Spectral hole burning
Spectral hole burning is the frequency-selective bleaching of the absorption spectrum of a material, which leads to an increased transmission (a "spectral hole") at the selected frequency. Two basic requirements must be met for the phenomenon to be observed: Particular case. Most molecules and atoms always return from the excited state to the initial ground state. In some situations, however, this may not happen. For example, some organic dye molecules can undergo a photochemical reaction, which alters the whole chemical structure of the molecule. If such a photochemically active molecule absorbs light, then with a probability  of a few percent it will not return to the initial, educt state, but will rather switch over to a new product ground state. Often the homogeneous absorption spectrum of the new product is much different from that of the educt, and the corresponding inhomogeneous bands do not overlap. Spectral hole width can be expressed as follows: where formula_2 is spectral hole width, formula_3 is homogeneous linewidth, formula_4 is the centre frequency and formula_5 is the saturation intensity.