Self-organized nonlinear gratings for ultrafast nanophotonics

As devices utilizing femtosecond-duration laser pulses become more commonplace, there is a need for next-generation nonlinear-photonics technologies that enable low-energy femtosecond pulses to be converted from one wavelength to another with high efficiency. However, designing nonlinear materials to operate with femtosecond pulses is challenging, because it is necessary to match both the phase velocities and group velocities of the light. Here, we show that femtosecond laser pulses can generate self-organized nonlinear gratings in nanophotonic waveguides, thereby providing a nonlinear optical device with both quasi-phase-matching and group-velocity matching for second-harmonic generation. We use nonlinear microscopy to uniquely characterize the self-organized nonlinear gratings and demonstrate that these waveguides enable simultaneous ${\chi }^{\left(2\right)}$ and ${\chi }^{\left(3\right)}$ nonlinear processes for laser-frequency-comb stabilization. Finally, we derive the equations that govern self-organized grating formation for femtosecond pulses and uncover the crucial role of group-velocity matching. In the future, nanophotonics with self-organized gratings could enable scalable, reconfigurable nonlinear photonics.