Crystal growth, optical characterisation and laser operation of yb3+ in monoclinic double tungstates

Identificador:  TDX:850

Handle: https://hdl.handle.net/20.500.11797/TDX850

Autores:  Mateos Ferré, Xavier

Resumen:
The interest and originality of the scientific work of this thesis focuse on the crystal growth, the compositional, structural, morphologycal and optical analysis of the monoclinic phase of the double tungstates K(RE3+)(WO4)2, where RE3+ denotes ions like Y3+, Gd3+, Yb3+, Er3+ and Lu3+. These crystalline materials have been studied without doping and doped with Yb3+ and Er3+ ions for the generation of laser radiation. <br/>The obtention of the monoclinic phase of these double tungstates has been realized by means of the Top-Seeded Solution Growth (TSSG) method using the potassium pyrotungstate, K2W2O7, as solvent. This was due to the advantages that it offers in front of other solvents. Single crystals with very high optical quality and suitable size for later chemical and physical characterizations have been obtained. <br/>The optical characterization of these materials has been the most extended part of this work and focused, on the one hand, on the host (double tungstates without doping) and, on the other hand, on the active ions. Concerning the host, the determination of the optical spectral range of transparency (that extends from 0.32 to 5.1 micrometers), the orientation of the principal optical axes with respect to the crystallographic axes and the measurement of the refractive indices along the three principal optical directions and for a wavelength range from 450 nm to 1500 nm (refractive index around 2) have been determined.<br/>On the other hand, the optical characterization of the active ions focused on the ytterbium and erbium spectroscopy and the laser experiments realized on ytterbium ions. The spectroscopy concerns, in first place, in the polarized optical absorption of the two ions at room temperature and cryogenic temperature, 6 K (it is remarkable the absorption coefficient of ytterbium, that derived in an absorption length of only 13.3 micrometers for the case of KYb(WO4)2, and in a very high absorption cross-section of 14x10-20 cm2 at 980 nm for the polarization labelled Nm). This absorption length allows think in the possibility of Thin-Disk laser designs and waveguides, and due to the high absorption cross-section, this ion has been used as sensitizer of the erbium emissions. Moreover, the range of absorption wavelength for ytterbium allows the possibility of exciting by means of diode. In second place, the spectroscopy involved the luminescence of these ions under the point of view of the optical emission also at room temperature and low temperature and the measurement of the lifetime. The emission of ytterbium is interesting also for the broadening of the emission lines, that is very interesting for the generation of laser pulses in the pico- and femtosecond regimes. The erbium emissions that have been studied deeper have been the 1530 and 550 nm emissions.<br/>The generation of laser radiation from ytterbium ions has been realized in continuous-wave mode for the case of KLu(WO4)2 doped with ytterbium and in pulsed mode (Quasi-CW) for the case of KYb(WO4)2. Ytterbium ions have been excited by means of a Titanium-Sapphire laser and a diode.