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

Abstract

Measurements of photoinduced light absorption in Ca₃Mn₂Ge₃O₁₂ revealed some unusual features: a saturation with the pumping intensity and a broad straggling of relaxation times with a predominance of very long times. These experimental facts cannot be understood in terms of photoinduced absorption centers associated with impurities or lattice defects, but are naturally explained within the notion of random electric fields of active charges. Active charges are produced by light pumping via the dissociation of coupled pairs 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. On the other hand, the random fields of active charges promote hopping of holes and hence the relaxation of photoinduced effects. A broad distribution of the magnitudes of random fields gives rise to a very broad range of the hole hopping rates. There is also a much faster annihilation process immediately conditioned by light pumping. The simultaneous action of these relaxation channels, depending on the number of active charges, pumping intensity and temperature, explains the entire experimental picture qualitatively and in part quantitatively. Photoinduced dichroism as well as birefringence, observed under polarized pumping, are caused by an anisotropic distribution of photoproduced holes over polarization directions.