Doppler-free spectroscopy of the lowest triplet states of helium using double optical resonance Ali Dia, Marie Abboud, Pierre-Jean Nacher and Geneviève Tastevin Eur. Phys. J. D, 75 8 (2021) 223 Published online: 11 August 2021 DOI: 10.1140/epjd/s10053-021-00233-1