LiYF4 Crystals Research Articles

Quantitatively Deciphering the Local Structure and Luminescence Spectroscopy of Pr3+-Doped Yttrium Lithium Fluoride.

Praseodymium (Pr3+)-doped LiYF4 nanophosphors have garnered significant interest for their potential applications in lasers, phosphors, and quantum memories. However, there remains a lack of comprehensive research on the local coordination environment and luminescence spectroscopy of Pr3+:LiYF4 nanocrystals. This study presents the first investigation of the ground-state structure of Pr3+:LiYF4 crystals by employing the crystal structure prediction method, and a [PrF8]5- ligand complex with S4 local symmetry is determined. The complete energy levels of the Pr3+ ions in LiYF4 nanocrystals are unveiled by using our newly developed well-established parametrization matrix diagonalization method. A novel set of free-ion and crystal-field parameters is derived through a good simulation with 45 experimental energy levels. Many of the emissions of Pr3+-doped LiYF4 are successfully reproduced based on Judd-Ofelt theory, and these transitions are comparable to the experimental ones. Moreover, two new prominent emission bands with their peaks at 675 and 849 nm originating from 1I6 → 3F4 and 1I6 → 1G4 transitions, respectively, are predicted by us for the first time. This study could provide a feasible method to search for practical laser transition channels of solid-state lasers based on Pr3+: LiYF4 nanophosphors.

High-power 2.3 µm Tm:YLF laser with intracavity upconversion pumping by a Nd:ASL laser at 1051 nm.

A Tm:LiYF4 laser operating on the 3H4 → 3H5 transition is embedded in a high-power diode-pumped Nd:ASL laser for intracavity upconversion pumping at 1.05 µm. This leads to a record-high output power at 2.3 µm for any bulk thulium laser pumped by an upconversion process. The continuous-wave Tm:LiYF4 laser delivers 1.81 W at 2.3 µm for 32 W of laser-diode pump power, making this kind of pumping competitive with direct diode pumping. The intracavity pumping process allows for counteracting the low absorption inherent to upconversion pumping and to dispatch the thermal loads on two separate laser crystals. The proposed laser architecture also features a relatively weak heating of the Tm:LiYF4 crystal and an increased tolerance to Tm3+ absorption. This laser design opens a new paradigm that holds great promise for high-power 2.3-µm solid-state lasers based on thulium ions.

Tm,Ho:YLF waveguide lasers at 2.05 µm.

In this work, we report on the first, to our knowledge, 2.05-µm laser based on femtosecond-laser direct written (FsLDW) Tm,Ho:YLF cladding waveguides. A channel waveguide with a 90-µm diameter "fiber-like" low-index cladding is fabricated in a 6 at.% Tm3+, 0.4 at.% Ho3+:LiYF4 crystal by FsLDW. Pumped by Ti:sapphire laser at 795.1 nm, the fabricated waveguide supports efficient lasing oscillation at 2050 nm with a maximum output power of 47.5 mW, a minimum lasing threshold of 181 mW, and a slope efficiency of 20.1%. The impacts of cavity conditions and polarizations of the pump light on the obtained lasing performance are well studied. The experimental results obtained in this study demonstrate the great potential of utilizing Tm,Ho:YLF and FsLDW for the development of durable mid-infrared lasers featuring compact designs.

"Rabi oscillations and ENDOR study of trivalent gadolinium ion in LiYF4 crystal "

"Rabi oscillations and ENDOR study of trivalent gadolinium ion in LiYF4 crystal "

Orange Sm:LiYF4 lasers emitting at 605 nm

We report on polarized spectroscopy and orange laser operation under 2ω-OPSL and GaN-diode pumping of Sm:LiYF4 crystals. The Samarium laser delivers 12 mW at 605 nm with a threshold of 51 mW and a linear polarization.

Orange surface waveguide laser in Pr:LiYF4 produced by a femtosecond laser writing.

Depressed-cladding surface channel waveguides were inscribed in a 0.5 at.% Pr:LiYF4 crystal by femtosecond Direct Laser Writing. The waveguides consisted of a half-ring cladding (inner diameter: 17 µm) and side structures ("ears") improving the mode confinement. The waveguide propagation loss was as low as 0.14 ± 0.05 dB/cm. The orange waveguide laser operating in the fundamental mode delivered 274 mW at 604.3 nm with 28.4% slope efficiency, a laser threshold of only 29 mW and linear polarization (π), representing record-high performance for orange Pr waveguide lasers.

