We report on a detailed spectroscopic study of heavily Er3+-doped LiYF4 epitaxial layers with the goal of developing mid-infrared waveguide lasers. Layers with a doping level up to 11 at.% Er3+ were grown on (001) oriented undoped bulk LiYF4 substrates using LiF as a solvent. The absorption spectrum of Er3+ ions was measured. Under excitation at 973 nm, the layers exhibited intense and strongly polarized mid-infrared luminescence spanning from 2.65 to 2.90 μm related to the 4I11/2 → 4I13/2 Er3+ transition. The peak stimulated-emission cross-section at the expected laser wavelength was calculated to be 0.88×10-20 cm2 at 2809 nm for π-polarization. By means of low-temperature (12 K) spectroscopy, the experimental crystal-field splitting of Er3+ multiplets was determined. The luminescence dynamics from Er3+ excited states were studied. For the 11 at.% Er3+ doping, the luminescence lifetimes of the 4I13/2 and 4I11/2 manifolds amounted to 5.54 ms and 2.66 ms, respectively.
Tm-based laser with emission at 2.3 μm could be used to detect atmosphere pollutants and various molecules. Within this work we investigated if oxide compounds could be efficient for this laser emission as complex energy transfers could occur such as non-radiative relaxation and upconversion. Several oxides hosts are investigated, namely CaGdAlO4 (CALGO), Y2O3, CaYAlO4 and CaYAl3O7 to host Tm3+ for laser effect at 2.3 μm. We focus on the optical characterizations and spectroscopic analysis of these materials, determining intrinsic optical features such as lifetime and broadening of the emission bands.
Trivalent titanium ions (Ti3+) are known for their broadband emission in the visible and near-IR. Zinc aluminate spinel, or gahnite (ZnAl2O4) is known as a host matrix for transition-metal ions. We report on the structure and spectroscopic properties of transparent zinc aluminosilicate glass-ceramics (GCs) nucleated by TiO2 and based on Ti3+-doped ZnAl2O4 nanocrystals. The initial glasses were melted under different redox conditions. After heat-treatments at the temperatures in the range of 720 to 1100 °C, transparent GCs were obtained. The materials were studied by the DSC method, XRD analysis, Raman, absorption and luminescence spectroscopy. The main crystalline phase in GCs is ZnAl2O4 with a cubic structure. The crystals are 5 - 21 nm in size. Their unit cell parameters vary with the heat-treatment temperature due to the titanium ions entering the gahnite nanocrystals. The volume fraction of gahnite nanophase increases with the heat treatment temperature. At 1000 – 1100 °C, TiO2 (rutile) crystals with a size of 11 - 37 nm also appear. In GCs obtained from glasses melted under reducing conditions, broadband absorption is observed in the visible and near IR due to the Ti3+ ions in Oh positions in ZnAl2O4 crystals, the absorption of Ti3+ - Ti4+ pairs and the appearance of Ti3+ self-doped rutile. By changing the redox conditions of the glass synthesis, one can control the content of titanium ions in various oxidation states and the spectral properties of GCs.
We report on the structure and spectral-luminescent properties of a composite ZnO-Er2O3-Yb2O3 optical ceramic. The ZnO ceramic codoped with 1 wt% Er3+ and 4 wt% Yb3+ was prepared by uniaxial hot pressing of oxide powders at 1180 °C in vacuum. The maximum total transmittance of the ceramic is ~40%. The ceramic is a composite material comprised of hexagonal ZnO microcrystals (mean size: 10-15 μm, a = 3.251 Å and c = 5.201 Å) and cubic sesquioxide Er2O3 and Yb2O3 nanocrystals (mean size: ~130 nm, a = 10.450 Å and 10.555 Å, respectively). Its texture is dominated by the ZnO prism planes (100). The absorption spectrum of the ceramic demonstrates bands characteristic of Er3+ and Yb3+ ions in the Er2O3 and Yb2O3 crystals, respectively. The ceramic exhibits intense red upconversion luminescence. The X-ray induced luminescence spectrum contains two intense emission bands. The more intense one with a maximum at ~390 nm is due to the near-band-edge transitions and its decay time is ~1 ns. The broad band in the green represents defect emission. Its decay deviates from the single-exponential law revealing several characteristic times of ~6, 60 and 250 ns. The free carrier concentration of the composite ceramic is ~7.65×1018 cm-3, which is significantly higher than that for the undoped ZnO one.
