Various rare-earth doped solids can be cooled by anti-Stokes fluorescence, but only a few, particularly ytterbium (Yb)-doped LiYF4 (YLF), showed the potential to reach the cryogenic temperature regime (below 123 K). We propose to adopt cubic Yb-doped KY3F10 (Yb:KYF) for reaching sub-100 K cooling temperatures. The temperature-dependent spectroscopy of Yb:KYF and the comparison with Yb:YLF indicate its high potential to achieve lower cooling temperatures. The calculated figure-of-merit of laser cooling of Yb:KYF is higher than that of Yb:YLF by a factor of five at 100 K. This is because Yb:KYF has a significantly shorter mean fluorescence wavelength of 991 nm compared to the value of 1004 nm for Yb:YLF at 100 K. We grew Yb:KYF crystals by the Czochralski method with varied growth parameters, and experimentally compared their laser cooling performance with an Yb:YLF also grown at our institute. We observed efficient laser cooling in the Yb:KYF crystals at room temperature. Laser-induced thermal modulation spectroscopy tests determined their external quantum efficiencies to be higher than 98.5% and background absorption coefficients to be as low as 1.0•10-4 cm-1. The minimal achievable temperature (MAT) of our best Yb:KYF sample was calculated to be ≈90 K, attractive to be used in optical cryocoolers.
Crystalline materials with suppressed impurity concentrations are essential elements for efficient solid-state laser cooling based on anti-Stokes fluorescence. So far, fluoride single crystals doped with rare earth ions have been demonstrated as efficient laser cooling media. We report on our growth activities on high purity rare-earth-doped fluoride single crystals for this specific application. We grew a variety of fluoride crystals doped with ytterbium by the Czochralski method. These crystals are studied by temperature-dependent spectroscopy to fully reveal their potential as laser cooling media. The cooling efficiency of the grown crystals is directly evaluated in a laser-induced cooling setup in vacuo.
We report on UV-pumping of a visible Tb:LiLuF4 laser emitting at 544 and 588 nm. Pumping with a frequency-doubled Ti:sapphire laser at 359 nm significantly improves the absorption efficiency compared to conventional in-band pumping at ~488 nm and cross relaxation from the excited level 5D3 efficiently populates the upper laser level 5D4. In this way, optical efficiencies of 29% and 12%, respectively, are obtained with respect to the incident pump power. A passively Q-switched Tb:LiLuF4 laser at 544 nm using a Co2+-doped MgAl2O4 as a saturable absorber yields 23-µJ pulses at 3.5 kHz with a pulse duration of ~200 ns.
We report our recent progress of solid-state lasers, the titanium-doped sapphire laser and the praseodymium-doped LiYF4 (Pr3+:LiYF4) laser, directly pumped by InGaN diode lasers of green and blue. While the titanium-doped sapphire laser can be pumped by both blue and green diode lasers because of its broad absorption spectrum, we find 450-nm diode laser pumping inefficient for its power scaling owing to the onset of an additional absorption loss induced by the blue pump laser. The degradation of the performance of a Ti:sapphire laser induced by a 450-nm diode pumping is experimentally revealed. Furthermore, we demonstrate a passively mode-locked Ti:sapphire laser pumped by two green diode lasers of 1-W, and 74 fs pulses at a repetition rate of 120 MHz with an averaged output power of 45 mW are obtained. The Pr3+:LiYF4 laser, one of the most promising candidates as a visible laser, has become possible to be pumped directly by blue diode lasers of 440-nm wavelength. We demonstrate the power scaling at 523 and 640 nm oscillation, a passive Q-switching at 640 nm with a Cr4+:YAG saturable absorber by utilizing polarization-combined blue diode lasers as the pump source.
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