[en] Single-crystalline Tm³⁺-doped calcium fluoride (Tm³⁺:CaF₂) films were grown onto (100) oriented undoped bulk CaF₂ substrates by Liquid Phase Epitaxy method using LiF as a solvent. The growth was performed at the temperatures of 853–865 °C. Crack-free transparent films with a high doping concentration of ∼6 at.% and a thickness up to ∼100 μm were obtained. The film structure and morphology were characterized by confocal microscopy, X-ray diffraction and scanning electron microscopy. High resolution X-ray diffraction and reciprocal space mapping confirmed the high crystallinity and the low mosaicity of the films. Energy-Dispersive X-ray spectroscopy revealed a uniform distribution of Tm³⁺ ions in the films. The Tm³⁺:CaF₂ films exhibit a broadband emission at ∼2 μm and are of interest for waveguide lasers. The use of CaF₂–LiF binary system is also promising for fabrication of other rare-earth-doped CaF₂ layers.

[en] We report on the first demonstration of a diode-pumped passively mode-locked Tm,Ho:LiYF₄ laser. Stable continuous-wave mode-locking is achieved by employing a single-layer graphene as a saturable absorber (SA). For a reflection-type graphene-SA, the laser delivers 5.2 ps pulses at ~2051 nm with an average output power of 40 mW at a repetition rate of 89.86 MHz. With a transmission-type graphene-SA and smaller output coupling, the output power is scaled up to 80 mW while the laser operated in a double-pulse regime. (letter)

[en] Cubic crystals are often favored due to their interesting isotropic properties -such as the refractive index, the dilatation, etc-that simplify their use in laser systems. Among all Yb-doped high-symmetry materials, Yb:CaF₂ is a very attractive one for high-energy high-power lasers. However, in such systems, thermal loads become important and thermal anisotropic effects can occur even for simple cubic crystals impacting severely the laser system design. In this context, we report here, a study of thermally induced polarization anisotropy in Yb-doped CaF₂ laser crystals. Both thermal lens and thermally induced depolarization are precisely studied for different crystal orientations, namely the [110] and the [111] orientation. The study is performed with a pump/probe setup adapted to improve the signal-to-noise ratio under high power pumping. We observed both, classical-thermally induced depolarization and thermal lensing in accordance with previous work, but also, for certain configurations, atypical depolarization figures and astigmatic lenses. These results were also confronted with numerical simulations. The comparison between the [110] and the [111] oriented crystals represents a crucial study to determine the origins of these unusual effects and to allow a better understanding of thermal anisotropy in cubic laser-crystals.

[en] Highlights: • Spectroscopic study of Dy-doped and Tm–Dy doped CaF₂ crystals. • Excellent energy transfer from Tm³⁺ ions to Dy³⁺ ions thanks to a clustering effect in CaF₂. • Promising perspectives of Tm³⁺-Dy³⁺ doped CaF₂ for of MIR laser applications around 3 μm. • Laser modeling of Dy³⁺ and Tm³⁺-Dy³⁺ doped CaF₂ in a waveguide configuration. We report here on a spectroscopic study of Dy³⁺-doped and Tm³⁺-Dy³⁺ doped CaF₂ as promising candidates to develop crystalline waveguide laser sources around 3 μm. The advantages of Tm³⁺ ions as sensitizers to improve the excitation of Dy³⁺ ions in CaF₂ is demonstrated: an energy transfer efficiency from Tm³⁺ to Dy³⁺ ions of 99.6% has been reached for Dy³⁺ concentration as high as 2 at.%, by considering a Tm³⁺ ratio set at 5 at.%. Moreover, the behavior of such doped crystals in a laser waveguide configuration has been modeled and the modeling results show that it seems possible to achieve promising laser perspectives around 3 μm, with laser thresholds in the watt level for singly doped Dy³⁺:CaF₂ and around 0.2–0.3 W for codoped Tm³⁺-Dy³⁺:CaF₂, presenting both laser efficiencies in the order of 30%. Finally, the saturation of the absorption which is observed in the modeling for such codoped Tm³⁺-Dy³⁺:CaF₂ waveguide at Dy³⁺ concentration below 1% is discussed and its origin is explained.

