Predicting physical response of an artificially structured material is of particular interest for scientific and engineering applications. Here we use deep learning to predict optical response of artificially engineered nanophotonic devices. In addition to predicting forward approximation of transmission response for any given topology, this approach allows us to inversely approximate designs for a targeted optical response. Our Deep Neural Network (DNN) could design compact (2.6 × 2.6 μm2) silicon-on-insulator (SOI)-based 1 × 2 power splitters with various target splitting ratios in a fraction of a second. This model is trained to minimize the reflection (to smaller than ~ −20 dB) while achieving maximum transmission efficiency above 90% and target splitting specifications. This approach paves the way for rapid design of integrated photonic components relying on complex nanostructures.

Graphene has extraordinary electro-optic properties and is therefore a promising candidate for monolithic photonic devices such as photodetectors. However, the integration of this atom-thin layer material with bulky photonic components usually results in a weak light-graphene interaction leading to large device lengths limiting electro-optic performance.In contrast, here we demonstrate a plasmonic slot graphene photodetector on silicon-oninsulator platform with high-responsivity given the 5 µm-short device length. We observe that the maximum photocurrent, and hence the highest responsivity, scales inversely with the slot gap width. Using a dual-lithography step, we realize 15 nm narrow slots that show a 15-times higher responsivity per unit device-length compared to photonic graphene photodetectors. Furthermore, we reveal that the back-gated electrostatics is overshadowed by channel-doping contributions induced by the contacts of this ultra-short channel graphene photodetector. This leads to quasi charge neutrality, which explains both the previously-unseen offset between the maximum photovoltaic-based photocurrent relative to graphene's Dirac point and the observed non-ambipolar transport. Such micrometer compact and absorption-efficient photodetectors allow for short-carrier pathways in nextgeneration photonic components, while being an ideal testbed to study short-channel carrier physics in graphene optoelectronics. Introduction.Graphene has become a complementary platform for electronics and optoelectronics because of its remarkable properties and versatility(1). A variety of applications exploit graphene's peculiar features to include modulators(2), plasmonic optoelectronics(3-6), photovoltaic devices (7), ultrafast lasers (8), and photo-detection(9, 10). For photo conversion applications the linear and gap-less band structure of graphene results in wavelength-independent absorption (11,12).Moreover, graphene's carrier can be tuned via electrostatically doping, thus modulating light absorption. Due to its superb carrier mobility (13,14), graphene-based absorption enables ultrafast conversion of photons or plasmons to electrical currents or voltages. However, the light-graphene interaction, and consequently the responsivity of graphene-based devices, is usually rather weak due to the geometrical mismatch between graphene's atom-thin thickness and the diffractionlimited optical mode area of photonic components.The first-generation of graphene-based free-space photodetectors (PDs) uses metal-graphenemetal structures(14); choosing different work-functions for the source-and drain contacts results in an asymmetric band structure, thus enabling non-biased band-bending for charge polarity separation, leading to near-zero dark current. Interdigitated metallic contacts, are typically adopted Corresponding AuthorVolker Sorger, sorger@gwu.edu Funding SourcesVS is funded by AFOSR (FA9550-17-1-0377) and ARO (W911NF-16-2-0194).

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with better understanding of the thermodynamics of the photon energy conversion processes re-shaped the landscape of energy conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon recycling schemes reduce the entropy production in the optical energy conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, reduce the thermal emission losses, and achieve non-contact radiative cooling of solar cells as well as of optical and electronic circuitry. Light-matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third-and fourth-generation energy conversion devices, including up-and downconversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy conversion technologies amplifies the role of cost-driven and environmental considerations.This roadmap on optical energy conversion provides a snapshot of the state-of-the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap, where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts and investors.

Electro-optic modulators transform electronic signals into the optical domain and are critical components in modern telecommunication networks, RF photonics, and emerging applications in quantum photonics and beam steering. All these applications require integrated and voltage-efficient modulator solutions with compact formfactors that are seamlessly integratable with Silicon photonics platforms and feature near-CMOS material processing synergies. However, existing integrated modulators are challenged to meet these requirements. Conversely, emerging electro-optic materials heterogeneously integrated with Si photonics open a new avenue for device engineering. Indium tin oxide (ITO) is one such compelling material for heterogeneous integration in Si exhibiting formidable electro-optic effect characterized by unity order index at telecommunication frequencies. Here we overcome these limitations and demonstrate a monolithically integrated ITO electrooptic modulator based on a Mach Zehnder interferometer (MZI) featuring a high-performance half-wave voltage and active device length product, VpL = 0.52 V•mm. We show, how that the unity-strong index change enables a 30 micrometer-short pphase shifter operating ITO in the index-dominated region away from the epsilon-bear-zero ENZ point. This device experimentally confirms electrical phase shifting in ITO enabling its use in multifaceted applications including dense on-chip communication networks, nonlinearity for activation functions in photonic neural networks, and phased array applications for LiDAR.

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