1.53 µm luminescence properties of Er3+-doped K–Sr–Al phosphate glasses

Trivalent erbium (Er³⁺)-doped K–Sr–Al phosphate glasses were prepared and studied their spectroscopic properties as a function of Er₂O₃ concentration. Judd–Ofelt analysis has been carried out for 1.0 mol% Er₂O₃-doped phosphate glass and in turn radiative properties have been evaluated for the excited levels of Er³⁺ ion. The radiative lifetime for the ⁴I_13/2 level was found to be higher for the present glass when compared to other Er³⁺-doped glasses. The Er³⁺-doped glasses exhibit intense near infrared emission at 1.53 µm corresponds to ⁴I_13/2→⁴I_15/2 transition as well as green emission at 546 nm corresponding to ⁴S_3/2→⁴I_15/2 under 980 nm and 488 nm excitations, respectively. The emission cross-section spectrum for 1.0 mol% of Er₂O₃-doped glass has been evaluated using McCumber theory. The gain cross-section has been evaluated as a function of population inversion, which revealed that the lasing action would be achieved at 1.53 µm for a population inversion about 40%. Decay curves for the ⁴I_13/2 level were measured and lifetimes have been determined for the studied glasses. The results indicate that the present glasses could be useful for laser as well as optical amplifiers at 1.53 µm.     

Effect of BaF2 Content on Luminescence of Rare‐Earth Ions in Borate and Germanate Glasses

Rare‐earth‐doped oxyfluoride germanate and borate glasses were synthesized and next studied using spectroscopic methods. Influence of fluoride modifier on luminescence properties of rare earths in different glass hosts was examined. The excitation and emission spectra of Pr³⁺ and Er³⁺ ions in the studied glasses were registered. The emission spectra of Pr³⁺ ions in germanate and borate glasses are quite different and depend strongly on the glass host. In samples doped with Er³⁺ ions emission bands located around 1530 nm corresponding to the main ⁴I_13/2→⁴I_15/2 laser transition were registered, independently of the glass host. Quite long‐lived near‐infrared luminescence of Er³⁺ ions was observed for germanate glasses with low BaF₂ content, while in borate glass systems influence of barium fluoride on luminescence lifetimes is not so evident. The Judd–Ofelt calculations were used in order to determine quantum efficiencies of excited states of rare‐earth ions in germanate and borate glasses.     

Near‐Infrared and Upconversion Luminescence in Er:Y2O3 Ceramics under 1.5 μm Excitation

Results of the spectroscopic characteristics and upconversion luminescence in Er³⁺ doped yttria (Y₂O₃) transparent ceramics prepared by a modified two‐step sintering method are presented. The near‐infrared (1.5 μm) luminescence properties were evaluated as a function of Er³⁺ concentration. Judd–Ofelt intensity parameters, radiative rates, branching ratios, and emission lifetimes were determined and compared with results reported for Er³⁺‐doped Y₂O₃ single crystal and nanocrystals. Following pumping at 1.532 μm, weak blue (~0.41 μm, ²H_9/2 → ⁴I_15/2), strong green (~0.56 μm, ²H_11/2, ⁴S_3/2 → ⁴I_15/2), and red (~0.67 μm, ⁴F_9/2 → ⁴I_15/2) emission bands were observed as well as weak near‐infrared emissions at 0.8 μm (⁴I_9/2 → ⁴I_15/2) and 0.85 μm (⁴S_3/2 → ⁴I_13/2) at room temperature. The upconversion luminescence properties under ~1.5 μm pumping were further investigated through pump power dependence and decay time studies. Sequential two‐photon absorption leads to the ⁴I_9/2 upconversion emission, whereas energy‐transfer upconversion is responsible for the emission from the higher excited states ²H_9/2, ²H_11/2, ⁴S_3/2, and ⁴F_9/2. The enhanced red emission with increasing Er³⁺ concentration most likely occurred via the cross‐relaxation process between (⁴F_7/2 → ⁴F_9/2) and (⁴I_11/2 → ⁴F_9/2) transitions, which increased the population of the ⁴F_9/2 level.     

