Programmable on-chip nonlinear photonics

Nonlinear optics¹ plays a central role in many photonic technologies, both classical^2-5 and quantum^6-8. However, the function of a nonlinear-optical device is typically determined during design and fixed during fabrication⁹, restricting the use of nonlinear optics to scenarios in which this inflexibility is tolerable. Here we present a photonic device with highly programmable nonlinear functionality: an optical slab waveguide with an arbitrarily reconfigurable two-dimensional distribution of χ⁽²⁾ nonlinearity. The nonlinearity is realized using electric-field-induced χ⁽²⁾ (refs. ^10-16), and the programmability is engineered by massively parallel control of the electric-field distribution within the device using a photoconductive layer and optical programming with a spatial light pattern. To showcase the versatility of our device, we demonstrate spectral, spatial and spatio-spectral engineering of second-harmonic generation by tailoring arbitrary quasi-phase-matching grating structures¹ in two dimensions. The programmability of the device makes it possible to perform inverse design of grating structures in situ, as well as real-time feedback to compensate for fluctuations in operating and environmental conditions. Our work shows that we can break from the conventional one-device-one-function paradigm, potentially expanding the applications of nonlinear optics to situations in which fast device reconfigurability is desirable-such as in programmable optical quantum gates and quantum light sources^7,17-19, all-optical signal processing²⁰, optical computation²¹ and adaptive structured light for sensing^22-24.