A photon avalanche (PA) effect that occurs in lanthanide-doped solids gives rise to a giant nonlinear response in the luminescence intensity to the excitation light intensity. As a result, much weaker lasers are needed to evoke such PAs than for other nonlinear optical processes. Photon avalanches are mostly restricted to bulk materials and conventionally rely on sophisticated excitation schemes, specific for each individual system. Here we show a universal strategy, based on a migrating photon avalanche (MPA) mechanism, to generate huge optical nonlinearities from various lanthanide emitters located in multilayer core/shell nanostructrues. The core of the MPA nanoparticle, composed of Yb 3+ and Pr 3+ ions, activates avalanche looping cycles, where PAs are synchronously achieved for both Yb 3+ and Pr 3+ ions under 852 nm laser excitation. These nanocrystals exhibit a 26th-order nonlinearity and a clear pumping threshold of 60 kW cm −2 . In addition, we demonstrate that the avalanching Yb 3+ ions can migrate their optical nonlinear response to other emitters (for example, Ho 3+ and Tm 3+ ) located in the outer shell layer, resulting in an even higher-order nonlinearity (up to the 46th for Tm 3+ ) due to further cascading multiplicative effects. Our strategy therefore provides a facile route to achieve giant optical nonlinearity in different emitters. Finally, we also demonstrate applicability of MPA emitters to bioimaging, achieving a lateral resolution of ~62 nm using one low-power 852 nm continuous-wave laser beam.
Separation and recycling of rare-earths using ionic liquids as extractant are becoming a promising approach to replace traditional volatile organic solvents in recent years. Generally, the addition of some special salts could improve the extraction efficiency of rare-earths by numerous non-functional ionic liquids. However, knowledge behind the nature of the salt effect is limited. Here, we found that the enhancement in the extraction of rare-earth ions, Pr 3+ ions, using non-functional ionic liquid, [A336][NO 3 ] (Tricaprylmethylammonium nitrate) was driven by the synergism of Hofmeister bias and complexation behaviors of salt anions with Pr 3+ ions. Molecular dynamic simulations offered a new insight into the interaction mechanism of the ionic liquid with Pr 3+ ions at liquid/liquid interface. It was revealed that salt anions could perform as a bridge to connect Pr 3+ ions and the ionic liquid, so that promoted the extraction of Pr 3+ ions. Therefore, the strong complexation ability of salt anions with Pr 3+ ions and poor hydration of salt anions faciliated the transport of Pr 3+ ions across liquid/liquid interface.
Distinctive upconversion or downshifting of lanthanide nanocrystals holds promise for biomedical and photonic applications. However, either process requires high-energy lasers at discrete wavelengths for excitation. Here we demonstrate that co-sensitization can break this limitation with ultrawide excitation bands. We achieve co-sensitization by employing Nd 3+ and Ho 3+ as the co-sensitizers with complementary absorptions from the ultraviolet to infrared region. Symmetric penta-layer core-shell nanostructure enables tunable fluorescence in the visible and the second near-infrared window when incorporating different activators (Er 3+ , Ho 3+ , Pr 3+ , and Tm 3+ ). Transient spectra confirm the directional energy transfer from sensitizers to activators through the bridge of Yb 3+ . We validate the features of the nanocrystals for low-powered white light-emitting diode-mediated whole-body angiography of mice with a signal-to-noise ratio of 12.3 and excitation-regulated encryption. This co-sensitization strategy paves a new way in lanthanide nanocrystals for multidirectional photon conversion manipulation and excitation-bandwidth-regulated fluorescence applications.
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