Abstract

Laser materials able to give efficient tunable or ultrashort-pulse emission in near infrared under direct lamp or diode laser pumping are in high demand as substitutes for Ti:sapphire. Materials activated with Cr³⁺ in sites of weak crystal field (c.f.) that leaves the ⁴T₂ level below ²E level could posses such qualities since ⁴T₂ → ⁴A₂ broad band emission can have fairly long lifetime (60-100 μsec). This condition could be met in materials with large host cations (such as Sc³⁺, radius r₆=0.89 A) in sites with octahedral coordination; such as cubic Sc₂O₃, attractive also due of the high heat conductivity. In Sc₂O₃ Sc³⁺ ions occupy two crystallographic sites with six-fold oxygen coordination: an axially distorted octahedral site of C_3i symmetry and a site of C₂ symmetry. EPR of weakly Cr³⁺-doped Sc₂O₃ crystal evidenced a unique center of C_3i symmetry [1]; consistent with the preference of Cr³⁺ for octahedral complexes. Nevertheless, the presence of Cr³⁺ centers with C₂ symmetry cannot be completely excluded. The optical spectroscopy of Cr: Sc₂O₃ crystals [2,3] revealed complex absorption and emission spectra which were attributed to Cr³⁺ in C_3i symmetry (the majority spectrum labeled as center I) and to an unknown center (II) [3] that could be a Cr³⁺ center of C₂ symmetry or a Cr ion of different valence. The high melting temperature of Sc₂O₃ (~2420°C) makes the crystal growth from melt very difficult.

© 2009 IEEE