  In dielectric laser-driven accelerators (DLA), careful tuning of drive-laser
wavelength and structure periodicity is typically required in order to hit the
resonant condition and match the phase velocity of the accelerating wave to the
electron beam velocity. By aggressively detuning (up to 30 mrad) the angle of
incidence of the drive laser on a double grating DLA structure, we show that it
is possible to recover resonant phase matching and maximize the energy
modulation of an externally injected 6 MeV beam in a 800 nm period structure
driven using a 780 nm laser. These results show that it is possible to power
DLA structures away from their design working point, and excite accelerating
fields in the gap with phase profiles that change by a relatively large amount
period-to-period. This flexibility is a key feature of DLAs and a critical
element in the realization of phase-modulation based ponderomotive focusing to
demonstrate MeV energy gain and large capture in a single DLA stage.
