5I7 Level Research Articles

Influence of Pr3+ co-doping on the luminescent properties of CGA:2 at.% Ho single crystal fibers

Ho3+, Pr3+ co-doped CaGdAlO4 (Ho,Pr:CGA) crystal fibers were successfully grown by the micro-pulling-down (μ-PD) technique for the first time. Detailed investigations were conducted on the polarized absorption spectra, fluorescence spectra, and lifetime decay curves. The 2 at.% Ho, 0.4 at.% Pr:CGA crystal fiber exhibits a broad absorption band at 1147 nm with full width at half maximum (FWHM) of 25.5 nm and 64.5 nm for σ and π polarization, respectively. Increasing the concentration of Pr3+ led to a decrease in emission intensity at 2001 nm and an increase in emission intensity at 2861 nm. The FWHMs of the crystal fiber containing 2 at.% Ho and 0.4 at.% Pr:CGA at 2861 nm were 44.5 nm for σ polarization and 24.3 nm for π polarization. Additionally, the lifetime ratio (5I7/5I6) decreases from 25.18 to 2.85 in the crystal fibers containing 2 at.% Ho:CGA and 2 at.% Ho, 0.4 at.% Pr:CGA, respectively. These findings suggest that the presence of deactivated Pr3+ ions can significantly reduce the lifetime gap between the 5I6 and 5I7 energy levels, potentially mitigating the self-termination phenomenon on the ∼3 μm emission.

2-W Ho:LaF3 laser intracavity pumped with a Tm:YLF laser

In this paper, we explore the laser properties of a 2.1 µm Ho:LaF3 laser intracavity pumped by a LD-pumped Tm:YLF laser. Ho:LaF3 crystals, with a doping concentration of 2%, have been verified as a novel type of laser gain material, whose luminescence lifetime of the 5I7 level was up to 25.7 ms. The Ho:LaF3 laser achieved an average output power of 2.07 W with a slope efficiency of 16.2%. The dual-wavelength output centered at 2089 nm and 2093 nm and exhibited excellent spot quality with M2x and M2y values of 1.19 and 1.23, respectively. Our results effectively demonstrate the impressive capability of Ho:LaF3 crystals for generating high-power 2.1 µm mid-infrared lasers.

Efficiency improvement for Ho3+/Pr3+co-doped 3 μm fiber laser in AlF3-based glass fiber

Over the past few decades, significant advancements have been made in the development of 3 μm fiber lasers utilizing Ho3+/Pr3+ doped fluoride fiber, particularly in terms of slope efficiency and output power. While achieving an efficiency of ∼15 % for the 3 μm laser in AlF3 fiber with a Ho3+/Pr3+ doping ratio of ∼10:1, further improvements are necessary to approach the Stokes efficiency limit of ∼40 %. Our study focused on the influence of the Ho3+/Pr3+ doping ratio on 3 μm laser efficiency. We conducted experiments on Ho3+/Pr3+ co-doped glass and fiber with doping ratios ranging from 1:0.01 to 1:1. Luminescence decay lifetime tests of the Ho3+: 5I6 and 5I7 energy levels in the Ho3+/Pr3+ doped AlF3 glass samples revealed the primary energy transfer progress that affected the 3 μm laser efficiency. When pumped by a 1150 nm Raman fiber laser, a maximum slope efficiency of 29.3 % at 3 μm was obtained in a 1mol%Ho3+/0.6mol%Pr3+ co-doped AlF3 Fiber.

