Efficient isolation of multiphoton processes and detection of collective states in dilute samples

A novel technique to sensitively and selectively isolate multiple-quantum coherences in a femtosecond pump-probe setup is presented. Detecting incoherent observables and imparting lock-in amplification, even weak signals of highly dilute samples can be acquired. Applying this method, efficient isolation of one- and two-photon transitions in a rubidium-doped helium droplet beam experiment is demonstrated and collective resonances up to fourth order are observed in a potassium vapor for the first time. Our approach provides new perspectives for coherent experiments in the deep UV and novel multidimensional spectroscopy schemes, in particular when selective detection of particles in dilute gas-phase targets is possible.