The mechanism of upconversion at the nanoscale is still under discussion. In this paper, we report on the experimental results of anti-Stokes luminescence kinetics in the upconversion nanoparticles of β-NaYF: 20%Yb; 0.6%Tm. The parameters of the luminescence kinetics were found to be unambiguously dependent on the number of excitation quanta n, which are necessary for certain transitions between the energy states of thulium ions. The observed correlation has been explained by means of the long-lasting energy migration between the ytterbium ions. The spread in time between the luminescent maxima of the corresponding thulium transitions not only shows the nonlinear character of upconversion, but also reveals the time scale of energy migration as well. From these, we derive that the conventional Förster formalism applied to the estimation of energy transfer efficiency in UCNP-fluorophore pairs can provide misleading results.
Photopolymerization of nanocomposite materials using near infrared light is one of the unique technologies based on the luminescent properties of lanthanide-doped upconversion nanoparticles (UCNPs). We explored the UCNP-triggered radical polymerization both in oligomer bulk and on the nanoparticle surface in aqueous dispersion. Core/shell UCNPs NaYF4:Yb3+ and Tm3+/NaYF4 with emitting lines in the ultraviolet and blue regions were used to activate a photoinitiator. The study of the bulk photopolymerization in an initially homogeneous reaction mixture showed the UCNP redistribution due to gradient density occurring in the volume, which led to formation of UCNP superlattices and spheres “frozen” in a polymer matrix. We also developed a strategy of “grafting from” the surface, providing polymer shell growth directly on the nanoparticles. The photosensitization of the endogenous water-soluble photoinitiator riboflavin by the resonance energy transfer from UCNPs was demonstrated in the course of monomer glycidyl methacrylate polymerization followed by photocrosslinking with poly(ethylene glycol) diacrylate on the nanoparticle surface.
The article demonstrates the fundamental possibility of creating microstructures for various functional purposes using the capabilities of the two-photon femtosecond polymerization method. The developed technological approach for creating a micro-optical holder for standard single-mode and multimode fibers is demonstrated. This type of holder can be used to manufacture a unit for optical matching of optical fibers with sensitive optical elements. The possibility of optical matching of fibers with a superconducting single-photon detector, an array system of bolometric superconducting microbridges, and spherical self-formed microlenses by near infrared-sensitive photopolymerization has been experimentally shown. The device manufacturing process was carried out in a combined single laser micromachining facility using femtosecond laser radiation for lithography with submicron resolution, texturing of the sensor surface, and the formation of microstructures for installing single-mode optical fibers.
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