalaev et al, em uma série de trabalhos teóricos 38,39 e experimentais 40,41 , investigaram a importância da fractalidade nas superfícies SERS-ativas, sugerindo que flutuações de campo estão presentes em agregados fractais de partículas, cujos dipolos individuais sofrem intenso acoplamento. Estes são sítios com intensificações gigantes do campo elétrico, onde espécies moleculares apresentam fatores de intensificação do espalhamento Raman da ordem de 10 10 vezes.…”

Microscopic theory of surface-enhanced Raman scattering in noble-metal nanoparticles

“…8,9,10,11 A combined effect of SP local fields from different particles acting on a molecule trapped in a gap can result in a giant (up to 10 14 ) enhancement of the Raman scattering crossection. 12,13,14,15,16,17 Other mechanisms contributing to SERS can involve electron tunneling between a molecule and a nanoparticle. 18 The conventional description of EM enhancement is based on classical Mie scattering theory.…”