On the nonlinear rovibrational excitation of phononic frequency combs in molecules

The mechanical analog of optical frequency combs, phononic frequency combs (PFCs), has recently been demonstrated in mechanical resonators via nonlinear coupling among multiple phonon modes. However, the requisite strong nonlinear couplings need not be readily present for exciting phononic combs in molecules. To overcome this limitation, this paper introduces an alternative route for generating phononic combs in polar molecules. Theoretically, we investigated the radiation and phononic spectra generated from CO molecules possessing relatively large permanent dipole moment with density matrix formalism. By considering the rovibronic excitation of the ground-state CO molecule while avoiding the electronic excitation, the contribution of the permanent dipole moment and electric dipole polarizability to creating PFCs is demonstrated and distinguished. The finding could motivate the possible extension of combs to molecular systems to offer new avenues in molecular sciences.