Optical spectroscopy of the Er3+ ions in heavily doped BaY1.8Lu0.2F8:Er mixed crystals

Here we present the study of spectral and kinetic characteristics of Er3+ ions in the heavily doped BaY1.8Lu0.2F8 mixed crystals, which are homologous to well-known fluoride host BaY2F8 with high value of crystal field splitting.Absorption and luminescence spectra were registered, and the luminescence decay was studied in the IR spectral range for BaY1.8Lu0.2F8 crystals doped with Er3+ ions at concentrations of 20.0 at. % and 30.0 at. %. Based on the Judd-Ofelt theory, the intensity parameters were determined from the absorption spectra with the use of two approaches, the standard theory (J-O), and the intermediate configuration interaction (ICI). It is shown that investigated BaY1.8Lu0.2F8:Er3+ crystals propose the value of stimulated emission cross-section at the wavelength around 2.7 μm higher than that for Er-doped YAG and at the level of Er-doped LiYF4 crystals. In addition, it was shown that for BaY1.8Lu0.2F8 crystals with a high doping level of Er3+ (more than 20.0 at. %), the radiative lifetime of the 4I13/2 state becomes shorter than the lifetime of the 4I11/2 state, which speaks for success on efficient laser oscillation at a wavelength of about 2.7 μm (the 4I11/2 → 4I13/2 transition).

High-efficiency 261-nm continuous-wave laser by single-blue-laser-diode-pumped Pr3+:LiYF4 crystal

High-power continuous-wave (CW) deep ultraviolet lasers attract growing research interest in the field of optoelectronics. Herein, we perform a CW deep ultraviolet laser at 261 nm by the intracavity frequency doubling of a single-blue-laser-diode-pumped Pr3+:LiYF4 (Pr:YLF) laser. The excellent mode matching between the pump and fundamental-frequency laser beams is realized by optimizing the pump focusing and laser resonator systems. On that basis, the maximum CW fundamental-frequency laser obtained is 1.41 W at 522 nm, corresponding to a slope efficiency of 59.12%. Moreover, a deep ultraviolet laser is further demonstrated at 261 nm using a β-BaB2O4 (BBO) frequency-doubling crystal, yielding a maximum output power of 1.17 W and an optical–optical conversion efficiency of 33.4% with respect to the pump power. Such a result is the highest optical–optical conversion efficiency of the CW DUV lasers from Pr3+-doped crystals, to the best of our knowledge. The finding in this work manifests the development potential of all-solid-state deep ultraviolet lasers and provides a feasible avenue for achieving high-efficiency deep ultraviolet laser output.

Growth and characterization of optical and thermal properties of LiGdF4 single crystal

The opportunity of growing of several centimeters in length LiGdF4 single crystals by Bridgman–Stockbarger technique was demonstrated and the optically perfect LiGdF4 single crystals with 8 mm and 55 mm in length were grown. The phase composition, thermal conductivity, basic optical and spectral-kinetic properties of LiGdF4 single crystals were studied for the first time. For LiGdF4 crystal, the resulting linear thermal expansion coefficients α is 11% larger, than for both LiYF4 and LiYbF4 crystals. The obtained thermal conductivity dependence k(T) is more pronounced compared to LiLuF4 and LiYF4. The absorption spectrum of the LiGdF4 crystal shows absorption bands corresponding to the 8S7/2 – (6Dj, 6Ij, and 6Pj) transitions of Gd3+ ions. The measured room temperature lifetime of 6Pj multiplet of Gd3+ ions was equal to 4.25 ± 0.05 ms. The values of the temperature gradient of the LiGdF4 refractive index do not strongly depend on the wavelength.

Laser cooling in Yb:KY3F10: a comparison with Yb:YLF.

