Optical Nonlinearities of Color Centers in Alkali Halides

Two different classes of nonlinear optical phenomena are present in color centers in alkali halides. One is the direct consequence of applying very intense electromagnetic fields, which results for instance in two and also three photon absorption with selection rules different from those of the much more usual one photon absorption. The other concerns the color centers with axial symmetry, whose dipole can be oriented in various directions under appropriate optical excitation. In this case the intensity of the exciting light plays a role only in the kinetics. Indeed the absorption of one photon not only excites centers like F2, F2 +, FA, and FH, but also reorients in the excited state their optical axes, producing at the same time a variation of the absorption itself. In this context the newly discovered bistability of the FH(OH−) center in KBr will be described in detail with the latest developments.