Resumen de Estudio de la espectroscopia y acción laser de los sistemas Nd3+: SBN e Yb3+SBN durante transiciones de fase
Ferroelectric materials are materials that exhibit spontaneous polarization in absence of an external electric field. The origin of the spontaneous polarization is a certain atomic or molecular organization and is based on the relative displacement between the ions in the crystal lattice of the material, so ferroelectric materials generally present a non-centrosymmetric structure.
This gives these materials their excellent electro-optical, nonlinear, pyroelectric and piezoelectric properties, which together with its ferroelectric domain structure (regions with different directions of the spontaneous polarization) leads to many practical applications. (Fejer y col. 1992; Zhu y col. 1997; Byer 2000; Wooten y col. 2000; Gopalan y col. 2006) Among all the ferroelectric materials, strontium barium niobate (SrxBa1- xNb2O6, with 0 x 1, commonly abbreviated SBN) is especially interesting because of its excellent photorefractive, electro-optical and pyroelectric properties. (Lenzo y col. 1967; Glass 1969) It presents a spontaneous distribution of ferroelectric domains, which, together with its good nonlinear properties, converts SBN in an excellent frequency converter. (Molina y col. 2008) Moreover, these good intrinsic properties of the SBN matrix can be enhanced by appropriate doping, which in this matrix is particularly easy thanks to its high degree of structural disorder (it supports high levels of doping and the structure of the material remains apparently unaffected). For example, the Ce 3+ or Rh 3+ doped SBN is a reference in the field of photorefractive materials, (for example, it was the first material in which solitons of this type were observed (Duree y col.
1993)), and in the field of optical recording of information. As well, it has recently been used to fabricate three-dimensional nonlinear photonic structures, known as photonic quasi-crystals. (Xavier y col. 2009) At certain temperature, ferroelectric materials suffer a transition to a nonpolar phase (known as paraelectric phase). In SBN, this transition occurs at unusually low temperatures (20 to 200 °C, depending on the value of x), which, from a fundamental point of view, makes this material be a reference in the study of the ferroelectric phase transition. (Glass 1969; David y col. 2004) This phase transition also shows a pronounced thermal hysteresis, which from the applied point of view, is the basis for the development of bistable optical devices. (Sanchez y col. 2004) This thesis work has focused on studying the intrinsic properties of the SBN system combined with the properties of two rare earth ions, which are widely used due to their laser properties: the ion Nd 3+ and Yb 3+ ion.
Nd 3+ ions present excellent properties as optical ion and is widely used as laser ion in the near infrared region of the spectrum when introduced into solid matrices. (Kaminskii 1990) This is due to the ability of Nd 3+ to operate as a four-levels laser system, facilitating the population inversion, to its relatively large energy separation between initial and final level of the laser emission and to its relatively long lifetimes of the emitting level compared with the relaxation times of the final laser level. For these reasons, a broad characterization of the spectroscopic and laser properties of large numbers of Nd 3+ -doped systems exists. (Guy y col. 1998; Koechner 1999) Regarding Yb 3+ ions, from some years ago exists a renewed good faith attempts at development and study of inorganic materials doped with this ion, owing to their applications for the manufacture of solid state lasers sintolengths in the region of wavelength around 1 micron, both continuous and pulsed, scintillators, optical sensors and bistable devices. (Boulon 2008) The good properties of both ions incorporated into the SBN matrix had been the result of previous studies in our group. In the case of the Nd 3+ :SBN system, this investigations had led to the development of an autosum and autodoubling of frequencies laser with a wide frequency tuning range, due to the combination of the good properties of Nd 3+ ion with the nonlinearity and structural disorder of the SBN matrix. (Romero y col. 2000; Romero 2002; Ramirez, Romero, y col. 2005) The system Yb 3+ :SBN was also spectroscopically characterized, having been proved its use as optical probe to follow the ferroelectric phase transition of SBN, and been demonstrated its suitability as laser system, but without empirical demonstration of laser action.
Therefore, in this thesis we tried to obtain a solid-state laser based on Yb 3+ :SBN as well as extending the application of Yb 3+ and Nd 3+ as optical probes to the study of the effect of the ferroelectric phase transition on processes non-studied yet, as well as the effect of the size-reduction, to the order of nanometers, on the properties of SBN.