Numerical design of an efficient Ho3+-doped InF3 fiber laser at ∼3.2 ​μm

In this work, we theoretically unlock the potential of Ho3+-doped InF3 fiber for efficient ∼3.2 ​μm laser generation (from the 5F4,5S2→5F5 transition), by employing a novel dual-wavelength pumping scheme at 1150 ​nm and 980 ​nm, for the first time. Under clad-coupled 1150 ​nm pumping of 5 ​W, ∼3.2 ​μm power of 3.6 ​W has been predicted with the optical-to-optical efficiency of 14.4%. Further efficient power scaling, however, is blocked by the output saturation with 980 ​nm pumping. To alleviate this behavior, the cascaded 5I5→5I6 transition, targeting ∼3.9 ​μm, has been activated simultaneously, therefore accelerating the population circulation between the laser upper level 5F4,5S2 and long-lived 5I6 level under 980 ​nm pumping. As a result, enhanced ∼3.2 ​μm power of 4.68 ​W has been obtained with optical-to-optical efficiency of 15.6%. Meanwhile the ∼3.9 ​μm laser, yielding power of 2.76 ​W with optical-to-optical efficiency of 9.2%, is theoretically achievable as well with a moderate heat load, of which the performance is even better than the prior experimentally and theoretically reported Ho3+-doped InF3 fiber lasers emitting at ∼3.9 ​μm alone. This work demonstrates a versatile platform for laser generation at ∼3.2 ​μm and ∼3.9 ​μm, thus providing the new opportunities for many potential applications, e.g., polymer processing, infrared countermeasures, and free-space communications.

Polarized spectroscopic properties of Tm3+/Ho3+:YPO4 single crystal: A potential gain medium for tunable and ultrafast pulse laser at 2.0 μm

YPO4 crystals co-doped with Tm3+ and Ho3+ ions (Tm3+/Ho3+:YPO4) have been prepared using the high temperature solution method. The crystal structure, electronic band and polarized spectroscopy were characterized. It has intense and broad absorption bands around 800 nm with a full width at half maximum of 14.31 nm and 22.95 nm for π- and σ-polarizations, respectively. Based on Judd-Ofelt theory, relevant parameters about the radiative properties were systematically analyzed. The broad emission bands of Tm3+ and Ho3+ from 1600 to 2300 nm allow wide tunability of laser oscillation. The energy transfer efficiency from Tm3+: 3F4 to Ho3+: 5I7 level in YPO4 crystal was calculated to be 54.59 %. The results show that Tm3+/Ho3+:YPO4 is expected to be applied in tunable and ultrafast pulse laser at 2.0 μm.

Enhanced 2.8 μm emission of Ho,Pr:CaYAlO4 crystal

Ho,Pr:CaYAlO4 (Ho,Pr:CYA) crystal doped with 1 at.% Ho3+ and 0.1 at.% Pr3+ was successfully grown using the Czochralski method. Polarized absorption spectra, emission spectra, and fluorescence lifetime measurements were performed. The impact of Pr3+ co-doping on the ∼3 μm emission of Ho3+ ions was analyzed. The energy transfer efficiency from Ho3+:5I7 to Pr3+:3F2 energy level was calculated to be 60.66 %, indicating that Pr3+ effectively mitigated the self-termination effect in Ho,Pr:CYA crystals. The absorption cross-section at 645 nm was found to be 1.53 × 10−20 cm2 for σ polarization and 2.62 × 10−20 cm2 for π polarization. The emission cross-section at 2842 nm was calculated to be 0.86 × 10−20 cm2 for σ polarization and 1.33 × 10−20 cm2 for π polarization at 2908 nm, with a full width at half maximum (FWHM) of 139 nm and 88 nm, respectively. The fluorescence lifetimes of the 5I6 and 5I7 levels were measured to be 0.17 ms and 2.01 ms, respectively. These results collectively suggest that Ho,Pr:CYA crystals hold great potential as a candidate for 2.8 μm laser applications.

Upconversion luminescence through dynamically regulating the depletion of Ho3+: 5I6 level for speed sensing

In this work, a strategy for dynamically adjusting the upconversion luminescence (UCL) color of NaGdF4:Yb3+/Ho3+/Ce3+/Sc3+ is reported based on a phosphor wheel. It has been demonstrated that the rotation-dependent UCL mainly originated from the regulation of depletion mode for the Ho3+: 5I6 level. Due to the dominant linear decay, a high-pure red UCL is observed under the steady-state excitation. However, as the proportion of the steady-state excitation decreases, the green–red emission intensity ratio gradually increases, followed by the color conversion from red to green. An approximate physical model is proposed to understand the dependence of IG/IR on rotation speed. We not only report a UCL material that shows potential application in velocity sensing but also provide new insights into wheel-based dynamic UCL regulation.

