Field evanescent

The penetration depth of this evanescent field, (defined to be the depth at which the evanescent field decays to 1/ of its original value,) is given... 

Attenuated total reflection, on which atr—ftir is based, occurs when the rarer medium is absorbing and is characterized by a complex refractive index (40). The absorbing characteristics of this medium allow coupling to the evanescent field such that this field is attenuated to an extent dependent on k. The critical angle in the case of attenuated total reflection loses its meaning, but internal reflection still occurs. Thus, if the internally reflected beam is monitored, its intensity will reflect the loss associated with the internal reflection process at the interface with an absorbing medium. 

The extent of the interaction between the evanescent field and the absorbing medium is formally described by the effective thickness, of this... 

Fig. 24. Attenuated total reflectance of thin film of thickness d and refractive index on a substrate of refractive index at the surface of internal reflection element (IRE) of refractive index Decay of evanescent field beyond thickness of thin film indicated.

The reduction of dimensions also reduces volumes which are accessible to the detector. Thus, detection principles related to geometric dimensions of the detector cell ai e not ideally suited for coupling to microsystems, whereas surface sensitive principles, such as electrochemical methods or optical methods utilizing the evanescent field of a waveguide, or methods which can be focussed on a small amount of liquid, such as electrochemiluminescence (ECE), ai e better suited. This is why electrochemiluminescence detectors ai e combined to microsystems. Moreover ECE has found wide applications in biochemistry because of its high sensitivity, relatively simplicity and feasibility under mild conditions. 

Hayazawa, N Inouye, Y. and Kawata, S. (1999) Evanescent field excitation and measurement of dye fluorescence using a high N.A. objective lens in a metallic probe near-field scanning optical microscopy J. Microsc., 194, 472-476. 

Figure 7.2 A schematic diagram of nanometer position sensing. Light from the evanescent field scattered by the microparticle is measu red with a quadrant photodiode detector, whose differential outputs correspond to the x and y displacements and the total intensity depends exponentially on the distance z between the particle and the glass plate.

Using this photon force measurement technique, radiation pressure induced by a focused laser beam and an evanescent field [12, 14, 19, 20] was investigated for polymer latexes and metallic particles. Electrostatic forces of charged particles in... 

Wada, K., Sasaki, K. and Masuhara, H. (2000) Optical measurement of interaction potentials between a single microparticle and an evanescent field. Appl. Phys. Lett., 76, 2815-2817. 

Schwotzer G. et. al., Optical sensing of hydrocarbons in air or in water using UV absorption in the evanescent field of fibers, Sensors Actuators B 1997 38 150-153. 

Stewart G., Jin W., Culshaw B., Prospects for fibre-optic evanescent-field gas sensors using absorption in near-infrared, Sensors and Actuators B 1997 38-39 42-47. 

Piraud C., Mwarania E., Wylangowski G., Wilkinson J., O Dwyer K., Schiffrin J., An optoelectrochemical thin-film chlorine sensor employing evanescent fields on planar optical waveguides, Anal. Chem. 1992 64 651. 

As the mode propagates within the waveguide by total internal reflection, its exponentially decaying evanescent tail extends into both cover and substrate layers over a distance that is characterised by the penetration depth, dp. The extent to which the evanescent field penetrates the cover layer is of vital importance to the operation of evanescent-wave-based sensors. The penetration depth can be calculated from Equation (1) and is typically of the order of the wavelength of the propagating light. 

The tuneable nature of the evanescent field penetration depth is critical to the effective operation of this sensor as it facilitates surface-specific excitation of fluorescence. This means that only those fluorophores attached to the surface via the antibody-antigen-labelled antibody recognition event... 

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

In addition, typical methods of sensing are total internal reflection fluorescence or monitoring of fluorescence resonance energy transfer6,7. The second class is a direct optical detection principle which relies either on reflectometry or refractometry. The latter is connected to evanescent field... 

Figure 2. Regular reflectance Replication of Snellius law for reflected and refracted radiation at interface in dependence on the refractive indices of the media adjacent to this interface, demonstrating total internal reflectance and evanescent field, exciting fluorophores close to the waveguide or even surface plasmon resonance.

One of the possibilities to interrogate the effect of the evanescent field on the propagation of the guided radiation in the waveguide is an interferometric approach. This approach uses so-called Mach-Zehnder interferometers5,11 shown in Figure 3. 

The waveguide is split into two arms one arm (the reference arm) is shielded and not affected by the environment. At the interface of the other arm, the evanescent field will be influenced by any sensing effect. Thus, the two partial beams will propagate in a different way within the two arms, in dependence on the difference in the refractive indices adjacent to the two arms. 

Interferometric sensors frequently have also been applied to biosensor measurements. Thereby, the evanescent field technique (Mach-Zehnder interferometer) has been compared with other optical detection principles regarding information on layer structure and in case of biosensing30. The... 

Nearly all integrated optical sensors4 6 rely on evanescent field sensing. This principle can be explained using Figure 7. 

Figure 7. Evanescent field sensing the black part of the field profile is the operational evanescent field.

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