Sensing evanescent wave

The major advantage of TIRF is that fluorophores outside the evanescent wave (typically more than 200 nm away from the surface) are not excited. Hence, TIRF has an intrinsic sectioning capability. Of interest is that the section capability (z-resolution) is far better than for confocal microscopy systems, which typically have a z-resolution of about 1 /mi. In addition and in contrast to confocal microscopy, TIRF does not cause out-of-focus bleaching because only the molecules at the surface will sense the evanescent wave. However, in comparison with confocal microscopy, a clear limitation of TIRF is that only one z-plane can be imaged the molecules immediately adjacent to the surface. As a consequence,... 

Kvasnik F., McGrath A.D., Distributed chemical sensing utilizing evanescent wave interactions, Proc. SPIE-Int. Soc. Opt. Eng. 1990 1172 75. 

Matejec V., Kasik I., Chomat M., Ctyroky J., Berkova D., Huttel I., Modified inverted graded-index fibers for evanescent-wave chemical sensing, Proc. SPIE 3860 (1999), Boston, pp.443-451. 

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. 

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. 

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

The fabrication and characterization of a fiber optic pH sensor based on evanescent wave absorption was presented by Lee63. The unclad portion of a multi-mode optical fibre was coated with the sol-gel doped with pH sensitive dye. The sensitivity of the device increased when the multiple sol-gel coatings were used in the sensing region. The dynamic range and the temporal response of the sensor were investigated for two different dyes -bromocresol purple and bromocresol green. 

Blue R., Stewart G., Fibre-optic evanescent wave ph sensing with dye doped sol-gel films, Intern. J. Optoelectron. 1995 10 211-222. 

The cavity-enhanced evanescent-wave sensing method described in Sect. 5.2 and applied to gases in Sect. 5.3 can also be employed for the detection of chemicals in liquid solution. In addition, the intracavity enhancement provided by the high-g... 

In conclusion, when a WGM is excited in a dielectric microresonator, its evanescent component provides a convenient probe of the microresonator s surroundings. Various ways to implement evanescent-wave sensing have been devised, but the emphasis of this chapter has been on microcavity-enhanced absorption spectroscopy. The techniques described here have broad applicability, can even be used with broadband sources, and lend themselves well to further enhancement methods. We are looking forward to continuing our development of these sensors. 

Farca, G., Cavity Enhanced Evanescent Wave Chemical Sensing Using Microresonators, PhD dissertation, Oklahoma State University, 2006... 

P. Kumar, R.C. Willson, J.J. Valdes and J.P. Chambers, Monitoring of oligonucleotide hybridization using light-addressable potentiometric and evanescent wave fluorescence sensing, Mater. Sci. Eng. C, 1(3) (1994) 187-192. 

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