The birthplace of the digital computer, ENIAC, is using this technology in the rebirth of analog computing. A study by researchers shows that metamaterials can be designed to do “photonic calculus” as a light wave goes through them.
The researchers’ theory has its roots in analog computing. The predecessors of modern computers were mechanical calculators, which used physical elements, ranging from sliding rulers to complex arrays of gears and drive shafts, to represent, store and manipulate numerical information. In the most complicated examples, a user might set the starting values to be computed on a wheel or dial, and then crank a handle a certain number of times. Intervening sets of gears transformed the starting values in a step-wise fashion until the results were produced.
Both mechanical and electronic analog computers were particularly suited to calculating large tables of information. Where the paper-and-pencil method would require tedious and repetitious steps performed on each of the table’s elements, with each step prone to human error, analog computers could perform those steps in parallel, producing the results all at once.
By swapping their mechanical gears and electrical circuits for optical materials that operate on light waves, however, it may once again be analog computers’ time to shine, but this time at the micro- and nanoscale.
“Compared to digital computers, these analog computers were bulky, power hungry, and slow,“ Engheta said. “But by applying the concepts behind them to optical metamaterials, one day we might be able to make them at micro- and nanoscale sizes, and operate them at nearly speed of light using little power.”
“The thickness of our structures can be comparable with the optical wave length or even smaller,” said Vincenzo Galdi of the University of Sannio. “Implementing similar operations with conventional optical systems, such as lenses and filters, would require much thicker structures."
Metamaterials are composites of natural materials, but are designed in such a way that they manipulate electromagnetic waves in ways that are more than the simple sum of their parts. Objects made from natural materials have atoms and molecules that are arranged in certain patterns dictated by the laws of physics and chemistry. Those patterns give natural materials their electromagnetic properties, which in turn determine how they influence the properties of waves. By going to another layer of organization — making patterns of multiple materials at length scales smaller than the waves passing through them, like a series of nanoscopic gold cubes embedded in glass — metamaterial designers can alter waves in ways not possible by simple surfaces or lenses.
For example, a pen sticking out of a glass of water looks bent because the water surface refracts or “bends” the light; light traveling from the pen to the viewer from below the water line has a different angle of refraction than the light traveling from above. Unlike natural materials, however, metamaterials can be designed to produce negative angles of refraction. If the water in the glass exhibits negative refraction, the image of the pen below the water line would not just be bent, but flipped as if viewed in a mirror.
To begin, the researchers created a computer simulation of an ideal metamaterial, one that could perfectly change the shape of the incoming wave profile into that of its derivative. With the ideal metamaterial as a guide, the researchers then constrained their simulations to specific materials suitable for existing fabrication techniques, such as silicon and aluminum-doped zinc oxide.
Once built, these metamaterials could be put to use doing specific computational tasks that are best suited to an analog approach. Taking derivative of an algebraic function, for example, is a task that digital computers must perform by brute force, essentially scanning the curve and evaluating the difference between every pair of neighboring points. Even though modern digital computers can scan the 2-D profile very quickly, the time to complete the task increases along with the size of the profile. A computational metamaterial designed to calculate derivatives with light waves, by contrast, could complete the task nearly instantaneously regardless of the size of the profile, as it would operate on all points at once.
Future research will entail constructing and testing these computational metamaterials in laboratory settings. If successful, the researchers will draw up plans for metamaterials that can perform other mathematical operations, or even solve equations. By encoding the input wave with a mathematical function, then feeding the outgoing wave back to the input, the intervening metamaterial would ultimately produce a wave that would reveal the desired variables within that function.
This story is reprinted from material from The University of Pennsylvania, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.