Optical spectroscopy and decoherence studies of Yb^(3+):YAG at 968 nm

Abstract

The ^2F_(7/2) ↔ ^2F_(5/2) optical transitions of Yb^(3+) doped into Y_3Al_5O_(12) (YAG) were studied for potential quantum information and photonic signal processing applications. Absorption and fluorescence spectroscopy located the energy levels of the ground ^2F_(7/2) and excited ^2F_(5/2) manifolds, allowing inconsistencies between previous assignments of crystal field splittings in the literature to be resolved. These measurements reveal an unusually large splitting between the first and second levels in both the ground and excited multiplets, potentially providing for reduced sensitivity to thermally induced decoherence and spin-lattice relaxation. Spectral hole burning through two-level saturation was observed, determining the excited state lifetime to be 860 μs and resolving ambiguities in previous fluorescence measurements that were caused by the large radiation trapping effects in this material. Optical decoherence measurements using two-pulse photon echoes gave a homogeneous linewidth of 18 kHz for an applied magnetic field of 1 T, narrowing to 5 kHz at 2.5 T. The observed decoherence was described by spectral diffusion attributed to Yb^(3+)−Yb^(3+) magnetic dipole interactions. Laser absorption determined an inhomogeneous linewidth of 3.6 GHz for this transition in this 0.05%-doped crystal, which is narrower than for any other rare-earth-ion transition previously studied in the YAG host. The temperature dependence of the transition energy and linewidth of the lowest ^2F_(7/2) to lowest ^2F_(5/2) transition centered at 968.571 nm measured from 4 K to 300 K was well described by phonon scattering at higher temperatures, with an additional anomalous linear temperature-dependent broadening at temperatures below 80 K. Two magnetically inequivalent subgroups of Yb^(3+) ions were identified when a magnetic field was applied along the 〈111〉 axis, as expected for the D_2 sites in the cubic symmetry crystal, with ground and excited state effective g-values of g_g = 3.40 (3.34) and g_e = 1.04 (2.01), respectively. Together with the convenient diode laser wavelength of this transition, our study suggests that Yb^(3+):YAG is a promising material system for spectral hole burning and quantum information applications.

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