Abstract

There continues to be great current interest in the development and characterization of solid state gain media for mid-IR lasers operating in the 3-5 μm region. Ho³⁺ doped crystals and glasses with low maximum phonon energies provide an attractive class of gain media for the ~3.9 µm region [1-3]. For example, Tabirian et al. [3] reported pulsed mid-IR lasing at 3.9 μm using a Ho: BaY₂F₈ (BYF) crystal. BYF has a maximum phonon energy of ~ 415 cm⁻¹, which reduces non-radiative decay and improves the emission efficiency of the 3.9 μm laser transition (⁵I₅➔⁵L₆). In this work, comparative studies were performed on the material preparation and infrared spectroscopy of Ho³⁺- doped KPb₂Cl₅ (KPC) and KPb₂Br₅ (KPB) for applications as mid-IR gain media. KPC and KPB are non-hygroscopic crystals with narrow phonon spectra not exceeding ~200 and ~140 cm⁻¹, respectively. The maximum phonon energies of chloride and bromide crystals further reduce non-radiative relaxation compared to fluorides and enhance long wavelength IR emissions from trivalent rare-earth ions. The investigated Ho-doped KPC and KPB materials were synthesized through careful purification of commercial starting materials including directional freezing, zone-refinement, and halogination. The halogination process further removed oxidic impurities and enhanced the quality of the crystals. Under 890 nm laser excitation into the ⁵I₅ level, IR emissions centered at 1.2, 1.7, 2.0, 2.9, and 3.9 μm were observed in both crystals with transition assignments corresponding to ⁵I₆ ➔⁵I₈ , ⁵I₅ ➔⁵I₇ , ⁵I₇ ➔⁵I₈ , ⁵I₆ ➔⁵I₇, and ⁵I₅ ➔⁵I₆, respectively (fig. 1). Further spectroscopic studies were conducted on the 3.9 μm emission arising from the ⁵I₅ ➔⁵I₆ Ho³⁺ transition. The Ho-doped KPC and KPB crystals showed a peak emission cross section of ~ 0.6 × 10⁻²⁰ cm⁻² at ~3.96 μm, which is slightly lower than values reported for Ho-doped fluorides. However, the significantly longer ⁵I₅ emission lifetimes in the Ho-doped KPC and KPB crystals with values of ~5.0 and 3.8 ms, respectively, resulted in higher σ-τ products compared to Ho-doped fluorides. The obtained spectroscopic results indicate that Ho: KPC and Ho: KPB are promising gain media for 3.9 µm laser applications.

© 2011 Optical Society of America