On the Nonlinear 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, for exciting phononic combs in molecules, the requisite strong nonlinear couplings need not be readily present. To overcome this limitation, this paper introduces an alternative route for the generation of phononic combs in polar molecules. Theoretically, we investigated the radiation and phononic spectra generated from CO molecule possessing relatively large permanent dipole moment with density matrix formalism. By considering 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 the creation of PFCs is demonstrated and distinguished. The finding could motivate the possible extension of combs to molecular systems to offer new avenues in molecular sciences.