Near-infrared-to-blue energy upconversion in LaF<SUB>3</SUB>:Ho<SUP>3+</SUP>

When the <SUP>5</SUP>F<SUB>5</SUB> levels of Ho<SUP>3+</SUP> in LaF<SUB>3</SUB> were resonantly excited with a red dye laser beam of 640 nm, blue and green emissions were detected from the <SUP>5</SUP>G<SUB>5</SUB>, the <SUP>5</SUP>F<SUB>3</SUB>, and the <SUP>5</SUP>S<SUB>2</SUB>(<SUP>5</SUP>F<SUB>4</SUB>) levels at 416, 485, and 546 nm, respectively. We found that excited ions in the <SUP>5</SUP>F<SUB>5</SUB> state undergo nonradiative relaxation to the <SUP>5</SUP>I<SUB>6</SUB> state and are then upconverted to the 5Gs state by the absorption of a second photon, as well as by energy-transfer interaction. The upconversion efficiency into the <SUP>5</SUP>F<SUB>3</SUB> state was estimated as ~1.1 &#215; 10-4. We also observed energy-upconverted emissions from the <SUP>5</SUP>F<SUB>3</SUB> and the <SUP>5</SUP>F<SUB>3</SUB> levels when the material was excited with an 800-nm near-infrared laser. The absorption of a second laser photon from the <SUP>5</SUP>I<SUB>6</SUB> state was found to be responsible for the upconverted green emission