Surface evanescent wave

As a rule these methods are based on the concept of evanescent surface waves caused by total internal reflection at a solid/liquid interface. In order to understand the so-called evanescent spectroscopy, a simplified theory of total internal reflection is given below. More complete treatments can be found in various specialized monographs and reviews 145 147>. 

One can distinguish between methods in which absorption of the evanescent surface wave in different wavelength regions is measured (these are often called attenuated total reflection methods), and methods which use the evanescent wave to excite other, spectroscopic phenomena, like fluorescence and Raman scattering or light scattering. As the methods of conventional fluorescence spectroscopy have been shown to be exceptionally successful in studies of proteins and other biopolymers, their evanescent surface-sensitive counterparts will be reviewed first. 

Fluorescence from labeled adsorbed protein has also been excited with the evanescent surface wave created by integrated optics. Both optical fiber I60) and flat rectangular waveguides193) have been used. Interesting use of optical fiber as a remote protein sensor was demonstated the excitation light was sent down the fiber whose tip was immersed in protein solution, evanescently excited fluorescence was collected by the same fiber and delivered to a scanning monochromator 160). 

Prieve, D. and Walz, J. (1993). Scattering of an evanescent surface wave by a microscopic dielectric sphere. Appl. Opt., 32 1629 -1641. 

Finally, the same transducer may be used in a number of diffa%nt operation modes, which are chosen according to the objective desired. For example, an optical fiber can be used either as an extrinsic transducer, with a bioreactive component immobilized at its tip, or as an intrinsic transducer, exploiting the interaction between the immunoproteins at its surface and the evanescent surface waves. 

The most common principle used in optical biosensors for detection in the real part of the refractive index is evanescent wave detection, where the transducer optics are modified by changes in optical parameters of the medium in contact with the sensor surface via the interaction with the evanescent light wave penetrating into the ambient medium. As the evanescent wave decays exponentially from the surface, the most sensitive detection is just at the transducer surface. 

Fig. 22 Spectral dependence of the effective index of a surface plasmon on the interface of gold-water and the effective index of the evanescent light wave produced by a plane light wave incident on the gold film from an optical prism (BK 7 glass) under nine different angles of incidence...

A photochromic film in a planar waveguide can bind light in a confined manner, allowing the use of evanescent fields as sensitive tools for probing the optical properties of these films. A surface wave that propagates at the interface between an ultrathin photochromic layer, such as a SAM, and a given medium can probe small photochemical changes at this interface. The... 

An optical transduction method that is often used with ultrathin hlms, such as LB hlms, is that of surface plasmon resonance [30, 31]. Surface plasma waves are collective oscillations of the free electrons at the boundary of a metal and a dielectric. These can be excited by means of evanescent electromagnetic waves. This excitation is associated with a minimum in the intensity of the radiation reflected from the thin him system, called surface plasmon resonance (SPR). The sensitivity of SPR is noteworthy, and changes in refractive index of 10 may be monitored thus the technique compares favorably with ellipsometry. The method has been used with LB hlms to provide both gas detectors [29] and sensors for metal ions in solution [32]. 

If one includes the employment of evanescent light waves in spectroscopic studies in the near-field area. Knoll and coworkers are among the first who reported on near-field Raman spectroscopy , though this approach was termed at that time (the early 1990s) as surface plasmon field-enhanced Raman spectroscopy and microscopy [104],... 

Some optical phenomena (e.g., evanescent waves) within the microscale optical environment are used to construct fiber-optic biosensors [4], In fiber-optic biosensors for photometric detection, the light between the sample and the source or detector is transported along the interior of the fibers following the principle of total reflection. The total reflection in a fiber is not perfect and some electromagnetic radiation penetrates the sheath covering the fiber. This is called the evanescent wave, and its intensity diminishes exponentially with the perpendicular distance from the interface as the surface wave in SPR. 

Lastly in this section we note that Kent et al. (1992) have used x-ray evanescent (XEWIF) wave-induced fluorescence to study the adsorption of poly(dimethyl siloxane) at the air/liquid interface, the liquids being bromohep-tane (a good solvent) and bromocyclohexane (a theta solvent at 29 °C). The signal is rather too noisy to allow a definitive and quantitative comparison with theory. In the good solvent the surface region was inferred to have a pol5nner... 

Note in particular that Za = 0 for Sx = 1.32. This case is particularly interesting because this constitutes the condition for a free surface wave as discussed earher. In other words, a surface wave will propagate even when the impressed voltage is zero. Also note that it is the sum of the individual evanescent waves that constimtes a surface wave that is unique and satisfies the boundary condition at the elements of the array, not the individual modes. 

From a physical point of view we note that the field associated with the surface wave on the latter is composed of an assembly of evanescent waves that is attenuated as we move away from the periodic structure. In fact these evanescent waves have fallen to rather low values at a distance of a quarter wavelength. Since this is about the typical spacing between adjacent slotted surfaces in many applications, we may conclude that the surface wave frequency for a single periodic surface is only lightly affected by the presence of a groundplane or a second slotted surface. 

In the angular range of the surface plasmon excitation there are still 2 contributions to the detected fluorescence light the first one originates from chromophores in the bulk solution that are close enough to be reached by the evanescent SP wave, i.e., within a layer of c. 150 - 200 nm above the metal surface (cf Figure 4). The second contribution... 

The atom mirror is the key element of matter-wave optics. An electromagnetic mirror for neutral atoms was suggested by Cook and Hill (1982). The idea was to use the radiation force of an evanescent laser wave outside a dielectric surface to repel slow atoms. This evanescent-wave atomic mirror was realized experimentally by Balykin et al. (1988a). 

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