Photoionization spectral hole-burning and inversion symmetry of sites in doped insulators

The linear Stark shift of optical frequencies in impurity centers lacking i nversion site symmetry results in various external electric field effects o bserved in persistent spectral hole-burning (PSHB) phenomena. This paper re views some new effects of inversionless symmetry of centers in internal ele ctric fields observed in PSHB of doped insulators with photoionization hole burning. (1) In multisite R-line ((4)A(2)-E-2) spectra of Cr-3 (+) ions, s ubstituting Ge4 + in the inversionless position in the crystal lattice of L i2Ge7O15 (LGO), the two-photon self-gated photoionization PSHB was observed in the R-spectra of one site only. This drastic site dependence of PSHB de monstrates extremely strong ( similar to eV) relative shifts of the positio n within the host energy gap of the (4)A(2), E-2 levels of Cr-3 (+) ions, l ocated at complex centers. These centers differ in the nature and the posit ion of the nearby charge-compensating defect, where the Coulomb field produ ces different large linear Stark shifts of the electronic states of Cr-3 (). (2) Photon-gated PSHB in f-d spectra of Eu-2 (+) multisites in CaS cryst al was observed with unusually broad hole wings in the case of lines belong ing to perturbed inversionless sites. This efficient off-resonant PSHB is d ue to strong linear Stark shifts of the f-d transition frequencies of Eu-2 (+) ions in time-varying Coulomb fields resulting from photoionization and recharging of Eu-2 (+) -EU3 + ions during the hole-burning process in optic ally excited crystals. (C) 1999 Elsevier Science B.V. All rights reserved.