Optics Kathy Geise April 2007 Evanescent Waves Optics Kathy Geise April 2007

Evanescent Wave Third quarter E&M D. Griffiths, Introduction to Electrodynamics, Third Edition, page 413. When light passes from an optically dense medium into an optically less dense one (n1>n2), the propagation vector bends away from the normal. If the light is incident at the critical angle, θC, total internal reflection occurs. When θI > θC there is no light ray refracted, just a reflected one.

Evanescent Wave But the fields are not zero in medium 2! We get an evanescent wave. It is rapidly attenuated. It transports no energy into medium 2. The evanescent fields can be detected. Place a second interface a short distance away. Analogous to quantum mechanical tunneling, the wave crosses the gap and reassembles.

Evanescent Wave This is evanescent wave simulation. Ki shows the incident beam's wave vector, and Kr shows total reflected beam's wave vector. The evanescent wave is propagating along surface and decaying along normal to the surface at sub wavelength scale. http://users.ece.gatech.edu/~sungwon/f11.htm

Evanescent Wave Measurement                                                                                   Measurement of the amplitude of the evanescent field generated via TIR at a clean glass/air interface. The interface lies approximately in the z=0 plane. The evanescent field can be detected out to a distance of approximately one micron before noise starts to become a significant problem. http://www-optics.unine.ch/former/microoptics/subwavelength_measurements/subwavelength_measurements.html

Total Internal Reflection Microscopy TIRM is an optical technique. Measures instantaneous separation between a microscopic sphere and a flat plate. Changes as small as 1 nm can be detected. Measures the intensity of light scattered by the sphere when it’s illuminated by an evanescent wave. Some of the evanescent wave is scattered. Called Frustrated Total Internal Reflection FTIR. The amount of light scattered by the sphere is sensitive to its proximity to the interface.

TIRFM Total Internal Reflection Fluorescent Microscopy. Total Internal Reflection Fluorescence Microscope epi- (TIRFM) diagram 1. Specimen 2. Evanescent wave range 3. Cover slip 4. Immersion oil 5. Objective 6. Emission beam (signal) 7. Excitation beam http://www.olympusmicro.com/primer/java/tirf/reflect/index.html

Fiber Optic Sensors In an evanescent wave fiber optic sensor, the cladding of the optical fiber is removed and it is placed in direct contact with the sample. Thus the sample poured in is in direct contact with the evanescent wave. Any materials which absorb the particular wavelength of light being carried by the fiber will take energy out of the evanescent wave. This in turn drains energy from the interior of the fiber and the output at the detector is reduced. http://www.photonics.cusat.edu/Research_Fiber%20Sensors_EW.html

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