Intense broadband 3.1 μm emission in Er3+-doped fluoroaluminate-tellurite glass for mid-infrared laser application

Er³⁺ single doped fluoroaluminate-tellurite glasses were made by employing a conventional melt-quenching technique. A strong fluorescence around 3.1 μm was achieved from Er³⁺-doped fluoride glasses, under a 980 nm laser diode pump, which was assigned to the Er³⁺: ⁴S_3/2 → ⁴F_9/2 radiation transition process. The up-conversion and mid-infrared spectra of emission for fluoroaluminate-tellurite glasses with various concentrations of Er³⁺ ions dopant was researched. In addition, the calculated fluorescence lifetime value about 3.1 μm reaches 0.48 ms. The findings indicate that fluoroaluminate-tellurite glasses doped with Er³⁺ have prospects of being developed into 3.1 μm mid-infrared fiber and laser materials.     

Near-infrared luminescent properties and natural lifetime calculation of a novel Er3+ complex

In this communication, a novel Er³⁺ complex Er(PT)₃TPPO [PT = 1-phenyl-3-methyl-4-tert-butylbenzoyl-5-pyrazolone, TPPO = triphenyl phosphine oxide] is successfully synthesized and characterized by elemental analysis and single-crystal X-ray diffraction. Its optical properties and the energy transfer process from the ligand PT to the Er³⁺ ion are investigated, the typical near-infrared (NIR) luminescence (centered at around 1530 nm) is attributed to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ion which results from the efficient energy transfer from PT to Er³⁺ ion (an antenna effect). The wider full width at half maximum (78 nm) peaked at 1530 nm in the emission spectrum and the Judd–Ofelt theory calculation on the radiative properties suggest that Er(PT)₃TPPO should be a promising candidate for tunable lasers and planar optical amplifiers.     

Broadened effect of Dy around 3 μm of Yb/Er/Dy: PbF2 crystal for broadband tunable lasers

It is well known that the laser crystal with mid-infrared (MIR) broadened and enhanced luminescence is of great significance for various applications, such as atmospheric monitoring, medical surgery and compact, and efficient coherent sources. Herein, we exploit the sensitization and deactivation effects of Yb³⁺/Dy³⁺ ions to achieve broadening and enhancement ~3 μm emission of Er³⁺: ⁴I_11/2→⁴I_13/2 transition in Yb³⁺/Er³⁺/Dy³⁺: PbF₂ crystal. The energy transfer (ET) mechanism between Yb³⁺, Er³⁺, and Dy³⁺ was studied. A broadened and enhanced emission with a full width at half maximum of 265 nm was obtained at ~3 μm due to the fact that Er³⁺ and Dy³⁺ ions were used as the emission center at the same time. On the one hand, the incorporation of Dy³⁺ ion can solve the self-termination bottleneck effect of Er³⁺ ion, reducing the lifetime of Er³⁺: ⁴I_13/2 level. On the other hand, Dy³⁺ ion can simultaneously serve as the emission center of 3 μm, broadening and enhancing the emission of 3 μm. The experiments show that the corresponding ET efficiency Er³⁺: ⁴I_13/2→Dy³⁺: ⁶H_11/2 level is as high as 98.0%, indicating that Dy³⁺ ion can be used as an effective deactivating ion, benefitting to achieve broadening and enhancing MIR emission around 3 μm. Hence, the Yb³⁺/Er³⁺/Dy³⁺: PbF₂ crystal is an attractive laser medium for MIR broadband tunable laser applications.     

