Patrice Gadenne

Infrared optical absorption and fractal structure of granular media at percolation

The independence of the infrared optical properties (reflectance, transmittance, absorption ϵ2/λ) on the wavelength λ, over a large spectral range (1<λ<10 μm), is proved to be a characteristic behaviour of granular metallic media (discontinuous gold and Au-Al2O3 cermet thin films) at the percolation threshold. Near the threshold, considering as a basis the ramified morphology of the medium, we give a qualitative account of our experimental results: the transmittance is treated by a real-space renormalization, the optical absorption in the conducting regime is analyzed using a Drude formalism leading to a very low value of the effective electron relaxation time.

First AFM observation of thin cermet films close to the percolation threshold using a conducting tip

Abstract Optical, electrical and magnetic properties of cermet thin films (metallic nanograins embedded in an insulating matrix) are strongly dependent on their morphology. The size, shape and distribution of metallic grains are the main parameters as well as roughness of the surface of the samples. Usual techniques such as transmission electron microscopy, scanning electron microscopy, atomic force microscopy, while very useful to characterize some of them, are inefficient for others and have to be used in a complementary way. Until now, it was impossible to point out metallic nanograins from the matrix using a commercial setup. We have developed a new technique of local contact resistance measurements with a conducting tip AFM called “Resiscope”. In this paper, we show the first electrical mapping obtained with our set-up on gold-alumina and nickel-alumina cermet, with different filling factors, very close to percolation threshold and we briefly correlate these observations to magnetic properties.

Morphological analysis of discontinuous thin films on various substrates

We study and characterize the morphology of granular gold films deposited on amorphous and polycrystalline substrates using some of the more efficient tools of the mathematical morphology. This analysis shows the occurrence of characteristic lengths in the morphology which are strongly dependent on the nature of the substrate. This implies that a completely random model such as the theory of percolation can only be considered as a first approximation to interpret the dielectric and optical properties of granular thin films.

Determination of the “optical” thickness and of the filling factor of discontinuous Au films from photometric and ellipsometric measurements

Abstract The “optical” thickness of discontinuous Au films is determined from a suitable combination of photometric and ellipsometric measurements. The value of the metal filling factor which can be deduced from a comparison with the “mass” thickness is found to be in good agreement with the value directly obtained from structure studies.

Levy's distributions of local fields intensities in metal-dielectric systems

Abstract Levys distributions are involved in many physical phenomena. Their main characteristics are a very wide spreading and a long tail. The aim of our study is to use these Levys distributions to understand optical measurements on metal-dielectric systems. Giant fluctuations of the local electric field intensity in metal-dielectric granular films in visible and infrared range have been recently studied. By using previously developed methods, we calculate the intensities of the local fields at each point of the sample surface for Au/TiO 2 cermets. We study the field intensities distributions, which show up non-negligible height in giant field values. We interpret these distributions in terms of Levys law. In such cases, because of the long tail decay, the most important characteristic is the decisive participation of rare events. We then conclude that high-intensities values are high enough to govern the statistical properties when the metal concentration is around 10%.

Surface-Plasmon-Enhanced Nonlinearities in Percolating 2-D Metal—Dielectric Films: Calculation of the Localized Giant Field and Their Observation in SNOM

The surfaces of percolating random 2-D metal—dielectric films consist of several spectral resonances, which have been calculated and afterward observed by near-field optical microscopy. These films show anomalous optical properties which are investigated in the first section. Nonlinear electrical and optical properties of metal—dielectric film percolation composites, though recognized very early, were not well understood. It is only recently that calculation of local fields in semicontinuous films allows us to define the enhancement factors of optical nonlinearities. These calculations are outlined from basic principles in the second section and compared with experimental results. An insightful approach to the same problem is to use a network description to represent the random system and discretize the equations satisfied by the scalar potential of the electrical field. We recall in the third section how such discretization leads to a Hamiltonian which is paradigmatic in the theory of Anderson localization. The imaging and spectroscopy of localized optical excitation in gold-on-glass percolation films was performed using near-field optical microscopy (SNOM), and the fourth section recalls the basic features of the experimental technique and describes the first experimental observation of “hot spots” in a nanometer-scale area.

Localized plasmon-enhanced optical response: harmonic generation and polarization effects

It is now known that plasmon oscillations supported by nanostructured metal thin films of fractal morphology, can result in large local fields and strong enhancement of optical phenomena, for example Raman scattering. The localized plasmons, acting like nano-antennas, can concentrate very large electromagnetic energy in nanometer- sized areas, hot spots, and provide particularly strong enhancement of optical responses, in a very broad spectral range. Our new experimental results show up position dependence of the hot spots on the polarization state of the light. Moreover as expected from recent theoretical predictions, on this kind of thin percolating films, there is a dramatic enhancement of the second harmonic generation (2(omega) ) out of the specular directions. This unusual diffuse SHG could be connected to possible chirality of the percolating metallic films, which is expected to manifest itself as change in the hot-spot distribution for the left and right circularly polarized incident light.

Far Infrared Optical Properties of Ferromagnetic Cermets

Abstract Presented is an extension of the Onsager theory, initially applicable to electric permanent dipoles, to the calculation of the effective magnetic permeability of cermets including monodomain grains of a ferromagnetic metal. Taking into account both the orientational alignment of the ferromagnetic moments of the metal grains and their induced magnetic moment, the magnetisation is calculated by considering the local magnetic field. The behaviour of the magnetic permeability μ is predicted, which is particle size and temperature dependent. The optical thickness β=( 2лd λ ) √ϵμ and the admittance g=√ϵ/μ are then calculate by introducing μ and the dielectric function ϵ given by an effective medium theory. It leads to a new expression of the reflection and transmission coefficients taking into account both the electrical and magnetical side of the optical properties of ferromagnetic cermet. As an example, the influence of the magnetic permeability on the absorption of a Ni-Al2O3 thin film is presented at near normal incidence, up to 1000 μm.

Plasmon-enhanced absorption by optical phonons in cermets

Cermet is mixture of nano-sized metallic grains and insulating matrix. When concentration in metal is large enough, the conducting particles are strongly interacting between each other. As predicted by V. Shalaev for fractal metallic clusters and experimentally verified, in the vicinity of the percolation threshold, the local electromagnetic field can be very large because of plasmon resonance in the metallic grains which may happen in a wide spectrum of frequencies. In this enhanced field region, the micro-crystallites of the matrix are immersed in these huge fields. In the spectral range of the optical phonon of the matrix there can be a very large absorption band because of the possible coupling between phonon of the matrix and resonance of the metallic grains. We experimentally observe this huge absorption in gold alumina cermets. The theoretical model is in good agreement with experimental results.

Scanning near-field optical microscopy: local probes and enhanced electromagnetic fields

Enhanced electromagnetic fields are investigated, both theoretically and experimentally, on two model systems using high spatial resolution. Strong field enhancements at the apex of a tungsten tip illuminated by an external light source are studied as a function of the incident polarization. The surface of percolating random metal- dielectric films consist of several spectral resonances, which have been calculated and are observed here in near field with 10 nm lateral resolution.