Laser cooling by anti-Stokes fluorescence is a technology to realize all-solid-state optical cryocoolers. We grew Yb3+-doped KY3F10 (Yb:KYF) crystals as novel laser cooling media and compare their cooling performance to Yb3+-doped LiYF4 (Yb:YLF) crystals also grown in our institute. We present temperature-dependent absorption and emission cross sections as well as the fluorescence lifetime of Yb:KYF, and calculate its material figure-of-merit for laser cooling. Yb:KYF exhibits a higher figure-of-merit than Yb:YLF at temperatures below 200 K. This is because, in contrast to Yb:YLF, the excitation transition from the second-highest Stark level of the ground state is best-suited for cryogenic cooling in Yb:KYF. Thus, it has the potential to achieve unprecedentedly low temperatures below the boiling point of liquid nitrogen. In this work, we observe the first laser cooling of Yb:KYF, and obtain a background absorption coefficient of ∼10-4 cm-1, which is among the lowest ever reported for Yb3+-doped fluoride crystals. A simple model calculation predicts that our Yb:KYF and Yb:YLF crystals can potentially be cooled down to ≈100 K in a high-power cooling setup. Our Yb:KYF crystals still leave room for further improvement through the optimization of the growth process and the use of purer raw materials.

Simultaneous red, green and blue laser operations in a Pr3+:LiYF4 crystal

We report on the first, to the best of our knowledge, simultaneous continuous-wave red, green and blue laser operations in praseodymium-doped crystalline materials. At an absorbed pump power of 4.7 W (or 10 W of incident pump power), the maximum output power was 665 mW, which included 607 nm (red), 523 nm (green) and 479 nm (blue) three wavelengths, and the optical conversion efficiency with respect to the absorbed pump power was 14.1%.

LiYF4 crystals embedded silicate glass with high electrochemical performance as an anode for lithium-ion batteries

To compensate for the loss of irreversible capacity of lithium-ion batteries, the silicate glass-ceramic containing LiYF4 crystals (YLFGC) is prepared by melting-annealing and then used as an active material to improve this situation. At the current density of 50 mA g−1, the specific capacity is 319.2 mAh g−1 while the efficiency is 99.4%. LiYF4 crystals provide more sites for the lithium-ion storage, on the other hand, it plays a synergistic effect with the disordered glass to promote the intercalation/deintercalation of lithium-ion. At the current density with 1 A g−1, there occurs an obvious phenomenon of capacity recovery from ca. 100 mAh g−1 to 200 mAh g−1 and finally stabilizes at 178 mAh g−1 with the efficiency stabilizing at 99 %, and the cycling stability of the anode is enhanced by the presence of LiYF4 crystals. This study provides a new material for improving the performance of anode for lithium-ion batteries.

High crystalline quality of hydrophilic LiYF4: Tm3+/Yb3+ particles prepared by fluorination of layered yttrium hydroxides under assistance of DTPA

Up-conversion emissions can be achieved by doping Tm3+ in hydrophilic LiYF4 crystal particles, with Yb3+ as a sensitizer. They are considered to be excellent up-conversion materials used in bioimaging. However, the crystalline quality of hydrophilic LiYF4 particles prepared by traditional methods is not satisfied and lowers the fluorescent performance. Herein, some additives were employed in preparation of LiYF4 crystals via fluorination transformation from layered yttrium hydroxides (LYHs). The reaction mechanism of additives was discussed in detail. Especially, diethylenetriaminepentaacetic acid (DTPA) can effectively promote the crystallization and Oswald ripening of LiYF4 crystals. The as-prepared LiYF4: Tm3+/Yb3+ up-conversion phosphors have a marvelously intense emission at 793 nm and would be beneficial to the application in bioimaging.

Differential absorption saturation in laser cooled Yb:LiYF4

The cooling efficiency for optical refrigeration by anti-Stokes Fluorescence was measured in a 10% Yb3+:LiYF4 crystal by Differential Luminescence Thermometry (DLT) and Thermal Lens Spectroscopy (TLS) over a range of pump intensities. Results showed that background and coolant ion absorption coefficients saturated at low intensities, and saturated differently as a function of wavelength. For wavelengths longer than 1000 nm, a higher pump intensity led to improved cooling efficiency through a differential saturation phenomenon. In this range, background absorption saturated more easily than the absorption of coolant ions so that parasitic heating was diminished, yielding higher cooling efficiency for higher pump intensities. This saturation phenomenon therefore offers the prospect of improved performance of electronic devices such as imaging arrays in space which could operate with reduced noise at low temperatures through laser cooling. The measured cooling efficiency was independent of pump intensity at a wavelength of 1000 nm where the saturation intensity of background absorption was found to be the same as that of Ytterbium in LiYF4. A novel experimental method for determining the saturation intensities of impurity and coolant ions is described, and consequences for laser cooling and radiation-balanced laser operation of the observed saturation effects are discussed and analyzed.