Emission and Absorption Cross Sections at 810 and 860 nm Bands of Er3+ in LiNbO3 Crystal

We have measured the unpolarized and polarized emission spectra of ⁴I_9/2→⁴I_15/2 (810 nm) and ⁴S_3/2→⁴I_13/2 (860 nm) electronic transitions of Er³⁺ in LiNbO₃ crystal under different incident directions and polarization states of excitation beam. From the measured emission spectra, the emission and absorption cross‐section spectra were calculated based upon McCumber theory. It is found that Er³⁺ electronic transition shows interesting excitation beam direction effect in polarization dependence, spectral shape, and cross‐section value. Both transitions are highly π‐polarized as the excitation beam was aligned perpendicular to the optical axis of crystal while being highly σ‐polarized as the excitation beam was oriented parallel to the optical axis of crystal. The spectral shape in the case of the perpendicular excitation is very different from that in the case of parallel excitation. The cross‐section value in the perpendicular excitation case is at least 1.5 times larger than that in the parallel excitation case. These excitation direction effects are independent of the polarization state of excitation light, and are attributed to the selective Er³⁺ site excitation. In addition, the Er³⁺ 860 nm emission lifetime was measured to be 27 ± 5 μs and the quantum efficiency of the emission is 2.5%.     

Enhancement of up-conversion emission and emerging cool white light emission in co-doped Yb3+/ Er3+: Li2O-LiF-B2O3-ZnO glasses for photonic applications

A series of co-doped (Yb³⁺/Er³⁺): Li₂O-LiF-B₂O₃-ZnO glasses were prepared by standard melt quenching technique. Structural and morphological studies were carried out by XRD and FESEM. Phonon energy dynamics have been clearly elucidated by Laser Raman analysis. The pertinent absorption bands were observed in optical absorption spectra of singly doped and co-doped Yb³⁺/Er³⁺: LBZ glasses. We have been observed a strong up-conversion red emission pertaining to Er³⁺ ions at 1.0 mol% under the excitation of 980 nm. However, the up-conversion and down conversion (1.53 µm) emission intensities were remarkably enhanced with the addition of Yb³⁺ ions to Er³⁺: LBZ glasses due to energy transfer from Yb³⁺ to Er³⁺. Up-conversion emission spectra of co-doped (Yb³⁺/Er³⁺): LBZ glasses exhibits three strong emissions at 480 nm, 541 nm and 610 nm which are assigned with corresponding electronic transitions of ²H_9/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 respectively. Consequently, the green to red ratio values (G/R) also supports the strong up-conversion emission. The Commission International de E′clairage coordinates and correlated color temperatures (CCT) were calculated from their up-conversion emission spectra of co-doped (Yb³⁺/Er³⁺): LBZ glasses. The obtained chromaticity coordinates for optimized glass (0.332, 0.337) with CCT value at 5520 K are very close to the standard white colorimetric point in cool white region. These results could be suggested that the obtained co-doped (Yb³⁺/Er³⁺): LBZ glasses are promising candidates for w-LEDs applications.     

Realizing particle population inversion of 2.7 μm emission in heavy Er3+/Pr3+ co-doped low hydroxyl fluorotellurite glass for mid-infrared laser

In this work, the population bottleneck of Er³⁺: ⁴I_11/2 → ⁴I_13/2 was overcome for the first time in heavy Er³⁺/Pr³⁺ co-doped TeO₂–BaF₂–La₂O₃–LaF₃ (TBLL) low hydroxyl fluorotellurite glasses. Infrared emission spectra and fluorescence lifetime decay curves reveal that Pr³⁺ ions could deplete the electrons from the Er³⁺: ⁴I_13/2 level faster than those from the Er³⁺: ⁴I_11/2 under 980 nm excitation. Specifically, the energy transfer (ET) efficiency of the Er³⁺: ⁴I_13/2 → Pr³⁺: ³F_3,4 process (ET1) reached 96.27%, while that of the Er³⁺: ⁴I_11/2 → Pr³⁺: ¹G₄ process (ET2) is only 2.17% in the Er³⁺/Pr³⁺ co-doped glass. Additionally, the energy transfer mechanism of Er³⁺ and Pr³⁺ ions was investigated using the Dexter theory, where the energy transfer microscopic parameters C_D-A are 13.21 × 10⁻⁴⁰ cm⁶/s and 0.89 × 10⁻⁴⁰ cm⁶/s for the ET1 and ET2 processes, respectively. Finally, a numerical simulations laser model was developed to discuss the laser properties of the Er³⁺/Pr³⁺ co-doped TBLL fibers. The simulation results indicate that a 2.7 μm laser with a maximum output power of 2.26 W and slope efficiency of 13.89% could be achieved when the fiber background loss is reduced to 0.5 dB/m. The above results suggest that the Er³⁺/Pr³⁺ co-doped TBLL glass has great potential applications in mid-infrared fiber lasers.     

