This paper reviews fundamentals of optical affinity biosensors based on plasmonic nanostructures and discusses recent advances in the development of this technology, including plasmonic nanostructures and surface plasmon phenomena, advances in sensor instrumentation, and functional coatings. Examples of applications for both the detection of chemical and biological substances as well as the investigation of biomolecular interactions are also given.

We report a phenomenon of focusing a radially polarized beam from the visible range by a silver film with no hole on the optical axis and double-sided concentric corrugations. The axes of symmetry of grooves and the illuminating beam coincide. An Ag lens of 100 nm thickness, five grooves, of which the outermost has $5\mu \mathrm{m}$ diameter, at $\lambda =400\mathrm{nm}$ transmits 22% of electric energy and focuses light into a $0.2{\lambda }^2$ spot area at a focal length close to $2\lambda$, while at $\lambda =500\mathrm{nm}$ the results are 11%, $0.16{\lambda }^2$ and $\lambda$, respectively. This Ag lens focuses without contribution of evanescent waves a far-field source into a far-field spot. The nanolens acts like a refractive optical system of high numerical aperture close to unity.

The future of plasmonic devices depends on effective reduction of losses of surface plasmon-polariton waves propagating along metal–dielectric interfaces. Energy dissipation is caused by resistive heating at the skin-deep-thick outer layer of metal and scattering of surface waves on rough metal–dielectric interfaces. Fabrication of noble metal nanolayers with a smooth surface still remains a challenge. In this paper, Ag layers of 10, 30, and 50 nm thickness deposited directly on fused-silica substrates and with a 1 nm wetting layer of Ge, Ti, and Ni are examined using an atomic-force microscope and four-probe resistivity measurements. In the case of all three wetting layers, the specific resistivity of silver film decreases as the thickness increases. The smallest, equal 0.4 nm root mean squared roughness of Ag surface of 10 nm thickness is achieved for Ge interlayer; however, due to Ge segregation the specific resistivity of silver film in $\mathrm{Ag}/\mathrm{Ge}/{\mathrm{SiO}}_2$ structures is about twice higher than that in $\mathrm{Ag}/\mathrm{Ti}/{\mathrm{SiO}}_2$ and $\mathrm{Ag}/\mathrm{Ni}/{\mathrm{SiO}}_2$ sandwiches.

Titanium nitride is a well-known conductive ceramic material that has recently experienced resumed attention because of its plasmonic properties comparable to metallic gold and silver. Thus, TiN is an attractive alternative for modern and future photonic applications that require compatibility with the Complementary Metal-Oxide-Semiconductor (CMOS) technology or improved resistance to temperatures or radiation. This work demonstrates that polycrystalline TiN_x films sputtered on silicon at room temperature can exhibit plasmonic properties continuously from 400 nm up to 30 μm. The films’ composition, expressed as nitrogen to titanium ratio x and determined in the Secondary Ion Mass Spectroscopy (SIMS) experiment to be in the range of 0.84 to 1.21, is essential for optimizing the plasmonic properties. In the visible range, the dielectric function renders the interband optical transitions. For wavelengths longer than 800 nm, the optical properties of TiN_x are well described by the Drude model modified by an additional Lorentz term, which has to be included for part of the samples. The ab initio calculations support the experimental results both in the visible and infra-red ranges; particularly, the existence of a very low energy optical transition is predicted. Some other minor features in the dielectric function observed for the longest wavelengths are suspected to be of phonon origin.

Reduction of surface plasmon-polariton losses due to their scattering on metal surface roughness still remains a challenge in the fabrication of plasmonic devices for nanooptics. To achieve smooth silver films, we study the dependence of surface roughness on the evaporation temperature in a physical vapor deposition process. At the deposition temperature range 90 to 500 K, the mismatch of thermal expansion coefficients of Ag, Ge wetting layer, and sapphire substrate does not deteriorate the metal surface. To avoid ice crystal formation on substrates, the working temperature of the whole physical vapor deposition process should exceed that of the sublimation at the evaporation pressure range. At optimum room temperature, the root-mean-square (RMS) surface roughness was successfully reduced to 0.2 nm for a 10-nm Ag layer on sapphire substrate with a 1-nm germanium wetting interlayer. Silver layers of 10- and 30-nm thickness were examined using an atomic force microscope (AFM), X-ray reflectometry (XRR), and two-dimensional X-ray diffraction (XRD2).