Probing the Excitations of a Lieb-Liniger Gas from Weak to Strong Coupling.

We probe the excitation spectrum of an ultracold one-dimensional Bose gas of cesium atoms with a repulsive contact interaction that we tune from the weakly to the strongly interacting regime via a magnetic Feshbach resonance. The dynamical structure factor, experimentally obtained using Bragg spectroscopy, is compared to integrability-based calculations valid at arbitrary interactions and finite temperatures. Our results unequivocally underlie the fact that holelike excitations, which have no counterpart in higher dimensions, actively shape the dynamical response of the gas.