Spectroscopic and energy transfer mechanism of Er3+, Pr3+-codoped ZBYA glass

Absorption spectra, emission spectra and the rate parameters of the energy-exchange processes relevant to the ⁴I_11/2→⁴I_13/2 laser transition in Er³⁺/Pr³⁺- codoped ZBYA(ZrF₄–BaF₂–AlF₃–YF₃) glass were presented. Intensive 2.7 μm emission was obtained in the codoped glass and the optimized concentration ratio of Pr³⁺ to Er³⁺ was found to be 0.1:1. With the presence of Pr³⁺ ions, the intensities of the green and near-infrared emission were dramatically reduced to 1/15 and 1/21, respectively. The Er³⁺/Pr³⁺-codoped sample was found to have higher predicted spontaneous transition probability (16.57%) along with larger calculated emission cross section (14.6×10⁻²¹ cm²). These results suggest that the 2.7 μm emission of Er³⁺ ions could be achieved in ZBYA glass and codoping with Pr³⁺ could greatly improve the mid-infrared emission performance.     

Two micrometer fluorescence emission and energy transfer in Yb3+/Ho3+ co-doped lead silicate glass

Enhanced 2.0 μm and visible up-conversion emissions from Ho³⁺ via Yb³⁺ sensitization in lead silicate glasses have been obtained under the excitation of 980-nm laser diode. The possible energy transfer mechanism has been analyzed based on the photoemission spectroscopy and lifetime measurement. The lifetime of Ho³⁺: ⁵I₇ laser upper level has also been measured. Based on the absorption spectra, Judd–Ofelt parameters, spontaneous emission probability, the absorption, emission cross sections, and gain coefficients have been calculated and analyzed. The results indicate that the Yb³⁺/Ho³⁺ co-doped lead silicate glass has potential application in mid-infrared wavelengths.     

Preparation and mid-infrared 2.7 µm luminescence property of high content Er3+-doped (Y0.9La0.1)2O3 transparent ceramics pumped at 980 nm

High content Er³⁺ doped (Y_0.9La_0.1)₂O₃ transparent ceramics have been prepared by conventional ceramic process. Absorption spectra, mid-infrared, up-conversion and near-infrared emission spectra of Er³⁺ pumped at 980 nm have been investigated. The mechanisms of energy transfer processes have been discussed. Large values of Judd–Ofelt parameter Ω₂ (5.73 × 10^–20 cm²) and spectral quality factor X (3.71) have been obtained. The greatly enhanced green up-conversion emission in the high Er³⁺ doped sample is considered important for the applications in up-converters. The much enhanced mid-infrared 2.7 µm and up-conversion emissions, as well as the depressed near-infrared 1.5 µm emission demonstrate the efficient population inversion of Er³⁺:⁴I_11/2 → ⁴I_13/2 in high Er³⁺-doped ceramics for the 2.7 µm emission. These results suggest that high Er³⁺-doped (Y_0.9La_0.1)₂O₃ transparent ceramics are promising host materials for the applications of mid-infrared lasers and infrared-to-visible up-converters.     

