Reversible luminescence modulation and temperature-sensing properties of Pr3+/Yb3+ codoped K0.5Na0.5NbO3 ceramics

In this work, we have prepared a novel (K_0.5Na_0.5)_0.99-xPr_xYb_0.01NbO₃ (abbreviated as KNN:xPr³⁺/0.01Yb³⁺, x = 0.0006, 0.0008, 0.001, 0.002, 0.003, and 0.004) ceramics, which possess visible UC emissions, photochromic (PC) and optical thermometric properties. Under the excitation of a 980-nm diode laser, all the samples show the featured emissions of Pr³⁺ ions and the UC emission intensity is greatly dependent on the Pr³⁺ doping content. The optimal UC luminescence intensity is obtained at x = 0.001. All the prepared samples show a strong PC reaction, and a large luminescence quenching degree (ΔR_t) of 74.94% is found. The optical thermometric properties of both the irradiated and unirradiated KNN:0.001Pr³⁺/0.01Yb³⁺ ceramics in the temperature range of 123-573 K have been investigated via measuring the temperature-dependent UC emission spectra of green emissions, which originate from the two ³P₁ and ³P₀ thermally coupled levels. It has been found that the prepared samples have both excellent PC behaviors and temperature-sensing performances. These results suggest that the KNN:xPr³⁺/0.01Yb³⁺ ceramics are promising candidates for the applications in PC reaction and thermometers.     

The upconversion luminescence modulation and its enhancement in Er3+‐doped Na0.5Bi0.5TiO3 based on photochromic reaction

A large and reversible upconversion (UC) luminescence modulation has been found in the Na_0.498Bi_0.498TiO₃:0.002Er (NBT:0.002Er) based on the photochromic reaction. The dependence of luminescence modulation of the ceramics on the wavelength of irradiation light and sintering temperature was investigated. It was found that the optimized sintering temperature and irradiation wavelength were 1130°C and 405 nm, respectively. The highest ΔR_t (defined as: ΔR_t = (R₀ – R_t)/R₀×100(%), where R₀ and R_t are the initial emission intensity and that after different irradiation time, respectively) value of 44.9% was obtained for the ceramics sintered at 1130°C after irradiation at 405 nm. More importantly, for the poled ceramics, ΔR_t value was promoted to a high value of 75.5%, which was 168% of that of the unpoled ones. The mechanism of luminescence modulation and its enhancement via electric field poling were discussed. This study demonstrated that electric field poling was an effective strategy to enhance the PC reaction in the NBT ceramics.     

Photoluminescence Properties of Er/Pr‐Doped K0.5Na0.5NbO3 Ferroelectric Ceramics

Er/Pr‐doped K_0.5Na_0.5NbO₃ ceramics have been fabricated and the effects of Pr³⁺ on their photoluminescence properties have been investigated systematically. The visible upconversion emissions, near‐infrared and mid‐infrared downconversion emissions of Er³⁺ ions under the excitation of 980 nm have been studied in detail. The effects of Pr³⁺ on PL properties and energy‐transfer processes have also been elucidated. By selecting an appropriate excitation source, simultaneous visible downconversion emissions of Er³⁺ and Pr³⁺ ions can be realized, and the emission colors of the ceramics can be tuned via the concentration of Pr³⁺ ions in a wide range from yellowish green to yellow. Our results also reveal that the photoluminescence emissions of the ceramics can be enhanced by the alignment of polarization of the ferroelectric host.     

Green and red upconversion luminescence of Er-doped K0.5Na0.5NbO3 ceramics

Er³⁺ doped K_0.5Na_0.5NbO₃ (KNN) lead-free piezoelectric ceramics were synthesized by the solid-state reaction method. The upconversion emission properties of Er³⁺ doped KNN ceramics were investigated as a function of Er³⁺ concentration and incident pumping power intensity. Bright green (~555 nm) and red (670 nm) upconversion emission bands were obtained under 980 nm excitation at room temperature, which are attributed to (²H_11/2, ⁴S_3/2)→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions, respectively. The upconversion emission intensity can be adjusted by changing Er³⁺ concentration, and the mechanism of upconversion processes involve not only a two-photon absorption but also a three-photon absorption. In addition to the admirable intrinsic piezoelectric properties of KNN, this kind of material may have potential application as a multifunctional device by integrating its upconversion and piezoelectric property.     

