Infrared Emission Properties of Ho Doped KPb 2 Cl 5
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Infrared emission properties of Ho doped KPb2Cl5 U.Hömmerich1,*, O.Oyebola1, E.Brown1, S.B.Trivedi2, A.G.Bluiett3, and J.M. Zavada4 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: [email protected] ABSTRACT We report on the optical properties of Ho doped KPb2Cl5 (Ho: KPC) for potential applications as an infrared (IR) solid-state gain medium. The investigated crystal was synthesized from commercial starting materials of PbCl2, KCl, and HoCl3 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 5I5 → 5I6 transitions of Ho3+ and was further evaluated for possible applications in mid-IR lasers. The decay time of the 5 I5 excited state was measured to be 5.0 ms at room-temperature. The long 5I5 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 cm2 at 3.96 µm. A drawback for laser applications is the long decay time of the lower 5I6 state with a value of 4.8 ms . Since the 3.96 µm transition terminates in the 5I6 level, its long lifetime will lead to population bottlenecking, which limits possible mid-IR lasing to pulsed and quasi-cw operation. INTRODUCTION
Holmium (Ho3+) 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 Ho3+ leads to several important IR laser transitions centered at ~2 µm (5I7 → 5I8), ~2.9 µm (5I6 → 5I7), and ~3.9 µm (5I5 → 5I6). Lasing at the mid-IR wavelength of 3.9 µm, however, is hampered by strong non-radiative decay of the 5I5 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 nonradiative relaxations and to achieve high emission efficiencies at mid-IR wavelengths. In this work, we report on the IR emission properties of Ho doped KPb2Cl5 (KPC). KPC has recently emerged as a promising new low-phonon energy host for solid-state lasers [10-14]. KPC is nonhygroscopic and has a maximum phonon energy of only ~200 cm-1 [10]. Results of
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