Resonant excitation and all-optical switching of femtosecond soliton molecules

The emergence of confined structures and pattern formation are exceptional manifestations of nonlinear interactions found in a variety of physical, chemical and biological systems(1). Facilitated by optical nonlinearities, solitons enable ultrashort temporal confinement of light and stable propagation despite the presence of dispersion. Such particle-like structures can assemble in stable arrangements, forming 'soliton molecules'(2,3). Recent work has revealed oscillatory internal motions of these bound states, akin to molecular vibrations(4-9), raising the question of how far the 'molecular' analogy reaches, that is, whether further concepts from molecular spectroscopy apply and whether such intramolecular dynamics can be externally driven or manipulated. Here, we probe and control ultrashort bound states in an optical oscillator, using real-time spectral interferometry and time-dependent excitation. For a frequency-swept pump modulation, we analyse the nonlinear response and resolve anharmonicities in soliton interactions that lead to generation of overtones and sub-harmonics. Applying stronger stimuli, we demonstrate all-optical switching between states with different binding separations. These results could be applied to rapid pulse-pair generation and may stimulate the development of future instruments for ultrafast science. By driving ultrafast soliton molecules with an all-optical external perturbation and monitoring their response in real time, a form of spectroscopy of soliton molecules akin to optical spectroscopy of chemical bonds is introduced.