Metasurfaces: From microwaves to visible

Electromagnetic interactions of molecules with metal surfaces

The nature and properties of surface-enhanced vibrational spectra of molecules adsorbed on surfaces that can enhance the absorption and the emission of electromagnetic radiation are discussed. Examples of surface-enhanced resonant Raman scattering, surface-enhanced Raman scattering in the near-infrared and surface-enhanced infrared are given. The rough surfaces used are metal island films coated with a nanometric organic film using the Langmuir-Blodgett technique or deposited by vacuum evaporation.

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Surface enhanced vibrational spectroscopy

We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Fluorescence microscopy of nanoscale silver oxide (Ag2O) reveals strong photoactivated emission for excitation wavelengths shorter than 520 nanometers. Although blinking and characteristic emission patterns demonstrate single-nanoparticle observation, large-scale dynamic color changes were also observed, even from the same nanoparticle. Identical behavior was observed in oxidized thin silver films that enable Ag2O particles to grow at high density from silver islands. Data were readily written to these films with blue excitation; stored data could be nondestructively read with the strong red fluorescence resulting from green (wavelengths longer than 520 nanometers) excitation. The individual luminescent species are thought to be silver nanoclusters that are photochemically generated from the oxide.

https://science.sciencemag.org/content/sci/291/5501/103.full.pdf

Photoactivated fluorescence from individual silver nanoclusters.

For interpreting the optical properties of aggregated metal films, a modification of Schopper’s model is proposed, based on the assumption that an aggregated film consists of particles in the shape of rotational ellipsoids. Instead of the distribution of the depolarizing factor f in gaussian fashion, which is assumed in Schopper’s model, the logarithmic normal distribution of the axial ratio γ of the ellipsoidal particles was assumed and the parameter σ (standard deviation of the distribution function) was introduced. Besides, the contributions of the dipole interaction between particles and the size-dependent optical constants of the particles were taken into account. Using this modified model, the observed values of silver films continuously measured by ellipsometry during deposition were analyzed and the two parameters, the mean axial ratio γ¯ and the standard deviation σ, were obtained as functions of film thickness. The results showed that as film grows, both γ¯ and σ increase monotonically. Good coincidence of the observed and the calculated absorption spectra from our model seems to show the validity of our modification.

Optical Properties of Aggregated Silver Films

Optical properties of a silver-island film are interpreted by taking account of retarded dipole–dipole interactions between island particles instead of the static-field interaction. The discrepancies between the values of the structural parameters evaluated from electron micrographs and those evaluated optically with the assumption of the static-field dipole–dipole interaction can be thus interpreted. The retarded dipole–dipole interactions as well as the size effect also broaden the width of the plasma-resonance absorption peaks.

Effect of retarded dipole–dipole interactions between island particles on the optical plasma-resonance absorption of a silver-island film

An aggregated silver film shows absorption bands in the visible and the near-ultraviolet spectral regions caused by the plasma resonances of conduction electrons in the island particles composing the film, for electric fields parallel and perpendicular to the substrate. We derive the relation between the peak wavelengths of absorption bands in terms of the model based on a two-dimensional distribution of island particles and propose the method for the determination of the inter-island dielectric constant ∊a. The method is applied to aggregated silver films and the thickness dependence of ∊a is found.

Determination of the Inter-Island Dielectric Constant in Aggregated Silver Films, by Measurement of Optical Plasma Resonance Absorption

The changes of the optical properties of aggregated silver films were investigated with an automatic recording ellipsometer immediately after deposition in vacuum and after admission of air into the vacuum chamber. The optical properties change for several minutes after the vapor stream is cut off from the substrate. By analyzing the changes with the model based on the two-dimensional distribution of island particles in the shape of ellipsoids of rotation (modified RE model), we found that these changes correspond to the decrease of the mean axial ratio of the island particles composing the film and the standard deviation of the distribution function of the axial ratio. That is, the flat island becomes globular after deposition. Such a morphological change of the particle is interpreted in terms of mass transport accompanied by surface self-diffusion. The changes followed by exposure to air can be interpreted by the formation of a layer of dielectric pellicle, such as an adsorption layer or a sulfide layer surrounding the island particles.

Changes of the Optical Properties of Aggregated Silver Films after Deposition

A method for obtaining the optical constants and thickness of thin metal films during deposition using an automatic recording ellipsometer is described. The restored azimuth ψ, the differential phase change on reflection Δ and the transmittance T have been continuously meaured during the film deposition. The optical constants and the packing factor are calculated from these three quantities and are given as functions of film thickness for various deposition conditions.

http://iopscience.iop.org/article/10.1143/JJAP.8.559/pdf

Sadafumi Yoshida

Papers

Optical Properties of Aggregated Silver Films

Effect of retarded dipole–dipole interactions between island particles on the optical plasma-resonance absorption of a silver-island film

Determination of the Inter-Island Dielectric Constant in Aggregated Silver Films, by Measurement of Optical Plasma Resonance Absorption

Changes of the Optical Properties of Aggregated Silver Films after Deposition

Continuous Ellipsometric Determination of the Optical Constants and Thickness of a Silver Film during Deposition