Figure 2: Upconversion emission from 28 mol% Yb³⁺-doped ZrO₂.

(a) Photograph showing the sample to be white powder (scale bar, 5 mm). (b) Photograph of the powder sample excited by a collimated 976-nm laser beam (1.6 W, 760 W cm⁻²; scale bar, 5 mm). The sample glows. (c) Optical image of two irregularly shaped particles from the powder sample (scale bar, 200 μm). (d) Optical image of the same region, showing that one particle within the 976-nm laser beam (1.2 W, 570 W cm⁻²; scale bar, 200 μm) glows and the other particle outside the laser beam remains dark. (e) X-ray diffraction pattern of the sample calcined at 800 °C. The pattern can be indexed to the cubic phase (JCPDS #: 81–1551) of ZrO₂. (f) Photographs of a sphere of 0.8 mm in diameter that glows under the 976-nm laser excitation (1.6 W, 760 W cm⁻²; scale bar, 5 mm). The sphere was fabricated by melting the powder sample in an arc melting furnace. (g) Photographs of a disk-shaped pellet of 8 mm in diameter that glows under the laser excitation at the same power (scale bar, 5 mm). The pellet was fabricated from the powder sample by mechanical pressing.