Doppler-free three-photon spectroscopy on narrow-line optical transitions

Abstract

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 $\sim 470$Hz linewidth without a confining potential.