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

Abstract

The linear Stark shift of optical frequencies in impurity centers lacking inversion site symmetry results in various external electric field effects observed in persistent spectral hole-burning (PSHB) phenomena. This paper reviews some new effects of inversionless symmetry of centers in internal electric fields observed in PSHB of doped insulators with photoionization hole burning. (1) In multisite R-line ( 4A 2– 2E ) spectra of Cr 3+ ions, substituting Ge 4+ in the inversionless position in the crystal lattice of Li 2Ge 7O 15 (LGO), the two-photon self-gated photoionization PSHB was observed in the R-spectra of one site only. This drastic site dependence of PSHB demonstrates extremely strong (∼eV) relative shifts of the position within the host energy gap of the 4A 2, 2E levels of Cr 3+ ions, located at complex centers. These centers differ in the nature and the position of the nearby charge-compensating defect, where the Coulomb field produces 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 crystal was observed with unusually broad hole wings in the case of lines belonging to perturbed inversionless sites. This efficient off-resonant PSHB is due 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+–Eu 3+ ions during the hole-burning process in optically excited crystals.