Self-organized nonlinear gratings for ultrafast nanophotonics

Abstract

We present the first demonstration of automatically quasi-phase-matched second-harmonic generation using femtosecond pulses. The high-confinement geometry of silicon-nitride nanophotonic waveguides provides group-velocity matching, which enables efficient second- harmonic generation over a broad bandwidth. We confirm the presence of self-organized gratings by using nonlinear optical microscopy to record the first direct images of photo-induced nonlinear gratings. Furthermore, we demonstrate how these waveguides enable simultaneous 𝜒⁽²⁾ and 𝜒⁽³⁾ nonlinear processes, which we utilize to stabilize a laser frequency comb. Finally, we derive the equations that govern self-organized grating formation for femtosecond pulses and explain how nanophotonic waveguides could enable scalable, reconfigurable nonlinear optical systems.