Understanding Evanescent Waves
An evanescent wave is a type of oscillating electromagnetic field that does not propagate as a true electromagnetic wave. Instead, it decays exponentially with distance from the interface where it is formed. It carries energy but does not result in net energy flow away from the interface in the direction of decay, meaning it's a non-radiative field.
How Evanescent Waves Form
Evanescent waves are created when light undergoes total internal reflection (TIR) at the boundary between two transparent media. This occurs when light traveling in a denser medium strikes an interface with a rarer medium at an angle greater than the critical angle. Although the light is reflected, a small portion of the electromagnetic field penetrates into the rarer medium, forming the evanescent wave before decaying within a few wavelengths from the surface.
Practical Example: Frustrated Total Internal Reflection
A classic example of an evanescent wave phenomenon is Frustrated Total Internal Reflection (FTIR). If a second denser medium is brought within the evanescent field's decay distance (typically a few hundred nanometers) of the first interface, the evanescent wave can 'tunnel' through the gap and excite a propagating wave in the second medium. This effect allows light to pass through a barrier it normally couldn't, demonstrating the wave's presence beyond the initial reflection point.
Importance and Applications
Evanescent waves are crucial in various technologies and scientific fields. They are fundamental to fiber optics, where they contribute to wave guiding. In microscopy, techniques like Total Internal Reflection Fluorescence (TIRF) microscopy use evanescent waves to illuminate only fluorophores very close to a surface, providing high-resolution imaging of cellular processes without background noise. They are also employed in biosensors, optical waveguides, and photonics research.