Improving upconversion luminescence efficiency in Er³⁺-doped NaYF₄ nanocrystals by two-color laser field

Improving upconversion luminescence efficiency of lanthanide-doped nanocrystals is always a hot topic for scientists because of its very important application in photonics, photovoltaics, biological imaging, security printing, and therapeutics. Two-color laser field has shown to be a well-established strategy to further improve the upconversion luminescence efficiency. Here, we first propose a two-color laser field combining the 850 and 980 nm lasers to improve the green and red upconversion luminescence efficiency in Er³⁺-doped NaYF₄ nanocrystals. In this work, an important advantage for our strategy is that the population is directly pumped to the radiation energy level of the upconversion luminescence by a cooperation excitation process, but not the spontaneous decay from other higher energy levels, and so the higher luminescence enhancement efficiency can be obtained. These studies also provide a clear physical picture for the physical control mechanism of the upconversion luminescence efficiency improvement, which can pave a way to properly design the laser fields in the future study of upconversion luminescence generation and control.