Accessing finite-momentum excitations of the one-dimensional Bose-Hubbard model using superlattice-modulation spectroscopy

We investigate the response to superlattice modulation of a bosonic quantum gas confined to arrays of tubes emulating the one-dimensional Bose-Hubbard model. We demonstrate, using both time-dependent density-matrix renormalization-group and linear response theory, that such a superlattice modulation gives access to the excitation spectrum of the Bose-Hubbard model at finite momenta. Deep in the Mott insulator, the response is characterized by a narrow energy-absorption peak at a frequency approximately corresponding to the on-site interaction strength between bosons. This spectroscopic technique thus allows for an accurate measurement of the effective value of the interaction strength. On the superfluid side, we show that the response depends on the lattice filling. The system can either respond at infinitely small values of the modulation frequency or only above a frequency threshold. We discuss our numerical findings in light of analytical results obtained for the Lieb-Liniger model. In particular, for this continuum model, bosonization predicts power-law onsets for both responses.