Doped LiYF4 Research Articles

Theoretical evaluation on the possibility of laser cooling and simultaneous temperature sensing in Er3+ doped LiYF4 single crystal

In our previous work, a novel laser cooling method was developed using anti-Stokes fluorescence from thermal equilibrium levels of Er3+, with its efficacy proven in Er3+ doped ZBLAN glass. This study has extended the method to Er3+ doped LiYF4 single crystal, where anti-Stokes fluorescence from 2H11/2 is utilized by excitation at the 4S3/2 level of Er3+ for optical in situ temperature control. The non-radiative and radiative transition probabilities among 4f levels of Er3+ were assessed, and the populations of all levels relevant to laser cooling and temperature sensing were calculated. Results demonstrate that the Er3+ doped LiYF4 crystal achieves effective cooling within a specific temperature range, exhibiting high temperature sensitivity.

Luminescent characteristics at ∼2.9 μm in Lu3+ modified Dy3+/Tm3+ co-doped LiYF4 single crystal

The single crystals specimens comprising 0.3 mol% Dy3+, 0.5 mol% Tm3+ and γ mol% Lu3+ doped LiYF4 (γ = 0, 0.3, 0.5, and 1.5) were meticulously synthesized using the Bridgman method, with the specific aim of catering to applications in the ∼2.9 μm laser domain. The microstructure of the obtained single crystal was investigated by conducting X-ray diffraction (XRD) measurements. The Judd-Ofelt (J-O) intensity parameters of the Dy3+ ion and the impacts of Lu3+ doping on fluorescence characteristics of the 6H13/2 (Dy3+) level were studied. The Dy3+/Tm3+ doped LiYF4 single crystal exhibits ∼52 % enhancement in emission intensity at ∼2.9 μm (Dy3+: 6H13/2→6H15/2) through a favorable introduction of Lu3+ ions into the single crystal at 808 nm LD excitation. The Tm3+/Dy3+/Lu3+ triply doped single crystal exhibited a maximum emission cross section of 2.480 × 10−20 cm2 at 2904 nm. The fluorescent lifetimes of the Dy3+: 6H13/2 level in the single crystals were determined to be located in the range of 2.78 ms–3.33 ms. The microscopic mechanism of energy transfer (ET) between Tm3+ and Dy3+ ions were investigated and the ET coefficients from 3H4 level of Tm3+ ion to 6F5/2 of Dy3+ ion (ET1) and 3F4 of Tm3+ to 6H11/2 of Dy3+ (ET2) in the triply doped single crystal were calculated, yielding values of 3.422 × 10−39 cm6/s and 5.768 × 10−39 cm6/s, separately. The results clearly demonstrated that the mid-infrared luminescence at ∼2.9 μm from Dy3+ ions can be significantly enhanced by the strategic incorporation of appropriate Lu3+ ions into Tm3+/Dy3+: LiYF4 single crystal.

NIR activated Er3+/Yb3+ doped LiYF4 upconverting nanocrystals for photothermal treatment, optical thermometry, latent fingerprint detection and security ink applications

This work reports the upconversion (UC) emission studies on Er3+/Yb3+ doped LiYF4 nanoparticles for radiation-to-heat conversion, optical thermometry using thermally coupled and uncoupled levels, latent fingerprint detection and security ink applications. LiYF4:Er3+/Yb3+ nanoparticles have been synthesized using the thermal decomposition reaction method using oleic acid and 1-octadecane as reaction medium. Crystal phase, crystallinity and size have been investigated using XRD and TEM techniques. UC emission ability of the prepared phosphor was tested using a CW laser source having 980 nm excitation. Under 980 nm excitation, synthesized nanoparticles have shown emission bands around 522 nm, 550 nm and 668 nm corresponding to transitions from 2H11/2, 4S3/2, and 4F9/2 levels to 4I15/2 level of Er3+ ions, respectively. Further, the number of photons involved in the UC process has been calculated using a pump power UC study. Decay time analysis of the most intense green (550 nm) and red (668 nm) emission bands is performed to study the emission dynamics. A study on the radiation-to-heat conversion phenomenon has revealed photothermal conversion efficiency of 29.5% which is important for photothermal therapy. The green and red UC emissions from the sample under stimulation at very low pump power (35 mW) are used to study the optical temperature sensing characteristics. The findings of this study suggest that the Er3+/Yb3+ co-doped LiYF4 phosphor may be particularly useful for creating NIR to visible upconversion and temperature sensing. Moreover, latent fingerprint detection and security ink for anti-counterfeiting applications are also demonstrated.

