Low-Phonon-Energy Rare-Earth Doped Laser Gain Materials: Crystals vs. Glasses

Development of new solid-state mid-infrared (mid-IR) laser sources for wide range of applications in remote sensing, free-space communications, materials processing, and medicine remains to be a challenge, and demands advanced laser material development. RE
³⁺
ions possess promising emission transitions in the mid-IR spectral region but require host materials with low maximum phonon energies to circumvent competing nonradiative multi-phonon relaxation (MPR), thus preserving high efficiency of mid-IR emission [1–3]. RE
³⁺
doped low-phonon fluoride
$(300-450\ \text{cm}^{-1})$
and ternary chloride crystals
$(200-250\ \text{cm}^{-1})$
are well studied as laser materials for mid-IR. Recently, interest in chalcogenide glasses increased significantly due to their chemical and mechanical durability, wide mid-IR transparency, sufficiently low phonon energies. Among RE
³⁺
ions, trivalent dysprosium Dy
³⁺
, holmium Ho
³⁺
, and erbium Er
³⁺
are known to be the most common laser active ions in solid host materials capable of producing laser output in the 3–5
$\upmu\mathrm{m}$
spectral range [1].