Numerical analysis of cascade lasing in Ho:ZBLAN fiber lasers with efficient output at ∼750 nm

Recent advances in blue-emitting laser diodes (LDs) have enabled visible laser operation in rare-earth-doped fluoride fibers. Therefore, investigation of the power scaling of visible fiber lasers enabled by direct blue diode pumping is significant. In this study, we numerically investigated the performance of a continuous-wave (CW) Ho:ZBLAN fiber laser operating at approximately 750 nm for the 5F4,5S2→5I7 transition using a cascade lasing process. Direct diode pumping at 450 nm is used in conjunction with double-clad fiber geometry. Simultaneous lasing for the mid-IR 5I7→5I8 transition effectively depopulated the 5I7 level and significantly enhanced efficiency and power scalability. A slope efficiency of 49% was associated with the calculated CW output power of 14.2 W at a launched pump power of 30 W. This study demonstrates the significance of depopulating the lower laser level in the development of an efficient Ho:ZBLAN 750 nm fiber laser and offers an alternative approach for developing high-power visible laser sources.

The deactivation effects of Nd3+ ion for 2.85 μm laser in Ho3+/Nd3+ co-doped fluorotellurite glass

The 2.85 μm band has garnered significant attention for its wide range of applications in the mid-infrared region, and Ho3+ doped fluorotellurite fiber shows great promise as a gain medium for the 2.85 μm fiber laser. To achieve efficient population inversion for Ho3+ ions at 2.85 μm, Ho3+/Nd3+ co-doped fluorotellurite glasses with low hydroxyl were synthesized. The deactivation effect of Nd3+ ions to Ho3+: 5I7 levels was investigated through emission spectra and lifetime decay curves under 890 nm excitation. The results show that Nd3+ ions can effectively quench the Ho3+: 2.05 μm emission and help the Ho3+: 5I6 → 5I7 transition to overcome the bottleneck of particle population inversion. Ultimately, the particle population inversion corresponding to 2.85 μm luminescence was realized in the Ho3+/Nd3+ co-doped fluorotellurite glass, and indicates that a maximum of 1.64 W laser at 2.85 μm with a slope efficiency of 8.72 % can be realized under 890 nm pump by numerical simulations.

Optical spectroscopy of Ho3+,Pr3+ co-doped YScO3 crystal

YScO3 single crystal co-doped with Ho3+ and Pr3+ ions has been successfully grown by the Czochralaki method. The absorption spectrum, fluorescence spectrum and fluorescence decay curve were measured at room temperature. The absorption cross section of Ho,Pr:YScO3 at 649 nm and 1130 nm was calculated to be 0.93 × 10−20 cm2 and 0.32 × 10−20 cm2, respectively. Judd-Ofelt (J-O) theory was performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. Ho,Pr:YScO3 exhibited strong emission at 2040 nm and 2890 nm with a emission cross section of 0.80 × 10−21 cm2 and 1.22 × 10−20 cm2, respectively. The fluorescence lifetime of the Ho3+:5I6 and 5I7 levels were calculated to be 0.31 ms and 5.89 ms, respectively. All the results indicate that the Ho,Pr:YScO3 crystal could be a potential laser material for 2.9 μm laser operation.

