Evanescent radiation

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... 

Sasaki, K., Hotta, J., Wada, K and Masuhara, H. (2000) Analysis of radiation pressure exerted on a metalbc particle within an evanescent field. Opt. Lett., 25, 1385-1387. 

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. 

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. 

This expression is useful for estimating of the radiation loss of an adiabatic taper with characteristic length L. Figure 13.7 shows the distribution of the evanescent electromagnetic field intensity in the vicinity of the taper for 0 = 0.2 pm 1,yao = 0.4 pm 1, L = 500 pm, and / 4 pm In agreement with illustration in... 

Fig. 13.6, the radiating and guiding components are split off near the focal point shown as a small circle. The interference between these components gives rise to the oscillations of the evanescent field and to the appearance of quasiperiodic dips, which are clearly seen in Fig. 13.7. 

The sensor systems outlined in the present chapter use evanescent electromagnetic radiation to monitor various analytes in aqueous solutions. Therefore, as a beginning, the basic properties of evanescent electromagnetic waves and the so-called TIR phenomena are summarized. Afterwards, two types of waveguide modes will be briefly discussed guided and leaky modes, which both generate evanescent waves at a solid/liquid boundary. 

As mentioned in Sect. 15.2, sometimes a 4th thin layer (M) of metal or die is incorporated between the substrate and waveguide film to decrease the radiation loss into the substrate of the substrate radiation modes. These modes are referred to as leaky modes and the obtained structure is the MCLW. This configuration is also broadly used in evanescent wave sensor systems. 

Shevchenko, A. Lindfors, K. Buchter, S. C. Kaivola, M., Evanescent wave pumped cylin drical microcavity laser with intense output radiation, Opt. Commun. 2005, 245, 349 353... 

Although the probability of absorption of TIR evanescent energy by a fluorophore of given orientation decreases exponentially with distance z from a dielectric surface, the intensity of the fluorescence actually viewed by a detector varies with z in a much more complicated fashion. Both the angular pattern of the emitted radiation and the fluorescent lifetime are altered as a function of z by the proximity of the surface. 

R. M. Weis, K. Balakrishnan, B. A. Smith, and H. M. McConnell, Stimulation of fluorescence in a small contact region between rat basophil leukemia cells and planar lipid membrane targets by coherent evanescent radiation. J. Biol. Chem. 257, 6440-6445 (1982). 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

Attenuated total reflectance infrared (ATR-IR) is used to study films, coatings, threads, powders, interfaces, and solutions. (It also serves as the basis for much of the communication systems based on fiber optics.) ATR occurs when radiation enters from a more-dense material (i.e., a material with a higher refractive index) into a material that is less dense (i.e., with a lower refractive index). The fraction of the incident radiation reflected increases when the angle of incidence increases. The incident radiation is reflected at the interface when the angle of incidence is greater than the critical angle. The radiation penetrates a short depth into the interface before complete reflection occurs. This penetration is called the evanescent wave. Its intensity is reduced by the sample which absorbs. 

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. 

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]. 

From an experimental point of view it is important to recognize that the profile of (Ey2 as a function of z is proportional to the profile of the intensity of electromagnetic radiation in the proximity of the interface in medium 2. Such a profile will determine the surface sensitivity of the evanescent wave the depth of penetration is smaller if ... 

The extent of the interaction between the evanescent field and the absorbing medium is formally described by the effective thickness, effective thickness is the thickness of the absorbing phase that would have to be passed through by the incident beam in a transmission experiment to give the same energy loss as in the attenuated total reflection experiment. The exact expressions for effective thickness can be very complex. However, for a single attenuated total reflection of an incident beam of radiation of electric field E that occurs at an interface between two bulk phases (ie, phase 2 is not a thin film), d is given by... 

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