Photoinduced light absorption and dichroism of Ca3Mn2Ge3O12 garnet as a probe of electronic processes and intrinsic electric fields

Measurements of photoinduced light absorption in Ca3Mn2Ge3O12 revealed some unusual features: a saturation with the pumping intensity and a broad stra ggling of relaxation times with a predominance of very long times. These ex perimental facts cannot be understood in terms of photoinduced absorption c enters associated with impurities or lattice defects, but are naturally exp lained within the notion of random electric fields of active charges. Activ e charges are produced by light pumping via the dissociation of coupled pai rs of charges (consisting of a Mn-hole coupled with a compensating negative impurity or a negatively charged vacancy) which exist in the ground state. Such active charges create electric fields in a larger volume than coupled pairs, thus enhancing the probability for forbidden optical transitions. O n the other hand, the random fields of active charges promote hopping of ho les and hence the relaxation of photoinduced effects. A broad distribution of random-field magnitudes gives rise to a very broad range of hole hopping rates. There is also a much faster annihilation process immediately condit ioned by light pumping. The simultaneous action of these relaxation channel s, depending on the number of active charges, pumping intensity, and temper ature, explains the entire experimental picture qualitatively and in part q uantitatively. Photoinduced dichroism as well as birefringence, observed un der polarized pumping, are caused by an anisotropic distribution of photopr oduced holes over polarization directions. (C) 2001 American Institute of P hysics.