Size‐Dependent Photon Avalanching in Tm3+ Doped LiYF4 Nano, Micro, and Bulk Crystals

AbstractPhoton avalanche (PA) is a highly nonlinear mode of upconversion that is characterized by 100–1000‐fold increase in luminescence intensity upon minute increments of pumping power. The practical realization of numerous possible nano‐bio‐technology applications utilizing the PA phenomenon will require information on its susceptibility to the material volume and surface. Here, these parameters are investigated via experimental and theoretical PA. The two‐color, highly nonlinear PA emission at 475 and 800 nm is clearly observed in bulk single crystal, individual microcrystals, and ensembles of colloidal core and core–shell nanoparticles of LiYF4 host doped with either 3 or 8% of thulium ions. The properties of PA emission, such as PA nonlinearity, PA gain, PA intensity, and luminescence kinetics in these materials show dependence on crystal volume and surface quenching. Theoretical simulations provide understanding of key physical processes that influence PA performance. Moreover, photon avalanche single beam super‐resolution imaging is realized for the first time in 3% Tm3+ doped LiYF4 core–shell nanoparticles. The obtained insights and predictions form a solid background for further development and applications of new optimized PA materials.

Bulk-like emission in the visible spectrum of colloidal LiYF4:Pr nanocrystals downsized to 10 nm.

Though Pr3+ doped LiYF4 (LiYF4:Pr3+) bulk crystals are a well-known laser gain material with several radiative transitions, their nanocrystal counterparts have not been investigated with regards to these. Through downsizing to the nanoscale, novel applications are expected, especially in composite photonic devices. For example, nanocrystals in stable colloidal form with narrow size distribution are highly desirable to reduce scattering in such composites. Herein, we synthesized monodispersed LiYF4:Pr3+ nanocrystals having a size of 10 nm resulting in colorless clear stable colloidal dispersions and conducted an extensive optical characterization for the first time. We observed unexpected yet intense emission with excited state lifetimes comparable to bulk crystals in the visible spectrum through excitation at 444 nm and 479 nm. In macroscopic bulk crystals, this emission is only exploitable through excitation of a different, subjacent energy level. A comprehensive comparison to the bulk crystals provides deeper insight into the excitation mechanism and performance of these nanocrystals. The presented results pave the way for developing application-oriented LiYF4:Pr3+ nanocrystals as emitters with tailored properties for quantum optics or biomedical applications.

Luminescence properties of LiYF4:Yb3+, Er3+ phosphors: A study on influence of synthesis temperature and dopant concentration

Yb3+ and Er3+ doped LiYF4:RE3+ (RE = Yb, Er) phosphors have been synthesized using solid state reaction of pre-converted rare earth fluorides and lithium fluoride. Structural and morphological properties of the synthesized fluoride phosphors are examined. Upconversion emission studies performed in the developed phosphors showed blue, green and red emission bands under 980 nm excitation. Effect of Yb3+ ions concentration and synthesis temperature on the upconversion emission bands has been investigated. As the synthesis temperature increased the phosphors exhibited intense upconversion luminescence. The colour co-ordinate analysis confirmed the strong green emission under upconversion process. Luminescence in synthesized phosphors have also been investigated with respect to 378 nm excitation. The prepared phosphors have shown good luminescent properties and could be a potential candidate for luminescent based device fabrications.