We report on the synthesis and a detailed spectroscopic study of an Er3+ -doped “mixed” (Lu,Y,La)2O3 transparent sesquioxide ceramic. Nanopowders of the composition (Lu0.63Y0.25La0.05Er0.07)2O3 were obtained by a glycine-nitrate self-propagating high-temperature synthesis. The ceramics were obtained by vacuum sintering at 1750 °C for 3 hours. La2O3 acted as a sintering additive and increased the disorder of the sesquioxide lattice. The ceramic had a cubic (C-type) structure. At 1.05 μm, its transmission was 78.9%. The ceramic exhibited intense mid-infrared emission owing to the 4 I11/2 → 4 I13/2 Er 3+ transition. The corresponding peak stimulated-emission cross-section σSE is 1.12×10-20 cm2 at 2717 nm. The luminescence spectrum exhibited a considerable inhomogeneous broadening as compared to the parent compounds, Er:Lu2O3 and Er:Y2O3. The luminescence lifetimes of the 4 I11/2 and 4 I13/2 manifolds were measured (1.99 ms and 3.35 ms, respectively), representing a favorable ratio for mid-IR laser operation. The crystal-field splitting of Er3+ multiplets in C2 sites was determined at low temperature.
We report on the growth, structure and spectroscopy of an Er3+ -doped Na5Y9F32 (5NaF∙9YF3) crystal featuring significant inhomogeneous spectral broadening. Single-crystals of Na5Y9F32 doped with 0.22 – 9.63 at.% Er3+ were grown by the Czochralski method. Er:Na5Y9F32 exhibits a cubic fluorite-type structure (a = 5.4881(2) Å for 5.59 at.% Er3+ doping). The most intense Raman band of this material is found at ~404 cm-1 . Er3+ ions in Na5Y9F32 exhibit a broad and smooth emission band owing to the 4 I11/2 → 4 I13/2 transition with a maximum stimulated-emission cross-section of 0.42×10-20 cm2 at 2708 nm. According to the Judd-Ofelt analysis, the radiative lifetime of the 4 I11/2 multiplet is 10.0 ms and the luminescence branching ratio β( 4 I11/2 → 4 I13/2) is 17.6%. The luminescence lifetimes of the 4 I11/2 and 4 I13/2 Er3+ states were studied as a function of the doping concentration. For 5.59 at.% Er doping, they are 7.72 ms and 6.69 ms, respectively, representing a favorable ratio for mid-infrared laser operation.
Transparent ceramics of cobalt-doped zinc aluminium spinel (gahnite), Co2+:ZnA2O4, are synthesized by hot pressing at 1520 °C for 4 h in the presence of zinc fluoride, ZnF2, as a sintering additive. The effect of the ZnF2 content (3–10 wt%) on the microstructure, Raman spectra, optical absorption and luminescence of ceramics is studied. The ceramics feature clean grain boundaries, the absence of pores and a narrow grain size distribution (mean grain size: 70-100 μm) resulting in high in-line transparency close to the theoretical limit. The obtained ceramics are suitable for fabrication of saturable absorbers of erbium lasers.
Ytterbium-doped transparent ceramics based on cubic garnets are promising for thin-disk lasers. 3.6 at.% Yb:Lu3Al5O12 transparent ceramics were fabricated by a solid-state reaction at 1800 °C in vacuum using Yb:Lu2O3 and Al2O3 nanopowders produced by laser ablation and their spectroscopic properties were studied. The stimulated-emission crosssection is 2.46×10-20 cm2 at 1030.2 nm. The Stark splitting of the Yb3+ multiplets was also determined. A compact CW Yb:Lu3Al5O12 ceramic laser pumped by a fiber-coupled 968 nm InGaAs laser diode generated 5.65 W at ~1031 nm with a slope efficiency of 67.2%. Using quasi-CW pumping, the peak power reached 8.83 W.
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