Infrared emission properties of Ho doped KPb 2 Cl 5 U.Hömmerich 1,* , O.Oyebola 1 , E.Brown 1 , S.B.Trivedi 2 , A.G.Bluiett 3 , and J.M. Zavada 4 1 Hampton University, Department of Physics, Hampton, VA 23668 2 Brimrose Corporation of America, 19 Loveton Circle, Baltimore, MD 21152 3 Elizabeth City State University, Dept. of Chemistry and Physics, Elizabeth City, NC 27909 4 North Carolina State University, Dept. of Electrical and Computer Engineering, Raleigh, NC 27695 *e-mail: uwe.hommerich@hamptonu.edu ABSTRACT We report on the optical properties of Ho doped KPb 2 Cl 5 (Ho: KPC) for potential applications as an infrared (IR) solid-state gain medium. The investigated crystal was synthesized from commercial starting materials of PbCl 2 , KCl, and HoCl 3 followed by several purification steps including directional freezing, zone-refinement, and chlorination. The Ho: KPC crystal was subsequently grown by Bridgman technique. Following optical excitation at 885 nm, several IR emission bands were observed at room-temperature with average wavelengths at 1.07, 1.18, 1.35, 1.65, 2.00, 2.89, and 3.96 μm. The emission at 3.96 μm originated from the 5 I 5 → 5 I 6 transitions of Ho 3+ and was further evaluated for possible applications in mid-IR lasers. The decay time of the 5 I 5 excited state was measured to be 5.0 ms at room-temperature. The long 5 I 5 lifetime is favorable for laser applications and indicates that non-radiative multi-phonon relaxations are small in Ho: KPC. Based on a Judd-Ofelt analysis, the emission quantum efficiency was determined to be near unity resulting in a peak emission cross-section of 0.62x10 -20 cm 2 at 3.96 μm. A drawback for laser applications is the long decay time of the lower 5 I 6 state with a value of 4.8 ms . Since the 3.96 μm transition terminates in the 5 I 6 level, its long lifetime will lead to population bottlenecking, which limits possible mid-IR lasing to pulsed and quasi-cw operation. INTRODUCTION Holmium (Ho 3+ ) doped crystals and glasses continue to be of interest for applications as solid-state gain media with laser transitions ranging from the visible (0.55 μm) to the mid-infrared (3.9 μm) spectral region [1-9]. The favorable energy level structure of Ho 3+ leads to several important IR laser transitions centered at ~2 μm ( 5 I 7 → 5 I 8 ), ~2.9 μm ( 5 I 6 → 5 I 7 ), and ~3.9 μm ( 5 I 5 → 5 I 6 ). Lasing at the mid-IR wavelength of 3.9 μm, however, is hampered by strong non-radiative decay of the 5 I 5 excited state and has only been observed in fluoride laser hosts [7-9]. Compared to fluorides, chloride crystals offer a narrower phonon spectrum extending to only ~200-300 cm -1 [1]. The small maximum phonon energy of chlorides provides an effective means to reduce non- radiative relaxations and to achieve high emission efficiencies at mid-IR wavelengths. In this work, we report on the IR emission properties of Ho doped KPb 2 Cl 5 (KPC). KPC has recently emerged as a promising new low-phonon energy host for solid-state lasers [10-14]. KPC is non- hygroscopic and has a maximum phonon energy of only ~200 cm -1 [10]. Results of absorption, emission, and lifetime studies of Ho: KPC are presented and the 3.96 μm emission ( 5 I 5 → 5 I 6 ) is evaluated for applications in mid-IR lasers. Mater. Res. Soc. Symp. Proc. Vol. 1111 © 2009 Materials Research Society 1111-D07-05