Study Looks Inside 'Beating Heart' of Lasers

TORONTO, June 12 -- A research breakthrough offers the first-ever glimpse inside a laser while it's operating and could lead to more powerful and efficient lasers for fiber optic communication systems.

University of Toronto researchers have conducted a study that enabled them to see "the inner workings of a laser in action," said Ted Sargent, a professor in the Edward S. Rogers Sr. Department of Electrical and Computer Engineering. "We've produced a topographical map of the landscape that electrons see as they flow into these lasers to produce light," he said. The findings could influence laser design, change the diagnosis of faulty lasers and potentially reduce manufacturing costs. The study, which will appear in the June 9 issue of the journal Applied Physics Letters offers direct experimental insight into how lasers function, said Sargent.

INSIDE LOOK: For the first time, researchers looked directly at the forces electrons experience as they travel to the light-producing active region of a laser. They studied how the design and realization of the laser determines the fraction of electrons that contribute usefully to the generation of light.

Lasers are created by growing a complex and carefully designed series of nanometer-sized layers of crystals on a disk of semiconductor material known as a wafer, Sargent said. Ridges are etched into the crystal surface to guide laser light, thin metal layers are added on top and bottom and the wafer is then cut into tiny cubes or chips. During the laser's operation, an electrical current flows into the chip, providing the energy to generate intense light at a specific wavelength used in fibre-optic communications.

In their study, researchers focused on the "beating heart" portion of the laser (called the active region), where electronic energy is converted into light. Using a technique called scanning voltage microscopy, they examined the surface of an operating laser, picking up differences in voltage. These differences translate to a topographical image of the laser's energy surface, allowing researchers to visualize the forces an electron experiences along its path into the active region, Sargent said.

'TOPO' MAP: A nanoscale electrical image of a laser in cross-section. Current from the scanning probe microscope tip flows most readily into the bright regions. The image thus serves as a map of the propensity of the many layers of the semiconductor to admit the flow of electrons. The technique can resolve features, such as the light-producing quantum wells (QWs), on the length scale of twenty nanometers.

The team used its newly acquired information about the inside operations of the laser to determine the fraction of electric current that contributed to producing light. The balance of electrons are undesirably diverted from the active region: such current leakage wastes electrons and heats the device up, degrading performance.

"We used direct imaging to resolve a contentious issue in the field: the effectiveness of electronic funnelling into the active region of a ridge-waveguide laser," saud Dayan Ban, a doctoral candidate who made the measurements. "Previously, uncorroborated models had fueled speculation by yielding divergent results. Now we know where the electrons go." Ban is now a researcher at the Institute for Microstructural Sciences of the National Research Council of Canada.

WHERE ELECTRONS GO: Electrical potential map of the laser ridge. A depression in the n-InP/n-substrate side of the laser revealed to researchers the forces responsible for guiding current into the active (light-producing) region of the device.

"Direct imaging of the functions that drive the action of a living laser could transform how we think about laser 'diagnosis and therapy,'" said Sargent, referring to the measurement and optimization of laser structures and their determination of the devices' inner workings. At present, designers use a variety of computer simulations to

Prof. Edward (Ted) H. Sargent, Nortel Networks-Canada Research Chair in Emerging Technologies, University of Toronto.

model how lasers work, but the University of Toronot research may determine which simulations are the most accurate design tools. "With accurate models," said Sargent, "the designs we can create are more likely to result in devices that meet design requirements."

Co-investigator St. John Dixon-Warren, a physical chemist from Bookham Technology, a UK-based optical components manufacturer located in Kanata, Ontario, said their research could also help diagnose faulty lasers. "If a particular laser fails," says Dixon-Warren, "the kind of measurements that we are taking could provide some idea of why it failed and the design could then be modified."

Sargent said the findings could have larger implications for the creation of optical circuits for fiber optic communication. "If we could fully develop these models and fully understand how lasers work, then we could start to build optical circuits with confidence and high probability of success," he said. "Optical chips akin to electronic integrated circuits in computers must be founded on a deep and broad understanding of the processes at work inside current and future generations of lasers."

The research was supported by Nortel Networks Optical Components (recently acquired by Bookham Technology), the Natural Sciences and Engineering Research Council of Canada, the Ontario Research and Development Challenge Fund, the Canada Foundation for Innovation, the Ontario Innovation Trust and the Canada Research Chairs Program.

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