Optical and electrical transport properties of semicontinuous metallic films

Semicontinuous metallic films posses unique optical and electrical transport properties. The strong localization and large enhancement of electric fields demonstrated by these films make them interesting in terms of a fundamental understanding of these phenomena as well as potentially useful in a large variety of applications. The field distributions and electrical transport properties of these films depend on the structural geometry and morphology of the samples and therefore also on the metal concentration, with distinctive properties near the percolation threshold where an insulator-to-metal transition occurs. This dissertation focuses on an experimental investigation of the optical, electrical transport and structural properties of semicontinuous silver-glass composites near the percolation threshold. Particular emphasis is placed on the optical enhancement observed in these films in terms of their near-field optical intensity distribution. The samples were synthesized by pulsed laser ablation and the percolation threshold was identified in terms of structural properties using transmission electron microscopy as well as electrical resistivity and optical transmission through in-situ measurements during sample synthesis. The optical intensity distributions were measured using near-field optical microscopy (NSOM) and found to depend strongly on metal concentration and wavelength. The degree of localization was found to increase as metal concentrations was varied above and below the percolation threshold and also found to increase with increase in wavelength. Comparisons of the experimental results with theoretical calculations of the local field distributions obtained using the block elimination method showed good agreement.; In addition, composite systems involving the coupling of enhancement from semicontinuous films with two other resonance phenomena are also studied viz. semicontinuous films (1) with random subwavelength apertures and (2) deposited on spherical microcavities. Extremely large transmission was obtained in the former system and a large concentration of fields near the subwavelength apertures was observed by NSOM. Localization effects due to the presence of the semicontinuous coating on the dielectric microcavities were observed, although evidence of mutual reinforcement of the enhancing abilities of the combined film-microcavity system was not conclusively obtained.