Examination of Judd-Ofelt calculation and temperature self-reading for Tm3+ and Tm3+/Yb3+ doped LiYF4 single crystals

To validate the reliability of Judd-Ofelt results and the influence of involving absorption transition number, the Judd-Ofelt calculations, in which various transitions were adopted, were carried out for Tm3+ doped LiYF4 single crystal. It was found that introducing more transitions into the calculation procedure might get more reliable results. In order to clarify the feasibility of temperature self-reading in Tm3+/Yb3+ doped LiYF4 single crystal during laser operation, the temperature sensing properties of the single crystal were studied. It was found that the fluorescence intensity ratio of 3F2+3F3→3H6 to 3H4→3H6 can be used for achieving better temperature detection, and the temperature sensitivity was found much better than that in other materials.

Ligand sensitized strong luminescence from Eu3+-doped LiYF4 nanocrystals: a photon down-shifting strategy to increase solar-to-current conversion efficiency.

Lanthanide (Ln)-doped nanocrystals generally display low luminescence quantum efficiency due to forbidden nature of the 4f-4f transition besides possessing low absorption cross sections (∼10 M-1 cm-1). Considering the demand for these materials, particularly for light emission and bioimaging applications, it is very important to improve their quantum efficiency. This work demonstrates a strategy to enhance Si solar cell efficiency via sensitization of Eu3+ ions luminescence from colloidal nanocrystals. We have for the first time developed a simple ligand exchange approach to attach 4,4,4-trifluoro-1-phenyl-1,3 butanedione (TPB) to the surface of Eu3+-doped LiYF4 nanocrystals (NCs). Owing to the good overlap between the emission of the TPB ligands and the energy levels of Eu3+ ions, an efficient energy transfer takes place from the ligand to Eu3+ ions upon ultraviolet (UV) excitation of the ligand, leading to intense red emission. The sensitization of Eu3+ ions greatly enhanced the quantum yield of Eu3+ ions (∼31%) compared to the ∼5% obtained via direct excitation of Eu3+ ions (λexi = 394 nm) in Eu3+-doped LiYF4 NCs. A device was fabricated by embedding the nanocrystals on a Si solar cell to capture the UV photons and convert them into visible ones, which subsequently creates charge carriers inside the cell. Upon exposure to UV light, the nanocrystal embedded Si solar cell shows overall enhancement in the photocurrent upon excitation under UV radiation.

Near and Mid-Infrared Emission Characteristics of Er³⁺/Tm³⁺/Ho³⁺-Doped LiYF₄ Single Crystals Excited by Laser Diode.

Yttrium lithium fluoride (LiYF₄) single crystals triply doped with Er³⁺/Tm³⁺/Ho³⁺ are synthesized by a vertical Bridgman method. Absorption spectra, emission spectra, and decay curves are measured to investigate the luminescent properties of the crystals. Compared with Er³⁺ singly doped and Er³⁺/Tm³⁺ and Er³⁺/Ho³⁺ doubly doped LiYF₄ crystals, an intense emission around 2.7 µm can be obtained in the triply doped LiYF4 crystal under excitation of 980 nm laser diode. Meanwhile, the near infrared emission at 1.5 µm from Er³⁺ in the triply doped crystal is effectively reduced. The possible energy transfer processes and the luminescent mechanisms for enhancing emission at 2.7 µm and quenching emission at 1.5 µm in the Er³⁺/Ho³⁺/Tm³⁺ triply doped crystals are proposed. The large energy transfer efficiency of 82.0% and excellent optical transmission indicate that this Er³⁺/Tm³⁺/Ho³⁺ triply doped crystal can be considered as a promising material for a mid- infrared laser at 2.7 µm.

Growth and Optical Spectra of Tb³⁺/Sm³⁺/Ce³⁺ Triply Doped LiYF₄Single Crystals for White-Light LEDs.