[en] Highlights: • Liquid Phase Epitaxy growth of 20 at.% Yb:LiYF₄ films on oriented LiYF₄ substrates. • The films crystallize in tetragonal system with a = 5.157 Å and c = 10.714 Å. • Polarized Raman spectra are measured: the maximum phonon energy is 444 cm⁻¹. • The stimulated-emission cross-section is 0.88 × 10⁻²⁰ cm² at 993.9 nm in π-polarization. • The crystal-field splitting of Yb³⁺ multiplets is resolved at 12 K. Highly doped (20 at.%) Yb³⁺:LiYF₄ single-crystalline thin films are grown on (001)-oriented bulk undoped LiYF₄ substrates by Liquid Phase Epitaxy using lithium fluoride (LiF) as a solvent. The growth temperature lies around 741 °C (0.5–1 °C supercooling) and the growth rate is 1.8–2.6 μm/min. The single-crystalline nature of the films is confirmed by X-ray diffraction and polarized Raman spectroscopy. The film morphology is studied and discussed. The polarized spectroscopic properties of Yb³⁺ ions are reported, indicating a stimulated-emission cross-section of 0.88 × 10⁻²⁰ cm² at 993.9 nm in π-polarization and a radiation trapping free lifetime of the ²F_5/2 state of 2.00 ms indicating weak concentration-quenching. The crystal-field splitting of Yb³⁺ multiplets is resolved at 12 K. Highly-doped Yb³⁺:LiYF₄/LiYF₄ homoepitaxies are promising for waveguide and thin-disk lasers at ~1 μm.

[en] A detailed spectroscopic study of various Yb³⁺:CaF₂ crystals with dopant concentrations ranging from 0.03 to 2 at% Yb³⁺ has been performed. By using site-selective laser excitation, the low-temperature (7 K) excitation and emission spectra recorded in the case of the lower doped samples (0.03% and 0.05%) are found to be dominated by three types of sites with excited state fluorescence lifetimes of about 2, 8 and 10 ms. Confronted with crystal field calculations, these sites have been identified as Yb³⁺ ions in trigonal (C_3v), tetragonal (C_4v) and cubic environments, respectively. At higher dopant concentrations (>0.1 at%), evidence is found of a third kind of site, whose contribution increases up to be dominant as dopant concentration is increased above about 1 at%Yb³⁺ and which gives rise to the lasing center which has been recently discovered and operated

[en] Highlights: • 0.94 at.% Ho³⁺-doped Y₂O₃ and Y₃Al₅O₁₂ transparent ceramics. • Vacuum sintering at 1780 °C from laser-ablated nanopowders using ZrO₂ and TEOS. • A detailed comparative study of microstructure, optical and spectroscopic properties. • The rates of multiphonon relaxation are determined for both ceramics. • The crystal-field splitting of Ho³⁺ ions is analyzed using the barycenter plot. Ho:Y₂O₃ and Ho:Y₃Al₅O₁₂ transparent ceramics are fabricated by vacuum sintering at 1780 °C for 20 h from laser-ablated nanopowders (Ho:Y₂O₃ and Ho:Y₂O₃ + Al₂O₃) using ZrO₂ and TEOS as sintering aids, respectively. The sesquioxide ceramic exhibits smaller grain size (∼5.4 μm), smaller content of pores (down to 6 ppm) and higher optical transmission (up to 82.2% at ∼1 μm) than the garnet one. A detailed comparative spectroscopic study of the two ceramics is performed. For the ⁵I₇ → ⁵I₈ Ho³⁺ transition, the maximum stimulated-emission cross-sections and the luminescence lifetimes are σ_SE = 0.57 × 10⁻²⁰ cm² at 2087.1 nm and τ_lum = 10.92 ms (for the Ho:Y₂O₃ ceramic) and σ_SE = 1.70 × 10⁻²⁰ cm² at 2090.8 nm and τ_lum = 7.04 ms (for the Ho:Y₃Al₅O₁₂ one) The rates of multiphonon relaxation are determined for both ceramics. For the Ho:Y₂O₃ ceramic, they do not exceed those for single-crystals. The crystal-field splitting of Ho³⁺ multiplets (⁵I₇ and ⁵I₈) is determined at 12 K and analyzed using the barycenter plot. Due to its broadband emission properties, the Ho:Y₂O₃ ceramic is suitable for mode-locked lasers emitting above 2 μm.