Evanescent wave, optics

As mentioned previously, photoisomerization of azobenzenes leads to refractive index or optical thickness changes that can be probed by means of evanescent wave optics. 

Figure 4. Two configurations for evanescent wave optics, (a) Top total internal reflection of a plane wave at the base of a glass prism. Bottom the reflectivity R recorded by a detector as a function of the angle of incidence shows the increase to unity at 6, the critical angle for total reflection, (b) ATR setup for the excitation of surface plasmons (PSPs) in Kretschmann geometry. Top a thin metal film (thickness 50 nm) is evaporated onto the base of the prism and acts as resonator driven by the photon field. Bottom the resonant excitation of the PSP wave is seen in the reflectivity curve as a sharp dip at coupling angle 6g.

Khijwania, S. K., Srinivasan, K. L., and Singh, J. P. 2005. An evanescent-wave optical fiber relative humidity sensor with enhanced sensitivity. Sensors and Actuators B Chemical 104 217-222. 

A pTAS must miniaturize the steps listed above (and any we may have omitted). There are two types of methods to do this. First, try to miniaturize the existing components used to accomplish these steps. Second, find new methods that accomplish the same result in a novel way appropriate to a pTAS. In the first family would be efforts to make microvalves and pumps, that move solutions around, mix them, and thereby miniaturize conventional chemistry. Homogeneous immunoassays involve efforts to find non-solution equivalents, such as controlled release of encapsulated reagents [14], or virtual separation using an evanescent wave optical signal [15], and are thus in the second category. 

Kasevich, M., Weiss, D., and Chu, S. (1990). Normal-incidence reflection of slow atoms from an optical evanescent wave. Optics Letters, 15, 607-609. 

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... 

A novel fiber optic sensor concept using antibody-antigen reactions at a glass-liquid interface was reported by Daehne146. The reaction of antibodies immobilized onto the surface of fused silica fiber optic or planar waveguides with antigens in solution was detected by interaction with the evanescent wave. By detecting in-line fluorescence, the measurement of human IgG is described. 

MacCraith B.D., Ruddy V., Potter C., O Kelly B., McGilp J.F., Optical waveguide sensor using evanescent wave excitation of fluorescent dye in sol-gel glass. Electron. Lett. 1991 27 1247. 

Bluenstein B., Walczak I., Chen S.Y., Fiber-optic evanescent-wave immunosensors for medical diagnostics, Trends Biotechnol 1990 161-168. 

Matejec V., Chomat M., Pospisilova M., Hayer M., Kasik I. Optical fiber with novel geometry for evanescent-wave sensing, Sensors Actuators (1995) B 29, pp. 416-422. 

Gupta B.D., Sharma A., Singh C.D., Fiber optic evanescent-wave absorption sensors based on uniform and tapered fibers, Proc. 2nd Europtrode (1994), Firenze, p.189. 

Shiverlanke L.C., Anderson G.P., Golden J.P., Ligler F.S., The effect of tapering the optical fiber on evanescent-wave measurements, Analytical Letters 1992 25 1183-1199. 

Biirck J., Mayer J., Zimmermann B., Aache H.-J., Integrated optical sensor for chemical analysis based on near-inlfared evanescent wave absorbance measurements, SPIE, 1995 2508 243-252. 

PLANAR OPTICAL SENSORS AND EVANESCENT WAVE EFFECTS... 

Planar Optical Sensors and Evanescent Wave Effects... 

Evanescent Wave Effects and Fluorescence-Based Optical Sensors... 

Perhaps the most effective demonstration of the advantages of evanescent wave interrogation is provided by the optical biosensor platform depicted in Figure 5. The platform consists of a multimode slab waveguide on the upper surface of which antibodies have been immobilised. 

The absorption-based platforms described previously employed evanescent wave interrogation of a thin sensing layer coated onto a planar waveguide. A sensitivity enhancement strategy for optical absorption-based sensors based on planar, multimode waveguides was developed recently by us18. The objective was to apply this theory to the development of low-cost, robust and potentially mass-producible sensor platforms and the following section outlines the assumptions and predictions of this theoretical model. 

Figure 6 illustrates the platform under consideration in this analysis. The principle of sensor operation is as described previously for absorption-based optical sensors employing evanescent wave interrogation of the sensing layer. 

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