We report on the realization of an on-chip waveguide platform capable of creating arbitrary two-dimensional refractive index profiles in situ and in real-time. The device exhibits complex multimode dynamics which we train to perform machine learning. We tune the refractive index profile in situ using a backpropagation algorithm to perform audio and image classification with up to 50-dimensional inputs. The two-dimensional programmability is realized by sandwiching a photoconductive film and a lithium niobate slab waveguide between two flat electrodes. While applying voltage between the electrodes, we program the effective index of the waveguide by projecting different light patterns onto the photoconductive film. The effective index increases by 10^-3 in illuminated regions via the electro-optic effect, free from any measurable memory effects or cyclic degradation. In conclusion, we developed a photonics platform with versatile spatial programmability that opens new avenues for optical computing and photonic inverse-design.
Resonant metasurfaces present an excellent platform for a variety of mid-IR devices, ranging from linear passive to nonlinear active, with everything in between. The key attractions of such metasurfaces are: their ultra-thin (sub-wavelength) format, strong field enhancement, and high-Q response. I will describe three applications currently under development in our group. First, I will demonstrate how a multi-resonant Si metasurface can be used for making perfectly efficient diffraction gratings. We analytically show that at least four independent resonances are required. Experimental realizations of such gratings will be presented, and high-contrast between targeted and parasitic diffraction orders will be demonstrated. Second, I will present an experimental demonstration of a highly nonlinear high-Q Si metasurfaces whose optical properties rapidly change while the pulse is “trapped” by the structure. The metasurfaces were designed to exhibit sharp resonances in the 3–4 μm spectral range. Third harmonic generation spectroscopy and pump–probe spectroscopy revealed the enhancement of coherent nonlinearities and free-carrier-induced by orders of magnitude compared with unpatterned silicon film without compromising the bandwidth. We show that a phenomenon of photon acceleration (PA) manifests in tunable harmonics generation. The PA phenomenon paves the path towards high-efficiency broadband nonlinear photonics. Finally, I will discuss the possibility of making a efficient generator/modulator of the polarization state of light using thermally tunable high-Q metasurfaces. Our experimental results indicate that such photonic structures enable compact polarimeters and ellipsometers.
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