New Mid-IR Lasers Based on Rare-Earth-Doped Sulfide and Chloride Materials

Applications in remote-sensing and military countermeasures have driven a need for compact, solid-state mid-IR lasers. Due to multi-phonon quenching, non-traditional hosts are needed to extend current solid-state, room-temperature lasing capabilities beyond ∼ 4 (micro)m. Traditional oxide and fluoride hosts have effective phonon energies in the neighborhood of 1000 cm-1 and 500 cm-1, respectively. These phonons can effectively quench radiation above 2 and 4 (micro)m, respectively. Materials with lower effective phonon energies such as sulfides and chlorides are the logical candidates for mid-IR (4-10 (micro)m) operation. In this report, laser action is demonstrated in two such hosts, CaGa2S4 and KPb2Cl5. The CaGa2S4:Dy3+ laser operating at 4.3 (micro)m represents the first sulfide laser operating beyond 2 (micro)m. The KPb2Cl5:Dy3+ laser operating at 2.4 (micro)m represents the first operation of a chloride-host laser in ambient conditions. Laser action is also reported for CaGa2S4:Dy3+ at 2.4 (micro)m, CaGa2S4:Dy3+ at 1.4 (micro)m, and KPb2Cl5:Nd3+ at 1.06 (micro)m. Both host materials have been fully characterized, including lifetimes, absorption and emission cross sections, radiative branching ratios, and radiative quantum efficiencies. Radiative branching ratios and radiative quantum efficiencies have been determined both by the Judd-Ofelt method (which is based on absorption measurements), and by a novel method described herein which is based on emission measurements. Modeling has been performed to predict laser performance, and a new method to determine emission cross section from slope efficiency and threshold data is developed. With the introduction and laser demonstration of rare-earth-doped CaGa2S4 and KPb2Cl5, direct generation of mid-IR laser radiation in a solid-state host has been demonstrated. In KPb2Cl5, predictions indicate that laser operation to 9 (micro)m may be possible, a wavelength previously considered unreachable in a room-temperature, solid-state host