Non-volatile, on/off control of the quasi-phase matching interaction in a ferroelectric nonlinear optical crystal

Quasi-phase matching (QPM) in ferroelectric crystals enables highly efficient interactions in nonlinear processes and has driven breakthroughs in both classical and quantum photonics. However, these devices have remained inherently static: once fabricated, their function is fixed. Here, we demonstrate the first binary on/off operation of QPM gratings through cyclic periodic polarization-switching of a ferroelectric nonlinear optical material, overturning this long-standing paradigm. Using coercive-field engineering in KTiOPO4, we achieve reliable multi-cycle writing and erasing of QPM gratings without degradation. The devices combine high efficiency (deff ≈ 10 pm/V), non-volatility (zero standby power), and fast switching, reaching kHz operation in bulk crystals and offering a clear path to MHz reconfigurability in thin films with sub-100 V drive. These findings introduce a new class of nonlinear photonic devices, with potential use in tunable quantum sources, programmable frequency converters, and adaptive photonic processors.