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A wave for the future in faster computing

While most of us use our computers for texting friends, watching videos, or reading the news, virtually everything we do on our electronic devices is actually manipulated by mathematical operations. Information, explained electrical and computer engineering professor Hossein Mosallaei, is carried and processed through devices called logic gates.

For instance, a text message sent from one person to another is first translated from letters into numbers (binary bits), which are then translated into an electronic signal that moves from the sender’s cell phone to the cell tower to the recipient’s phone. From there it is translated back into bits, and then again into letters. Every manipulation of the data along the way stems from the output of the preceding mathematical operations.

The metasurface that electrical and computer engineering professor Hossein Mosallaei and his team created includes two layers that indpenednetly control the phase and amplitude of a light wave. Image courtesy of Hossein Mosallaei.

Electronic communication is a relatively efficient mode of information transfer—but, according to Mosallaei, another form of information transfer has the potential for a 1,000-fold increase in speed and bandwidth over our current technologies. That method is called photonic communication.

Because light waves propagate at 1,000 times greater frequencies than electrons, they have the capacity for fundamentally offering fascinating features. Until recently, one major roadblock stood in the way of realizing photonic computers operating on analogue waves. In a paper recently released online in the Journal of the Optical Society of America B, Mosallaei and his team members have whisked that roadblock aside.

The new work, Mosallaei said, “lays both the theoretical and physical foundation for an entirely new form of computing that will be faster, will have the capacity to transfer 1,000 times more data, and will open up a range of applications we can’t presently imagine.”

Whereas electronic signals are passed along via binary numerical code, photonic signals are light waves and one can perform analogue processing on them. Just as logic gates do mathematics on electronic signals, novel devices can do math on light waves—but only if those devices can independently manipulate the various parts of the wave.

A wave is characterized by three things: its amplitude, or how tall each peak is; its phase, or where those peaks occur on a horizontal axis; and its polarization, or the directionality of the particles contained in the wave. If you want to perform mathematics on light waves, you must control each of these elements independently.

In previous research, Mosallaei’s team created a nanoscopic device that was capable of tailoring only phase and concentrate the light, and it couldn’t do anything to the amplitude or polarity. This time around, the team has paired that earlier device with other devices, which each performs a different function. When stacked on top of each other, the result is a surface that manipulates all three elements virtually simultaneously. “This is the first time we can control amplitude, phase, and polarization to our desire and independent from each other,” said Mosallaei.

In the new paper, the team stacked these metasurfaces in such a way that when a light wave passes through the surface, what comes out on the other side is the wave’s derivative. “We can use it to get the derivative of any function,” said Mosallaei, whose team is now working to create similar structures that output other mathematical solutions, including sums and integrals. “The surface does the mathematical operation on the wave.”

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