Liangliang Zhang

Current imaging and temperature sensing techniques utilizing persistent phosphors face a trade‐off between charging efficiency and charging wavelength, particularly hinders biomedical applications, due to the limitations of conventional charging modes. Here, we introduce the upconversion charging (UCC) technique to balance high‐efficiency and long‐wavelength charging, using β ‐Ga ₂ O ₃ :Cr ³⁺ as a representative persistent phosphor to realize effective UCC at wavelengths within the bio‐window. Spectroscopic studies reveal that the phosphor can be charged not only via ultraviolet photoionization but also through two‐photon UCC with the Cr ^{3+ 4} T ₂ / ² E level acting as an intermediate state, thereby extending the charging wavelength into the bio‐window. Quantum tunneling is found to dominate the long‐lasting persistent luminescence (PersL) output, broadening the time window for readout imaging and temperature signals. Three linearized thermal response parameters based on temperature‐dependent Cr ³⁺ PersL spectra are synergistically optimized via multiple linear regression analysis, achieving orders‐of‐magnitude improvements in relative sensitivity and temperature uncertainty. The potential of UCC‐induced PersL for simultaneous imaging and temperature sensing is demonstrated. Our work opens new perspectives for the long‐wavelength high‐efficiency charging and multifunctional applications of persistent phosphors.

Er3+/Yb3+ codoped rare-earth sesquioxides are widely used red upconversion (UC) phosphors under near-infrared (NIR) 980 nm excitation. Besides the old mechanism, a newly identified round-trip energy transfer (RTET) between Er3+ and Yb3+ accelerates relaxation from the green-emitting to the red-emitting state, making the red and green UC lifetimes comparable. Here, we show that the effectiveness of RTET is closely related to multiphonon relaxation (MPR) of Er3+; that is, slow MPR is conducive to RTET. The contributions of the two mechanisms are studied in Gd2O3 and Sc2O3, which have the slowest and fastest MPR, respectively, in rare-earth sesquioxides. The effect of RTET is verified based on the spectral and temporal behavior of down-shifted and upconversion luminescence. Upon NIR pulse excitation, RTET contributes 94 ± 9.3% in Gd2O3, leading to nearly identical red and green lifetimes, but drops to 70 ± 6.2% in Sc2O3, showing a lifetime of red UC emission significantly longer than that of green UC emission due to the nonnegligible effect of the old mechanism. Upon NIR CW excitation, the old mechanism and RTET make a main contribution in Sc2O3 and Gd2O3, respectively. These findings provide new insights into controlling UC lifetimes for time-gated optical applications.

Noncontact optical temperature measurement has attracted considerable attention due to its crucial applications across various fields. However, the demand for highly sensitive optical thermometric materials remains pressing. This study introduces an ultrasensitive charge transfer band-based optical temperature sensing system utilizing Eu³⁺ monodoped MgNb₂O₆ phosphor, effectively circumventing the limitations associated with traditional thermally coupled energy levels. By utilizing the luminescence intensity ratio based on excitation and emission spectra, we introduced three measurement modes; the optical thermometer exhibited a maximum relative sensitivity of 4.24% K^–1 (300 K) and an absolute sensitivity of 2.99% K^–1 (570 K), surpassing the sensitivity of most optical thermometers of the same type. Furthermore, by employing the shift of charge transfer band peak position as a measurement parameter, we developed a novel optical thermometer with a sensitivity (S_w) of 0.064 nm·T^–1. This material, exhibiting high brightness and high-purity red emission due to the efficient energy pathway provided by the ultraviolet absorption of the matrix, emerges as a promising candidate for LED applications. Additionally, ink and PDMS flexible films made of MgNb₂O₆:Eu³⁺ phosphor were prepared to illustrate potential applications in anticounterfeiting and encryption. All the studies demonstrate that the MgNb₂O₆:Eu³⁺ material is a promising candidate in the field of multimodal high-performance and multifunctional optical thermometers.

