This paper shows a way to program the nonlinear behavior of light inside tiny optical waveguides after the devices are made. The team built controllable patterns of second-order nonlinearity — called χ(2) (chi-two) — inside etched waveguides. In plain terms, χ(2) is a material property that lets three light waves interact, for example to convert one color of light into another. Making that property programmable means the device’s optical function can be changed later, not just fixed when it is fabricated.

To do this the researchers created what they call programmable χ(2) holograms. They shine structured light onto photoconductive electrodes — electrodes made from a material whose conductivity changes where light hits it — and that produces spatially varying electric fields. Those fields induce χ(2) locally in the waveguide. By changing the projected light pattern, they can change the induced nonlinear pattern. The paper reports that the quasi-phase matching pattern, which keeps interacting light waves in sync, can be updated in about one second.

This programmable nonlinearity gives control over three-wave mixing, the family of processes enabled by χ(2). The authors demonstrate control of which wavelengths (spectral control), which transverse modes of the waveguide (modal control), and which polarizations of light are mixed. They also used closed-loop, in situ optimization — meaning the device is tuned while it is running — to correct for phase mismatch in more complicated structures. The team shows this works even for width-modulated waveguides and for spiral waveguides longer than 10 centimeters.

The approach also enables useful on-chip measurements. Because the nonlinear pattern can be changed in place, the device can perform direct dispersion measurements of different transverse modes and can be used for on-chip wavefront phase tomography, a way to map the phase of light across the waveguide. More broadly, programmable χ(2) could let designers build cascaded nonlinear systems and reduce the impact of fabrication errors. That is, instead of throwing away devices that deviate from their intended geometry, the nonlinear response can be retuned after fabrication while keeping the advantages of etched waveguides such as tight light confinement and engineered dispersion.