Infrared to visible up-conversion luminescence of SrF2:Ho particles upon excitation of the 5I7 level of Ho3+ ions

The visible and near-infrared (NIR) up-conversion luminescence (UCL) of a concentration series of fluoride phosphors SrF2:Ho (CHo = 1.3, 3.5, 4.8, 7.4, 8.9, 10.8, 13.0, 13.6 mol.%) upon excitation of the 5I7 level of Ho3+ ions are presented. The dependences of the UCL intensity corresponding to the 5F3→5I8, 5S2(5F4)→5I8, 5F5→5I8, 5I4→5I8, 5F3→5I6, 5I5→5I8, 5F5→5I7, 5I6→5I8 transitions on the concentration of Ho3+ ions were revealed. The UCL energy yield, color coordinates, and correlated color temperature (CCT) values were estimated. The maximum values of UCL energy yields of 0.03% (380–780 nm) and 0.09% (380–1100 nm) were registered for SrF2:Ho (3.5 mol.%) sample. All studied fluoride phosphors are characterized by a red glow of UCL. This glow corresponds to CCT of 6711–7474 K upon incident power density of 28 W/cm2. The UCL mechanisms of SrF2:Ho phosphors were proposed. The SrF2:Ho phosphors can be used as laser beam viewers of the two-micron radiation of thulium and holmium commercial lasers.

Improved 2 µm broadband luminescence in Tm3+/Ho3+ doping tellurite glass.

Tm3+/Ho3+ doping tellurite glasses (TeO2-ZnO-La2O3) were prepared by applying melt-quenching technique, and the ∼2.0 µm band luminescence characteristics were examined. A broadband and relatively flat luminescence at 1600 to 2200 nm was observed in the tellurite glass co-doped by 1.0 mol% Tm2O3 and 0.085 mol% Ho2O3 under the excitation of 808 nm laser diode (LD), which is the result of spectral overlapping of 1.83 µm band of Tm3+ ions and 2.0 µm band of Ho3+ ions. Further, about 103% enhancement was acquired after the introduction of 0.1 mol% CeO2 and 7.5 mol% WO3 at the same time, which is primarily caused by the cross-relaxation between Tm3+ and Ce3+ ions together with the enhanced energy transfer from the Tm3+:3F4 level to Ho3+:5I7 level due to the increase in phonon energy. Spectral characteristics associated with the radiative transition of Ho3+ and Tm3+ ions on the basis of Judd-Ofelt theory, and the fluorescence decay behaviors after the addition of Ce3+ ions and WO3 component were analyzed to understand the broadband and luminescence enhancement. The findings in this work indicate that tellurite glass with optimal Tm3+-Ho3+-Ce3+ tri-doping combination and appropriate amount of WO3 is a prospective candidate for broadband optoelectronic devices operated in the infrared bands.

Effective enhancement of ∼2.86 µm mid-infrared emission of Na5Y9F32 single crystal co-doped Ho3+/Pr3+ introduced by Gd3+ ions

The optical spectral properties of high-quality Na5Y9F32 single crystals triply incorporated Ho3+/Pr3+/Gd3+ ions prepared successfully via a Bridgman technique were characterized with the help of their absorption spectra, emission spectra, and fluorescence lifetimes. It was discovered that the emission intensity at ∼2.86 µm of Na5Y9F32 co-doped Pr3+/Ho3+ single crystal was boosted obviously by introducing of Gd3+ ions, and it reached its maximum when the concentration of Gd3+ ion dopant was ∼0.7 mol%. The release of more luminescent centers due to the destruction of [Ho3+-Ho3+] ion quenching clusters and variation of the symmetry of crystal field around Ho3+ ions are responsible for the augmentation of ∼2.86 µm fluorescence emission. In addition, the introduction of Gd3+ ion reduced the fluorescence lifetime of Ho3+: 5I7 level, while the Ho3+:5I6 level remained slightly changed, which is beneficial to improve the particle inversion of Ho3+ ion between the 5I6 and 5I7 energy levels to enhance the ∼2.86 µm emission. The results showed that the triply doped fluoride single crystal can become a potentially prospective material for ∼2.86 µm mid-infrared lasers.

