Abstract

The concept of modes, or eigenfunctions, is fundamental for all wave phenomena in physics like optics, acoustics and quantum mechanics. In optics and photonics, the concept of modes is well suited to describe emission and absorption, coherence and interference, propagation and dispersion. The concept of modes consists of two aspects: first, the modes are solutions for the propagation of the light; second, the number of photons in the different modes describes the transport of energy or information. The energy density of the black body radiation, given by Planck's law, is the product of the density of modes in free space and the average number of photons per mode, given by the Bose-Einstein distribution. The transverse shape of the modes is determined by diffraction and boundary conditions. The longitudinal extension is given by the coherence length.

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