Evanescent Wave Scattering

Evanescent Wave Scattering (German: "Scattering falling waves" or "Scattering evanescent waves") describes a physical effect from the field of optics .

Basics

If electromagnetic radiation hits an interface between media with a different refractive index , the wave is reflected or refracted at this interface . If the light runs from the optically denser ( ) into the optically thinner ( ) medium and the sine of the angle of incidence is greater than the ratio of the refractive indices of the two media , so-called total reflection occurs . The light is completely reflected at the interface. However, this does not mean that there is no electromagnetic field in the space behind the interface, it simply means that there is no wave propagation perpendicular to the interface. The electromagnetic field behind the interface falls exponentially with the distance from the interface (evanescent) and oscillates over time with the frequency of the incident light. ${\displaystyle n_{1}}$ ${\displaystyle n_{2}<n_{1}}$ ${\displaystyle \sin(\alpha )>{\frac {n_{2}}{n_{1}}}}$

If you now bring an object very close to the interface, so close that it "feels" the evanescent field, this object acts as a disturbance at which the field can be scattered. A (propagating) wave then emanates from this object, which draws its energy from the evanescent field. Because of the conservation of energy, part of the incident light must flow into the scattered wave. Therefore it is also called frustrated total internal reflection ( english frustrated total reflection ).

The scattering object

The scattering object can be a very fine metal tip that can be moved across the boundary surface using a special mechanism. If you measure the intensity of the scattered light as a function of the position of the tip above the surface, you get an image of the optical properties of the surface on a microscopic scale. Such devices are referred to as an optical scanning near-field microscope . What is special about such microscopes is that, in contrast to normal light microscopes, they can resolve structures that are smaller than the light wavelength used.

application

The scattering of evanescent light waves is used, for example, in internal total reflection fluorescence microscopy (TIRFM).