Three-Photon Doppler-Free Spectroscopy: Experimental Evidence

We report here the first experimental demonstration of Doppler-free three-photon spectroscopy. The Doppler broadening is eliminated by insuring that the sum of the momenta of the three photons simultaneously exchanged between the atom and the laser beams is equal to 0. This is accomplished by making a proper choice of the angles between the three exciting beams. The observation has been made on the sodium $3S_{\frac{1}{2}}-3\mathrm{}{P}_{\frac{1}{2}}$ transition with absorption of two photons and induced emission of one.