Transparent sol-gel nano-glass-ceramics comprising Ln3+-doped YF3 nanocrystals (about 11 nm) in a silica matrix were reported in 2011, showing bright up-conversion under 980 nm diode excitation, including UV emissions generated by four- and five-infrared-photon processes in Yb3+–Tm3+ co-doped samples. The source paper left open three coupled development issues: raising the absolute quantum efficiency of these intrinsically weak high-order IR-to-UV channels and determining how far it can be raised; determining the behaviour, efficiency limits, and spectral changes under high-density or pulsed excitation (saturation, quenching, thermal effects), since the published measurements used moderate continuous-wave pumps in the far-from-saturation regime; and integrating the materials into short-wavelength solid-state devices, which requires dopant, nanocrystal-size, and partition-fraction optimization, quantitative quantum-yield measurements, and long-term photostability studies.

Background

Up-conversion (UC) luminescence is an anti-Stokes process in which a material absorbs two or more low-energy photons and emits one higher-energy photon. In lanthanide (Ln) ions, the optically active 4f electron shells give long-lived excited states arranged in ladders of nearly equally spaced levels, which makes sequential multi-photon excitation possible. The workhorse scheme sensitizes the process with ytterbium (Yb3+), which has a single, strongly absorbing transition near 980 nm, and transfers the energy stepwise to an activator such as thulium (Tm3+). After three or four transfers Tm3+ occupies its 1G4, 1D2, and 1I6 levels, whose radiative decay produces blue and, at the highest levels, ultraviolet (UV) light near 290–360 nm; UV emission therefore requires the cooperative accumulation of four or five infrared photons per emitted photon. Because each intermediate step competes with spontaneous decay and with cross-relaxation losses (energy transfer between nearby lanthanide ions that drains the excited state non-radiatively), the efficiency of such high-order channels falls steeply with photon order and their absolute quantum efficiency (emitted UV photons per incident infrared photon) is intrinsically small. Oxyfluoride nano-glass-ceramics address a second bottleneck: they are transparent silicate glasses in which thermal treatment precipitates fluoride nanocrystals a few nanometres across, small enough to avoid scattering visible light. Fluorides have low maximum phonon energies, which suppresses multiphonon relaxation (non-radiative decay by simultaneous emission of lattice vibrations) that would otherwise depopulate the excited 4f levels; the glass matrix retains the chemical and mechanical robustness and the processing flexibility of glass. The sol-gel route builds the precursor glass by hydrolysis and condensation of molecular precursors at low temperature, then a controlled heat treatment nucleates the fluoride phase; a key figure of merit is the partition fraction, the fraction of Ln3+ ions that enter the nanocrystals rather than stay in the oxide glass. A further experimental observable is the pump-power dependence: in the unsaturated regime the UC intensity grows as a power of pump power whose exponent (the slope in a log-log plot) equals the number of absorbed infrared photons, while at high excitation density the intermediate levels saturate and the slope drops, accompanied by concentration quenching (extra non-radiative loss at high dopant concentration, where nearby ions exchange energy instead of emitting) and sample heating. Building on earlier work on UC materials and their applications (see the review of Zhou et al. 2015 and the sol-gel glass-ceramic reviews of Gorni et al. 2018 and Secu et al. 2021), Yanes and co-workers reported in 2011 the first sol-gel-derived transparent nano-glass-ceramics containing Ln3+-doped orthorhombic YF3 nanocrystals of about 11 nm, confirmed by X-ray diffraction and electron microscopy, with Eu3+ and Sm3+ ions used as structural probes (their sharp emission lines fingerprint the local site symmetry and crystallographic environment around the ion); a large fraction of the optically active ions partitioned into the nanocrystals, and Yb3+–Tm3+ co-doped samples showed intense UV UC assigned to four- and five-photon processes. The mechanisms were analyzed through the dependence of the emission intensity on Yb3+ concentration and pump power. Saturation behaviour has been characterized in the successor Yb3+–Tm3+ system Ba(Y,Gd)F5:Yb,Tm nano-glass-ceramics (del-Castillo and Yanes 2018), and IR-to-UV up-conversion efficiency gains have been achieved in colloidal core/shell nanoparticles (Shen et al. 2013; Qiu et al. 2014) rather than in bulk glass-ceramics. The paper closed by identifying what its measurements could not yet deliver.

Problem Statement

For transparent sol-gel nano-glass-ceramics comprising Ln3+-doped YF3 nanocrystals in a silica matrix — as reported by Yanes and co-workers in 2011, where Yb3+–Tm3+ co-doped samples show UV up-conversion from four- and five-infrared-photon processes under 980 nm continuous-wave excitation — the source paper leaves three coupled development questions: (1) over the design space of this system — dopant concentration, nanocrystal size and distribution, sample geometry, and excitation regime — what is the highest absolute IR-to-UV up-conversion quantum efficiency these channels can reach, with the configuration attaining it identified, and what limits it: the intrinsic competition between radiative decay, non-radiative decay, and cross-relaxation per absorbed photon, or the per-incident bottleneck set by pump absorption, coupling, and scattering; (2) at what excitation densities or pulse conditions do population saturation, quenching, and thermal effects appear under high-density or pulsed excitation, and what efficiency limits and spectral changes do they impose, given that the published measurements at moderate continuous-wave pump powers operated far from saturation; and (3) what dopant concentrations, nanocrystal size and distribution, and partition fraction should be targeted, and what quantum-yield determination and long-term photostability measurements are needed, to translate these materials into short-wavelength solid-state lasers or comparable UV devices?

These three questions form one materials-development program for the same nano-glass-ceramic system rather than three independent problems: the efficiency that can be reached depends on the high-excitation-density behaviour, and the device translation depends on both, so the program is addressed as a whole rather than through any single question. Answering it requires, for question (1), a stated efficiency ceiling together with the configuration that attains it and the dominant limiting mechanism; for question (2), the excitation densities or pulse conditions at which population saturation, quenching, and thermal effects set in, with their efficiency and spectral consequences; and for question (3), explicit target values for dopant concentration, nanocrystal size and distribution, and partition fraction, together with a named quantum-yield determination and photostability measurement approach.

References

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