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 s ingle crystals. Hole burning occurs by two-step photoionization, can b e strongly gated with IR irradiation, and takes place by excited-state absorption from the metastable 4f(b)5d excited state to the conductio n band followed by electron transport to Eu3+ centers which are the do minant traps. A complex hole structure consisting of a narrow feature (200 MHz), and broader features (approximate to 5 and approximate to 1 00 GHz) is observed. A mechanism is described for the occurrence of th ese unusually broad features. Time-varying internal electric fields wh ich 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 betwee n Eu ions, is demonstrated. This mechanism is identified from the freq uency 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 s ingle-photon process.