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, that must be met for the observation of this phenomenon, are: 1) The spectrum is inhomogeneously broadened and 2) the material undergoes, subsequent to light absorption, a modification which changes its absorption spectrum. Typical materials include dye molecules dissolved in suitable host matrices and the frequency selective irradiation is usually realized by a narrow band laser.
In most cases, molecules and atoms always return from the excited state back to the initial ground state. There are situations, however, when this is not always the case. For example, some organic dye molecules can undergo a photochemical reaction, which alters the whole chemical structure of the molecule. If such photochemically active molecule absorbs light, then with a probability of a few % it will not return to the initial state called educt, but rather switches over to a new ground state called product. Often the homogeneous absorption spectrum of the product is much different from the educt, so that the corresponding inhomogeneous bands do not even overlap.
http://www.iupac.org/publications/pac/pdf/1995/pdf/6701x0191.pdf http://www.physics.montana.edu/faculty/rebane/Research/Tutorials/Hole_burning/HoleBurning_00.htm