Singly Ho3+-doped tantalum tellurite glass and optical fiber for 2 μm fiber lasers

Singly Ho3+-doped TeO2–Ta2O5–ZnO (TTZ) glass and optical fiber for 2 μm fiber lasers have been investigated. The glass-forming region of the ternary TTZ glass system is determined to study its glass-forming ability and to select an appropriate glass matrix for doping Ho3+ ions. Upon the excitation of a 1940 nm laser diode (LD), an intense emission at 2.06 μm with a full width at half maximum (FWHM) of 61 nm is observed from the Ho3+-doped TTZ glass. The measured lifetime of the Ho3+: 5I7 level in TTZ glass shows a gradual decrease from 3.5 to 0.98 ms as the dopant concentrations increase from 0.2 to 2 mol%. A large emission cross-section (σeMC ∼ 4.56 × 10−21 cm2) and figure of merit (FOM, ∼ 6.83 × 10−24 cm2·s) are obtained. The Ho3+-doped TTZ glass is further fabricated into an optical fiber with an average transmission loss of 0.12 dB/cm at 1310 nm. The results show that singly Ho3+-doped TTZ glass and optical fiber are promising candidates for efficient 2 μm fiber lasers.

Crystal growth and spectroscopic analysis of Ho,Pr:Lu2O3 crystal for a 2.9 µm mid-IR laser

The EFG technique was used to successfully grow a crystal of Ho,Pr:Lu2O3. The crystal's spectroscopic features were investigated. Ho,Pr:Lu2O3 has an absorption cross-section of 0.47 × 10−20 cm2 at 647 nm and 0.20 × 10−20 cm2 at 1147 nm. The fluorescence characteristics of the grown crystal at 2.9 µm were examined using the J-O theory. Calculations were made for the intensity parameters Ωt (t = 2, 4, 6), radiative transition rates, branching ratios, and radiative lifetime. Ho,Pr:Lu2O3 demonstrated significant emission at 2104 and 2893 nm, with stimulated emission cross sections of 2.96 × 10−21 cm2 and 4.24 × 10−21 cm2, respectively. Ho3+ 5I6 and 5I7 levels were found to have fluorescence lifetimes of 0.55 ms and 3.23 ms, respectively.

Highly sensitive response of luminescence chromaticity to laser power in Lu2Mo4O15:Yb3+/Ho3+ upconverting materials.

Power-based upconversion luminescence color regulation (PUCR) is especially suitable for developing dynamic luminescence anti-counterfeiting owing to its straightforward usage. However, it remains a challenge to achieve visually remarkable Ho3+-based PUCR. Herein, favorable PUCR behavior is achieved by codoping Yb3+ and Ho3+ into the Lu2Mo4O15 lattice. It has been demonstrated that the ultrashort lifetime of the Ho3+:5I6 level and the anomalous three-photon nature of green emission are essential. The former causes high-purity red emission at low power, while the latter enables power-responsive tuning from red to green. Compared with Er2Mo4O15:4% Tm3+ we recently reported that Lu2Mo4O15:90% Yb3+/1% Ho3+, thanks to the high solubility of Yb3+ ions, showed a ∼25-fold enhancement in emission intensity. This new material is potentially applicable in dynamic luminescence anti-counterfeiting.

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Up-conversion luminescence spectra of CaF2-SrF2-HoF3 single crystals were investigated upon excitation by laser radiation with a wavelength of 1940 nm to the 5I7 level of Ho3+ ions. The CaF2-SrF2-HoF3 single crystals were grown by the vertical directional crystallization method. For these excitation conditions, quantitative characteristics of the up-conversion luminescence of Ho3+ ions were determined: energy yield, chromaticity coordinates, and correlated color temperatures. The possibility of increasing the efficiency of solar cells due to the conversion of infrared radiation in CaF2-SrF2-HoF3 crystals was considered.