Photoluminescence,thermoluminescence and reversible photoluminescence modulation of multifunctional optical materials Pr3+ doped KxNa1-xNbO3 ferroelectric ceramics

It is highly significant to develop multifunctional optical materials to meet the huge demand of modern optics. Usually, it is difficult to realize multiple optical properties in one single material. In this study, we choose ferroelectric (K_xNa_1-x)NbO₃:Pr³⁺ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) as hosts, and the rare earth ions Pr³⁺ are doped in them. For the first time, the integration of photoluminescence, photochromism, luminescence modulation and thermoluminescence and has been achieved in the Pr³⁺ doped (K_xNa_1-x)NbO₃:Pr³⁺ ferroelectric ceramics. Upon 337- or 448-nm light irradiation, all samples show strong red emissions centered at 610 nm. The photochromic reaction increases with the increasing K⁺ content in the (K_xNa_1-x)NbO₃:Pr³⁺ ceramics. A strong photochromic reaction has been found in the (K_0.5Na_0.5)NbO₃:Pr³⁺ ceramics. Accordingly, a large and reversible photoluminescence modulation (ΔR_t = 50.71%) is achieved via altering 395-nm-light irradiation and 200 °C thermal stimulus. All the prepared ceramics show a visible thermoluminescence when stimulated at 200 °C. The mechanisms of luminescence modulation and thermoluminescence are discussed. Present study could provide a feasible paradigm to realize multiple optical properties in one single material.     

Reversible luminescence modulation of Ho‐doped K0.5Na0.5NbO3 piezoelectrics with high luminescence contrast

A significant luminescence modulation behavior based on photochromic reactions was observed in Ho³⁺‐doped (Na_0.52K_0.48)_0.92Li_0.08NbO₃ ceramics, fabricated by the conventional solid‐state reaction method. Under visible light irradiation (407 nm) for 20 second, the samples changed pale gray from initial pale green, and returned to their original color by a thermal stimulus of 230°C for 10 minutes, showing typical photochromic phenomenon. Under 453 nm excitation, the samples exhibited strong green emission at 551 nm. Interestingly, their green emission intensity can be effectively tailored by controlling photochromic reaction processes (irradiation wavelength and time), and the luminescent modulation ratio (ΔR_t) reaches up to 77%. And, the ΔR_t value has no any obvious degradation after 10 cycles by alternating visible light irradiation and thermal stimulus, showing excellent reversibility. These results make it potential applications in many fields as a kind of multifunctional material.     

Dual-mode optical ratiometric thermometer based on rare earth ions doped perovskite oxides with tunable luminescence

Rare earth ions doped luminescent materials have drawn considerable attention as they can generate both upconversion and downshifting emissions. Here, the rare earth ions Pr³⁺/Er³⁺ codoped perovskite oxide Bi₄Ti₃O₁₂ is proposed as a dual-mode temperature sensor and anti-counterfeiting material based on its up/down-conversion luminescence. Under 481 nm excitation, the intensity ratio of green emission (～523 nm in Er³⁺) and red emission (～611 nm in Pr³⁺) brings about a very high absolute sensitivity (S_a) of 2% K^?1 at 568 K and a maximum relative sensitivity (S_r) of 1.03% K^?1 at 478 K in the temperature range of 298–568 K. In addition, the upconversion green emissions of Er³⁺ yield a relatively-high S_r of 1.1% K^?1 at 298 K with 980 nm excitation, which can provide self-calibration coupled with down-conversion luminescence temperature sensing mode. Besides, this phosphor also shows tunable luminous colors for the potential application in the anti-counterfeiting field under various excitation wavelengths.     