Structural,optical, and spectroscopic properties of Er3+-doped TeO2–ZnO–ZnF2 glass-ceramics

Transparent fluorotellurite glass-ceramics have been obtained by heat treatment of precursor Er-doped TeO₂–ZnO–ZnF₂ glasses. ErF₃ nanocrystals nucleated in the glass-ceramics have a typical size of 45 ± 10 nm. Based on the Judd-Ofelt theory, the main radiative parameters for the ⁴I_13/2 → ⁴I_15/2 transition have been obtained. The split of the absorption and emission bands and the reduction of the Ω₂ parameter, as compared to the glass, confirm the presence of Er³⁺ ions in a crystalline environment in glass-ceramic samples. The analysis of the ⁴I_13/2 decays suggests that a fraction of Er³⁺ ions remains in a glass environment while the rest forms nanocrystals. For the glass-ceramics, intense red and green upconversion emissions were observed with an enhancement of the ⁴F_9/2 → ⁴I_15/2 red one compared to the glass sample. The temporal evolution of the red emission together with the excitation upconversion spectra suggests that energy transfer processes are responsible for the enhancement of the red emission.     

Optical Property of Dy3+‐and Ce3+‐Doped Si–B–Na–Sr Glasses

Novel Dy³⁺ and Ce³⁺ doped Si–B–Na–Sr (SBNS) glasses were synthesized by melt‐quenching technique. Excited by 327 nm, the 0.5Dy³⁺‐and 0.5Ce³⁺‐doped SBNS exhibits white emission with Commission Internationale de L'Eclairage coordinates of (0.308, 0.280). Basic optical characterizations have been performed by measuring the absorption and emission spectra and calculating Judd–Ofelt intensity parameters, radiative probability, luminescence branching ratio, cross sections, and effective bandwidth. The Judd–Ofelt parameters Ω₂, Ω₄, and Ω₆ indicate a high asymmetrical environment and covalent environment in the optical glass. The emission color of Ce³⁺ and Dy³⁺ codoped transparent glass can be tuned from blue to white through energy transfer from Ce³⁺ to Dy³⁺ ions. The resulting glass may have potential application in white‐light‐emitting source.     

Er3+ cross-section spectra of Er3+/Pr3+:Gd3Ga5O12 single-crystal: Pr3+-codoping effect

Fluorescence and absorption spectra at 530 nm (²H_11/2→⁴I_15/2), 560 nm (⁴S_3/2→⁴I_15/2), 660 nm (⁴F_9/2→⁴I_15/2), 980 nm (⁴I_11/2→⁴I_15/2), 1530 nm (⁴I_13/2→⁴I_15/2), and 2710 nm (⁴I_11/2→⁴I_13/2) of Er³⁺ in Gd₃Ga₅O₁₂ single-crystal codoped with Pr³⁺ have been measured. Judd-Ofelt analysis yields the intensity parameters Ω₂ = (0.68 ± 0.03) × 10⁻²⁰ cm², Ω₄ = (0.60 ± 0.07) × 10⁻²⁰ cm², and Ω₆ = (0.90 ± 0.17) × 10⁻²⁰ cm². A comparison with previously reported values of Er³⁺-only doping case shows that Pr³⁺-codoping causes slight change of both Ω₂ and Ω₄, while onefold increase of Ω₆. From calculated radiative rates and measured fluorescence spectra, Er³⁺ emission cross-section spectra were calibrated at first. Then, the absorption cross-section spectra were calculated using McCumber relation. In parallel, the absorption cross-section spectra were also obtained from the measured absorption spectrum, and compared with those obtained from the McCumber relation. The comparison shows that both methods give consistent result of absorption cross-section spectrum. Further comparison with Er³⁺-only doping case shows that Pr³⁺-codoping causes considerable change of Er³⁺ cross-section value. In spectrally mixing regions of Er³⁺ and Pr³⁺, Pr³⁺ emission affects little the determination of Er³⁺ emission cross-section as Pr³⁺ fluorescence is much weaker than Er³⁺ fluorescence due to low Pr³⁺ concentration.     

