Vacuum Radiative Level Shift and Spontaneous-Emission Linewidth of an Atom in an Optical Resonator

The center frequency and linewidth of the ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ resonance line of barium atoms placed near the center of a concentric optical resonator are studied as functions of cavity tuning. Shifts in the transition center frequency, due to radiative level shifts, and changes in linewidth, due to enhanced and suppressed spontaneous emission, are observed. A QED calculation which explicitly includes the resonator mode density gives good agreement with the data.