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Points of Lights

Don't Blink Or You Might Miss The Next Big (Quantum) Thing

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SMU Professor of Electrical Engineering Gary Evans recently received some good news: Journal reviewers said they thought his proposal for solving one of the most perplexing problems in the emerging field of integrated photonics sounded impossible. "To me, that's extremely promising when the reviewers don't think its' possible." “To me, that’s extremely promising when reviewers don’t think it’s possible. When that’s happened, it’s been fun showing the reviewers that the conventional wisdom is incorrect,” Evans says. Photonics is the science of processing or transmitting information using light. Fiber-optic systems – perhaps the field’s bestknown application – transform telephone conversations into laser-generated signals that travel through thin glass wires to machines that decode the signals at the other end.

A photon is a light quantum, the smallest measurable unit of light. Integrated photonics researchers seek to create circuits that use photons to do what electrons do in electric integrated semiconductor circuits. Evans and Jerome Butler, University Distinguished Professor of Electrical Engineering, think they have hit on a solution to the problem of integrating an optical isolator with other components in a photonic circuit. In electric semiconductor circuits, diodes act as isolators by letting electrons flow in only one direction. “Isolation is crucial when you put about 1 billion devices on a single chip of silicon,” Evans says. The two researchers want to integrate an optical isolator with a tiny semiconductor laser that would let light travel in one direction within a photonic semiconductor circuit and keep it from reflecting back into the laser, where it could create instabilities in the laser’s output.

It is understandable that their peers might be skeptical, Evans says. Researchers around the world have been trying to create integrated photonic isolators since the 1970s and no one has overcome the problem of reflection in photonic circuits. Evans had a similar experience when he worked with lasers at RCA Labs in Princeton, N.J., before joining SMU. In 1984 all semiconductor lasers were edge-emitting, meaning they generated light from the edge of the chip rather than the surface. Evans and his team proposed a surface-emitting laser to the Air Force. “Their reviewers said we could never get light out, much less create a laser,” he recalls, adding that his team wrote a proposal and nevertheless received funding from the Air Force starting in 1985. In only seven years, Evans’ group got light out of the system and demonstrated surface-emitting lasers with performance efficiencies as good as edge-emitting lasers. When he came to SMU in 1992, the Air Force continued to fund Evans’ work, which resulted in a spin-off company, Photodigm in Richardson, Texas.

Photodigm conducts research for the government and manufactures a range of lasers, most of them edge-emitting lasers that have been improved using processes developed for surface-emitting ones, says Evans, who serves as co-founder, vice president and chief technology officer. Another co-founder is Jay Kirk, the Electrical Engineering Department’s lab manager and Evan’s former colleague at RCA. Electrical Engineering Chair and Associate Professor Marc P. Christensen is on the company’s technical advisory board, as is Butler, who worked closely with Evans when he was at RCA and helped lure him to SMU.

Evans has since expanded into medical photonics, working with SMU and Drexel University colleagues on a photodynamic therapy system to treat cancer of the esophagus. Similar laser-based systems are used commercially, but they are large and water-cooled. The team hopes to create a machine that’s portable and cheap enough for use in every doctor’s office. Their design uses arrays of semiconductor lasers, each no bigger than a grain of sand, inserted into the esophagus via a balloon catheter. The patient is given a photosensitive drug that kills cancer cells during a chemical reaction triggered by the lasers.