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

We demonstrate coherent Doppler-free three-photon excitation of the ¹S₀↔³P₀ optical clock transition and the ¹S₀↔³P₁ intercombination transition in free-space thermal clouds of ⁸⁸Sr 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 ¹S₀↔³P₁ transition enables high-contrast Ramsey spectroscopy with interrogation times comparable to the 21μs natural lifetime using a single near-resonant laser source. Three-photon spectroscopy on the ¹S₀↔³P₀ clock transition, using only laser frequencies nearly resonant with the ¹S₀↔³P₀ and ¹S₀↔³P₁ transitions, enables a reduction in Doppler broadening by two orders of magnitude and a corresponding ~470Hz linewidth without a confining potential.