Cascaded linear and non-linear operations serve as the backbone of integrated photonic applications, enabling diverse functions from routing to computing. In computing, while incoherent processors have shown excellent speed and parallelization in wavelength, space and time, they require electro-optical components to perform the mathematical operations. Achieving independent modulation and direct summation of multi-wavelength carrier signals within a single waveguide in an entirely optical manner remains a significant challenge. This talk will highlight our recent work in exploiting spatial-degrees-of-freedom of in-plane modes using standing waves, beginning with a wavelength-addressable modulator with non-volatile multi-level operation. I will talk about a recent demonstration of a new photonic framework where the non-local thermo-optic effect is combined, enabling direct addition and all-optical encoding of signals carried in different wavelengths.
Electro-optical computing systems are promising for future energy-efficient computing, while scalable, dual electrical-optical functionality and low energy-consumption electrical-optical devices are not yet implemented on-chip. In this work, with a tailored design for better heat confinement, we implement scalable phase-change devices with dual electrical-optical functionality. They show ultralow programming energy for both electrical and optical switching in a pico-joule scale, which is orders lower than other integrated electro-optical phase change devices, multilevel switching until 20% contrast, good cyclability, and readout in both electrical and optical domains.
Cascaded linear and non-linear operations form the basis for all integrated photonic applications ranging from routing to computations. Because of the weak interaction between photons, achieving substantial optical nonlinearities at low power remains challenging. In this work, we demonstrate a new photonic framework where signals carried in different wavelengths can be added and encoded in a new wavelength with low optical loss. Using this approach, we can realize highly nonlinear all-optical neurons operated fully in the near-infrared domain at low power.
Nonlocal metasurfaces, which have uniform geometric arrangements but respond to incident angles differently, are uniquely capable of processing images. Phase change materials are promising functional building blocks to make metasurfaces reconfigurable owing to the significant refractive index contrast between amorphous and crystalline states. This presentation shows a novel tunable nonlocal metasurface based on the lossless phase change material Sb2Se3. It demonstrates the integration of two on-demand switchable functions, bright field imaging and second-order spatial differential imaging, on a single device.
There is a growing interest in the use of chalcogenide phase-change materials (PCMs) for reconfigurable metasurfaces to realize next-generation compact adaptive optical systems. However, the application of the classic PCM composition such as Ge2Sb2Te5 for near infrared metasurfaces has been limited due to its high absorption in the crystalline state. Here, by using an ultralow-loss and high-index phase-change material Sb2Se3, we show reconfigurable metasurfaces can manipulate light efficiently in near infrared region with comparable efficiencies in both the amorphous and crystalline states of the material.
As integrated photonics is seeing huge progress and renewed research interest, the focus on components that enable demonstrations of components such as tensor cores are becoming vitally important. In this talk I shall present recent progress on work in photonics that aims to enable some of the components required to effectively create on-chip active components for aspects related to computing (Feldmann et al, Nature, 2021; Li et al, IEDM 2020; Rios et al, Sci Adv 2019) as well as for emerging photonic signal processing applications.
Phase change materials, and other functional nanomaterials typically require energy to be applied to them in order to have tunable properties. Typically, they can be tuned either optically, or electrically. However, a fundamental issue is that the interaction size scales of optics and that of electronics are very different - electronics function efficiently (energy/speed-wise) when dimensions are smaller than the wavelengths of light; unfortunately these are smaller than the typical interaction length-scales for optics. This has meant that efficient electro-optical coupling between electronic and photonic switching has been challenging. In this talk, I will talk about recent work within our group of collaborators to integrate concepts from plasmonics to bridge this length-scale disparity in integrated photonics, and present our recent work in this area. Although applied to phase change materials, these concepts are broadly applicable to other functional materials.
Phase change materials are increasingly becoming important functional materials for applications in emerging integrated optics. Since the demonstration of a photonic phase change memory device in 2015, several new applications i this area have emerged ranging from lossless routing to on-chip photonics synapses. More recently the use of these materials in unconventional computing has seen an emerging interest, especially in the areas of optical abacuses and other forms of brain-inspired computing. There have also been advances in non-von Neumann approaches to carry out large-scale matrix multiplications. In this talk, I shall cover these topics and present a future view of these materials, not only in computation, but also in displays and holographic projections.
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