• fluorides
• Laser materials
• rare-hearth
• spectroscopy

Rare-earth doped crystals have been extensively studied since the beginning of the last century, but this does not mean, however, that this class of materials exhausted things to say. The advances on laser field prompted to a deeper investigation in laser active media. The enhancement of the efficiency and attainable power, indeed, requires a more thorough understanding of the processes that occur in the active medium itself. It is well known that the characterization of a material by optical spectroscopy techniques is fundamental in order to assess its perspectives of application

In the first part of the thesis, I report the spectroscopic characterization of an appealing crystal, Ba2NaNb5O15 (BNN), doped with different rare-earth ions: Er3+, Tm3+, and Ho3+. BNN is a ferroelectric crystal having good electro-optical and non linear optical properties, which make it interesting for technological applications. The experimental results are discussed in order to evaluate the potentialities of this material as active medium in solid state laser devices. The broadening of the spectral features, in particular, suggests that BNN is particularly suitable for diode laser pumping and tunable laser operation or for short pulse generation.

The second part of the thesis deals with one interesting phenomenon: the laser cooling of solids. Up to now, almost the totality of the works on laser cooling of solids have been devoted to the optimization of the cooling process, with only short descriptions of the investigated material. Indeed, the most used materials for laser cooling are still the fluorozirconate glasses ZBLAN and ZBLANP. The sixth and the eighth chapters report the spectroscopy and the laser cooling results of a new promising hosts for laser cooling applications: BaY2F8 (BYF) single crystals doped with Tm3+ any Yb3+ ions. One of the most important feature of this crystal is the very low phonon energy that makes this matrix suitable for laser application in the near IR region. BYF also has minimal thermal lensing and is a promising laser host material. Indeed, BYF has long been used as a laser material. The spectroscopic investigation together with the analysis of the optical and mechanical features, confirm the good properties of BYF for optical cooling applications. Laser cooling have been observed and measured in both crystals, with an efficiency that is comparable, and even slightly higher, with that of Yb-doped ZBLAN glasses. Finally, the last part of the thesis is devoted to the study of the influence of up-conversion and cross-relaxation processes in BaYF as a function of the Tm3+ concentration and how these mechanisms can affect the laser cooling efficiency. Then a model suitable to study the effect of multiple emission and re-absorption steps on laser cooling is described.