Doppler-free selective reflection spectroscopy of electric-quadrupole transitions

Electric-dipole-forbidden transitions play an important role in quantum sensing, quantum information, and fundamental tests in physics. As such, the development of novel and sensitive spectroscopic methods is of major interest. Here, we present a Doppler-free selective reflection experiment on the 6²S_1/2-5²D_5/2 electric-quadrupole transition of cesium vapor at the vicinity of a sapphire window. The spectrum is obtained by precision experiments overcoming limitations due to the small-signal amplitude of forbidden transitions. The observed sub-Doppler lines allow us to measure the collisional broadening of the electric-quadrupole line. The interaction of cesium atoms with the sapphire surface of the cell is also evidenced, but, due to its weak contribution, a quantitative analysis remains challenging. Our experiment paves the way for further studies of the Casimir-Polder interaction between exotic excited-state atoms and dielectric surfaces.