CC BY
35LS-I-12
Cobalt-doped transparent ceramics and glass-ceramics for saturable absorbers of erbium lasers
P. Loiko1, O. Dymshits2, A. Belyaev3, I. Alekseeva2, M. Tsenter2, V. Vitkin4, D. Shemchuk2, A. Zhilin2
1ITMO University, Center for Physics of Nanostructures, Saint-Petersburg, Russian Federation 2S.I. Vavilov State Optical Institute, Laboratory of Optical Glass-ceramics, Saint-Petersburg, Russian Federation
3G.G. Devyatykh Institute of Chemistry of High-Purity Substances IHPS of the Russian Academy of Sciences, Laboratory of High-purity Oxygen-free Glasses, Nizhny Novgorod, Russian Federation
4ITMO University, Faculty of Laser Photonics and Optoelectronics, Saint-Petersburg, Russian Federation
Cobalt ions (Co2+, electronic configuration: [Ar]3d7) embedded in tetrahedral (Td) sites are known for their absorption around 1.5 p,m due to the 4A2^-4T1(4F) transition. The high ground-state absorption cross-sections inherent for transition-metal ions in Td-sites and a spectral overlap between the Co2+ absorption and emission of eye-safe erbium (Er) lasers make Co2+-doped materials attractive for saturable absorbers (SAs) of such lasers. The SAs feature a decreased absorption (bleaching) with increasing incident laser fluence and they are used to generate ns pulses under passive Q-switching (PQS) operation regime. Among the known crystals with Co2+ ions in Td sites, the most widespread is the cubic spinel, MgAhO4. Co2+:MgAhO4 single-crystals are the state-of-the-art SAs for lasers based on Er3+,Yb3+-codoped phosphate glasses emitting at ~1.54 p,m.
The drawbacks of Co2+:MgAhO4 single-crystals are the complicated growth method, polarization-anisotropy of nonlinear properties and an abrupt drop of absorption at above 1.6 p,m, where the emission from Er crystalline lasers (Er3+:Y3AbO12) is observed. Thus, other materials such as transparent ceramics and glass-ceramics (GCs) based on various spinels and doped with Co2+ ions were proposed. The ceramics contain ^m-sized crystalline grains while the GCs contain nm-sized crystals uniformly distributed in the residual glassy matrix. The linear and nonlinear properties of ceramics resemble those of the single-crystals [1]; some specific spinels (e.g., Co2+:ZnAhO4) can be prepared in the ceramic form [2]. Transparent GCs based on spinel nanocrystals are produced in different systems. Their properties are dependent on both the composition of initial Co-doped glasses and the regime of their secondary heat-treatments leading to formation of nanocrystals.
In the present work, we review our achievements in developing SAs using transparent ceramics and GCs based on various spinels. Regarding Co2+-doped ceramics, we describe MgAhO4 and ZnAl2O4 ones sintered by Hot Isostatic Pressing (HIPing) [2]. We also focus on transparent GCs containing MgAl2O4, Mg(Al,Ga)2O4, Li(Al,Ga)5O8 or Co:y-Ga2O3 nanocrystals [3,4]. Furthermore, we propose GCs with novel host crystals for Co2+ ions in Td sites, such as ZnO (zinc oxide) or P-ZmSiO4 (P-willemite) [5].
The Co2+-doped ceramics and GCs are studied with respect to their microstructure (by SEM or TEM and X-ray diffraction), vibronic properties (by Raman spectroscopy), linear optical properties (absorption and luminescence) and nonlinear properties. The absorption saturation characteristics and the recovery times are determined. The laser-induced damage threshold of
ceramics and GCs is compared. Finally, SAs made of various Co2+-doped GCs are applied in PQS lasers based on Er3+,Yb3+-codoped glasses generating ns-long pulses with mJ-level pulse energies at ~1.54 pm [4].
Fig. 1. Transparent GCs based on Co2+:Li(Al,Ga)5O8 nanocrystals: (a) TEM image, inset - size distribution of the nanocrystals; (b) Absorption spectra of the initial glass and GCs, inset - energy levels of Co2+ ions (Td sites);
(c) absorption saturation curve at 1.54 ^m.
References
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