Yoad Yagil

Transmittance and reflectance in situ measurements of semicontinuous gold films during deposition

We have continuously measured in situ reflectance and transmittance of percolating gold films over the entire range of surface coverage P, in the IR regime (1.7 and 2.2 μm). The samples present similar optical and electrical behavior, when compared on a normalized thickness scale. In the fractal region, close to the percolation threshold, the optical properties show a linear dependence on the surface coverage parameter, in agreement with a renormalization argument previously suggested. A strong absorption (about 40%), whose origin is not well understood, is found in the fractal region. The interpretations previously proposed by other authors are not applicable. We conclude that geometrical effects must dominate the optical properties over a large range of surface coverage.

In-situ measurements of the optical properties of gold films near the percolation threshold

D.C. resistance, mass thickness, optical reflectance and transmittance at two infrared wavelengths (1.7 and 2.2 μm) are performed during the deposition process of thin gold films. A very large absorption is found in the vicinity of the percolation threshold well below and after it, while the optical properties are only weakly wavelength and surface coverage dependent. We conclude that the fractal morphology dominates the optical properties in this region.

THE ROLE OF MICROGEOMETRY IN THE ELECTRICAL BREAKDOWN OF METAL-INSULATOR MIXTURES

The breakdown phenomena of percolative metal-insulator composites and their extreme sensitivity to fine details of the microgeometry are discussed, for three different cases: the critical current of superconductor-insulator mixtures (or superconductor-normal metal); the dielectric breakdown of metal-insulator composites below the percolation threshold (insulating regime); and the electrical breakdown above the metal-insulator transition (metallic regime). Two experimental techniques for characterizing the microgeometry are described: (a) 1/f noise measurements, which provide the fourth moment of the current distribution; (b) the harmonic generation method, where the weakly nonlinear electrical response due to local Joule heating provides information on the fourth (and higher) moment of the current distribution.

Optical and microwave properties of metal-insulator thin films: possibility of light localization

High frequency response (optical, infrared and microwave) of thin metal-dielectric inhomogeneous films is studied. We propose a new approach based on a direct solution of Maxwells equations without having to invoke the quasi-static approximation. Our theory reproduces most of the known experimental results including the anomalous absorption near a percolation threshold. For the strong skin effect case or for superconducting-dielectric films we predict an electromagnetic wave localisation at the percolation threshold.

Nonlinear electrical response and breakdown of semicontinuous metal films

The weakly nonlinear electrical response, determined by 3rd harmonic generation, and the breakdown current Ib of percolating systems are measured in real materials: semicontinuous Ag and Au films. The 3rd harmonic is interpreted as a direct measure of the 4th moment of the current distribution, and thus provides the critical exponent κ (normally determined by 1/ƒ noise measurements). Observations of higher harmonics (5th and 7th), corresponding to higher moments of the current distribution, provide an estimation of higher critical indices. The breakdown current Ib was measured over three decades of film resistance. A new criterion for Ib is suggested, defined as the current at which a hot spot reaches the melting temperature of the metallic grains Tm. This criterion remains valid in the presence of nonlinear effects. The hot spot model yields a power law Ib ∞B-x, B=V3ƒ/I3 where the upper and lower bounds of x are found to be 0.5 (extreme case of continuum percolation) and 0.36 (lattice percolation lower bound), respectively, in excellent agreement with the measured data.

Field‐induced tuning of the optical properties of nonlinear composites near resonance

The bulk effective dielectric function of metal‐dielectric composites has a series of poles in the vicinity of which it is extremely sensitive to small changes in the properties of the components. It is possible to induce such a change in the nonlinear component of a composite, containing at least one nonlinear component, by applying an external electric field at a similar frequency. This causes a tuning of the effective dielectric function and a continuous alteration of the optical properties of the medium. We propose a general framework for calculating this effect in two component composites and also calculate it in a few three component microgeometries. It is found that the optical transmittance of a thin film of this kind can be changed from zero up to 80% using moderate electric fields.

Optical properties of semi continuous films on a substrate. Does an effective dielectric constant exist

We present recent results on the optical properties of semi continuous metal films, and discuss the limits of validity of effective medium approaches. The optical reflectance and transmittance of percolating gold films, close to the metal-insulator transition, were measured over an extended wavelength range: 2.5–500 μm. Comparing the measured data with the predictions of the scaling model for the optical properties yields excellent agreement, while effective medium approach is shown to be non-valid close to the percolation threshold. In particular, the short length scale determined by the anomalous diffusion relation is shown to be the relevant length scale. Determination of the optical response of the films on the substrate, and of the (hypothetical) free standing films, suggests that the optical properties cannot be described by any effective dielectric constant.

What is the relevant length scale when studying percolating clusters: a critical study

Percolation clusters are known to be well described by the percolation correlation length ξ. In fact, this length is defined by the geometry of the clusters, and thus can only give an account of zero frequency (time independent) physical properties, such as dc conductivity or fractal morphology of 2D or 3D clusters. Taking examples on optical properties of granular metal films close to the percolation threshold, we show that the relevant length scale L is not known well. It depends explicitly on the kind of measurement, and on the frequency ω at which the physical properties are observed. We conclude that, in the region where L(ω) ξ, an averaged beheviour can also be used.