Doppler-free optical spectroscopy on the Ba^+ mono-ion oscillator

Abstract

Laser fluorescence spectroscopy has been performed on the two-photon 6²S_1/2 to 5²D_3/2 transition in Ba⁺. Using signals from an individual laser-cooled ion in a rf trap, a laser limited linewidth of less than 3 MHz has been achieved with an effective wavelength of 2.07 μm. The absence of any sidebands in the spectra spaced at the 5.5-MHz oscillation frequency of the ion in the rf quadrupole trap indicates that the ion vibrational motion must be well into the Lamb–Dicke regime. An upper limit of 165 nm is given to the vibrational amplitude.