Y. Guyot

Optimization of spectroscopic properties of Yb3+-doped refractory sesquioxides: cubic Y2O3,Lu2O3 and monoclinic Gd2O3

Abstract Absorption and emission spectra are given for Yb3+-doped Y 2 O 3 , Lu 2 O 3 and Gd2O3 at room temperature. Y2O3 and Lu2O3 as close cubic matrices, show Yb3+ similar spectra different of Yb3+ in Gd2O3 monoclinic structure. Here, we use a new method to study and optimize the main spectroscopic properties with only one concentration gradient sample. Finally, assignments of Yb3+ Stark levels and Raman vibrations in Y 2 O 3 , Lu 2 O 3 and Gd2O3 single crystal are given.

Crystal growth, Yb3+ spectroscopy, concentration quenching analysis and potentiality of laser emission in Ca1−XYbXF2+X

Ca1−XYbXF2+X crystals were grown by two different methods: simple melting under CF4 atmosphere and the laser heated pedestal growth (LHPG) method under Ar atmosphere. Spectroscopic characterization (absorption, emission, Raman spectroscopy and decay curves) was carried out to identify Stark levels of Yb3+ transitions in the different crystallographic sites of the cubic structure in Ca1−XYbXF2+X crystals. The Yb3+ concentration dependence of the experimental decay time was analysed by using concentration gradient fibre in order to understand involved concentration quenching mechanisms. Under Yb3+ ion infrared pumping, self-trapping and up-conversion non-radiative energy transfer to unexpected rare earth impurities (Er3+, Tm3+) has been observed in the visible region and interpreted by a limited diffusion process within the Yb3+ doping ion subsystem towards impurities. The main parameters useful for a theoretical approach of laser potentiality have been given as τrad = 2.05 ms, N0 = 7.47 × 1021 cm−3 (32 mol%) and Nm = 6.39 × 1021 cm−3 (26.5 mol%).

Luminescence properties of Y2O3 single crystals doped with Pr3+ or Tm3+ and codoped with Yb3+, Tb3+ or Ho3+ ions

Abstract We have recorded fluorescence spectra and measured fluorescence lifetimes in yttria (Y2O3) single crystals doped with only Tm3+ or Pr3+ ions or codoped with Yb3+, Tb3+ or Ho3+ ions. The crystals were grown in the form of crystal fibers by using the laser heated pedestal growth (LHPG) technique. The codoped (Yb, Tm) and (Yb, Pr) crystals were studied for their potential as upconversion laser systems. Evidence is shown of cooperative and multi-step energy transfers giving rise to very efficient visible emissions after pulsed infrared excitation. As to the codoped (Tm, Ho) and (Tm, Tb) crystals, they are studied to examine the effect of the codopants on the infrared emission properties of the Tm3+ ions. Indeed, we found a very attractive fluorescence around the eye-safe wavelength of 1.55 μm which corresponds to a 3H4→3F4 transition of Tm3+. This transition occurs with a large enough stimulated emission cross section to be practical for laser application, and the Tb3+ or the Ho3+ ions are introduced to reduce the problem of bottlenecking due to a lower laser state lifetime longer than that of the upper state.

Growth, spectroscopic, and laser properties of Yb 3+ -doped Lu 3 Al 5 O 12 garnet crystal

We have grown high-quality LuAG:Yb3+ crystals with 0.75, 3.8, 10, 12, 15, 20, and 50 at. % concentrations by the vertical Bridgman method. With low-temperature spectroscopy the Stark sublevel structure of the 2F7/2 ground state and the 2F5/2 excited state has been determined. With room-temperature spectroscopy, the emission cross section was found to be 3×10−20 cm2, being 1.5 times the YAG:Yb3+ emission cross section. The luminescence quantum efficiency was measured in samples with different Yb3+ concentrations. Its value was found to be 90% for 3.8 and 10 at. %, 84% for 20 at. %, and 70% for 50 at. % Yb3+. The laser-emission tunability under diode pumping was found to extend from 1045 up to 1095 nm in a 12 at. % sample with 3.15 mm thickness.

Spectroscopy and laser operation of Pr, Mg:SrAl12O19

Pr3+‐doped SrAl12O19 has been investigated spectroscopically as a visible laser material. Its 3P0 fluorescence lifetime is rather long for an oxide, about 35 μs. This excited state exhibits only mild concentration and temperature quenching, so that concentrations high enough to give good ground state absorption into the 3P0 and 3P1 manifolds still give strong, long‐lived emission. The ground state absorption spectra indicate that excited state absorption into the 4f5d configuration is less likely to be significant at laser wavelengths in this host than in Y3Al5O12. The stimulated emission cross sections of major emission lines have been estimated, and are quite realistic for laser operation. Laser pumped laser operation has been demonstrated in the red (3P0→3F2) at room temperature and in the blue‐green (3P0→3H4) at cryogenic temperatures.

