Continuous demand for compact and efficient laser sources, specifically those operating in short-wavelength spectral range, have resulted in dynamic development of both semiconductor and diode pumped solid state lasers. Undoubtedly, semiconductor lasers are presently the most intensively investigated field of active materials and a number of impressive results has been achieved, including violet GaN laser diodes. Nevertheless, solid state lasers are constantly considered as irreplaceable in all applications requiring excellent optical parameters of the beam together with high power levels. The short-wavelength emission and lasing in solid state lasers is typically obtained via harmonic generation or up-conversion phenomena. The latter method, involving either stepwise absorption of photons or energy transfer processes, is specifically applicable to fiber geometry, where high intensity of radiation and waveguiding effect guarantee high up-conversion efficiency.
Trivalent thulium is the activator ion, which energy structure in certain conditions specifically favours a multi-photon or multi-ion pumping mechanisms resulting in emission within the UV-VIS part of spectrum. In low phonon glasses and fibers, luminescence from 1G4 (480 nm), 1D2 (455 nm), as well as 1I6 (287 nm) has been reported, typically involving ESA-type consecutive absorptions of 650 nm photons (3H6→3F2+3F3, 3F4→1G4, 3H4→1D2, 1G4→3PJ) . UV-VIS emission has been also observed under multi-wavelength pumping at 1112 nm, 1116 nm and 1127 nm . Several years ago laser action at 287 nm in Tm3+:ZBLAN fiber was obtained under consecutive ESA of 1064 nm delivered by an Nd3+:YAG laser  – which is to date the shortest wavelength of stimulated emission generated in an optical fiber. Laser experiments with thulium-activated fluoride fibers, however very promising, were all severely hindered by photodarkening effects accompanying excitation of UV-violet radiation in the fibers.
In this work we present our latest results on UV emission properties of bulk ZBLAN glasses doped with thulium and co-doped with ytterbium ions in different concentrations. In particular, we carefully examined the absorption characteristics, as well as concentration-dependant spectra of UV emission from the 1I6 and 1D2 levels obtained under direct and up-conversion excitation. The fluorescence dynamics profiles, recorded for all concentrations, together with excitation spectra enabled discussion of mechanisms responsible for upper levels populating. Moreover, the multi-ion processes resulting in non-radiative depopulation of excited states have been carefully examined and appropriate cross-relaxation rates have been determined, giving further impact to better understanding of the short wavelength optical properties of the investigated system.
This work has been supported by the National Science Centre, Poland, grant number 2011/03/B/ST7/01917.
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