The world's plastic production reached 269 million tons p.a. in 2015. The EU fraction of this was 50 million tons, with 40% of this attributed to product packaging. [1] Even though packaging exhibits the highest recycling rate for any plastic product, this recycling rate in the EU is less than 40%. [1,2] Overall, 29.7% of the plastic within the EU was recycled, with the remainder either burnt for energy recovery (39.5%) or dumped in landfills (30.8%). Recently adopted EU legislative proposals on waste management put pressure on the industry to find solutionsThe demand for more efficient and complete sorting techniques for plastic waste is growing, and one possible solution isbased on fluorescent labeling. Novel fluorescent labels based on trivalent lanthanide (Ln 3+ ) activated inorganic up-conversion (UC) materials offer a promising technological solution for plastic recycling. UC is a nonlinear, anti-Stokes process of combining two or more low energy near-infrared (NIR) photons to obtain the emission of a single higher energy photon. While Ln 3+ based UC materials possess one key disadvantage -low quantum yield, they also exhibit many unique features, such as high signal/noise ratio, tailored emission color, long photoluminescent lifetime, and low toxicity. These unique features endear them for a diverse range of applications and offer many new opportunities. Herein, we review the recent advances in the Ln 3+ activated inorganic micro-sized UC materials from the perspective of tailoring UC emission color and intensity for plastic recycling applications.

In photon upconversion (UC) based on triplet−triplet annihilation, the upconversion photoluminescent quantum yield (UC-PLQY) depends on the excitation power density in a way that can be described by a single figure of merit. This figure of merit, the threshold value, allows the excitation power density required for efficient UC-PLQY to be compared between different triplet−triplet annihilation systems. Here, we investigate the excitation power density dependence of two-photon UC processes in a series of four lanthanide-doped inorganic host materials (oxides, fluorides, and chlorides) all doped with 18 mol % Yb 3+ sensitizer ions and 2 mol % Er 3+ activator ions. We demonstrate that an analogous figure of merit, which we call the critical power density (CPD), accurately describes the UC power dependence of these samples. Better CPD values are obtained when the lifetime of the intermediate states is long. The UC-PLQY at the CPD is linked to the saturation UC-PLQY. Thus, a measurement of the UC-PLQY at this low power density can be used to estimate the theoretical saturation UC-PLQY in the absence of deleterious effects such as laser-induced heating. This is compared to another method to estimate the saturation based on the CPD model, namely, taking half of the level's PLQY under direct excitation. Our careful analysis of the upconversion spectra as a function of excitation power density gives several insights into the differing upconversion pathways in the hosts and proves to be a useful tool for their comparison.

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Efficient single-band NIR-to-NIR upconversion (UC) emission is strongly desired for many applications such as fluorescent markers, plastic recycling, and biological imaging. Herein, we report highly efficient single-band NIR-to-NIR UC emission in La2O3:Yb3+,Tm3+ (LYT) microcrystals. Under 980 nm laser excitation, LYT exhibits a NIR UC emission at ∼795 nm (Tm3+: 3H4 → 3H6) and blue UC emission at ∼476 nm; the NIR UC emission is dominant, with the intensity ratio of the NIR to blue I NIR/I blue > 100. Remarkably, a high absolute UC quantum yield (UCQY) of 3.4% is obtained for the single-band NIR UC emission of LYT at a relatively low excitation power density of 7.6 W/cm2. This value is much higher than the reported values of a single-band NIR UC for rare-earth-based UC materials in literature, such as the well-known benchmark UC materials of β-NaYF4:Yb3+,Er3+ (∼0.9%, with a excitation power density of 9 W/cm2) and Gd2O2S:Yb3+,Er3+ (∼1.9%, with a excitation power density of 20 W/cm2). The high absolute UCQY of single-band NIR UC emission combined with their facile preparation hints at their potential application in anti-counterfeiting, verified by the proof-of-concept demonstration of fluorescent labeling of a transparent IMT pattern.

Introducing porous material into optical cavities is a critical step toward the utilization of quantum-electrodynamical (QED) effects for advanced technologies, e.g. in the context of sensing. We demonstrate that crystalline,...