Triple NIR light excited up-conversion luminescence in lanthanide-doped BaTiO3 phosphors for anti-counterfeiting

Up-conversion luminescent (UCL) materials are excellent candidate for optical anti-counterfeiting and the exploitation of multi-wavelength NIR light triggered UC phosphors with tunable color emission is essential for reliable anti-counterfeiting technology. Herein, a series of lanthanide ions (Er³⁺, Er³⁺–Ho³⁺, and Yb³⁺–Tm³⁺) doped BaTiO₃ submicrometer particles are synthesized through a modified hydrothermal procedure. XRD and SEM measurements were carried out to identify the structure and morphology of the samples and their UCL properties under 808, 980, and 1550 nm NIR excitation are investigated. Er³⁺ singly doped sample exhibits Er³⁺ concentration-dependent and excitation wavelength-dependent emission color from green to yellow and orange. The corresponding UC mechanisms under three NIR light excitation are clarified. Pure red emission under 1550-nm excitation was obtained by introducing small amount of Ho³⁺ and the fluorescent lifetime test was used to confirm the energy transfer from Er³⁺ to Ho³⁺. In addition, Yb³⁺–Tm³⁺ co-doped sample shows intense blue emission from ¹G₄ → ³H₆ transition of Tm³⁺ under 980-nm excitation. As a proof of concept, the designed pattern using phosphors with red, green, and blue three primary color emissions under 1550, 808, and 980 nm NIR excitation was displayed to demonstrate their anti-counterfeiting application.     

Two-wavelength excitation–driven robust operation of green up-conversion emission in Er3+-doped oxyfluoride glass–ceramic

Efficient optical modulation enables a significant improvement of optical conversion efficiency and regulation of optical response rate, showing great potential for optoelectronics applications. However, the weak interaction between photons poses a strong obstacle for manipulating photon–photon interactivity. Here, upon simultaneous excitation of 850 and 1550 nm, a fast–slow optical modulation of green up-conversion (UC) luminescence in oxyfluoride glass ceramics containing NaYF₄:Er³⁺ nanocrystals can be achieved. Compared with the sum of luminescence intensity excited by the two single-wavelengths, green UC luminescence excited by simultaneous two-wavelength presents an obvious increase by approximate six times. Interestingly, the response rate of green UC luminescence relies on the pump strategy of two-wavelength excitation, showing as high as two times of the fast–slow response difference. The fast–slow optical modulation of green UC luminescence under two-wavelength excitation is promising for emerging applications in all-optical switching.     

Novel strategy for the enhancement of anti-counterfeiting ability of photochromic ceramics: Sm3+ doped KSr2Nb5O15 textured ceramics with anisotropic luminescence modulation behavior

Sm³⁺ doped KSr₂Nb₅O₁₅ (KSN-Sm) textured ceramics with anisotropic photochromic and luminescence modulation behaviors provided a new strategy for the enhancement of anti-counterfeiting ability. The KSN-Sm textured ceramics were fabricated by the tape casting technology, which exhibited obvious grain-orientation, with Lotgering factor f_(00l) of 0.62. The textured sample possessed evident difference of reflectivity, photochromic, luminescent and luminescence modulation properties among various grain-orientated directions. The difference of luminescent emission intensity was over than 30 % and the luminescence modulation ratios △R_t are 75.3 % and 63.3 % along paralleled and vertical [00l] orientations, respectively. These optical anisotropies were attributed to the different refractive indexes, distributions of photochromic centers and energy transfer rates at various orientations. This work is hopeful to achieve the multidirectional data recording and enhancement of anti-counterfeiting ability of photochromic ceramics by the anisotropic properties of textured ceramics.     

Tunable photoluminescence properties of Pr3+/Er3+-doped 0.93Bi0.5Na0.5TiO3–0.07BaTiO3 low-temperature sintered multifunctional ceramics

Pr³⁺/Er³⁺-doped 0.93Bi_0.5Na_0.5TiO₃–0.07BaTiO₃ ceramics have been fabricated at a low sintering temperature of 960°C using a sintering aid of Li₂CO₃. The effects of energy transfer between Pr³⁺and Er³⁺on their photoluminescence properties have been investigated. Our results reveal that the down-conversion emissions of Pr³⁺are weakened and the lifetimes are shortened by the co-doping of Er³⁺. As a result, when both Pr³⁺and Er³⁺are excited simultaneously, with increasing the concentration of Er³⁺, the green emissions from Er³⁺increase but the red emissions from Pr³⁺decrease. Moreover, the emission color of the ceramics can be reversibly changed between red, yellow and yellowish green by using excitation sources of different wavelengths. Strong up-conversion green emissions with short lifetimes arisen from Er³⁺have also been observed for the ceramics under the excitation of 980 nm. Owing to the Li₂CO₃ sintering aid, the low-temperature sintered ceramics also exhibit reasonably good ferroelectric and piezoelectric properties, and hence should be promising for multifunctional applications such as electro-optical coupling devices.     

