Off-resonant spectral hole burning in CaS:Eu by time-varying Coulomb fields

Persistent spectral hole burning has been observed on the zero-phonon lines of the main site and several perturbed sites of Eu2+ 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(b)5d excited state to the conduction band followed by electron transport to Eu3+ centers which are the dominant traps. A complex hole structure consisting of a narrow feature (200 MHz), and broader features (approximate to 5 and approximate to 100 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 Eu2+ 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.