Abstract

We report observations consistent with photon avalanche upconversion¹,² in 5% Er:YLiF₄. Intense cw pumping at 558 nm leads to characteristic Er³⁺ fluorescence at 415 nm. The pump light is chosen to be resonant with a transition between two excited states of the Er³⁺ ion, but not with transitions from the ground state. In the usual upconversion schemes,³ absorption of pump light by ions in the ground state is a necessary step in the upconversion process and so the wavelength of the pump light must lie in an absorption band of the activated medium. The distinguishing feature of photon avalanche upconversion is the absence of direct optical pumping of ions in the ground state. Instead, ions are excited to an intermediate level by nonradiative transfer of energy from nearby ions in a higher lying level. The optical pumping serves to excite ions from the intermediate level to the higher lying level.

© 1989 Optical Society of America