Polarized spectroscopy of Sm3+ ions in monoclinic KGd(WO4)2 crystals for lasers emitting in the red

Abstract

Single-crystals of potassium gadolinium double tungstate KGd(WO4)2 doped with Sm3+ ions up to 20 at.% were grown from the flux using K2W2O7 as a solvent and a detailed polarization-resolved spectroscopic study of Sm3+ ions was performed with the goal of developing novel materials emitting in the visible spectral range. Polarized absorption spectra were measured and 4f-4f transition intensities of Sm3+ ions were calculated using a modified Judd-Ofelt theory yielding the intensity parameters of Ω2 = 8.027, Ω4 = 7.210 and Ω6 = 2.322 [10−20 cm2] and α = −0.017 [10−4 cm]. Polarized stimulated-emission properties of Sm3+ ions were studied. For the 4G5/2 → 6H9/2 transition in the red, the maximum σSE is 0.55 × 10−20 cm2 at 649.0 nm for light polarization E || Np (luminescence branching ratio: 40.2 %). The radiative lifetime of the 4G5/2 state is 734 μs. The luminescence dynamics from this manifold was analyzed using the Inokuti-Hirayama model. The possible role of 4f – 4f excited-state absorption from the 4G5/2 level in preventing laser operation in the orange and deep red is analyzed. A 0.8 at.% Sm:KGd(WO4)2 laser generated an output power of 37.8 mW at 649 nm, with a laser threshold of 87 mW and a slope efficiency of 17.6 %.