Surface Kretschmann 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). 

Fig. 2 Surface plasmon resonance device in the Kretschmann configuration, so is the refractive index of prism, ei is the refractive index of thin metal film (usually Au or Ag), 62 is the refractive index of air, and 0 represents the critical angle...

Combination of Surface Plasmon Resonance (SPR) and Optical Waveguide Spectroscopy (OWS) was used for the simultaneous determination of refractive index and film thickness of the hydrogel layers in the Kretschmann configuration [24], The resulting angle scans from the SPR instrument were fit to Fresnel calculations and different layers were represented using a simple box model. A detailed description of this process has been published previously [18]. 

Fig. 1 Kretschmann configuration in SPR depicting the conversion of energy from light waves to surface plasmons via a gold/dielectric interface...

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. 

FlG. 12.2. Kretschmann configuration for investigation of surface wave at interface of metal (silver, in this case) and coating. 

Over recent years, internal reflectance infrared studies have tended to concentrate on the study of relatively thick films of conducting polymers or layers, (see, for example, the work of Pham and coworkers [49, 50], or Kvarn-strom, Nauer, Neugebauer and coworkers [51-54]) in which sensitivity was not a particular problem, or on the semiconductor-electrolyte interface, (see the work of Chazalviel and coworkers [35, 40, 41]), in which the SPP excitation approach is not appropriate. However, interest has focused again on this phenomenon with the surface-enhanced infrared absorption spectroscopy (SEIRAS) studies of Osawa and coworkers [19, 26, 27, 46, 55, 56], who have combined the application of the Kretschmann configuration with step-scan FTIR spectroscopy to study fast, reversible electrochemical processes on timescales down to microseconds [26, 46, 57-60]. 

More complicated dependences are observed when two layers are located on the surface of the ATR element. The optical properties of a hemicylin-drical IRE-thin (d < 50 nm) metal hhn-hlm system, called the Kretschmann configuration [84] (Fig. 2.36a), were actively investigated in the seventies and eighties (see, e.g.. Ref. [85]) regarding the possibility of SEW excitation at the metal-outer layer interface. However, even without exploiting this and surface-enhanced infrared absorption (SEIRA) (Section 3.9.4) effects, optical enhancement may be achieved in the ATR spectrum of a layer deposited on metal. Because of this, the Kretschmann configuration has found wide application in the investigation of nanolayers located on the metal surfaces, especially at the metal-solution interface (Section 4.6.3). 

Fig. 3. Schematic diagram of SPR imaging (SPRi). (a) SPR array in the Kretschmann configuration with light source and CCD camera, (b) SPR reflectance curve for a pure gold surface (solid lind), a reactant dot without analyte (dashed lindj and adsorbed analyte molecules on a reactant dot (dotted lInd). (c) The contrast of the SPR image is based on the different reflectance r >r >r. Reprinted from ref. 39 with permission.

The SPRi-Flex uses the Kretschmann configuration with a rotating mirror for scanning the SPR angle (at the SPR reflectivity dip). A broad monochromatic polarized light bundle illuminates the whole functional array surface of the SPRi Biochip (26). 

Figure 1(a) shows the Kretschmann configuration [9] for the excitation of plasmon surface polaritons (surface plasmons for short) [10] in the attenuated total reflection (ATR) mode. When a p-polarized laser beam is irradiated at the (internal) incident angle 9t from the prism of a refractive index np above 6c, a strong nonradiative electromagnetic wave, i.e. a surface plasmon is excited at the resonant angle which propagates at the metal /electrolyte interface. 

Figure 3 shows an attenuated total reflection (ATR) setup for the excitation of surface plasmons in the Kretschmann configuration combined with an electrochemical cell. 

Figure 17. Schematic experimental setup for surface-plasmon and surface-plasmon field-enhanced fluorescence microscopy in the Kretschmann configuration.

Figure 9. Geometries used for SPCE measurements. Top For SPE (Kretschmann configuration, KR). The excitation enters through the coupling prism. Bottom In reverse Kretschmann configuration, RK, the excitafitm directly reaches the sample and does not excite surface plasmons.

Figure 22. Comparison of the emission intensities of SlOl in PVA with surface plasmon excitation (KR/SPCE) and reverse Kretschmann configurations (RK/SPCE), (adopted from [30]).

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... 

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