Oxygen vacancies induce Ti ⁴⁺ clustering in m -ZrO ₂ , enabling UV-driven electron delocalization within clusters and subsequent energy transfer to isolated Ti ⁴⁺ ions.

A chemical unit co-substitution strategy, replacing Zr ⁴⁺ –Zr ⁴⁺ pairs with Al ³⁺ –Ta ⁵⁺ units in Ba ₃ ZrTa ₄ O ₁₅ :Cr ³⁺ , yields a fourfold enhancement in near-infrared (NIR) luminescence intensity and broadens the emission bandwidth from 239 to 290 nm.

The popular Er³⁺-Yb³⁺ upconversion (UC) system generates green and red emissions under near-infrared (NIR) 980 nm excitation. The green emission is a two-photon process, whereas for the red emission two and three photon process are both possible, depending on host. Fluoride hosts generally favor a three-photon process because fluorides have low phonon energies and multiphonon relaxation (MPR) involved in the usual two-photon process is suppressed. However, CaF₂:Er³⁺/Yb³⁺ exceptionally produces an intense two-photon upconverted red emission. Here, we show that the red UC emission in CaF₂ is generated mainly by an unusual two-photon UC mechanism that enables efficient relaxation from the UC pumped green emitting state to the red emitting state via round trip energy transfer (RTET) between Er³⁺ and Yb³⁺. The efficient RTET in CaF₂ is attributed to the unique characteristic of CaF₂ in which rare earth ions are prone to clustering. The contribution of RTET to red UC emission is evaluated for various Yb³⁺ and Er³⁺ concentrations based on analysis of spectral and temporal behavior of luminescence. The results indicate that RTET dominates red UC emission for a wide range of dopant concentrations in CaF₂:Er³⁺/Yb³⁺. Furthermore, the origin of the commonly observed decrease of red-to-green intensity ratio on increasing Yb³⁺ concentration companied by UC luminescence color change from yellow to green in (Ca, Sr)F₂ is revealed based on the RTET model.

X-ray-excited persistent luminescence (XEPL) phosphors have garnered considerable attention in flexible X-ray delayed imaging. However, the poor chemical stability of the corresponding phosphors limits their practical applications. In this paper, a garnet phosphor, Lu₃Ga₅O₁₂:Eu³⁺ (LGG:Eu³⁺), with excellent chemical stability and outstanding XEPL performance, is reported. After 50 cycles of X-ray radiation, there is no significant decrease in the luminescence intensity of LGG:Eu³⁺. In addition, the LGG:Eu³⁺ afterglow phosphor has shallow traps with a depth of 0.91 eV and an afterglow duration exceeding 50 min. A flexible film composed of LGG:Eu³⁺ and polydimethylsiloxane (PDMS) was prepared for X-ray delayed imaging. Our findings prove that LGG:Eu³⁺ is a chemically stable XEPL phosphor for flexible X-ray delayed imaging techniques.

Current optical storage technologies utilizing phosphor media face challenges in achieving rapid and precise data recording with visible or infrared light, primarily due to the constraints of traditional charging techniques. Here, we introduce a cutting-edge method termed up-conversion charging (UCC) to address these challenges, enabling rapid and high-resolution data storage in phosphors. Our study focuses on the unique two-step ionization and non-linear charging characteristics of UCC in storage phosphors, specifically in a gallate composition Gd₃Ga₅O₁₂:Cr³⁺. Remarkably, this technique enables data writing with high solution, requiring only 0.01 s of exposure per bit when utilizing a portable laser engraver equipped with visible-emitting diode lasers. The present strategy not only enhances recording efficiency but also ensures long-term data retention and superior rewritability. Moreover, we illustrate the versatility of UCC storage across various material systems through thermally- and optically-stimulated luminescence. Our outcomes highlight the transformative potential of the UCC method in advancing optical storage applications, offering significant improvements in the development of information storage solutions.