On-chip photonic-neural-network processors have potential benefits in both speed and energy efficiency but have not yet reached the scale to compete with electronic processors. The dominant paradigm is to build integrated-photonic processors using relatively bulky discrete components connected by single-mode waveguides. A far more compact alternative is to avoid explicitly defining any components and instead sculpt the continuous substrate of the photonic processor to directly perform the computation using waves freely propagating in two dimensions. In this talk, I will present our recent work [1] on experimentally realizing this approach with a device whose refractive index as a function of space, n(x,z), can be rapidly reprogrammed. This device combines photoconductive gain with the electro-optic effect in a lithium niobate slab waveguide. Using this device, we performed neural-network inference with up to 49-dimensional input vectors in a single pass.
[1]: T. Onodera*, M.M. Stein*, et al. arXiv:2402.17750 (2024)
On-chip photonic-neural-network processors promise benefits in both speed and energy efficiency but have not yet reached the scale to compete with electronic processors. The dominant paradigm is to build integrated-photonic processors using discrete components connected by single-mode waveguides. A far more compact alternative is to avoid discrete components and instead sculpt a complex and continuous microphotonic medium in which computations are performed by multimode waves controllably propagating in two dimensions. We show our realization of this approach with a device whose refractive index as a function of space can be rapidly reprogrammed. We demonstrate optical computations much larger and more error-resilient than previous photonic chips relying on discrete components. We argue that beyond photonic-neural-network processors, devices with such arbitrarily programmable index distributions enable the realization of a wide range of photonic functionality.
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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