Dual-mode dichromatic SrBi4Ti4O15: Er3+ emitting phosphor for anti-counterfeiting application

Fluorescent materials have been widely used for anti-counterfeiting of important documents and currencies, wherein their anti-counterfeit abilities could be improved through multi-mode excitation. Herein, dual-mode-excited double-colour-emitting Er³⁺doped SrBi₄Ti₄O₁₅ up-conversion (UC) phosphors (SBTO: Er³⁺) were synthesised, and their UC spectra included green (2H11/2/4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) emissions from Er3+ ions under 980 or 1550 nm excitation. However, the green emission colour of phosphors was independent of dopant concentration under 980 nm laser irradiation; whereas the final emission colour was dominated by red emission and significantly affected by contents of Er3+ under 1550 nm excitation. These observations demonstrated potential application in dual-mode double-colour anti-counterfeiting. The possible UC mechanisms and emission characteristics of the phosphors using different 980 and 1550 nm irradiation source were contrastively investigated, and some fluorescent security patterns were also designed to demonstrate the potential applications in anti-counterfeiting and concealing important information.     

Preparation,luminescence properties,and application of temperature sensing and anti-counterfeiting of La2MgTiO6:Yb3+/Ln3+/Mn4+

The doping of transition metal ions in the up-conversion (UC) luminescent material doped with Yb³⁺/Ln³⁺ is a facile way to increase their UC luminescence intensities and alter their colors. In this study, La₂MgTiO₆:Yb³⁺/Mn⁴⁺/Ln³⁺ (Ln³⁺ = Er³⁺, Ho³⁺, and Tm³⁺) phosphors showing excellent luminescence properties were prepared by a solid-state method. The sensitivity of the La₂MgTiO₆:Yb³⁺/Ln³⁺/Mn⁴⁺ phosphor was double that without Mn⁴⁺, because Mn⁴⁺ affects the UC emissions of Ln³⁺ via energy transfer between these ions. Moreover, Mn⁴⁺ also acts as a down-conversion activator, which can combine with UC ions to achieve multi-mode luminescence at different wavelengths. Under 980 nm excitation, these samples emit green light (from Er³⁺ and Ho³⁺) and blue light (from Tm³⁺). In contrast, under 365 nm excitation, they emit red light (from Mn⁴⁺). Further testing revealed that the La₂MgTiO₆:Yb³⁺/Mn⁴⁺/Ln³⁺ phosphors have potential applications in temperature sensing and anti-counterfeiting.     

Tunable upconversion luminescence in new Ho3+/Yb3+-doped SrBi4Ti4O15 photochromic ceramics for switching application

Upconversion (UC) luminescence modulation is quite important in controlling and processing light for active components of light sources, photoswitches, optical memories, and optical sensing devices. In this work, we reported one kind of novel phosphor, Ho³⁺/Yb³⁺-doped SrBi₄Ti₄O₁₅ ceramics, which displayed both strong UC luminescence and obvious photochromic (PC) reaction. The UC luminescence, PC effect, and the modulation of UC performance based on PC behavior were investigated in detail. By alternating visible light irradiation and thermal stimulus, the UC luminescence could be reversibly regulated. Meanwhile, the modulation was unveiled to tightly rely on the irradiation time and thermal treatment processes. Excellent reproducibility was also achieved. In addition, as an alternative method to thermal treatment, the manipulation of luminescence by electric field was also explored. Finally, the mechanism related to the UC luminescence manipulation was illustrated. The results indicate that these samples could be potentially utilized in optical data storage and anti-counterfeiting security fields.     

