Concentration dependence of fluorescence and lasing efficiency in Cr 2+ :ZnSe lasers Alphan Sennaroglu a, * , Umit Demirbas a , Adnan Kurt a , Mehmet Somer b a Laser Research Laboratory, Department of Physics, Koc ¸ University, Rumelifeneri Yolu, Sariyer, 34450 Istanbul, Turkey b Department of Chemistry, Koc ¸ University, Rumelifeneri Yolu, Sariyer, 34450 Istanbul, Turkey Received 11 August 2005; accepted 26 November 2005 Available online 6 January 2006 Abstract We investigated the effect of Cr 2+ concentration on the fluorescence lifetime, fluorescence efficiency, and lasing performance of Cr 2+ :ZnSe lasers. In the experiments, polycrystalline Cr 2+ :ZnSe samples with concentrations of 0.34 · 10 18 –66 · 10 18 ion/cm 3 were pre- pared by diffusion doping. Spectroscopic measurements performed with Cr 4+ :YAG and Tm 3+ :fiber lasers show that the fluorescence effi- ciency decreases monotonically with increasing active ion concentration. Lifetime measurements and lasing characterization were also performed with a pulsed 1570-nm KTP optical parametric oscillator. In gain-switched operation, the highest output energy was obtained with the Cr 2+ :ZnSe sample having a Cr 2+ ion concentration of 14 · 10 18 cm 3 . The power performance degrades with increasing con- centration due to larger passive losses at the lasing wavelength. Furthermore, the pulse build-up time and the output pulsewidth decrease with pump energy and ion concentration. Finally, with a single set of optics, tunable output could be obtained in the 2520–3050 nm wavelength range. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Solid-state lasers; Mid-infrared lasers; Solid-state spectroscopy; Transition metal ion-doped chalcogenides 1. Introduction First demonstrated by DeLoach et al. [1], chromium- doped zinc selenide (Cr 2+ :ZnSe) lasers provide broadly tunable radiation in the 2–3 lm range and can be used in numerous scientific and technological applications includ- ing vibrational spectroscopy, trace gas detection, atmo- spheric imaging [2–4], and so on. To date, extensive work has been performed on the development of Cr 2+ :ZnSe lasers and different modes of operation have been demon- strated to produce continuous-wave and pulsed laser out- put [1,5–10]. Due to the broad absorption band of the Cr 2+ :ZnSe medium extending from 1500 to 2000 nm, numerous lasers have been demonstrated as pump sources, including Co 2+ :MgF 2 [1,5,11,12], Tm:YALO [7], NaCl:OH  [13], laser diodes [14], and fiber lasers [15,16]. One drawback of these systems is that they are not as widely available as 1-lm solid-state pump lasers such as Nd:YAG. There- fore, an attractive alternative excitation scheme involves the use of 1064-nm-pumped nonlinear frequency convert- ers such as Raman lasers or optical parametric oscillators (OPO). As an example, a BaNO 3 Raman laser operating at 1598 nm has been used in previous studies to pump Cr 2+ :ZnSe lasers [17]. However, due to the hydroscopic nature of BaNO 3 , implementation of such a pump laser in practical systems faces several challenges. In our group, we have been exploring the possibility of exciting Cr 2+ :ZnSe lasers with 1064-nm-pumped KTP OPO’s at 1570 nm. The chemical stability and reasonably high dam- age threshold of the KTP crystal makes this source ideal for the construction of gain-switched Cr 2+ :ZnSe lasers. One important issue that needs to be addressed in this case has to do with the poor overlap of the pump wavelength 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.11.019 * Corresponding author. Tel.: +90 212 338 1429; fax: +90 212 338 1559. E-mail address: asennar@ku.edu.tr (A. Sennaroglu). www.elsevier.com/locate/optmat Optical Materials 29 (2007) 703–708