The Tb³⁺/Sm³⁺/Ce³⁺ triply doped LiYF4 single crystals were grown by a modified Bridgman method. The absorption spectra, excitation spectra, and fluorescence spectra of Tb³⁺/Sm³⁺/Ce³⁺ ions in LiYF₄ crystals were measured. The fluorescence spectra of several bands, mainly located at purplish blue ~413 nm (⁵D₃ --> ⁷F₅), yellowish green ~542 nm (⁵D₄ --> ⁷F₅), and red ~643 nm (⁴G₅/₂ --> ⁶H₉/₂), were observed under excitation of ultraviolet light. White light could be generated by the mixture of the multicolor lights. The luminous intensities varied slightly with the excitation wavelength from 300 nm to 400 nm and doping Tb³⁺/Sm³⁺/Ce³⁺ ion concentration. The chromatic- ity coordinates of the crystal could be modified by changing the excitation wavelengths and the concentrations of Tb³⁺/Sm³⁺/Ce³⁺ ions. A near-ideal white light emission could be obtained from 1.25 mol% Tb³⁺, 1.98 mol% Sm³⁺, 0.44 mol% Ce³⁺ triply doped LiYF₄ single crystal with chro- maticity coordinates of x = 0.3090, y = 0.3223, color temperature Tc = 6777 K, color rendering index Ra = 77 and color quality scale Qa = 76 under the excitation of a 374 nm light.

A 60W Tm:YLF Laser with Triple Tm:YLF Rods

A narrow linewidth stable high-power continuous-wave 3.5% Tm3+ doped LiYF4 (Tm:YLF) laser is reported. By using dual F–P etalons and three Tm:YLF rods in a cavity, laser output of ∼60 W at 1907.7 nm is obtained with a slope efficiency of 34.8%. The M2 factor is found to be ∼2.0 under an output power of 30 W. In addition, the relaxation oscillation and efficiency of the Tm:YLF laser are also studied. The relaxation oscillation of the Tm:YLF laser is improved obviously by setting the ratio of pump beam to oscillating laser beam as ∼1.5:1 and the efficiency is increased in comparison with the ratio of ∼1.3:1.

Temperature dependent infrared absorption, crystal-field and intensity analysis of Ce3+ doped LiYF4

Infrared absorption has been used to determine the crystal-field levels of the 2F7/2 excited multiplet of trivalent cerium doped into scheelite structure LiYF4 single crystals. A crystal-field analysis well accounts for a total of six experimentally observed energy levels and the ground state g-values as previously determined by electron paramagnetic resonance, whilst intensity simulations confirm the experimentally assigned level symmetries. Temperature dependent spectral line broadening measurements highlight the importance of coupling to low frequency phonon modes of the YF8 tetrahedron.

Tunable chemical release from polyester thin film by photocatalytic zinc oxide and doped LiYF4 upconverting nanoparticles.

Once manufactured or implanted, polyester release kinetics tend to be fixed with little modulation possible for optimal local chemical concentrations. Here, a typical implantable polyester was fabricated into thin films (∼50 μm thick) with additives of photocatalytic ZnO nanoparticles, lanthanide-doped LiYF4 nanoparticle upconverting nanoparticles, or a combination thereof and irradiated with either 6 mW ultraviolet (365 nm) light emitting diodes or 50 mW near-infrared (980 nm) laser diodes to induce polymer photooxidation. Irradiated polyester films with the aforementioned photoadditives had enhanced release kinetics up to 30 times more than nonirradiated, neat films with extended release times of 28 days. Near-infrared, ZnO-mediated photocatalysis had the highest light on/light off ratio release kinetics of 15.4, while doped LiYF4 upconversion nanoparticles paired with ZnO nanoparticles had the highest linear R(2) correlation of 0.98 with respect to duty cycle and release kinetics. Future applications of the technology will aim toward modulation of previously developed polymeric reagents/drugs for real-time, feedback-optimized release.

Rare earth doped LiYF4 single crystalline films grown by liquid phase epitaxy for the fabrication of planar waveguide lasers

This article reports the epitaxial growth process and the characterizations of thulium doped or praseodymium doped LiYF4 layers grown on pure LiYF4 substrates for the development of planar waveguide lasers. After having presented the key points of the growth process to achieve high quality crystalline layers, microstructural, chemical and optical characterizations of the epilayers are reported for various dopants compositions.

