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Down‐conversion (DC) is a phenomenon that can enable the observation of photoluminescent quantum yield (PLQY) values exceeding 100%. A comprehensive study of the DC properties of BaF2–based single crystals with different Er3+ doping levels (1–25 mol.%) is presented. The samples exhibit a PLQY of 110% in the 1550–1650 nm under 405 nm excitation. This remarkable PLQY is attributed to a cross‐relaxation process within the energy levels of Er3+. Furthermore, when considering all emitted photons in the 1000–1650 nm range, the PLQY reaches 153% for a sample doped with 5 mol.% of Er3+. By integrating the emission of this single crystal with a Ge diode, serving as an example of a photovoltaic device sensitive to short‐wave infrared light, a significant enhancement in short‐circuit current is demonstrated. In a broader context, the presented material holds promise for improving the spectral response of low‐bandgap photovoltaic devices. [ABSTRACT FROM AUTHOR]

A ratiometric optical thermometer based on triply‐doped La2O3:Yb3+/Er3+/Nd3+ microcrystals is reported with a relative sensitivity above 1% K−1 in the entire range from 300–700 K, and is between 1.8–0.7% K−1 over the range 290–833 K. The 825 nm upconversion (UC) emission from the Nd3+ 4F5/2 level relies on thermally‐assisted energy transfer from Yb3+; thus, unusually, the near‐infrared emission increases with increasing temperature in the relevant range. More typically, the two‐photon 660 nm UC from Er3+ 4F9/2 level decreases in intensity with increasing temperature due to increasing non‐radiative rates. The variation of fluorescent intensity ratio between these emissions is amplified by their opposite responses to temperature change leading to excellent sensitivity. Concurrently, the different pathways for the temperature response in the two emitting ions enable the high sensitivity to be maintained over an atypically broad temperature range. The wide separation in wavelength means that a standard silicon‐based monochrome camera with broad (inexpensive) band pass filters is sufficient to use this phosphor for thermography. The concept of combining thermally‐activated UC with classical Stokes‐shifted emission is demonstrated to provide combined features of excellent and broad‐range sensitivity plus excellent repeatability. Materials based on this concept are very promising for optical thermometry. [ABSTRACT FROM AUTHOR]

Thermal images generated from infrared radiation are useful for monitoring many processes; however, infrared cameras are orders of magnitude more expensive than their visible counterparts. Methods that allow visible cameras to capture thermal images are therefore of interest. In this contribution, thermal images of a surface coated with an inexpensive inorganic micropowder phosphor are generated from the analysis of a video taken with a smartphone camera. The phosphor is designed to have a temperature‐dependent emission lifetime that is long enough to be determined from the analysis of a 30 frames‐per‐second video recording. This proof‐of‐principle work allows temperatures in the 270–320 K range to be accurately determined with a precision better than 2 K, even in the presence of bright background illuminance up to 1500 lm m−2. In the broader context, this inspires further development of phosphors to bring time‐resolved sensing techniques into lifetime long enough ranges to allow smartphone‐based detection. [ABSTRACT FROM AUTHOR]

The main challenge for applying upconversion (UC) to silicon photovoltaics is the limited amount of solar energy harvested directly via erbium‐based upconverter materials (24.5 W m–2). This could be increased up to 87.7 W m–2 via spectral concentration. Due to the nonlinear behavior of UC, this could increase the best UC emission by a factor 13. In this paper, the combined use of quantum dots (QDs)—for luminescent down‐shifting—and photonic crystals (PCs)—for reshaping the emission—to achieve spectral concentration is shown. This implies dealing with the coupling of colloidal QDs and PC at the high‐density regime, where the modes are shifted and broadened. In the first fabricated all‐optical devices, the spectral concentration rises by 67%, the QD emission that matches the absorption of erbium‐based upconverters increases by 158%, and the vertical emission experiences a 680% enhancement. Remarkably, the PC redshifts the overall emission of the QDs, which could be used to develop systems with low reabsorption losses. In light of this, spectral concentration should be regarded as one of the main strategies for UC photovoltaics. [ABSTRACT FROM AUTHOR]

Thermal images generated from infrared radiation are useful for monitoring many processes; however, infrared cameras are orders of magnitude more expensive than their visible counterparts. Methods that allow visible cameras to capture thermal images are therefore of interest. In this contribution, thermal images of a surface coated with an inexpensive inorganic micropowder phosphor are generated from the analysis of a video taken with a smartphone camera. The phosphor is designed to have a temperature‐dependent emission lifetime that is long enough to be determined from the analysis of a 30 frames‐per‐second video recording. This proof‐of‐principle work allows temperatures in the 270–320 K range to be accurately determined with a precision better than 2 K, even in the presence of bright background illuminance up to 1500 lm m−2. In the broader context, this inspires further development of phosphors to bring time‐resolved sensing techniques into lifetime long enough ranges to allow smartphone‐based detection. [ABSTRACT FROM AUTHOR]