Evanescent penetration depth

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... 

At higher ethanol concentrations, ATR spectra should contain the contribution from bnUc species, becanse of the long penetration depth of the evanescent wave, 250 nm. To examine the bulk contribution, the integrated peak intensities of polymer OH peaks of transmission (Ats) and ATR (Aatr) spectra are plotted as a function of the ethanol concentration in Figure 5. The former monitors clnster formation in the bulk liquid, and the latter contains contributions of clusters both on the snrface and in the bulk. A sharp increase is seen in A tr... 

A more direct approach to the photoinduced ET dynamics involves monitoring the lifetime of the excited state at the interface. By illuminating the interface in TIR from the electrolyte phase containing the quencher species, the generation of excited state is limited to the characteristic penetration depth given by the evanescent wave (/ ) [127],... 

ATR is one of the most useful and versatile sampling modes in IR spectroscopy. When radiation is internally reflected at the interface between a high-refractive index ATR crystal (usually Ge, ZnSe, Si, or diamond) and the sample, an evanescent wave penetrates inside the sample to a depth that depends on the wavelength, the refractive indices, and the incidence angle. Because the penetration depth is typically less than 2 pm, ATR provides surface specific information, which can be seen as an advantage or not if surface orientation differs from that of the bulk. It also allows one to study thick samples without preparation and can be used to characterize highly absorbing bands that are saturated in transmission measurements. 

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. 

The background problem can be further overcome when using a surface-confined fluorescence excitation and detection scheme at a certain angle of incident light, total internal reflection (TIR) occurs at the interface of a dense (e.g. quartz) and less dense (e.g. water) medium. An evanescent wave is generated which penetrates into the less dense medium and decays exponentially. Optical detection of the binding event is restricted to the penetration depth of the evanescent field and thus to the surface-bound molecules. Fluorescence from unbound molecules in the bulk solution is not detected. In contrast to standard fluorescence scanners, which detect the fluorescence after hybridization, evanescent wave technology allows the measurement of real-time kinetics (www.zeptosens.com, www.affinity-sensors.com). 

The measured intensity modulation can then be used to recover the original optical phase change Aphase shift, shown in Fig. 9.14b, is directly proportional to the density of molecules on the surface, as long as the film thickness is much less than the evanescent field penetration depth of <5 162 nm. 

From the three-layer mode equation, (15.3), the values cutoff and the penetration depth of the evanescent wave in the cover media, dv c, can be found for the three-layer structure as ... 

In recent years, the measurement of thick adlayers has received an increasing interest in the field of so-called polyelectrolyte multilayer films. These films sometimes have a thickness of several micrometers. Such large thicknesses are obviously not easily monitored using conventional waveguide sensors because of the limited penetration depth into the adlayer. Simply, waveguide sensors loses their sensitivity when the adsorbed layer thickness exceeds 2 3 times the penetration depth of the evanescent field and cannot be used to monitor films thicker than 350 nm12. 

From Snell s Law, sin(0j) m = sin(0j) nr. We have TIR when sin(Oj) > nr/rii, while we will have refraction and reflection when sin(0j) < nr/ni. In practical cases properties of light, such as phase, polarization and intensity, can be modulated inside the wave guide by a given measurand, which is interacting, for instance, with a CIM lying within the penetration depth for the evanescent field of the light localized near the external guide surface. 

A. Evanescent Field, Penetration Depth, and Effective Thickness... 

Model catalysts have to be prepared directly on the IRE, which can be a challenging task. Thin metal films are an important type of model catalyst and can be made, for example, by physical vapor deposition or sputtering of the metal onto the IRE. Most suitable IRE materials for such applications are Ge and Si. The former has a high refractive index of 4.0, which results in a small penetration depth and therefore good discrimination against bulk solvent. The metal film should not be too thick, so that the evanescent field can reach the outer interface of the metal film. 

Quantitative simulation of spectra as outlined above is complicated for particle films. The material within the volume probed by the evanescent field is heterogeneous, composed of solvent entrapped in the void space, support material, and active catalyst, for example a metal. If the particles involved are considerably smaller than the penetration depth of the IR radiation, the radiation probes an effective medium. Still, in such a situation the formalism outlined above can be applied. The challenge is associated with the determination of the effective optical constants of the composite layer. Effective medium theories have been developed, such as Maxwell-Garnett 61, Bruggeman 62, and other effective medium theories 63, which predict the optical constants of a composite layer. Such theories were applied to metal-particle thin films on IREs to predict enhanced IR absorption within such films. The results were in qualitative agreement with experiment 30. However, quantitative results of these theories depend not only on the bulk optical constants of the materials (which in most cases are known precisely), but also critically on the size and shape (aspect ratio) of the metal particles and the distance between them. Accurate information of this kind is seldom available for powder catalysts. 

An important issue to consider when probing powders with ATR spectroscopy is the match between particle size and penetration depth of the evanescent wave, as outlined schematically in Fig. 1. For large particles (Fig. 7, case (a)), only the part closest to the IRE is probed by the evanescent field. For large spherical particles, the overlap between the particle and the evanescent field is reduced for geometrical reasons. As shown by Fig. 7(a), the point of contact (the point of highest density) of... 

The fact that ATR-IR spectroscopy uses an evanescent field and therefore probes only the volume very close to the IRE has important consequences for its application in heterogeneous catalysis, in investigations of films of powder catalysts. The catalyst particle size and packing affect the size of the detectable signals from the catalyst and bulk phase. Furthermore, if the catalyst layer is much thicker than the penetration depth of the evanescent field, diffusion of reactants and products may influence the observed signals. In fast reactions, gradients may exist within the catalyst layer, and ATR probes only the slice closest to the IRE. 

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