Materials Today

Volume 15, Issue 11, November 2012, Pages 473


Electronic goes photonic: Optical Materials

The currency of computing is the electron. However, a future all-optical computer, which uses photons instead, comes a small step closer thanks to the development of a new type of photonic switch by researchers at the University of Pennsylvania. The team has built the first all-optical photonic switch from cadmium sulfide nanowires and combined them into a logic gate, a building block of the computer chip.

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Ritesh Agarwal and colleagues have built on earlier research in which they demonstrated that cadmium sulfide nanowires show extremely strong light-matter coupling [Agarwal et al., Nature Nanotechnol (2012) 7, 640–645; doi: 10.1038/nnano.2012.144]. This phenomenon is key to the efficient manipulation of light just as electrical modulation by a gate electrode is crucial to handling electrons. The discovery will reduce the bulkiness of photonic components as well as dramatically cutting their energy demands.

“The biggest challenge for photonic structures on the nanoscale is getting the light in, manipulating it once it's there and then getting it out,” Agarwal explains. “Our major innovation was how we solved the first problem, in that it allowed us to use the nanowires themselves for an on-chip light source.”

By cutting a precisely defined gap into a nanowire and then pump-priming the first segment to emit laser light from its end and through the gap the team was able to demonstrate efficient absorption and transmission of photons. The fact that they used a single, albeit modified, wire was important to success because the ends exposed to each other at the gap are perfectly matched.

It is a relatively simple matter to make this nanowires gap act as a binary switch reflecting a 0 and a 1 state. “Once we have the light in the second segment, we shine another light through the structure and turn off what is being transported through that wire,” Agarwal adds. “That's what makes it a switch.”

The team has connected several of these switches into a Y-shaped configuration to make a logic gate the behavior of which manifests as a NAND operator. NAND is equivalent to “not and.” It gives a “0” output if both of its inputs are “1.” Sequences of NAND gates can themselves carry out all other Boolean logic functions and so form the basis of a generalized computer processor.

“The next step in our research is to find the right material system for optimized light-matter coupling to configure more efficient optical switches,” Agarwal told Materials Today. “Following which we will focus on better in-coupling schemes for the pump and probe optical beams to once again improve the switching process.”