Points of Lights
Don't Blink Or You Might Miss The Next Big (Quantum) Thing
By Deborah Wormser
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." ďTo me, thatís extremely promising when reviewers donít think
itís possible. When thatís happened, itís been fun showing the reviewers
that the conventional wisdom is incorrect,Ē Evans says.
Photonics is the science of processing or transmitting information
using light. Fiber-optic systems Ė perhaps the fieldís bestknown
application Ė 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
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. ďIsolation
is crucial when you put about 1 billion devices on a single chip of
silicon,Ē 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
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. ďTheir reviewers said we could never get light out, much less create
a laser,Ē he recalls, adding that his team wrote a proposal and
nevertheless received funding from the Air Force starting in 1985. In only seven years, Evansí 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 Evansí 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 Departmentís lab manager and Evanís former
colleague at RCA. Electrical Engineering Chair and Associate Professor
Marc P. Christensen is on the companyís 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 thatís portable and cheap
enough for use in every doctorís 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.