Hao Wu

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.

Controlling UV emission through upconversion luminescence (UCL) presents an exciting pathway for developing secure photonic technologies, yet conventional single‐wavelength excitation methods leave encoded features vulnerable to replication. Here, a dual‐wavelength excitation strategy is proposed, in which UV emission is generated via excited‐state absorption (ESA) upon simultaneous illumination with infrared and visible light. Using a model system based on Tm ³⁺ ‐doped Y ₃ Ga ₅ O ₁₂ , this approach enables distinct UV emissions (e.g., at 294, 350, or 365 nm) under concurrent 800 nm and blue‐violet excitation. Spectroscopic investigations reveal that 800 nm light pre‐populates long‐lived intermediate states ( ³ H ₄ and ³ F ₄ ), enabling low‐power visible photons to reach the UV‐emitting levels ( ¹ I ₆ or ¹ D ₂ ). Remarkably, this process operates at exceptionally low visible‐light power thresholds (∼0.05 mW∙cm ⁻² ), comparable to typical indoor lighting levels. Additionally, by co‐doping the material with Gd ³⁺ ions, we demonstrate the capability to further tailor the UV emission profiles, thereby offering increased flexibility for applications such as multi‐mode UV anti‐counterfeiting. These findings suggest that our dual‐wavelength excitation strategy could serve as a complementary addition to existing UV encoding techniques, providing an extra layer of photonic security.

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.

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.

Round-trip energy transfer (RTET) in the popular Er³⁺/Yb³⁺ upconversion (UC) system is a newly discovered mechanism for the red emission of Er³⁺ through Yb³⁺ as an intermediate ion. However, the importance of the RTET still remains a question. Here, we show in cubic Y₂O₃ that the new mechanism defeats conventional ones and dominates the red emission in both UC and down-shifting (DS) luminescence for a wide concentration range of Yb³⁺. The RTET enables the red UC emission to exhibit a similar time evolution as the green UC emission for Yb³⁺ concentration ≤10%, and its contribution to the red DS emission exceeds 90% for Yb³⁺ concentration ≥10%. As a unique feature, the temporal profile of the entire RTET process is presented for the first time. Our findings provide new physical insight into the regulation of color and temporal behavior of UC luminescence in the Yb³⁺/Er³⁺ system.

Deep traps are needed for electron‐trapping‐based long‐term optical storage due to its resistance to thermal erasure. Current electron trapping materials have the erasing temperatures hardly beyond 600 K, limiting its storage time. Herein, an electron trapping material containing deep traps is achieved by co‐doping Tb and Eu in Y₃Al₅O₁₂ via solid‐state reaction in reducing atmosphere. After 254 nm UV charging, strong thermoluminescence of Tb³⁺ with the glow curve peaking at 600, 693, and 765 K is observed. The comparative studies of the as‐made and the air‐annealed samples suggest that these new traps are related to the complex defects containing oxygen vacancy with adjacent Eu²⁺ and the charging process is the photoionization of Tb³⁺ with subsequent electron trapping. The 693 and 765 K thermoluminescence glow peaks in the charged sample show almost no decrease during 108 h storage in dark at room temperature. Images write‐in and read‐out via 808 nm laser stimulation are realized. Furthermore, the present phosphor also exhibits temperature‐sensitive afterglow spectra in the range of 553–803 K with the sensing span of 250 K wider than other afterglow thermometers. These findings indicate the great application potentials of Y₃Al₅O₁₂:Tb,Eu phosphor in long‐term optical storage and temperature sensing.