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Assistant professor of physics Swastik Kar and associate professor of mechanical and industrial engineering Yung Joon Jung are the first to create a device that integrates both optical and electronic signals to perform the most elementary computational operations. (Photo by TK)

New approach to chip design could yield light speed computing

Every second, your com­puter must process bil­lions of com­pu­ta­tional steps to pro­duce even the sim­plest out­puts. Imagine if every one of those steps could be made just a tiny bit more effi­cient. “It would save pre­cious nanosec­onds,” explained North­eastern Uni­ver­sity assis­tant pro­fessor of physics Swastik Kar.

Kar and his col­league Yung Joon Jung, an asso­ciate pro­fessor in the Depart­ment of Mechan­ical and Indus­trial Engi­neering, have devel­oped a series of novel devices that do just that. Their work was pub­lished Sunday in the journal Nature Pho­tonics.

Last year, the inter­dis­ci­pli­nary duo com­bined their expertise—Kar’s in graphene, a carbon-​​based mate­rial known for its strength and con­duc­tivity, and Jung’s in the mechanics of carbon nan­otubes, which are nanometer-​​sized rolled up sheets of graphene—to unearth a phys­ical phe­nom­enon that could usher in a new wave of highly effi­cient electronics.

They dis­cov­ered that light-​​induced elec­trical cur­rents rise much more sharply at the inter­sec­tion of carbon nan­otubes and sil­icon, com­pared to the inter­sec­tion of sil­icon and a metal, as in tra­di­tional pho­to­diode devices. “That sharp rise helps us design devices that can be turned on and off using light,” Kar said.

This finding has major impli­ca­tions for per­forming com­pu­ta­tions, which, in simple terms, also rely on a series of on-​​off switches. But in order to access the valu­able infor­ma­tion that can be stored on these switches, it must also be trans­ferred to and processed by other switches. “People believe that the best com­puter would be one in which the pro­cessing is done using elec­trical sig­nals and the signal transfer is done by optics,” Kar said.

This isn’t too sur­prising since light is extremely fast. Kar and Jung’s devices—which are the first to inte­grate elec­tronic and optical prop­er­ties on a single elec­tronic chip—represent a crit­ical break­through in making this dream com­puter a reality.

The com­pu­ta­tional mod­eling of these junc­tions were per­formed in close col­lab­o­ra­tion with the group of Young-​​Kyun Kwon, a pro­fessor at Kyung Hee Uni­ver­sity, in Seoul, Korea.

In the new paper, the team presents three such new devices. The first is a so-​​called AND-​​gate, which requires both an elec­tronic and an optical input to gen­erate an output. This switch only trig­gers if both ele­ments are engaged.

The second device, an OR-​​gate, can gen­erate an output if either of two optical sen­sors is engaged. This same con­fig­u­ra­tion can also be used to con­vert dig­ital sig­nals into analog ones, an impor­tant capa­bility for actions such as turning the dig­ital con­tent of an MP3 file into actual music.

Finally, Kar and Jung also built a device that works like the front-​​end of a camera sensor. It con­sists of 250,000 minia­ture devices assem­bled over a centimeter-​​by-​​centimeter sur­face. While this device would require more inte­gra­tion to be fully viable, it allowed the team to test the repro­ducibility of their assembly process.

“Jung’s method is a world-​​class tech­nique,” Kar said. “It has really enabled us to design a lot of devices that are much more scalable.”

While com­puters process bil­lions of com­pu­ta­tional steps each second, improving their capa­bility of per­forming those steps, Kar said, begins with the “demon­stra­tion of improving just one.” Which is exactly what they’ve done.

For more information, visit www.northeastern.edu

 

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