Sensitization effect of Nd3+ ions on Yb3+/Nd3+ co-doped oxyfluoride glasses and study of their optical,fluorescence, and upconversion abilities for visible laser and NIR amplifier applications

The development of laser technology has created intense demand for optical confinement materials with high performance. Herein the authors have been investigated Yb³⁺-singly doped and Yb³⁺/Nd³⁺-codoped SiO₂-based oxyfluoride glasses in terms of their optical absorption, and their near-infrared (NIR) and up-conversion (UC) emissions including emission decay profiles. Under 808 nm laser diode (LD) excitation, four NIR emission bands were observed i.e., (Nd³⁺: ⁴F_3/2 → ⁴I_9/2, Yb³⁺: ²F_5/2 → ²F_7/2, Nd³⁺: ⁴F_3/2 → ⁴I_11/2, and Nd³⁺: ⁴F_3/2 → ⁴I_13/2) in co-doped glasses. NIR emission cross-sections [(σ_emi) stimulated, (σ^M_emi) from Mc-cumber theory] were calculated for ²F_5/2 → ²F_7/2 (～1030 nm) transition of Yb³⁺ ion. σ_emi was found to be highest (26.27 × 10^?21 cm²) for the Yb³⁺: ²F_5/2 → ²F_7/2 transition in N2 glass. UC emission spectra recorded at 980 nm LD show bands centered at 500, 536, 595 & 610, and 664 nm, attributed to ⁴G_9/2 → ⁴I_9/2, ⁴G_7/2 → ⁴I_9/2& ⁴G_7/2 → ⁴I_11/2, ⁴G_5/2 → ⁴I_9/2, and ⁴G_9/2 → ⁴I_13/2 transitions, respectively. Decay profiles were analyzed for Yb³⁺: ²F_5/2 → ²F_7/2 (～1030 nm) and Nd³⁺: ⁴F_3/2 → ⁴I_11/2 (～1057 nm) transitions at 808 nm LD. Energy transfer (ET) process from Nd³⁺ to Yb³⁺ in present glasses were detailed.     

Characterization of luminescent properties of ZnO:Er thin films prepared by rf magnetron sputtering

ZnO:Er thin films were deposited on c-plane sapphire substrates by rf magnetron sputtering and annealed at 700 °C under air and H₂ atmospheres for the luminescent improvement. The effects of sputtering parameters and the annealing conditions on visible and 1.54 μm IR emissions were investigated. Structural and luminescent properties strongly depended on the deposition conditions and annealing atmospheres. By tuning the excitation wavelength, ZnO:Er thin films exhibited a strong emission band at around 465 nm and a weak emission at 525 nm originated from the energy transition of ⁴I_15/2–⁴F_5/2 and ⁴I_15/2–²H_11/2, respectively, while 1.54 μm IR emissions due to ⁴I_15/2–⁴I_13/2 transition.     

Intense 2.7 µm emission of Er3+/Pr3+ doped Ga5Ge20Sb10S65 chalcogenide glass

There are numerous vital usages for mid-infrared (MIR) lasers in satellite communication, biomedicine, military, remote sensing, and environmental monitoring. In this work, a progression of Er³⁺ ions doped, Er³⁺/Pr³⁺ ions co-doped Ga₅Ge₂₀Sb₁₀S₆₅ glasses were prepared, and their physical performances and structural characteristics were examined. To understand the non-phonon-assisted energy transfer mechanism, we recorded the up-conversion and infrared fluorescence emission spectra by pumping with a commercial 980 nm LD. Then the 2.7 µm strong fluorescence signal intensity can be obtained when the doped concentration of Pr³⁺ is proper. After the doping of Pr³⁺, fluorescence lifetime results revealed that the lifetimes of the Er³⁺:⁴I_13/2 level fell dramatically from 7.33 to 1.90 ms, which experienced a much more significant decrease in lifetimes than the Er³⁺:⁴I_11/2 level. The MIR fluorescence performances were assessed by the determined J–O parameters and relative emission cross sections. Additionally, the generally huge emission cross sections and the small pump energy show that it is possible to obtain population inversion with relatively small pump energy; thus the Er³⁺/Pr³⁺ glasses have great potential to be 2.7 µm laser materials.     