Ce3+ induced broadband enhancement around 2 μm in Tm3+/Ho3+ co-doped tellurite glass

Tellurite glasses doped with Tm3+, Ho3+ and Ce3+ ions were prepared via melt-quenching to realise broadband and fluorescence enhancement in near-infrared (NIR) band. Under the pumping of a commercial 808 nm laser diode (LD), the emission bands at 2.0 μm, 1.85 μm, 1.47 μm, and 705 nm were observed in the Tm3+/Ho3+ co-doping glass samples, which originated from the transitions of Ho3+:5I7→5I8 and Tm3+:3F4→3H6, 3H4→3F4, 3F2,3 → 3H6, respectively. The existence of 2.0 μm band fluorescence is due to the energy transfer from the Tm3+:3F4 level to the Ho3+:5I7 level. This band overlaps with the 1.85 μm band which forms a broadband fluorescence spectrum in the range of 1600–2200 nm. In glass samples co-doped with Tm3+/Ho3+ with 0.085 mol% Ho2O3 and 1 mol% Tm2O3, the full width at half maximum (FWHM) of this broadband spectrum (1600–2200 nm) was as high as ∼370 nm. After introducing 0.6 mol% CeO2, the emission intensity of broadband fluorescence increased by ∼50%, which was caused by the cross-relaxations between Ce3+ and Tm3+ ions. The lifetime of fluorescence decay was determined to prove the interactions among the doped rare-earth ions, the radiative parameters such as transition probability, branching ratio and radiative lifetime were calculated from the absorption spectra based on the Judd-Ofelt theory to better understand the observed luminescence phenomena. In addition, X-ray diffraction (XRD) confirmed the amorphous state structure of the synthesised glass samples, while Raman spectrum revealed the different vibrational structural units forming the glass network.

Vacuum-Assisted Strong Luminescence Thermal Enhancement in NaYF4:Ho3+/Yb3+ Upconverting Nanocrystals: A Conclusive Evidence for the Effect of Water Desorption

The luminescence thermal enhancement (LTE), in lanthanide-doped upconverting nanocrystals (UCNCs), has attracted extensive attention for its applications in anticounterfeiting, temperature sensing, and so on. Some mechanisms for LTE were proposed, including thermally induced water desorption from the surface of UCNCs, but conclusive evidence is absent for any of the mechanisms. Here, vacuumizing is employed in the experiment for the LTE in NaYF4:Ho3+/Yb3+ UCNCs, and significantly promoted LTE is observed. The 300–450–300 K temperature cycling experiment in vacuum shows continuous luminescence enhancement without luminescence intensity recovery as in the cooling process in moisture air, indicating the effect of water desorption because water readsorption is blocked in vacuum. It is found that the Ho3+ 5I6 level can be deexcited by direct long-range energy transfer to water adsorbed at the surface of nanocrystals due to resonant coupling between the Ho3+ 5I6 → 5I7 transition and the −OH vibration of water. As a result, water desorption suppresses the 5I6 depopulation and thus enhances the luminescence of Ho3+. Finally, the regulation of upconversion LTE is realized by adjusting the size of nanocrystals and Ho3+ or Yb3+ concentrations. Our findings indicate that the vacuumizing technique is an effective method to distinguish water-desorption-induced LTE. This work deepens the understanding of LTE and offers insights into the regulation of LTE.

Growth, spectroscopy and CW laser operation of Ho3+ doped CaY0.9Gd0.1AlO4 single crystal

Ho3+-doped CaY0.9Gd0.1AlO4 single crystal was successfully grown by the Czochralski technique for the first time. The polarized absorption spectra, polarized luminescence spectra and fluorescence decay curve of the crystal were investigated at room temperature. The spectroscopic parameters are calculated based on Judd–Ofelt theory, the values of Ω2,4,6 were calculated to be 3.16 × 10−20 cm2, 3.93 × 10−20 cm2, 0.91 × 10−20 cm2, respectively. The Ho:CaY0.9Gd0.1AlO4 crystal exhibits strong emission at 2013 nm for π- and 2032 nm for σ-polarizations with stimulated emission cross section of 6.64 × 10−20 cm2 and 1.90 × 10−20 cm2, respectively. The fluorescence lifetime of the 5I7 level was measured to be 6.60 ms. The continuous-wave (CW) laser operation of Ho:CaY0.9Gd0.1AlO4 crystal around 2 μm was demonstrated.