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Giving optical switches the 'contrast' of electronic transistors

In a bid to improve computer power and speed, the University of Pennsylvania (Penn) is experimenting with photonic computing.

Professor Ritesh Agarwal from Penn is focusing his research into finding the right combination and physical configuration of materials that can amplify and mix light waves in ways that are analogous to electronic computer components.

Prof Agarwal and his team claimed to have taken an important step by demonstrating precise control of the mixing of optical signals via tailored electric fields and obtaining outputs with a ‘near perfect’ contrast and ‘extremely large’ on/off ratios. Properties which are key to the creation of a working optical transistor.

"Currently, to compute '5+7,' we need to send an electrical signal for '5' and an electrical signal for '7' and the transistor does the mixing to produce an electrical signal for '12,'" Prof Agarwal said. "One of the hurdles in doing this with light is that materials that are able to mix optical signals also tend to have very strong background signals as well. That background signal would drastically reduce the contrast and on/off ratios leading to errors in the output."

Prof Agarwal explained that this has caused poor results for the computational qualities optical transistors need and that electric transistors have high standards for these qualities to prevent errors.

Finding a material that can serve in optical transistors has proved challenging because of the additional property requirements. Prof Agarwal explained that only ‘nonlinear’ materials are capable of this kind of optical signal mixing.

The team at Penn have said that in order to address this issue, they found a system which had no background signal – a nanoscale ‘belt’ made out of cadmium sulphide. They applied an electric field to this belt and explained that they were able to introduce optical nonlinearities to the system, enabling a signal mixing output that was otherwise zero.

"Our system turns on from zero to extremely large values, and hence has perfect contrast, as well as large modulation and on/off ratios," Prof Agarwal said. "Therefore, for the first time, we have an optical device with output that truly resembles an electronic transistor."

The team is now working towards demonstrating actual computation by integrating a photonic computer with optical interconnects, modulators and detectors.

Author
Bethan Grylls

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