Internal state cooling with a femtosecond optical frequency comb

A single femtosecond optical frequency comb is implemented to induce two-photon transitions between molecular vibrational levels to form ultracold molecules. The phase across an individual pulse in the pulse train is sinusoidally modulated with a carefully chosen modulation amplitude and frequency. Piecewise population transfer is fulfilled to the final state by each pulse in the applied pulse train providing a controlled population accumulation in the final, ultracold state. The dynamics of rovibrational cooling is discussed on an example of the KRb molecule.