Recoil-free spectroscopy of neutral Sr atoms in the Lamb-Dicke regime.

Recoil-free as well as Doppler-free spectroscopy was demonstrated on the 1S0-3P1 transition of Sr atoms confined in a one-dimensional optical lattice. By investigating the wavelength and polarization dependence of the ac Stark shift acting on the 1S0 and 3P1(m(J)=0) states, we determined the wavelength where the Stark shifts for both states coincide. This Stark-free optical lattice, allowing the purturbation-free spectroscopy of trapped atoms, may keep neutral-atom based optical standards competitive with single-ion standards.