A novel material platform based on metallic thin films with ultra-low losses in the visible and near-IR range is crucial for rapid development of a wide range of new generation plasmonic devices. For many years silver has been known as potentially the best plasmonic material with the naturally lowest ohmic losses at optical frequencies. However, the widespread implementation of the silver-based material platform for plasmonics and metamaterials is limited due to technological challenges in thin film synthesis and nanoscale features fabrication techniques. This review describes the main types of silver-based plasmonic devices from the thin films point of view required for their widespread practical application. Based on comparative analysis of more than 60-year-long history of previously reported data for the silver thin films, the authors formulate the qualitative and quantitative criteria of "ideal" silver film (which the authors name the silver "dream" plasmonic film) for plasmonic devices with ultra-low loss. This paper outlines on several well-known metrology issues in plasmonic metallic films characterization and summarize the set of methods for their properties careful and precise extraction. Finally, the detailed analysis of silver synthesis techniques and quantitative comparison of the achieved silver properties is carried out.

Silver-based plasmonics: golden material platform and application challenges [Invited]

Silver has unique optical properties for topical applications such as plasmonics. The two most widely used silver optical data sets are the Palik handbook compilation and that determined by Johnson and Christy. Unfortunately these are inconsistent making realistic modelling of the likely performance of silver in optical applications difficult, with modelling producing either highly optimistic or very pessimistic results, depending on application. By critical examination and duplication of the original experiments leading to the widely accepted literature values, we show that both data sets have drawbacks and conclude that there is a need for an improved data set for realistic simulation of experimentally obtainable properties.

Re-evaluation of literature values of silver optical constants

By employing a modified Otto's configuration for measuring surface plasmon resonance that has been proposed by Bliokh and his coworkers [Appl. Phys. Lett.89, 021908 (2006)APPLAB0003-695110.1063/1.2220540] we have obtained complex refractive indices of metals at several wavelengths. We demonstrate that the configuration has high potential for obtaining dispersion relations of metal conductors in bulk samples as well as in thin films from the visible to the near-infrared wavelength region. Furthermore, we show that the configuration enables us to obtain the complex refractive indices of metals or a thickness or refractive index of a dielectric layer on the metal at different points simultaneously. We have constructed a measurement system and carried out basic experiments. The experimental results agreed well with numerically simulated values or published ones.

Measurements of complex refractive indices of metals at several wavelengths by frustrated total internal reflection due to surface plasmon resonance

In this paper, a surface plasmon resonance (SPR) spectroscopic ellipsometry, based on Otto-Bliokh configuration, is developed for the measurement of thickness and optical constants of ultra-thin coatings. This technique combines sensitivity of surface plasmon with accessibility of optical constants and other advantages of ellipsometry. Surface plasmons (SP) are generated in the sample under test in total reflectance mode and SP geometric distribution over the sample surface is influenced by the coating thickness and optical properties on one hand, and by the air gap thickness on the other hand. Nanoscale control of the thickness of the air gap between a convex surface and the sample was assured using a micron-size beam spot irradiating the contact zone. The amplitude and phase change induced by SPR in the visible and near-infrared spectral range were obtained to determine the dispersion of optical constants and the thickness of the ultra-thin layer. The extracted optical constants were found to be in excellent agreement with the results obtained using TEM and XRR techniques. Both theoretical analysis and experimental results demonstrated high sensitivity and precision of the proposed technique for the analysis of coatings of both metals and dielectrics on metals.

High-precision measurement of optical constants of ultra-thin coating using surface plasmon resonance spectroscopic ellipsometry in Otto-Bliokh configuration

In order to carry out precise measurements of the thickness of a dielectric layer deposited on a metal surface, we have introduced an ellipsometric measurement technique (EMT) to the modified Otto’s configuration (MOC) that is used for observing surface plasmon resonance (SPR). For that purpose, we have measured the thickness of the Au layer by the EMT at four different locations on an elliptic fringe pattern obtained from the MOC basing on a four-layer structure model: prism (BK7)-air-Au-substrate (BK7). Then, we have measured that of a TiO2 layer deposited on the Au layer by the EMT basing on a five-layer structure model: prism (BK7)-air-TiO2-Au-substrate (BK7). We have found experimentally that the combination of EMT and MOC is effective for measuring the thickness of the dielectric layer on the metal.

Ellipsometric measurement technique for a modified Otto configuration used for observing surface-plasmon resonance.

A simple imaging geometry in which total internal reflection in a glass prism is frustrated by the proximity of a metal surface is implemented for observation of surface plasmon resonance. At a certain angle of incidence, the total internal reflection is completely suppressed at a certain distance between the metal and the prism surfaces. Using planar metal and spherical prism surfaces, the distance parameter is sampled in a single image. This allows a direct determination of the complex refractive index in bulk samples as well as in thin films. Our experimental data are in good agreement with previously published data.

R. Vander

Papers

Visualization of the complex refractive index of a conductor by frustrated total internal reflection