We demonstrate coherent Doppler-free three-photon excitation of the $^{ 1}S_{ 0}$$\leftrightarrow$$^{ 3}P_{ 0}$ optical clock transition and the $^{ 1}S_{ 0}$$\leftrightarrow$$^{ 3}P_{ 1}$ intercombination transition in free-space thermal clouds of $^{ 88}$Sr atoms. By appropriate orientation of the wavevectors of three lasers incident on the atoms, the first-order Doppler shift can be eliminated for all velocity classes. Three-photon excitation of the $^{ 1}S_{ 0}$$\leftrightarrow$$^{ 3}P_{ 1}$ transition enables high-contrast Ramsey spectroscopy with interrogation times comparable to the 21$\mu$s natural lifetime using a single near-resonant laser source. Three-photon spectroscopy on the $^{ 1}S_{ 0}$$\leftrightarrow$$^{ 3}P_{ 0}$ clock transition, using only laser frequencies nearly resonant with the $^{ 1}S_{ 0}$$\leftrightarrow$$^{ 3}P_{ 0}$ and $^{ 1}S_{ 0}$$\leftrightarrow$$^{ 3}P_{ 1}$ transitions, enables a reduction in Doppler broadening by two orders of magnitude and a corresponding $\sim470$Hz linewidth without a confining potential.
Categories: physics.atom-ph physics.optics quant-ph