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

Recoil-free as well as Doppler-free spectroscopy was demonstrated on the S-1(0)-P-3(1) 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 S-1(0) and P-3(1)(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.