DOI: 10.1007/s00340-002-0951-3 Appl. Phys. B 75, 841–846 (2002) Lasers and Optics Applied Physics B v.g. savitski 1, ✉ n.n. posnov 1 p.v. prokoshin 1 a.m. malyarevich 1 k.v. yumashev 1 m.i. demchuk 1 a.a. lipovskii 2 PbS-doped phosphate glasses saturable absorbers for 1.3-μm neodymium lasers 1 International Laser Center, #65/17 F. Skaryna Ave., 220027 Minsk, Belarus 2 S.I. Vavilov State Optical Institute, 197131, St. Petersburg, Russia Received: 23 January 2002/Revised version: 2 April 2002 Published online: 20 December 2002 • © Springer-Verlag 2002 ABSTRACT Passive mode locking and saturable absorber Q-switching of neodymium lasers at 1.3 μ m with PbS-doped phosphate glasses are demonstrated. Q-switched pulses of 120 ns (0.1 μ J) in duration (energy) and the average output power of 3 mW from a quasi-cw diode-pumped Nd 3+ :KGW laser and ultrashort pulses of a maximum of 250 μ J in energy and 150 ps in duration from a Nd 3+ :YAP laser were obtained. The bleaching decay rate of the samples was found to increase with the Quantum Dot’s size decreasing due to the enhancement of quantum confinement effects for smaller dots and stronger overlapping of the electron and trap state wave functions. PACS 42.60.Fc; 42.60.Gd; 78.47.+p 1 Introduction Glasses doped with IV–VI (PbS, PbSe) low- dimensional semiconductor structures that are Quantum Dots (QDs) have recently been employed as saturable absorbers for solid state lasers emitting in a spectral range from 1 –2.1 μ m. Over the last five years, mode-locking of Cr:forsterite at 1.2– 1.3 μ m [1] and Nd:YAG at 1.06 μ m [2] lasers, as well as Q-switching of flash-lamp- and diode-pumped Er-glass at 1.5 μ m [3–5], diode-pumped Nd 3+ :KGd(WO 4 ) 2 (KGW) at 1.06 μ m [2], flash-lamp-pumped Ho:YAGat 2.1 μ m [6] lasers have been demonstrated. The accent of low-dimensional semiconductors is that, due to the small sizes of the QDs’ quantum confinement effect, which appears for semiconduc- tor crystallites of sizes comparable with exciton Bohr radius (a B ), takes place. Quantum confinement, in return, manifests itself in the blue shift of the energy band gap, and the en- hancement of optical non-linearity in the range of the lowest in energy transition (first excitonic resonance) [7]. PbS is very attractive for QD-doped glass fabrication, since it has large bulk exciton Bohr radius (a B = 18 nm) and narrowband gap (0.41 eV) (for a recent review of the research of the lead salts QDs, see [8]). This allows us to obtain strong quantum con- finement with relatively large crystallites, and to move the position of the absorption peak resonance (first excitonic res- ✉ Fax: +375-17/232-6286, E-mail: savitski@eudoramail.com onance) in a wide spectral range from 3–1 μ m by varying the QDs’ size through the heat treatment of synthesized glasses. Nowadays, there is great interest in saturable absorbers for pico- and nanosecond pulse generation at 1.3 μ m for applica- tions in telecommunications and fiber-sensing. On the other hand, the Q-switched lasers emitting at 1.3 μ m are suitable for pumping Raman lasers, which are an alternative source for obtaining radiation at 1.5 μ m. This paper presents bleaching relaxation measurements of PbS-QD-doped phosphate glasses, as well as their use as passive shutters for mode-locking of a flash lamp pumped Nd 3+ :YAlO 3 (YAP) laser at 1.34 μ m and Q-switching of a quasi-cw diode-pumped Nd:KGW laser operating at 1.35 μ m. 2 Sample preparation The phosphate glass samples were prepared using a P 2 O 5 (56 mol %)-Na 2 O (25 mol %)-ZnO (5 mol %)-AlF 3 (2 mol %)-Ga 2 O 3 (12 mol %) glass system and the technique we used earlier to prepare PbSe QDs-doped glasses [9]. How- ever, to decrease the volatility of the chalcogenide compound in the process of glass synthesis, we used glass modifiers that allow us to get PbS in the batch melt as a result of a chemical reaction occurring in the melt. This was provided by modi- fying the glass composition by adding PbO and ZnS (up to 2.5 wt %). This last-minute formation of PbS allowed us to increase the homogeneity and concentration of the solid solu- tion of the semiconductor in glass samples, obtained after con- ventional room temperature quenching the glass. The samples studied contained 0.4–1.0 wt % of PbS and some extra lead oxide, while sulfur volatilized. Low glass synthesis (about 1100 ◦ C) and glass transition (about 400 ◦ C) temperatures of the glass matrix used also aimed to increase the concentration and uniformity of PbS distribution in the glass and to decrease necessary duration of secondary thermal processing for the formation of PbS QDs in the glass samples. The glass formed after quenching was transparent and slightly yellowish, typi- cal for lead-containing glasses. Secondary thermal treatment of these glass preforms at 390– 420 ◦ C led to their coloring from brownish to black, depending upon the temperature and duration of the annealing. Transmission electron microscopy and secondary X-ray diffractometry studies proved that the coloring corresponded to the nucleation and growth of PbS