Ultrasensitive optical thermometer based on abnormal thermal quenching Stark transitions operating beyond 1500 nm

Light with wavelength longer than 1500 nm has great potential to afford deep bio-tissue penetration due to its extremely weak photon scattering and undetectable autofluorescence in vivo. Here, in order to satisfy the requirements for thermometry during the tumor hyperthermia process, an ultrasensitive optical thermometer operating beyond 1500 nm is developed by employing the thermally coupled Stark sublevels of Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition based on fluorescence intensity ratio (FIR) technology in Yb³⁺ and Er³⁺ codoped BaY₂O₄. Compared with the typical upconversion (UC) material β-NaYF₄: Yb³⁺/Er³⁺ and Y₂O₃: Yb³⁺/Er³⁺, BaY₂O₄: Yb³⁺/Er³⁺ shows more intense red Er³⁺: ⁴F_9/2 → ⁴I_15/2 transition and 1.5 μm near-infrared (NIR) Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition induced by its larger phonon energy and higher quenching concentration of Er³⁺. An equivalent four-level model is proposed to investigate the temperature characteristics of the NIR emission, from which four Stark transitions are separated from the raw spectra, named α, β, γ, and δ respectively. Then, the NIR thermal sensing performance have been developed by utilizing the FIR of I_β to I_α and I_γ to I_α. More importantly, an ultra-high sensitivity for optical thermometry has been obtained through the combination of transition β and γ, especially in the physiological temperature region. Furthermore, the detection depth of NIR light in bio-tissues is assessed by an ex vivo test, demonstrating that the maximal detection depth of NIR emission can reach to 8 mm without any influence on optical thermometry. These findings indicate that Yb³⁺ and Er³⁺ codoped BaY₂O₄ is a remarkable contender for optical thermometry in deep tissue with ultra-high sensitivity.     

Enhanced upconversion luminescence and optical temperature sensing performance in Er3+ doped BaBi2Nb2O9 ferroelectric ceramic

A series of BaBi_2-xNb₂Er_xO₉ ceramic compositions with different Er³⁺ concentration (x = 0.0–8 mol %) is synthesized by a conventional solid-state reaction method. The upconversion (UC) light emission under 980 nm excitation with different pump powers and luminescence-based temperature sensing ability of BaBi_2-xNb₂Er_xO₉ composition have been examined. The formation of a Bi-layered perovskite phase of BaBi₂Nb₂O₉ is confirmed having an orthorhombic geometry and Fmmm space group. Shifts in the Raman modes indicate reduced interaction of Bi³⁺ ions with NbO₆ octahedron leading to relaxation of structural distortion with increasing Er³⁺ content. The maximum value for remnant polarization and coercive field of doped BaBi_2-xNb₂Er_xO₉ ceramic for (x = 0.08) Erbium concentration comes out to be 2.9524 μC/cm² and 49.8980 kV/cm. For an optimum content of x = 0.04, two strong UC green emission bands were observed at 549 nm via ⁴S_3/2 → ⁴I_15/2 transition and 527 nm via ²H_11/2 → ⁴I_15/2 transitions, and a weak red emission appears at 657 nm attributed to the ⁴F_9/2 → ⁴I_15/2 transition. Pump power dependence suggests that UC emission is a two-photon mechanism for red and green emission bands. Temperature sensing evaluated by the change in the fluorescence intensity ratio (I₅₂₇/I₅₄₉) indicates the highest sensitivity to be 0.00996 K^?1 at 483 K for an optimum concentration of Er³⁺ at x = 0.04 in BaBi_2-xNb₂Er_xO₉ composition and is useful for non-contact optical thermometry.