Dual-mode optical thermometry and multicolor anti-counterfeiting based on Bi3+/Er3+ co-activated BaGd2O4 phosphor

Here, Bi³⁺, Er³⁺ co-activated gadolinium phosphors with multimode emission properties are prepared, which can emits blue, green, and orange light under the excitation of ultraviolet, 980 and 1550 nm, respectively. Moreover, BaGd₂O₄:Bi³⁺, Er³⁺ can show multicolor luminescence under different excitation conditions, such as pump light source, ambient temperature, working current, and other factors. Based on the dynamic luminescence characteristics, the dynamic anti-counterfeiting experiments are designed based on the phosphor. At the same time, the material also shows multimode temperature sensing characteristics. Under the excitation of 980 nm laser, three strong up-conversion signals Er³⁺ ions are generated at 528 nm (²H_11/2), 555 nm (⁴S_3/2), and 668 nm (⁴F_9/2), which have different temperature dependences. Based on the fluorescence intensity ratio between thermal-coupled energy levels (²H_11/2/⁴S_3/2) and nonthermal-coupled energy levels (²H_11/2/⁴F_9/2) of Er³⁺ ions, respectively, the dual-mode temperature thermometer was constructed with high-temperature sensitivity. In addition, the fluorescence lifetime of Bi³⁺ ions also has a strong temperature dependence, which can be used as another temperature detection signal, greatly improving the stability of thermometers under harsh conditions. Therefore, the material has a bright prospect in the field of anti-counterfeiting and temperature sensing.     

Concentration effects on the upconversion luminescence in Ho3+/Yb3+ co-doped NaGdTiO4 phosphor

Ho³⁺/Yb³⁺ co-doped NaGdTiO₄ phosphors were synthesized by a solid-state reaction method. The upconversion (UC) luminescence characteristics excited by 980 nm laser diode were systematically investigated. Bright green UC emission centered at 551 nm accompanied with weak red and near infrared (NIR) UC emissions centered at 652 and 761 nm were observed. The dependence of UC emission intensity on excitation power density showed that all of green, red and NIR UC emissions are involved in two-photon process. The UC emission mechanisms were discussed in detail. Concentration dependence studies indicated that Ho³⁺ and Yb³⁺ concentrations had significant influences on UC luminescence intensity and the intensity ratio of the red UC emission to that of the green one. Rate equations were established based on the possible UC mechanisms and a theoretical formula was proposed to describe the concentration dependent UC emission. The UC luminescence properties of the presented material was evaluated by comparing with commercial NaYF₄:Er³⁺, Yb³⁺ phosphor, and our sample showed a high luminescence efficiency and good color performance, implying potential applications in a variety of fields.     

Effect of ligand field symmetry on upconversion luminescence in heat‐treated LaBGeO5:Yb3+, Er3+ glass

In this paper, we report upconversion (UC) luminescence enhancement in LaBGeO₅:Yb³⁺, Er³⁺ glass‐ceramics (GCs), surface crystallized glass‐ceramics (SCGCs) and ceramics compared with the as‐melt glass fabricated by the conventional melt‐quenching technique. Based on structural investigations, we find that the nucleation and crystallization of trigonal stillwellite LaBGeO₅:Yb³⁺, Er³⁺ nanocrystals occur first at the glass surface before the following volume crystallization. The local site symmetry around rare earth (RE) ions which was evaluated using the Eu³⁺ ions as a probe together with Judd‐Ofelt theory calculations exhibits a clear increase with the devitrification of the glass. Consequently, complete crystallization of the glass leads to largest enhancement in the UC emissions of the LaBGeO₅:Yb³⁺, Er³⁺ ceramics. We ascribe the enhancement of UC luminescence in the LaBGeO₅:Yb³⁺, Er³⁺ GCs, SCGCs, and ceramics to the structural ordering and the improvement of site symmetry surrounding RE ions that minimizes the rate of nonradiative relaxation process.     