Growth of rare earth (RE) doped concentration gradient crystal fibers and analysis of dynamical processes of laser resonant transitions in RE-doped Y2O3 (RE=Yb3+, Er3+, Ho3+)

Abstract New and original monocrystalline samples having a continuous longitudinal concentration gradient are used to study dynamical processes of resonant transitions in rare earth doped Y 2 O 3 . This fast and simple method allows to measure the influence of radiation trapping and impurity quenching on the excited-state lifetime as a function of the dopant concentration. Examples in Yb 3+ , Er 3+ and Ho 3+ rare earth ions are presented.

Growth and spectroscopic analysis of Yb3+-doped Y3Al5O12 fiber single crystals

Two major fiber crystal growth methods have been applied for the synthesis and spectroscopic characterizations of Yb3+-doped Y3Al5O12 (Yb:YAG) to show the feasibility of a flexible single crystal fiber laser. One is the micropulling-down (μ-PD) method and the other is the laser heated pedestal growth (LHPG) method. 500 mm length fiber with no Yb segregation has been grown by the μ-PD method. A detailed analysis of Yb:YAG spectroscopy is made to contribute to the discussion on the determination of energy levels. On the other hand, a combinatorial chemistry approach has been applied on concentration gradient crystal fibers grown by the LHPG method allowing the measurement of the intrinsic radiative lifetime and the analysis of concentration quenching processes of Yb3+ ions in YAG.

Potentiality of Pr3+- and Pr3++Ce3+-doped crystals for tunable UV upconversion lasers

Abstract Gain experiments based on the interconfigurational 4f5d to 4f 2 optical transitions in praseodymium-doped fluoride crystals were achieved. The measurements were performed under upconversion pumping of the 4f5d excited configuration via the 3 P J , 1 I 6 intermediate levels of the 4f 2 configuration, using a pump-probe set-up with three synchronized Q-switched nanosecond pulsed lasers. In spite of this two-step excitation, optical losses, due excited state absorption (4f5d→conduction band transition) leading to Pr 3+ ion photoionisation and color center formation, were too high to observe any amplification. However, these losses were found definitely weaker in fluoride crystals (Pr:LiYF 4 , Pr:LiLuF 4 ) than in oxides (Pr:YAlO 3 ) and, probing the 5d→4f emission of Ce 3+ under two-step excitation pumping of the Pr 3+ 4f5d states in co-doped Pr,Ce:LiLuF 4 crystals, no losses were detected at all.

Optical behavior of Eu3+-doped BaTiO3 nano-crystallites prepared by sol–gel method

Abstract Eu 3+ -doped BaTiO 3 nano-crystallites with grain sizes 32–38 nm have been obtained by a sol–gel method. It has been found that the luminescence characteristics are strongly dependent on the sintering temperature. Powders sintered at temperatures below 1000 °C demonstrate a luminescence behavior at room temperature associated with a lack of the inversion symmetry of Eu 3+ sites. Above 1000 °C nano-crystallites undergo a phase transition characterized by an inversion symmetry of Eu 3+ sites. The size effect of Eu 3+ :BaTiO 3 nano-crystallites on luminescence characteristics is observed.

Spectroscopic properties of new Yb3+-doped K5Bi(MoO4)4 crystals

This research work analyzes the spectroscopic properties of Yb 3+ ion in a new oxide single crystal belonging to the potassium bismuth molybdate family with the following concentrations: K 5 Bi 1-x -Yb x (Mo0 4 ) 4 (x =0.005, 0.05, 0.15). We are dealing with temperature and concentration dependences of both σ and π polarizations of the IR absorption and emission spectra. Raman spectra is used to interpret electronic Stark levels and vibronic levels. Concentration dependence of fluorescence lifetimes allows the measurement of the high radiative lifetime in the range 1.89-1.93 ms and shows a strong self-trapping process which has been seen up to 15% Yb 3+ . Competition with a non-radiative energy transfer by up-conversion process is also observed due to the presence of Er 3+ ion as unexpected impurity. Yb 3 + pairing and clustering has not been observed in the molybdate host in which the two distances between Bi 3+ next nearest crystallographic sites seem rather too large to give any observable anti-Stokes signal by cooperative luminescence.