Evanescent field distribution

In the near-field recording technique presented in Section 16.5.1, the evanescent field distributions near the particles were recorded as surface topography of a photosensitive film, and the topc aphy was read out with an AFM. This is good for the application of high-resolution imaging, because the evanescent distribution near a specimen can be converted to the topographical change of the film surface and detected with an AFM. 

The same observations are true for the evanescent wave of the modes in the surrounding medium. Moreover, these changes in the field distributions are accompanied by an increase of the effective refractive indices of the cladding modes, as will be clarified later. For this reason and by virtue of (3.1), the resonance wavelengths of the coated LPG are expected to blue-shift in response to an overlay thickness change. 

To use the OFRR as a biosensing device, the optical resonant mode is excited and the resonant frequency is measured continuously in real time. The conceptual measurement setup is illustrated in Fig. 14.3. Laser light from a distributed feedback (DFB) laser is delivered to the OFRR using fiber optic cable. One method that has been used to excite the resonant modes is to place a tapered fiber optic cable with a diameter less than 4 pm in contact with the OFRR. The evanescent field of the tapered fiber overlaps with the evanescent field outside of the capillary wall, which enables mode coupling between the two media24. 

Fig. 16.3 Simulation of transmission spectrum for a four resonator array. FDTD simulation showing the steady state electric field distributions when the device is excited at the (a) resonant wavelength and (b) nonresonant wavelength. Note that the color levels in this image are scaled to the maximum field intensity in each image not to each other. The field levels in (b) are roughly of 20 times greater magnitude than those shown in (a), (c) Output spectrum for a device consisting of a waveguide with four evanescently coupled side cavities adjacent to it. Here each resonator consists of a cavity with four holes on either side. Reprinted from Ref. 37 with permission. 2008 Optical Society of America...

Excitation of a fluorophore that is close to the surface of the waveguide can be achieved via the evanescent field. The resulting fluorescence emission is isotropically distributed, however, some small component of the emitted light... 

It was found that the surface of the photosensitive film was modulated by the intensity distribution of light induced by the fine structures of the specimen. The cilia of a paramecium were clearly observed. One could recognize that the end of each cilium branched into two cilia. Because the spatial resolution of the observed result is smaller than 100 nm, the evanescent wave distribution generated by the fine structures of the specimens were imaged with the near-field recording technique. 

In ATR-FTIR excitation occurs only in the immediate vicinity of the surface ol the reflection element, in an evanescent wave resulting from total internal reflection. The intensity of the evanescent field decays exponentially in the direction normal to the interface with a penetration depth given by (1.7.10.121, which for IR radiation is of the order of a few hundreds of nm. Absorption leads to an attenuation of the totally reflected beam. The ATR spectrum is similar to the IR transmission spectrum. Only for films with a thickness comparable to, or larger than, the penetration depth of the evanescent field, do the band intensities depend on the film thickness. Information on the orientation of defined structural units can be obtained by measuring the dichroic ratio defined as R = A IA, where A and A are the band absorbances for radiation polarized parallel and perpendicular with respect to the plane of incidence, respectively. From this ratio the second-order parameter of the orientation distribution (eq. [3.7.13]) can be derived ). ATR-FTIR has been extensively used to study the conformation and ordering in LB monolayers, bilayers and multilayers of fatty acids and lipids. Examples of various studies can be found... 

Figure 12.18 Electrochemical cell and silicon ATR element coated with dye-sensitised Ti02- The electric field distribution 8 in the evanescent wave is also shown.

Keywords Fiber optic sensors Chemical sensors Biosensors Evanescent field sensor Intrinsic fiber optic sensors Multimode fibers Modal power analysis Modal power distribution Spatial intensity modulation Optical far-held pattern... 

To this end we present a series of experiments with a surface architecture that is schematically presented in Figure 15. We choose the Biacore chip CMS with its carboxylated dextran layer as the coupling matrix. This functional polymer brush is known to extend in the swollen state about 100 nm into the buffer solution. This is schematically indicated to scale with the evanescent field of a surface plasmon mode in Figure 4. The architecture thus provides not only a quasi 3-dimensional (3D) surface architecture with an enhanced ligand (tensity compared to a mere 2D planar surface layer, it also optimizes the overlap of the evanescent wave with the distribution of the chromophore-labeled analyte bound to the dextran-coiq)led ligands.Moreover, it minimizes the reduction of the fluorescence intensity via quench processes by preventing the chromoph( es form being placed too close to the metal surface... 

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