Up- and Downconversion Dual-Mode Excitation Spectral Studies of Rare Earth Doped KLaF4 Nano Emitters for Biophotonic Applications

An innovative approach has been demonstrated to achieve site-selective, dualmode excitation, resulting in synergy between upconversion (UC) and downconversion (DC) photoluminescence (PL) in nanospectral emitters. The spectral signatures of rare-earth ions are highly sensitive to site-selective occupancy and crystal field effects. Here, dual excitation has been effectively used to analyze the complex energy transfers between the different rare earth ions in a single host and to demonstrate contrast detection from conventional and upconversion spectra and imaging. A series of photostable mono cubic phase KLaF4 with various combinations of Er3+/Eu3+/Yb3+-doped nanospectral emitters have been critically examined. The dual (near-infrared and visible) excitation results show a huge enhancement in UC/DC PL compared to individual laser excitations. The energy transfer between the activators and sensitizers produced distinct colors with a broad range of tunability. Additionally, this technique can be used for multiplexed imaging that is supported by the spectral spatial imaging of the Eu3+ spectroscopic agent. Therefore, the results may be easily extended to other fluorescent/rare earth-doped systems pumped by multiple-wavelength lasers (such as 375, 405, 532, 785, 808, 980, 1064, 1550 nm, etc.). This technique would open new possibilities for super-resolution microscopy, multiplexed bioimaging, color displays, spectrum converters in solar cells, and supercontinuum generation.