Evanescent Wave Illumination

When an electromagnetic wave (light) in a dielectric medium of index n1 is incident upon an interface of a different dielectric material of a lower optical density, n2 at an angle θ1 greater than the critical angle, θc = sin−1(n2/n1), total internal reflection (TIR) occurs. Although all of the incident energy is reflected, an electromagnetic field with exponentially decreasing intensity propagates in the less dense medium. This field is known as an evanescent wave and has a decay length or penetration depth, d, on the order of the wavelength of the incident light, λ0. The confined nature of the evanescent wave close to the interface is ideal for studying surface and near-surface phenomena pertaining to fluid mechanics and mass transport typically by fluorescence microscopy, which is known as total internal reflection fluorescence microscopy (TIRFM) or simply TIRF.