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

摘要

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