Red to blue tunable upconversion in Tm³⁺-doped ZrO₂ nanocrystals

Abstract

The effect of dopant concentration on the blue upconversion (UPC) emission of Tm³⁺ -doped ZrO₂ nanocrystals under different excitation wavelengths in the red region is reported. The UPC emissions are due to the f−f electronic transitions from excited states ¹G₄ and ¹D₂ of Tm³⁺. We observed a chromatic change in the UPC with tuning the excitation wavelength. The UPC emission bands at 475, 488, and 501 nm are observed under excitation at 649 nm, but bands centered at 454 and 460 nm are observed when the excitation wavelength is tuned to 655 nm. The UPC emission could be tuned from 501 to 454 nm (∼47 nm) by changing the excitation wavelength from 649 to 655 nm (∼6 nm). The pump power dependence of the emission bands at 475, 488, and 501 nm were investigated on excitation intensity at 649 nm, and the emission bands at 454 and 460 nm are investigated on excitation intensity at 655 nm, which confirms that all of these UPC emission lines are a two-photon absorption process.