Herbert Rabin

F-Aggregate Centers in Alkali Halide Crystals

Publisher Summary The development of the field of color centers in alkali halide crystals, spanning the last sixty to seventy years, has unfolded along what appears to be a continuously lengthening and broadening avenue. As the quantity of different centers has increased to a current total of some thirty in number there has been a simultaneous advancement in the detailed understanding of individual centers and their relationships to one another. A perusal of the reviews of color-center phenomena that have appeared over the years makes it quite obvious that the F center has been the focal point of color-center research. The reasons are partly historical and partly the design of nature. Not only does the F center give rise to the most prominent optical absorption band in a large number of the alkali halides, but the simplicity of the F center (a single electron trapped by a negative ion vacancy) offers a particularly attractive defect to study from an experimental as well as a theoretical point of view. The family of centers giving rise to optical absorption bands on the long-wavelength side of the F band has received far less attention than the F center. These centers, M, R, and N, are termed F-aggregate or F-associate centers because they bear a strong genetic relationship to the F center, and like the F center they are defects which have trapped electrons. This chapter also discusses special varieties of the F-aggregate centers such as the singly ionized M center, as well as the so-called “prime” centers, which are the result of adding an electron to the normal M , R , and N centers.

Linear and circular polarized laser radiation in optical third harmonic generation

Abstract Theory shows that linear polarized laser radiation produces a third harmonic in an isotropic medium while circular polarized radiation does not. Experimental confirmation of this effect has been obtained and possible applications are mentioned.

Nonlinear optics and the defect solid state

Abstract The condition for the generation of second harmonics in the dipole approximation is that the nonlinear crystal medium lack a center of inversion. Conversely, a center of inversion precludes optical second harmonic generation. It is well known, however, that lattice defects introduced into a crystal may alter the local point symmetry in the medium, and in fact may convert local inversion symmetry to that which corresponds to a noncentrosymmetric point group. It is thus clear that selected defects in nonlinear media may be of interest for studying nonlinear optical processes which depend on the square of the electric field strength. The specific case of second harmonic generation employing spatially ordered FA-centers in alkali halide crystals is considered.