Optical echo spectroscopy and phase relaxation of Nd³⁺ ions in CaF₂ crystals

Abstract

Accumulated photon echoes have been used to investigate the mechanisms of optical dephasing in CaF₂ crystals activated by Nd³⁺ ions. Tunable picosecond laser radiation, which permits the selective excitation of various Nd³⁺ optical centers in the ⁴ I _9/2→⁴ G _5/2, ² G _7/2 transition, is used. The optical phase relaxation times measured at temperatures from 9 to 50 K permit determination of the homogeneous widths of the transitions between the low-lying ⁴ I _9/2 Stark level and three excited ⁴ G _5/2, ² G _7/2 levels, and calculation of the constants of the inter-Stark relaxation transitions in the ground and excited multiplets for the rhombic N and M Nd³⁺ centers in CaF₂ crystals. An analysis of the temperature dependence of the homogeneous linewidth of the transitions between low-lying Stark levels of the ground and excited states shows that the mechanism of optical dephasing in the crystals investigated is described well by direct relaxation processes with resonant inter-Stark absorption of one phonon in the ground and excited states. At T=9 K, the homogeneous linewidth Γ_h in CaF₂ crystals is almost an order of magnitude smaller than Γ_h in disordered CaF₂-YF₃ crystals. This difference can be attributed to the significantly greater spectral phonon density of states in disordered crystals.