Otto configuration

The vibrational spectrum of Mn stearate has been measured using the Otto configuration by Hjortsberg, et al. UL Figure 12 shows the change in the reflectivity from the prism base due to the addition of two layers and 10 layers of Mn stearate. Clearly monolayers can be easily detected. 

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)). 

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). 

There are two main configurations with regards to the prism coupling technique Kretschmann configuration and Otto configuration. Optical excitation of SPs by the method of attenuated total reflection was demonstrated by Kretschmann and Otto... 

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.

One can "elongate" the wave vector of light by passing it through a medium (a prism) with a refractive index n > Otto configuration (Otto 1968)) (see Fig. 3.5a). In this case, the wave vector component kx does not change across both interfaces, prism/medium 1 and medium 1/medium 2, and is given by... 

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. 

Otto configuration (geometry) An ATR configuration represented by a prism suspended above the sample with the help of a distance holder. 

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