• Physics 17, 52
Novel metamaterial-based architectures supply a promising platform for constructing mass-producible, reprogrammable schemes that carry out computing duties with gentle.
The concept of an analog laptop—a tool that use steady variables quite than zeros and ones—could evoke out of date equipment, from mechanical watches to bombsight gadgets utilized in World Warfare II. However rising applied sciences, together with AI, could reap nice advantages from this computing method. A promising course entails analog computer systems that course of info with gentle quite than with electrical currents. As reported on the 2024 APS March Assembly by Nader Engheta of the College of Pennsylvania, composite media often known as metamaterials supply a robust platform for constructing analog optical computer systems. In latest work, his group demonstrated a metamaterial platform that may very well be mass produced and built-in with silicon electronics [1], in addition to an method for constructing architectures that may very well be reprogrammed in actual time to carry out completely different computing duties [2]. Metamaterial-based analog optical computer systems could at some point carry out sure duties a lot sooner and with much less energy than standard computer systems, says Engheta.
Metamaterials are artificial supplies made by assembling many items which might be every smaller than the wavelength of the sunshine they’re designed to govern. They are often tailor-made to show properties not present in naturally occurring supplies, most famously, a near-zero or unfavourable refractive index. These unique properties can allow distinctive functions, from subwavelength imaging to invisibility cloaking.
The design flexibility of metamaterials impressed a number of teams to discover methods for turning them into computing machines. In 2014, Engheta and collaborators put ahead a primary set of proposals. Their simulations recommended that metamaterials may understand a collection of mathematical operations, together with differentiation, integration, and convolution. The method entails taking an electromagnetic wave as an enter operate and manipulating it via interplay with the metamaterial in order that the output wave corresponds to a desired mathematical transformation of the enter.
5 years later, Engheta’s group realized this proposal experimentally. Working at microwave wavelengths, their scheme concerned a block of a metamaterial with a number of enter and output ports related by waveguides in a suggestions loop. The experiments demonstrated that, for a given enter, the system’s output was the answer of the so-called Fredholm integral equation, which is utilized in fields as numerous as fluid mechanics, antenna design, and quantum-mechanics perturbation principle. To select the metamaterial construction realizing the specified math, the researchers used “inverse design”—an iterative method for fixing optimization issues. The ensuing metamaterial had a nontrivial “Swiss-cheese” construction, with an inhomogeneous distribution of small islands with completely different dielectric properties—air holes, polystyrene, and microwave-absorbing supplies.
Since microwaves indicate cumbersome, impractical setups, a number of analysis teams got down to lengthen related ideas to optical frequencies, demonstrating a wide range of computing schemes. Most of those demonstrations used subwavelength-thin sheets of metamaterials, often known as metasurfaces, to govern gentle propagating in free area and transmitted via the sheet. Metasurface schemes, nevertheless, require subtle, custom-made fabrication processes, which limits the potential for mass manufacturing, says Engheta.
Engheta and his colleagues have now developed an on-chip platform which will overcome such limitations [1]. In contrast to metasurface schemes with free-space gentle propagation, the group’s metamaterial design channels gentle via structured waveguides on a silicon chip. The researchers inverse designed and constructed a micron-sized chip with a construction harking back to their 2019 microwave design: a set of waveguides feeding gentle into and out of a flat cavity containing a Swiss-cheese-like metamaterial. Such a construction could be merely ordered from industrial foundries, says Engheta. In comparison with its microwave cousin, the optical chip does easier math—it multiplies a vector by a matrix, an operation helpful for AI instruments akin to neural networks. To resolve equations, the scheme might want to incorporate suggestions waveguides linking outputs to inputs, as was performed within the microwaves, an engineering problem that the group plans to handle in next-generation chips.
In parallel to the optical work, Engheta is pushing the mathematical talents of analog computer systems utilizing proof-of-principle gadgets at decrease frequencies. The group’s newest end result added an essential new characteristic: reconfigurability—the flexibility of an equation solver to be reprogrammed to carry out completely different math. The scheme consisted of a 5 × 5 module of radio-frequency (45-MHz) components akin to amplifiers and part shifters. The system may very well be reconfigured by controlling the parameters of every of the weather. As an illustration, the researchers had their machine remedy two completely different issues: discovering the roots of a system of polynomials and performing the inverse design of a metastructure. Each issues are nonstationary, that’s, they require a sequence of steps with completely different math operations at every step.
Engheta envisions that this reconfigurability characteristic may finally be carried over to the silicon-photonics chips. One method for doing so would contain depositing a patterned layer of a “phase-change” materials on prime of the system’s waveguides. When heated, such a fabric modifications its index of refraction, affecting gentle propagation within the waveguides and thereby the mathematical operator that such propagation encodes.
The programmable, metamaterial silicon-photonics chip could be a boon for analog optical computing, says Engheta, processing info on the pace of sunshine with a fraction of the vitality wanted to energy the hundreds of thousands of operations {that a} standard digital processor must carry out to resolve the identical duties. “Right here, gentle goes via a waveguide labyrinth, and when it comes out, you get the reply in a single shot,” he says. And since photons, in contrast to electrons, don’t work together with one another, parallel operations may very well be carried out concurrently just by shining gentle at completely different wavelengths via the system. What’s extra, such a tool would have privateness advantages, as a result of it doesn’t require intermediate steps that retailer info right into a doubtlessly hackable reminiscence, says Engheta.
–Matteo Rini
Matteo Rini is the Editor of Physics Journal.
References
- V. Nikkhah et al., “Inverse-designed low-index-contrast buildings on a silicon photonics platform for vector–matrix multiplication,” Nat. Photon. (2024).
- D. C. Tzarouchis et al., “Programmable wave-based analog computing machine: A metastructure that designs metastructures,” arXiv:2301.02850.