Luminescence modulation of the Eu3+ doped Srn+1SnnO3n+1 (n=1, 2, 5, ∞) ceramics based on photochromism and its application in anti-counterfeiting technology

The structure and optical properties of Sr_n+1Sn_nO_3n+1 ceramics greatly depend on the n value. Thus, we fabricated four compositions, namely Sr_n+1Sn_nO_3n+1:Eu³⁺ (n = 1, 2, 5, ∞) ceramics, and their crystal structure, photoluminescence, photochromism and luminesce modulation properties have been investigated. It is found that excellent photochromism and luminesce modulation properties are found in Sr₂SnO₄:Eu³⁺ and Sr₃Sn₂O₇:Eu³⁺ ceramics. After 280-nm light irradiation, the Sr₂SnO₄:Eu³⁺ ceramics transform into gray purple from primal white. Meanwhile, luminescence intensity decrement ratio ΔI_dec of the colored Sr₂SnO₄:Eu³⁺ reaches a high value of 80.8% under optimized irradiation wavelength. The decreased luminescence intensity of Eu³⁺ can be completely recovered via 450-nm light irradiation. The ΔI_dec of Sr₃Sn₂O₇:Eu³⁺ ceramic reaches 53.1%, and the decreased luminesce intensity can not be covered by light irradiation, only can be covered by a high temperature stimulus at 400 °C. Finally, we successfully fabricated a flexible membrane using Sr₂SnO₄:Eu³⁺ and PDMS for anti-counterfeiting applications.     

Pr3+/Yb3+:PLZT ferroelectric ceramics for near‐infrared radiation at 1340 nm

The near‐infrared luminescence properties of Pr³⁺/Yb³⁺:PLZT ferroelectric ceramics have been examined for the first time. Independently, upon either 450 nm (Pr³⁺) or 980 nm (Yb³⁺) excitation, luminescence centered at 1340 nm was observed, which corresponds to the ¹G₄→³H₅ transition of Pr³⁺. Several spectroscopic parameters for the ¹G₄→³H₅ transition of Pr³⁺ ions were determined. The average product of emission cross section and radiative lifetime were relatively large for all x/65/35 PLZT samples (x=6‐10) studied, with values close to 105±2 (×10^?26 cm²·s). These spectroscopic investigations indicate that Pr³⁺/Yb³⁺:PLZT ferroelectric ceramics are promising candidate for efficient sources emitting near‐infrared radiation at 1340 nm.     

Bright red upconversion luminescence from Er3+ and Yb3+ co-doped ZnO-TiO2 composite phosphor powder

ZnO-TiO₂ composites co-doped with Er³⁺ and Yb³⁺ ions were successfully synthesized by powder-solution mixing method and their upconversion (UC) luminescence was evaluated. The effect of firing temperature, ZnO/TiO₂ mixing ratio, and dopant concentration ranges on structural and UC luminescence properties was investigated. The crystal structure of the product was studied and calculated in detail by means of X-ray diffraction (XRD). Also, the site preference of Er³⁺ and Yb³⁺ ions in the host material was considered and analyzed based on XRD results and UC luminescence characteristics. Brightest UC luminescence was observed in the ZnO-TiO₂:Er³⁺,Yb³⁺ phosphor fired at 1300 °C in which the system consisted of mixed phases; Zn₂TiO₄, TiO₂, RE₂Ti₂O₇ and RE₂TiO₅ (RE = Er³⁺ and/or Yb³⁺). Under the excitation of a 980 nm laser, the two emission bands were detected in the UC emission spectrum, weak green band centered at 544 and 559 nm, and strong red band centered at 657 and 675 nm wavelengths in accordance with ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively. The simple chemical formula equations, for explaining the site preference of Er³⁺ and Yb³⁺ ions in host crystal matrix, were generated by considering the Zn₂TiO₄ crystal structure, its crystal properties, and the effect of Er³⁺ and Yb³⁺ ions to the host crystal matrix. The UC emission intensity of the products was changed by varying ZnO/TiO₂ mixing ratios, and Er³⁺ and Yb³⁺ concentrations. The best suitable condition for emitting the brightest UC emission was 1ZnO:1TiO₂ doped with 3 mol% Er³⁺, 9 mol% Yb³⁺ fired at 1300 °C for 1 h.