A variety of optical sensing mechanisms exist to detect anaytes present in water and other solvents. The mechanisms include: luminescence, fluorescence, phosphorescence, absorbance, elastic scattering, Raman
scattering, surface plasmon polaritons (SPPs), guided-wave resonance, and reflection/transmission due to planar
interfaces. SPP-based sensing technology has gained considerable following during the last two decades. Its basis is a quantum-electromagnetic resonance arising from the interaction of light with free electrons at the interface of a
metal and a dielectric material. The classical analog of a train of SPPs is an SPP wave.

Although several techniques exist to launch an SPP wave guided by the interface of a metal and an isotropic homogeneous dielectric material, the commonest way is to use the Kretschmann configuration. Both the metal and the dielectric material in this configuration are layers of finite thickness. The metal film's thickness is about 50 nm, whereas the dielectric layer has to be much thicker. On the other side of the metal film is a dielectric coupling material (in the form of a prism), which is optically denser than the dielectric material. Quasi-monochromatic light
is launched at an angle to the thickness direction in the coupling material towards the metal film. The fraction of illuminating light that is neither reflected nor transmitted is absorbed. As the angle of incidence increases from 0 deg, a sharp peak in absorbance, accompanied by minuscule reflectance and transmittance, indicates the excitation of an SPP wave. This sharp peak occurs only for p-polarized light, and only one SPP wave of a certain frequency can be excited.

The basic characteristic of the excitation of only one SPP wave does not change when the isotropic homogeneous dielectric partner of the metal film is made anisotropic. A far more interesting possibility emerges when the anisotropic dielectric partnering material is a sculptured thin film (STF) that is periodically
nonhomogeneous in the thickness direction. Remarkable characteristics of SPP-wave propagation guided by this kind of an interface have been found recently, both theoretically and experimentally.

For application to optical sensing, the following conclusions are pertinent from the studies completed thus far: (i)
A converging light beam with sufficient angular spread can be used to excite more than one SPP-wave modes
simultaneously. (ii) The propagation speeds for all SPP-wave modes are different. (iii) The propagation lengths for
all SPP-wave modes are different. (iv) Different slices of the STF, most likely in the first structural period closest
to the metal, can be functionalized to promote attachment to different analytes for better discrimination.