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- ItemCzochralski growth of mixed cubic sesquioxide crystals in the ternary system Lu2O3-Sc2O3-Y2O3(Oxford [u.a.] : Wiley-Blackwell, 2021) Kränkel, Christian; Uvarova, Anastasia; Haurat, Émile; Hülshoff, Lena; Brützam, Mario; Guguschev, Christo; Kalusniaka, Sascha; Klimm, DetlefCubic rare-earth sesquioxide crystals are strongly demanded host materials for high power lasers, but due to their high melting points investigations on their thermodynamics and the growth of large-size crystals of high optical quality remain a challenge. Detailed thermal investigations of the ternary system Lu2O3-Sc2O3-Y2O3 revealing a large range of compositions with melting temperatures below 2200°C and a minimum of 2053°C for the composition (Sc0.45Y0.55)2O3 are presented. These reduced temperatures enable for the first time the growth of high optical quality mixed sesquioxide crystals with disordered structure by the conventional Czochralski method from iridium crucibles. An (Er0.07Sc0.50Y0.43)2O3 crystal is successfully grown and characterized with respect to its crystallographic properties as well as its composition, thermal conductivity and optical absorption in the 1μm range. © 2021 International Union of Crystallography. All rights reserved.
- ItemSub-6 optical-cycle Kerr-lens mode-locked Tm:Lu2O3 and Tm:Sc2O3 combined gain media laser at 2.1 μm(Washington, DC : Soc., 2021) Suzuki, Anna; Kränkel, Christian; Tokurakawa, MasakiWe present a combined gain media Kerr-lens mode-locked laser based on a Tm:Lu2O3 ceramic and a Tm:Sc2O3 single crystal. Pulses as short as 41 fs, corresponding to less than 6 optical cycles, were obtained with an average output power of 42 mW at a wavelength of 2.1 μm and a repetition rate of 93.3 MHz. Furthermore, a maximum average power of 316 mW with a pulse duration of 73 fs was achieved.
- ItemDiode-Pumped Laser Operation of Tb3+:LiLuF4 in the Green and Yellow Spectral Range(Weinheim : Wiley VCH, 2020) Castellano-Hernández, Elena; Kalusniak, Sascha; Metz, Philip W.; Kränkel, ChristianHere, a diode-pumped laser based on trivalent terbium (Tb3+) as the active ion is reported. Optical pumping of a Tb3+-doped lithium-lutetium-fluoride (LiLuF4) crystal with up to 200 mW from a diode laser emitting at a wavelength of 488.2 nm enables continuous-wave lasing directly in the green and in the yellow. At an emission wavelength of 542 nm, the laser reaches an output power of up to 43.8 mW with a high slope efficiency of 52% with respect to the absorbed pump power. The yellow laser at 587 nm exhibits a slope efficiency of 22% and the output power of 13.8 mW is only limited by the available pump power. Laser thresholds as low as 14 and 27 mW of absorbed pump power are observed for the green and yellow, respectively. The investigations toward further optimization of the laser performance reveal that highly Tb3+-doped materials are suitable for compact, efficient, and affordable diode-pumped solid-state lasers with direct emission in the visible spectral range. These results are of high relevance, as in particular for the yellow spectral range such systems are currently not available. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemSpectroscopy and 2.1 µm laser operation of Czochralski-grown Tm3+:YScO3 crystals(Washington, DC : Soc., 2022) Suzuki, Anna; Kalusniak, Sascha; Tanaka, Hiroki; Brützam, Mario; Ganschow, Steffen; Tokurakawa, Masaki; Kränkel, ChristianWe report on growth, temperature-dependent spectroscopy, and laser experiments of Tm3+-doped YScO3 mixed sesquioxide crystals. For the first time, cm3-scale laser quality Tm3+:YScO3 crystals with 2.2 at.% and 3.1 at.% doping levels were grown by the Czochralski method from iridium crucibles. We reveal that the structural disorder in the mixed crystals allows for broad and smooth spectral features even at cryogenic temperatures. We obtained the first continuous wave laser operation in this material at wavelengths around 2100 nm using a laser diode emitting at 780 nm as a pump source. A maximum slope efficiency of 45% was achieved using a Tm3 + (3.1 at.%):YScO3 crystal. Our findings demonstrate the high potential of Tm3+-doped mixed sesquioxides for efficient ultrafast pulse generation in the 2.1 µm range.
- ItemInfluence of diluted acid mixtures on selective etching of MHz- and kHz-fs-laser inscribed structures in YAG(Washington, DC : OSA, 2021) Hasse, Kore; Kip, Detlef; Kränkel, ChristianWe show that the inscription velocity of fs-laser written structures in YAG crystals can be significantly improved by the use of MHz repetition rates for the writing process. Using a 10 MHz inscription laser, record high writing velocities up to 100 mm/s are achieved. Also, the selective etching process is accelerated using a diluted mixture of 22% H3PO4 and 24% H2SO4. The diluted mixture enables selective etching of up to 9.6 mm long, 1 µm wide and 18 µm high microchannels in 23 days. The etching parameter D of 11.2 µm2/s is a factor of 3 higher than previously reported and the selectivity is even increased by an order of magnitude.
- ItemLaser cooling in Yb:KY3F10: a comparison with Yb:YLF(Washington, DC : Optica, 2022) Püschel, Stefan; Mauerhoff, Felix; Kränkel, Christian; Tanaka, HirokiLaser 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.