Near-infrared to visible upconversion and second harmonic generation in BaTiO3:Ho3+ and BaTiO3:Ho3+/Yb3+ phosphors

The present work shows the properties of BaTiO3:Ho3+ and BaTiO3:Ho3+/Yb3+ phosphors, prepared by the sol-gel method. The samples were characterized by powder X-ray diffraction (PXRD), Raman spectroscopy and scanning electronic microscopy (SEM). Upconversion (UC) and second harmonic generation (SHG) were monitored by photoluminescence. The UC mechanism was studied by time-resolved spectroscopy. Through PXRD, it was possible to determine that all synthesized phases crystallized in a cubic perovskite-type structure. Raman spectra corroborated the coexistence of both cubic and tetragonal phases. For the BaTiO3: Ho3+ samples excited at 1200 and 1150 nm, the emission spectra exhibited only one peak corresponding to the second harmonic of the emission wavelength. On the other hand, for the BaTiO3: Ho3+/Yb3+ samples excited at 975 nm, the emission spectra exhibited an intense green band at 554 nm, a medium intensity red band at 665 nm and a weak band at 760 nm. The study of the dependence of UC emission intensity on the pump power of the incident laser and the decay curves of red and green emissions were used to elucidate the UC mechanism, which corresponded to the process known as ground state absorption/energy transfer upconversion. (C) 2019 Elsevier B.V. All rights reserved.