Advances in the theoretical understanding of photon upconversion in rare-earth activated nanophosphors

Abstract

Photon upconversion in rare earth activated phosphors involves multiple mechanisms of electronic transitions. Stepwise optical excitation, energy transfer, and various nonlinear and collective light-matter interaction processes act together to convert low-energy photons into short-wavelength light emission. Upconversion luminescence from nanomaterials exhibits additional size and surface dependencies. A fundamental understanding of the overall performance of an upconversion system requires basic theories on the spectroscopic properties of solids containing rare earth ions. This review article surveys the recent progress in the theoretical interpretations of the spectroscopic characteristics and luminescence dynamics of photon upconversion in rare earth activated phosphors. The primary aspects of upconversion processes, including energy level splitting, transition probability, line broadening, non-radiative relaxation and energy transfer, are covered with an emphasis on interpreting experimental observations. Theoretical models and methods for analyzing nano-phenomena in upconversion are introduced with detailed discussions on recently reported experimental results.

Phase angle encoded upconversion luminescent nanocrystals for multiplexing applications

“…Inherited from the studies on fluorescent dyes and quantum dots, optical multiplexing strategies using UCNPs have been developed in the spectral domain 7,[24][25][26][27] , time domain 28 and spatial domain 29,30 . However, studies of the complex luminescence mechanisms involving sequential multi-step photophysical and energy transfer processes featuring complex luminescence kinetics are still at their infancy [31][32][33][34] , and may open possibility to develope unique multiplexing strategies in other domains.…”

Up- and down-conversion emissions from Er3+ doped K2YF5 and K2YbF5 crystals

“…Er 3 þ ions can absorb light at around 980 nm which corresponds to the emission of well-developed InGaAs laser diodes and emit in both the green ( $ 540 nm) and the red ( $ 650 nm) spectral regions. Due to the special structure of Er 3 þ energy levels, several efficient processes that provide the population of the higher-lying excited states can be easily implemented namely excited-state absorption (ESA), cross-relaxation (CR) and energy-transfer (ET) [2]. The use of an Er 3 þ -Yb 3 þ couple is beneficial for increasing the up-conversion efficiency due to a rather strong absorption of the Yb 3 þ ions at $ 1 μm.…”

“…Typically, absorption of UV/visible light by RE 3 þ ions leads to the emission of Yb 3 þ ions at $ 1 μm corresponding to the 2 F 5/2 -2 F 7/2 transition. Down-conversion is Er 3 þ and Er 3 þ /Yb 3 þ doped fluoride and oxyfluoride materials (in the form of single crystals, glass-ceramics or nanopowders) provide intense up-and down-conversion luminescence [1][2][3][4][5][6][7][8].…”

Characterization of Luminescent Materials with 151Eu Mössbauer Spectroscopy

“…The 4$f^n$ levels are depicted as solid black lines and the closely spaced levels of 4$f^{n-1}$ 5 d and 4$f^{n-1}$ 5 p are depicted as grey bands [20,21,22]. …”