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.
Ga doping of ZnO allows for generation of free electrons up to concentrations of about 1021 cm-3 without significant deterioration of the crystal structure. In this way, a metallic dielectric function is formed with a negative-to-positive crossover of the real part tunable from mid infrared up to telecommunication wavelengths. The losses are at least one order of magnitude lower than for traditional metals. We demonstrate surface plasmon polaritons with dispersion relations that can be engineered in a unique way by utilizing epitaxial multi-layer structures of different doping level.
Molecular-beam epitaxial growth far from thermal equilibrium allows us to overcome the standard solubility limit
and to alloy ZnO with CdO in strict wurtzite phase up to mole fractions of several 10%. In this way, a band-gap
range extending from 3.3 eV down to 2.3 eV can be covered. Strong improvement of the crystalline quality
indicated by a rocking curve width of only 45 arc sec is achieved when growing the ternary on ZnO substrates.
Despite very low growth temperatures (~150 °C), layer-by-layer growth indicated and controlled by RHEED
oscillations is accomplished. This enables us the fabrication of atomically smooth heterointerfaces and well-defined quantum well structures exhibiting prominent band-gap related light emission in the whole composition
range. Post-growth annealing increases the radiative efficiency up to two orders of magnitude and demonstrates
thermal stability of the structures with respect to phase separation even up to temperatures of about 500°C.
Low-energy shifts of the photoluminescence features reaching the order of 1 eV as well as a dramatic increase of
the lifetime from the sub-ns to the 100-μs time-scale uncover the presence of huge polarization-induced electric
fields of some 108 V/m in ZnCdO/ZnO single quantum well structures. Carrier injection by moderate optical
excitation in the 10 kW/cm2 screens these fields and recovers practically the bare quantum-confined energy
transitions. On appropriately designed structures, laser action from the UV down to the green wavelength
range is observed under optical pumping. The threshold at low temperature is only 60 kW/cm2 and increases
only moderately up to room temperatures. All these findings make ZnO-based heterostructures a promising
alternative to group-III-nitrides for opto-electronic applications in the short-wavelength range.
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