Fluctuations of the local density of states probe localized surface plasmons on disordered metal films

We measure the statistical distribution of the local density of optical states (LDOS) on disordered semi-continuous metal films. We show that LDOS fluctuations exhibit a maximum in a regime where fractal clusters dominate the film surface. These large fluctuations are a signature of surface-plasmon localization on the nanometer scale

Extinction calculations of multi-sphere polycrystalline graphitic clusters - A comparison with the 2175 AA peak and between a rigorous solution and discrete-dipole approximations

Certain dust particles in space are expected to appear as clusters of individual grains. The morphology of these clusters could be fractal or compact. In this paper we study the light scattering by compact and fractal polycrystalline graphitic clusters consisting of touching identical spheres. We compare three general methods for computing the extinction of the clusters in the wavelength range 0.1 - 100 micron, namely, a rigorous solution (Gerardy & Ausloos 1982) and two different discrete-dipole approximation methods -- MarCODES (Markel 1998) and DDSCAT (Draine & Flatau 1994). We consider clusters of N = 4, 7, 8, 27,32, 49, 108 and 343 particles of radii either 10 nm or 50 nm, arranged in three different geometries: open fractal (dimension D = 1.77), simple cubic and face-centred cubic. The rigorous solution shows that the extinction of the fractal clusters, with N < 50 and particle radii 10 nm, displays a peak within 2% of the location of the observed interstellar extinction peak at ~4.6 inverse micron; the smaller the cluster, the closer its peak gets to this value. By contrast, the peak in the extinction of the more compact clusters lie more than 4% from 4.6 inverse micron. At short wavelengths (0.1 - 0.5 micron), all the methods show that fractal clusters have markedly different extinction from those of non-fractal clusters. At wavelengths > 5 micron, the rigorous solution indicates that the extinction from fractal and compact clusters are of the same order of magnitude. It was only possible to compute fully converged results of the rigorous solution for the smaller clusters, due to computational limitations, however, we find that both discrete-dipole approximation methods overestimate the computed extinction of the smaller fractal clusters.Comment: Corrections added in accordance with suggestions by the referee. 12 pages, 12 figures. Accepted for publication in Astronomy & Astrophysic

Determination of silica aerogels nanostructure characteristics by using small angle neutron scattering technique

Small angle neutron scattering (SANS) technique has been widely employed in probing the microstructure of amorphous materials in the nanometer range (1 to 100 nm). In this study, small angle neutron scattering was used to study the structure of the silica aerogels and titanium containing silica aerogels by using SANS facility at MINT, Malaysia and BATAN, Indonesia. Besides scattering method, imaging technique such as transmission and scanning electron microscopy (TEM and SEM) can be used to provide real-space structure. However, microscopy image may include artifacts and may not be truly representative of the sample. While SANS does not provide real space structure directly, the technique does probe the sample in its entirety. In this work, the aerogels physical properties such as particle size and fractal dimension as a function of pH were studied. In a typical scattering experiment, an incident neutron beam is bombarded to the sample and is elastically scattered. The scattered intensity is measured as a function of the scattering angle which occurs at small angle of less than 100. Reactor was used as our neutron source. The monochromated neutron beam has a wavelength of 0.5 Ã…. The sample which is in powder form is filled into a quartz cell with a 2 mm pathlength. A complete data set consists of three measurements; scattering measured from the sample, scattering from the empty sample holder and scattering from the dark counts due to complete absorber in sample position. The scattered neutrons were detected by a 128 X 128 array area sensitive, gas-filled proportional counter, which is known as Position Sensitive Detector (PSD). A personal computer which is linked to the PSD neutron counting system is used for data collection. SANS neutron counting system programs include the display of scattered neutron data in two and three dimensional isometric view. The resulting 2D scattering pattern is reduced to 1D profile for further analysis. Plots of I(Q) vs Q were derived. Results show that as pH decreases fractal dimension decreases from 3.60 to 2.44. On the other hand, particle size increases from 9.87 nm to 11.26 nm with decreasing pH of the aerogels. Titanium containing silica aerogels has bigger fractal dimension and smaller particle size compared to silica aerogels

Measurement of the electromagnetic field backscattered by a fractal surface for the verification of electromagnetic scattering models

Fractal geometry is widely accepted as an efficient theory for the characterization of natural surfaces; the opportunity of describing irregularity of natural surfaces in terms of few fractal parameters makes its use in direct and inverse electromagnetic (EM) scattering theories highly desirable. In this paper, we present an innovative procedure for manufacturing fractal surfaces and for measuring their scattering properties. A cardboard–aluminum fractal surface was built as a representation of a Weiestrass–Mandelbrot fractal process; the EM field scattered from it was measured in an anechoic chamber. A monostatic radarlike configuration was employed. Measurement results were compared to Kirchhoff approximation and small perturbation method closed-form results that were analytically obtained by employing the fractional Brownian motion to model the surface shape. Matching and discrepancies between theories andmeasurements are then discussed. Finally, fractal and classical surface models are compared as far as their use in the EM scattering is concerned.Postprint (published version

Quantum interference in nanofractals and its optical manifestation

We consider quantum interferences of ballistic electrons propagating inside fractal structures with nanometric size of their arms. We use a scaling argument to calculate the density of states of free electrons confined in a simple model fractal. We show how the fractal dimension governs the density of states and optical properties of fractal structures in the RF-IR region. We discuss the effect of disorder on the density of states along with the possibility of experimental observation.Comment: 19 pages, 6 figure