Surface Otto configuration

In general, the system comprises the light source, detector, optical system (mostly prism), and a sensor chip (mostly thin gold film) (Fig. 4.23). The sensor chip, depending on the method, can stay in direct contact with the prism surface (Kretschmann configuration) or close to the surface (Otto configuration). 

Polymer membranes have also been used as a "sandwich". In this configuration, the liquid film is supported between two polymer membranes. Ward (18) used two silicone rubber membranes to contain a solution of ferrous ions in formamide. Ward noted that Bernard convection cells could be maintained if the complex were formed at the upper surface. Ward (19) used this same system and membrane configuration to study electrically-induced, facilitated gas transport. The silicone rubber membranes provided the mechanical support so the electrodes could be placed next to each liquid surface. Otto and Quinn (20) immobilized the liquid film in a horizontal layer between two polymer films. The polymer was described as an experimental silicone copolymer having high CO2 permeability as well as excellent mechanical properties. They were studying CO2 facilitated transport in bicarbonate solutions with and without carbonic anhydrase. 

FlG. 12.1. Experimental Otto configuration for (a) single prism ATR measurements and (b) double prism measurement of propagation length L. Prism are spaced a variable distance g above surface (from (6)). 

Akowuah EK, Gorman T, Haxha S. Design and optimization of a novel surface plasmon resonance biosensor based on Otto configuration. Opt Express 2009 17 23511. 

In practical terms there are two configurations, both based on the ATR technique available to optically excite SPR at the metal/dielectric (or emerging medium) interface. In the first, the Kretschmann configuration, the prism is in direct contact with the surface active (metal) medium. In the second, the Otto configuration, the prism is separated by a thin layer of a dielectric (inactive) medium at a distance of approximately one wavelength of excitation light from the metal film. The practical consequences of... 

Fig. 3.5 Geometry for obtaining surface poiariton excitations, (a) Otto configuration. A prism of refractive index n is suspended above medium 2. (b) Kretschmann configuration. The sampie fiim is deposited onto the prism surface.

The Otto configuration is well adapted for the cases when it is necessary to avoid contact between prism and surface. However, similar conditions can be realized if the sample film (medium 2) is deposited directly onto the prism surface. Such a geometry is known as a Kretschmann corfiguratim (Kretschmann and Raether 1968) (Fig. 3.5b). The film thickness in this case must be less than the penetration depth of the evanescent wave excited at the prism/film interface which is usually of the order of a few 100 A. 

For particulate samples, and even more for compact samples exhibiting a smooth surface, it is crucial to prevent specularly reflected radiation from being detected together with the diffusely reflected component. To avoid this, the solid angle into which the incident radiation is specularly reflected should be excluded. Advantageously, irradiation normal to the surface (Korte and Otto, 1988) or out-of-plane configurations are used (see Fig. 6.4-19). As an alternative, a razor blade is vertically positioned on the surface as a blocker for surface-reflected radiation (Messerschmidt, 1985). 

In the experimental study of surface excitons various optical methods have been used successfully, including the methods of linear and nonlinear spectroscopy of surface polaritons. A particularly large body of information has been obtained by the method of attenuated total reflection of light (ATR), introduced by Otto (1 2) (Fig. 12.1) to study surface plasmons in metals. Later the useful modification of ATR method also was introduced by Kretschmann (3) (the so-called Kretschmann configuration, see Fig. 12.2). The different modification of ATR method has opened the way to an important development in the optical studies of surface waves and later was used by numerous authors for investigations of various surface excitations. 

FIGURE 6.2 Typical configuration of different indicator-mediated fiber optic sensor tips, (a) Indicator is immobilized on the fiber surface, (b) indicator is immobilized on the surface of a membrane attached to the fiber, (c) indicator contained within a chamber created between a membrane and the fiber, (d) indicator contained in a chamber created by a tubular capillary at the end of the fiber, (e) indicator retained in a hollow open capillary at the end of the fiber. (Taken from Otto S. Wolfbeis, Fiber Optic Chemical Sensors and Biosensors, Vol. 1, CRC Press, Boca Raton, FL, 1990.)... 

ATR in Otto s configuration (Fig. 236d) can be applied to characterizing surfaces of carbon fibers. To obtain the spectra, a fiber cloth is pressed to an IRE. The optimum conditions for such a system were studied by Ohwaki and Ishida [182], It was shown that a multiple reflection Ge IRE, 5-polarization, and an angle of incidence of 30° provide the maximum spectral contrast. However, in terms of SNR, the use of unpolarized radiation at an angle of incidence of 35°-40° is more advantageous. 

E. A thin film on a flat surface of a massive (metal [78], dielectric [79], or semiconductor [80]) plate can be studied using Otto s optical configuration (Fig. 236d). When this geometry is used for the in situ investigation of films at... 

Attenuated total reflection (ATR) Attenuation of the reflected wave intensity in total internal reflection due to energy losses in the medium which covers the face where reflection occurs. This phenomenon is used to monitor the excitation of surface polaritons in Otto and Kretschmann configurations. 

---