We present a non-perturbative analysis of a new experimental technique for probing ultracold bosons in an optical lattice by periodic lattice depth modulations. This is done using the time-dependent density-matrix renormalization group method. We find that sharp energy absorption peaks are not unique to the Mott insulating phase at commensurate filling, but also exist for superfluids at incommensurate filling. For strong interactions the peak structure provides an experimental measure of the interaction strength. Moreover, the peak height of the second peak can be employed as a measure of the incommensurability of the system.
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