Abstract

Yellow and green upconversion fluorescence is observed at room temperature in Nd:YLiF₄ by pumping with a high power diode laser around 795 nm (12.6 × 10³ cm^-1). The upconversion fluorescence was found to be strongest with the pump light polarized parallel to the crystallographic a-axis and was found to be weaker than the ⁴F_3/2 → ⁴I_11/2 near infrared (NIR) fluorescence by about five orders of magnitude. At room temperature, Nd:YLiF₄ absorbs strongly between 780 nm and 815 nm because of the ⁴I_9/2 → ⁴F_5/2 and ⁴I_9/2 → ⁴F_3/2 transitions. The ⁴F_5/2 level relaxes non- radiatively to the metastable ⁴F_3/2 level, which is in turn responsible for the NIR fluorescence transitions occurring between 9.2 x 10³ and 11.6 x 10³ cm^–1. The green upconversion emission lies between 18.5 × 10³ and 19.3 × 10³ cm^–1, and the yellow upconversion emission lies between 16.7 × 10³ and 17.4 × 10³ cm^-1. Considerable structure is visible in the spectrum. A possible route to the observed upconversion is absorption of a second laser photon from the ⁴F_3/2 metastable level into the ²D(1)_5/2 and/ or ²P_1/2 levels. We will present power dependent and time resolved studies of these fluorescence bands and discuss the dynamics of the upconversion process.

© 1992 Optical Society of America