Operation of a fiber-coupled laser-cooler down to cryogenic temperatures.

We report on the optical cooling of a 7.5%Yb:LiYF4 crystal down to 125 K in a multi-pass Herriott absorption cell, coupled via a single mode polarization maintaining optical fiber to the laser source. This configuration, never exploited before, is more practical for potential applications, in particular for spaceborne cryogenics setups. Moreover, the temperature reached is exactly the one needed in many setup embarked in small and medium satellites. We evaluate the heat load on the crystal at the minimum attainable temperature, which is comparable to state of the art systems.

Effect of erbium concentration on the Verdet constant dispersion of LiY1.0-xErxF4 single crystal

The dispersion of the Verdet constant of LiY1.0-xErxF4 crystals was evaluated from 190 nm to 500 nm for different doping concentrations of Er ions. A 15% doping concentration yielded a high Verdet constant of 54.5 rad/(T·m) at 193 nm. This value can be explained by the contribution of the diamagnetic term associated with LiYF4 and the paramagnetic term of the Er ions. Although the LiYF4 crystal yielded a lower value of −36.6 rad/(T·m) at 193 nm from Er-doped LiYF4, it can be used in the vacuum–ultraviolet region because of its high transmittance at wavelengths longer than 120 nm.

A diode-pumped femtosecond Pr:YLF laser emitting at the near-infrared 915 nm

Femtosecond pulse generation from diode-pumped praseodymium (Pr) lasers in the near-infrared has been a long pursuit for the optics community. Utilizing the Pr:LiYF4 crystal in a novel dispersion arrangement scheme that includes a set of triple Gires-Tournois interferometer mirrors, mode-locked ultrashort pulses as short as 451 fs at 915 nm have been achieved when the crystal is directly pumped by a blue laser-diode. The pulse energy is 3 nJ and peak power is 7.2 kW at the repetition rate of 85.5 MHz. This is the first demonstration of diode-pumped Pr lasers arriving the femtosecond regime.

Laser emission from low-loss cladding waveguides in Pr:YLF by femtosecond laser helical inscription

Depressed cladding waveguides are fabricated in Pr:LiYF4 (YLF) crystal by femtosecond laser inscription following a helical scheme. With the optimized parameters, the propagation loss of the waveguide is around 0.12 dB/cm for multimode guiding. Under optical pumping with InGaN laser diodes at 444 nm, efficient waveguide lasers in the orange around 604 nm (π-polarized) are achieved with minimum lasing threshold of 119.8 mW, maximum slope efficiency of 16.6%, and maximum output power of 120.6 mW. Benefiting from their optimized performances, the waveguides produced in this work are promising for applications as compact orange laser sources.

Liyf4 Crystals Embedded Silicate Glass with High Electrochemical Performance as an Anode for Lithium-Ion Battery

Liyf4 Crystals Embedded Silicate Glass with High Electrochemical Performance as an Anode for Lithium-Ion Battery

Highly Polarized Upconversion Emissions from Lanthanide-Doped LiYF4 Crystals as Spatial Orientation Indicators.

Polarized emission, an inherent characteristic that correlated with structure and morphology, is very sensitive to orientation. For the upconversion (UC) emission of lanthanides, the mechanism of polarization is rarely discussed, and the highly polarized UC emissions are poorly developed. Herein, with the benefit of the strong anisotropic crystal field, well-resolved emissions from lanthanide-doped LiYF4 crystals were studied, and highly polarized UC emissions from Er3+ and Ho3+ were investigated. With multiple sub-energy level transitions, the UC emissions are classified into two sets, with transition dipoles being either parallel or perpendicular to the c-axis of the LiYF4 crystal. An optical three-dimensional orientation sensor was further investigated, in which the in-plane angle is referenced from the orientation of the transition dipoles. In contrast, the out-of-plane angle can be deduced from the change in the degree of polarization. This research deepens our understanding of the polarized photoluminescence, and it opens up an avenue toward unique UC orientation sensors.