Upconversion properties in hexagonal-phase NaYF4:Er3+/NaYF4 nanocrystals by off-resonant excitation

► Upconversion in NaYF4:Er3+/NaYF4 nanocrystals excited at 1620 nm were observed. ► An off-resonant absorption peak at around 1600 nm was observed. ► The off-resonant absorption is phonon assisted. ► Upconversion intensity excited at 1620 nm strongly depends on temperature. ► Upconversion under off-resonant excitation involves phonon-assisted transitions.