Mechanisms of the blue emission of NaYF₄:Tm³⁺ nanoparticles excited by an 800 nm continuous wave laser

Abstract

A thorough understanding of energy transfer and upconversion (UC) processes between trivalent lanthanide (Ln(3+)) ions is essential and important for improving UC performance. However, because of the abundant energy states of Ln(3+) ions, UC mechanisms are very complicated, which makes it a challenge to exclusively verify and quantitatively evaluate the dominant process. In this study, the fundamental excitation processes of Tm(3+)-doped NaYF4 nanocrystals under 800 nm continuous wave (CW) laser excitation were experimentally investigated on the basis of the quantum transition principle. An 800 nm CW laser combined with other wavelength CW lasers, including 471 nm, 657 nm, 980 nm, and 1550 nm lasers, were designed to study in-depth the excitation processes of UC luminescence via simultaneous two-wavelength laser excitation. The results indicate that the excited state absorption of (3)H6→(3)H4∼∼(3)H5→(1)G4 is the dominant pathway of the 481 nm and 651 nm emission bands, and two kinds of energy transfer UC pathways, uniformly expressed as (1)G4 + (3)H4→(1)D2 + (3)F4, play the primary roles in the 456 nm emission band.

Achieving high-efficiency emission depletion nanoscopy by employing cross relaxation in upconversion nanoparticles

“…The slight decrease of the depletion efficiency induced by the 810-nm beam with the Tm 3+ doping concentration above 10% (Fig. 1f ) could be due to the activation of the otherwise inefficient cross relaxations between Tm 3+ ions at adequately high Tm 3+ concentration, for example, 1 G 4 + 3 F 4 → 1 D 2 + 3 H 6 and 3 F 2,3 + 3 H 4 → 1 D 2 + 3 H 6 , as reported in a previous report 39 , which enhances the synergistic excitation effect of the 810-nm laser beam. The critical role of the Tm 3+ concentration-dependent CR1 process is also supported by the results of our numerical simulations (Supplementary Figs.…”

“…Notably the 1 D 2 → 3 F 2 transition has an even larger branching ratio than the 1 D 2 → 3 F 3 transition according to previous theoretical studies 41 – 44 . The 1 D 2 → 3 F 2 transition spectrally overlaps with other transitions, including 3 H 4 → 3 H 6 and 1 G 4 → 3 H 5 39 , 45 , 46 , making it barely distinguishable from others and thus causing negligence. The matching of the STED laser light at 810 nm with the emission spectrum (red-shifted relative to the absorption spectrum) of the 1 D 2 → 3 F 2 transition is supported by many previous reports that indicate the absorption spectrum of 3 F 2 → 1 D 2 transition is centered around 800 nm and could cover quite a few nanometers 39 , 46 – 49 .…”

“…Due to the matching of the 810-nm laser (full width at half maximum of 4 nm) with multiple transitions, including 3 H 6 3 H 4 , 3 H 5 1 G 4 , and 3 F 2 1 D 2 39 , 45 , 46 , three light absorption and three stimulated emission processes at 810 nm are all considered in this model, with different cross sections (Supplementary Table 2 ). Regarding the net effect of the 810-nm laser on the population density of the 3 H 4 state, fundamentally determined by the magnitude relationship between two items, namely, and , our spectroscopic result of the 1470-nm band ( 3 H 4 → 3 F 4 ) (Fig.…”

Intrinsic Time‐Tunable Emissions in Core–Shell Upconverting Nanoparticle Systems

“…The dual‐excitation experiments demonstrate that only the emissions from the 1 G 4 state of Tm 3+ are being populated through a multiple‐step absorption of the 800 nm photons. An increased probability of UCL from the 1 G 4 state after excitation with an 800 nm pumping source has been confirmed by the results in Tm 3+ ‐doped NaYF 4 nanoparticles . Cooperative UC was also excluded as a possible mechanism to sensitize Tb 3+ , being generally less efficient, as confirmed from the spectra shown in Figure S10.…”

Photon Upconversion Kinetic Nanosystems and Their Optical Response

“…Due to the ample energy levels of rare earth ions, often with ladder‐like arrangement, and efficient interionic energy transfer, upconversion luminescence can be generated in singly‐doped UCNPs by many single excitation wavelengths, spanning from NIR to visible range ‐ for instance, 532 nm, 808 nm and 1490 nm for Er 3 + ions, 532 nm for Ho 3 + ions, 800 nm and 1064 nm for Tm 3 + ions, and 473 nm and 609 nm for Pr 3 + ions . However, these upconversion systems generally have low efficiency due to the relatively small absorption cross‐sections of the rare earth ions at these wavelengths.…”