Switches for an Optical Transistor

Prototype of an optical transistor to precisely control the mixing of optical signals via tailored electric fields, and obtaining outputs with a near perfect contrast and extremely large on/off ratios. (Source: S. Dhara)

Current computer systems represent bits of infor­mation, the 1’s and 0’s of binary code, with elec­tricity. Circuit elements, such as tran­sistors, operate on these electric signals, producing outputs that are dependent on their inputs. As fast and powerful as computers have become, Ritesh Agarwal, professor in the Depart­ment of Materials Science and Engi­neering in the Univer­sity of Penn­sylvania’s School of Engi­neering and Applied Science, knows they could be more powerful. The field of photonic compu­ting aims to achieve that goal by using light as the medium.

Agarwal’s research on photonic compu­ting has been focused on 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. He and his colleagues have taken an important step: precisely control­ling the mixing of optical signals via tailored electric fields, and obtaining outputs with a near perfect contrast and extremely large on/off ratios. Those proper­ties 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,” 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.” With back­ground signals washing out the intended output, necessarily compu­tational qualities for optical tran­sistors, such as their on/off ratio, modu­lation strength and signal mixing contrast have all been extremely poor. Electric transistors have high standards for these qualities to prevent errors.

The search for materials that can serve in optical tran­sistors is complicated by addi­tional property require­ments. Only non­linear materials are capable of this kind of optical signal mixing. To address this issue, Agarwal’s research group started by finding a system which has no back­ground signal to start: a nanoscale belt made out of cadmium sulfide. Then, by applying an electrical field across the nanobelt, Agarwal and his colleagues were able to intro­duce optical nonlinearities to the system that enable 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,” Agarwal said. “Therefore, for the first time, we have an optical device with output that truly resembles an elec­tronic transistor.” With one of the key compo­nents coming into focus, the next steps toward a photonic computer will involve inte­grating them with optical inter­connects, modu­lators, and detectors in order to demonstrate actual compu­tation. (Source: PSU)

Reference: M.-L. Ren et al.: Strong modulation of second-harmonic generation with very large contrast in semiconducting CdS via high-field domain, Nat. Commun. 9186 (2018); DOI: 10.1038/s41467-017-02548-3

Link: Dept. of Materials Science and Engineering, Univ. of Pennsylvania, Philadelphia, USA

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