Intense 2.7 μm emission from Er3+/Tm3+/Pr3+ triply doped LiYF4 single crystal grown by Bridgman method

Laser crystals of Er3+ singly, Er3+/Tm3+, Er3+/Pr3+ doubly and Er3+/Tm3+/Pr3+ triply doped LiYF4 are grown by a vertical Bridgman method. The luminescent properties of the crystals were investigated. An intense emission at 2.7μm can be obtained by using 980nm laser diode excitation in the triply doped LiYF4 crystals. Meanwhile, the green and red up-conversion emission and near infrared emission at 1.5μm from Er3+ in the triply doped crystals were effectively restricted. The possible energy transfer processes and the luminescent mechanisms for the Er3+/Pr3+/Tm3+ triply doped crystals were proposed. Decay curve fitting using a Inokuti–Hirayama expression indicates a dipole–dipole energy transfer from Er3+ to Pr3+, and Er3+ to Tm3+, which is consistent with the expected cross-relation scheme. These results suggest that the Er3+/Tm3+/Pr3+ triply doped crystal may be considered as a promising material for a 2.7μm laser.

Growth of Ce:LiYF4 bulk single crystal with high Ce concentration by Cz method and the scintillation properties

AbstractCe 2% doped LiYF4 [Ce:LYF] bulk single crystal was grown by Czochralski method and the structural phase, crystallinity, optical properties and scintillation properties were investigated. Grown Ce:LYF bulk single crystal included some cracks in the initial part of crystal and meanwhile there were no visible cracks in the center and end parts. Full width of half maximum of X‐ray rocking curve on the plate crystal in center part of Ce:LYF bulk single crystal was 81.4 arcsec. Alpha‐ray position in the pulse‐height spectra under alpha‐ray irradiation was almost quarter that of Li‐glass(GS20). Decay times of the crystals in center and end parts were 69.9 and 58.3 ns, respectively (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stability of color centers in LiYF4 crystals at low temperatures

This paper presents the results of examining the stability of induced radiation defects in crystals at 15K. The following regularities have been established. Defects induced at low temperatures in pure and Nd3+ doped LiYF4 crystals are unstable: upon termination of radiation, even at 15K induced color centers are visibly destructed, exposure to the halogen lamp light increases the destruction speed significantly. Heating crystals up to 80K results in complete destruction of color centers induced at 15K.

Efficient continuous wave deep ultraviolet Pr3+:LiYF4 laser at 261.3 nm

We report continuous wave deep ultraviolet laser operation at 261.3 nm by intracavity frequency doubling of laser diode pumped Pr3+ doped LiYF4 lasers. Using two InGaN laser diodes with a maximum output power of about 1 W each, a 2.9 mm long Pr3+:LiYF4 crystal and a 5 mm long beta-barium-borate crystal, coherent radiation with an output power of 481 mW at 261.3 nm was achieved at an optical-to-optical efficiency of 26.1% with respect to the total pump power. Both, linear and nonlinear losses have been calculated to be approximately 0.6% by using a model for intracavity frequency doubling with focussed Gaussian beams.

Parameterizing intensity of [formula omitted] electric-dipole transitions in Pr3+ doped LiYF4

A parametrization method is reported to calculate the electric-dipole transition intensities within the 4f2 configuration in LiYF4:Pr3+ by considering the main perturbing component of 4f5d. On the basis of calculating the energy level of the 4f5d configuration, the main 4f5d components that can mix with the 4f2 transitional states are determined. We exhibit the calculated densities of the 4f5d states which can mix with the initial and the final states of 3P0 energy level in Pr3+ doped LiYF4. Whereafter a new set of phenomenological intensity parameters, Tkq, is put forward. In addition to the explicit static-coupling mechanism, the approach is extended by the non-explicit dynamic mechanism. Detailed description of the calculated method is brought out. The results of present fitting are compared with that obtained by the traditional parametrization scheme and obvious improvements are exhibited.

High-power diode-pumped Tm:YLF laser

A high-power, continuous-wave 3.5% Tm3+ doped LiYF4 (Tm:YLF) laser has been developed. Using two Tm:YLF rods in a single cavity, 55 W of laser output at 1910 nm was obtained with a slope efficiency of 49%. The M2 factor was found to be <3. With a single Tm:YLF rod, a maximum laser power of 30 W was obtained with a slope efficiency of 50%. The laser was tuned to the peak absorption wavelength of Ho:YAG of 1907.5 nm by an intracavity quartz etalon with an output power loss < 1 W.