Light-based analog computing could be far more efficient than digital computing on the micro and nanoscale

Light at the end of the tunnel for analog computing

Advances in metamaterials that can manipulate light could lead to the re-birth of analog computing.

The advent of all-purpose digital computers in the mid-20th Century was revolutionary as abstracting the input data allowed digital computers to be multi-purposed, meaning they could be reprogramed to perform multiple types of calculations, unlike their purpose-built analog predecessors.

Though analog computers still had an advantage in not having to translate, quantize and digitise the information they were calculating, their mechanical and electronic makeup could not compete with the advances in integrated electronic circuits that allowed digital computers to rapidly shrink their footprints and boost their speed.

But researchers at the University of Pennsylvania, The University of Texas at Austin and University of Sannio in Italy, have now shown that metamaterials – composites of natural materials designed to manipulate electromagnetic waves – can be designed to do "photonic calculus" as a light wave goes through them.

The researchers' theoretical material, outlined in the journal Science, can perform a specific mathematical operation on a light wave's profile, such as finding its first or second derivative, as the light wave passes through the material.

Shining a light wave on one side of such a material would result in that wave profile's derivative exiting the other side – a near instantaneous computational operation.

The researchers believe that by swapping their mechanical gears and electrical circuits for optical materials that operate on light waves 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," said Nader Engheta, professor of Electrical and Systems Engineering in Penn's School of Engineering and Applied Science.

"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."

To arrive at their metatmaterial the team 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.

They then constrained their simulations to specific materials suitable for existing fabrication techniques, such as silicon and aluminium-doped zinc oxide.

"The simulation results of the two were almost identical, so we're hopeful we'll be able to do photonic calculus in a physical experiment in the future," Engheta said.

The team believe metamaterials capable of other calculus operations, such as integration and convolution, could also be produced.

Viewing and manipulating this type of light wave profile is an everyday occurrence for applications like image processing, though it is typically done after the light wave has been converted to electronic signals in the form of digital information.

The researchers' proposed computational metamaterials could almost instantly perform such operations on the original wave, such as the light coming in through the lens of a camera, without conversion to electronic signals.

"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."

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