Persistent spectral hole burning has been observed on the zero-phonon lines of the main site and several perturbed sites of ${\mathrm{Eu}}^{2+}$ in CaS:Eu single crystals. Hole burning occurs by two-step photoionization, can be strongly gated with IR irradiation, and takes place by excited-state absorption from the metastable ${4f}^{6}5d$ excited state to the conduction band followed by electron transport to ${\mathrm{Eu}}^{3+}$ centers which are the dominant traps. A complex hole structure consisting of a narrow feature (200 MHz), and broader features ($\approx 5$ and $\approx 100\mathrm{GHz}$) is observed. A mechanism is described for the occurrence of these unusually broad features. Time-varying internal electric fields which occur during the hole burning due to photoionization and trapping can lead to burning of holes at frequencies non-resonant with that of the laser. In addition, a mechanism for hole erasure, tunneling between Eu ions, is demonstrated. This mechanism is identified from the frequency dependence of the hole erasure which follows the ${\mathrm{Eu}}^{2+}$ absorption, and the linear dependence of the photoconductivity and hole erasure efficiency on irradiation power, both of which indicate erasure in a single-photon process.

• Received 9 June 1997

DOI:https://doi.org/10.1103/PhysRevB.56.12992