Youchen Liu

Morphology, structure and optical properties in TiO 2 nanostructured films annealed at various temperatures

Amorphous TiO2 nanostructured thin films (TNFs) were deposited by the glancing angle deposition technique (GLAD) with an electron gun. All of the prepared TNFs were composed of discrete nanoscale columns, characterized by scanning electron microscope. With the annealing treatment, the pure anatase phase was transformed from amorphous TNFs. The morphological, structural and optical properties of TNFs under the annealing treatment were measured, and the evolvement mechanism was further analyzed. The optimum annealing temperature for the pure anatase precipitation of TNFs is about 400°C. The variation of morphology, chemical state and crystallization of TNFs also resulted in the shift of the transmittance spectra. The results show that appropriate post-annealing treatment can build fine nanostructures and pure anatase precipitation, which can support the applications and researches about the anatase phase TNFs powerfully.

Crystal growth, spectroscopic characterization, and laser operation of Tm/Mg: LiTaO 3 crystal

Tm, Mg co-doped LiTaO3 crystal is grown by the traditional Czochralski method. By analyzing the absorption and emission measurements of the Tm/Mg: LiTaO3 single crystal with the Judd-Ofelt analysis, the intensity parameters Ω2,4,6, transition probabilities, exited state lifetimes, branching ratios, and emission cross-sections were calculated. Non-photorefractive continuous wave laser operation with a Tm/Mg: LiTaO3 single crystal is demonstrated for the first time. We obtained 1.51 W output power at 1.92 μm with a slope efficiency of near 38.5%, which, to the best of our knowledge, are the largest output power and highest slope efficiency obtained for this crystal thus far. The long-term photorefractive effect was also quantitatively analyzed.

Optical properties of Dy3+ ion in PbF2 laser crystal

High-quality Dy:PbF2 crystal is grown by the Bridgman method in a nonvacuum atmosphere. By measuring the area under absorption bands, the experimental oscillator strengths are determined. The Judd–Ofelt (JO) intensity parameters Ωλ (λ = 2, 4, 6) are evaluated by the least-squares fit method. These phenomenological parameters are used to predict radiative transition probabilities, radiative lifetime and branching ratios for various excited levels of the Dy3+:PbF2 crystal. Photoluminescence spectra and lifetime of 6H13/2 levels of the Dy3+ ions have been measured. The laser transitions with most potential are identified and the utility of the PbF2 crystal as laser active material is discussed.

Growth and characterization of large-scale Ti:sapphire crystal using heat exchange method for ultra-fast ultra-high-power lasers

Large-scale titanium-doped sapphire (Ti:sapphire) crystal was grown in a 380 mm diameter molybdenum crucible using the heat exchange method (HEM) and characterized. The diameter of the core Ti:sapphire single crystal was estimated to be slightly larger than 200 mm. A Ti:sapphire crystal wafer 157 mm in diameter and 28 mm thick was obtained. Characterization based on X-ray diffraction spectroscopy, Raman spectroscopy, and absorption and fluorescence spectroscopy was carried out. The as-grown Ti:sapphire crystal had high intrinsic quality and a large figure of merit (FOM) without additional annealing. The high optical quality of the as-grown Ti:sapphire crystal indicated it is suitable for ultra-fast ultra-high-power lasers in chirped pulse amplification systems.

Optical characteristics of ZnO single crystal grown by the hydrothermal method

ZnO single crystals have been grown by the hydrothermal method. Raman scattering and Photoluminescence spectroscopy (PL) have been used to study samples of ZnO that were unannealed or annealed in different ambient gases. It is suggested that the green emission may originate from defects related to copper in our samples.

Low-phonon PbF2:Tm3+-doped crystal for 1.9 µm lasing

Tm-doped PbF2 crystal is successfully grown in a home-made Bridgman furnace. The laser properties of Tm3+ in PbF2 crystal at around 1900 nm are then evaluated based on the absorption and emission cross-section. Laser operation in Tm:PbF2 single crystal at 1900 nm with laser–diode pumping is demonstrated for the first time. With a 2 mol. % Tm3+ -doped sample, a maximum output power of 1.17 W is realized at a wavelength of 1900 nm for 6.8 W of absorbed pump power with a slope efficiency of 26%. We propose that the Tm:PbF2 crystal may be a promising new material for 1900 nm high peak power laser applications.