Internal reflection evanescent wave

The interactions of the incident and the internally reflected electromagnetic waves generate an exponentially decaying evanescent field, which penetrates the adjacent medium to a... 

PTM Photon tunneling microscopy [12] An interface is probed with an evanescent wave produced by internal reflection of the illuminating light Surface structure... 

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 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 configuration most often used in SPR instruments relies on the phenomenon of total internal reflectance and was developed by Kretchmann (Fig. 8).71,73 Total internal reflectance occurs when light traveling from a medium of higher refractive index toward a medium of lower refractive index reaches the interface and is reflected back completely into the higher refractive index medium. An important side effect of total internal reflection is the propagation of an evanescent wave across the interface into the medium of lower refractive index. 

Fig. 5.14. Schematic diagram of an ATR gallium arsenide crystal and the total internal reflection of a light ray. The sample is placed on top of the crystal and interacts with the evanescent wave producing the spectrum.

Incorrect conclusion 1 above is sometimes said to derive from the reciprocity principle, which states that light waves in any optical system all could be reversed in direction without altering any paths or intensities and remain consistent with physical reality (because Maxwell s equations are invariant under time reversal). Applying this principle here, one notes that an evanescent wave set up by a supercritical ray undergoing total internal reflection can excite a dipole with a power that decays exponentially with z. Then (by the reciprocity principle) an excited dipole should lead to a supercritical emitted beam intensity that also decays exponentially with z. Although this prediction would be true if the fluorophore were a fixed-amplitude dipole in both cases, it cannot be modeled as such in the latter case. 

In reflectance cells, in contrast, the IR beam is directed through an IR-transmit-ting crystal which has a surface which is in intimate contact with the sample solution. The system is arranged such that one or more internal reflections of the IR beam occur at the interface of the crystal and sample. At each reflection, an evanescent wave is generated which penetrates a short distance into the sample medium. 

Figure 2.38 Schematic representation of the evanescent wave resulting from the lota) internal reflection of an electromagnetic ray at an inlerface.

Let us consider the possibility of reflection of electrons by an evanescent laser wave formed due to total internal reflection of femtosecond laser pulses from a dielectric-vacuum interface [4] (Fig. lb). Such a laser field was considered elsewhere [7, 8] to effect the mirror reflection of atoms (references to the latest works on the mirror reflection of atoms can be found in Refs. 9 and 10). The light intensity distribution in the evanescent wave in the vacuum may be represented in the form [11]... 

The possibility of reflection of electrons by an evanescent wave formed upon the total internal reflection of femtosecond light pulses from a dielectric-vacuum interface is quite realistic. The duration of the reflected electron pulses may be as long as 100 fs. In the case of electrons reflecting from a curved evanescent wave, one can simultaneously control the duration of the reflected electron pulse and affect its focusing (Fig. lc). Of course, one can imagine many other schemes for controlling the motion of electrons, as is now the case with resonant laser radiation of moderate intensity [9, 10]. In other words, one can think of the possibility of developing femtosecond laser-induced electron optics. Such ultrashort electron pulses may possibly find application in studies into the molecular dynamics of chemical reactions [1,2]. 

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>. 

What is even more relevant to the present subject is that a thin dielectric layer of polymer can be situated between two media without grossly distorting the optical nature of the interface, i.e. total internal reflection would occur as if no polymer layer is present even if its refractive index is unmatched to both media. Such a layer will, however, affect the intensity of the evanescent wave and particularly its depth of penetration. ... 

---