  The emergence of confined structures and pattern formation are exceptional
manifestations of concurring nonlinear interactions found in a variety of
physical, chemical and biological systems[1]. Optical solitons are a hallmark
of extreme spatial or temporal confinement enabled by a variety of
nonlinearities. Such particle-like structures can assemble in complex stable
arrangements, forming "soliton molecules"[2,3]. Recent works revealed
oscillatory internal motions of these bound states, akin to molecular
vibrations[4-8]. These observations beg the question as to how far the
"molecular" analogy reaches, whether further concepts from molecular
spectroscopy apply in this scenario, and if such intra-molecular dynamics can
be externally driven or manipulated. Here, we probe and control such ultrashort
bound-states in an optical oscillator, utilizing real-time spectroscopy and
time-dependent external perturbations. We introduce two-dimensional
spectroscopy of the linear and nonlinear bound-state response and resolve
anharmonicities in the soliton interaction leading to overtone and sub-harmonic
generation. Employing a non-perturbative interaction, we demonstrate
all-optical switching between distinct states with different binding
separation, opening up novel schemes of ultrafast spectroscopy, optical logic
operations and all-optical memory.
