Light evanescent

Figure 43 Penetration of light (evanescent wave) into medium. A plasmon cloud forms in the medium near the metal film on the waveguide, and the reflected light has a minimum at a certain angle (inset). Physical and chemical events within the evanescent wave can change the angle.

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

The dynamics of polymers at surfaces can be studied via dynamic light scattering (DLS), as described in Section IV-3C. A modification of surface DLS using an evanescent wave to probe the solution in a region near the interface has... 

It is interesting to note the analogy of developments in light microscopy during the last few decades. The confocal microscope as a scaiming beam microscope exceeds by far the nomial fluorescence light microscope in resolution and detection level. Very recent advances in evanescent wave and interference microscopy seem to promise to provide even higher resolution (B1.18). 

While the spatial resolution in classical microscopy is limited to approximately X/2, where X is the optical wavelength (tlie so-called Abbe Limit [194], -0.2 pm with visible light), SNOM breaks through this barrier by monitoring the evanescent waves (of high spatial frequency) which arise following interaction with an... 

Figure C 1.5.6. Single Ag nanoparticles imaged with evanescent-wave excitation. (A) Unfiltered photograph showing scattered laser light (514.5 nm) from Ag particles immobilized on a polylysine-coated surface. (B) Bandpass filtered (540-580 nm) photograph taken from a blank Ag colloid sample incubated witli 1 mM NaCl and...

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.

Simply visualised, the infrared beam penetrates (of the order 0.3-3 pm, dependent on its wavelength) just beyond the ATR crystal-specimen boundary before it is reflected back and makes its way through the crystal to the detector. On this short path (of the evanescent wave) into the sample surface layer, light is absorbed, and the reflected beam carries characteristic spectral information of the sample. The decaying amplitude of the evanescent wave and the depth of penetration dp at which it has decreased to a proportion of 1 /e is defined by the Harrick equation (Equation (2)), where X is the wavelength of the incoming... 

Toomre, D. and Manstein, D. J. (2001). Lighting up the cell surface with evanescent wave microscopy. Trends Cell Biol. 11, 298-303. 

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

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. 

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