Laser cooling of trapped ytterbium ions using a four-level optical-excitation scheme

Abstract

We demonstrate laser cooling of ¹⁷²Yb⁺ ions in a rf quadrupole trap using an alternative optical-excitation scheme. The metastable 4f¹⁴5d²D_3/2 state of Yb⁺, to which ions decay if the resonance (cooling) transition is driven, is depleted by driving an optical transition at a wavelength of 935 nm, which is much more attractive than the 2.438 μm radiation used previously. This couples the metastable state to a fourth energy level from which the ions decay rapidly to the ground state. Fluorescence line shapes of the cooling transition indicate that small numbers of Yb⁺ ions were cooled in this way to kinetic temperatures below 1 K.