By Andrea Quong

An engineering team at University of California at Berkeley has invented a new kind of mirror that could pave the way for major advancements in semiconductor lasers, affecting everything from high-definition DVD players and optical mice for computers to laser printers, LED displays and telecommunications equipment. 

The research, which appeared in Nature Photonics this month, uses highly reflective mirrors that are 20 times thinner and can accommodate a broader spectrum of light than those currently used. The improvement could result in thinner, lighter lasers that could be packed into a smaller space, more easily manufactured, and eventually incorporated into tinier and tinier integrated optical devices.

“What we have here is a very novel mirror structure,” said U.C. Berkeley professor Connie J. Chang-Hasnain, who co-authored the study with Michael Huang and Ye Zhou, both graduate students in the electrical engineering and computer science department. “Instead of 80 layers, we have only one layer. All of a sudden we are talking about a reduction in mass that translates into speed that can be increased 100 times.”

The research was funded by the National Science Foundation and the Defense Advanced Research Projects Agency (DARPA), the R&D wing of the U.S. Department of Defense. The work is part of DARPA’s $6 million Advanced Precision Optical Oscillator program, which funds research to develop new technology to improve such things as radars that detect slow-moving targets, electronic warfare and communications systems, according to a DARPA spokeswoman in a written reply.

In practical application, this thinner, highly reflective mirror will make possible laser beams in shorter wavelengths like blue-green, which is used in high definition DVD players, and infrared, whose applications are primarily medical and military, said Ms. Chang-Hasnain, who directs U.C. Berkeley’s Center for Optoelectronic Nanostructured Semiconductor Technologies. “These are all wavelengths that are not accessible yet,” Ms. Chang-Hasnain said.

Long-distance fiber-optic communications rely on lasers that emit a single, pure tone. “But for today’s high bandwidth we put in hundreds of different channels,” she said. “If you have a tunable laser, it’s one size fits all.” Currently, laser wavelength is determined by the geometry of the laser, among other things.

Mirrors are an integral part of what makes up a laser. Roughly put, photons of a certain frequency are generated and bounced back and forth between the mirrors over what’s called a “gain medium.” When the medium is saturated, the light energy is transformed into a single wavelength of light – the laser beam.

But one of the more promising semiconductor laser technologies, the vertical-cavity surface-emitting laser, or VCSEL, requires mirrors that reflect 99.9 percent of the light. To achieve that degree of reflectivity, conventional high-grade mirrors are stacked in alternating vertical layers of about 40 pairs, adding up to a thickness of 5 microns. (A micron is a 1/25,000 of an inch).

“Many people have been thinking of ways to make a mirror that is highly reflective but at the same time extremely thin,” said Ms. Chang-Hasnain. “We decided to start with a simple structure and follow our basic intuition.

The mirror, called a "high-index contrast sub-wavelength grating" (HCG), is a single, grooved layer of aluminum gallium arsenide about a quarter of a micron thin--about 200 times thinner than a hair. The grooving design produces contrast between the high-and low-index substances of the gallium arsenic and air, replacing the need for multiple layers of mirrors. Others have also experimented with such grooved mirrors but haven’t succeeded in creating the reflectivity needed to create laser beams.

“We use total contrast. It’s the high contrast that causes the reflection,” Ms. Chang-Hasnain added. “The grooves are high-index material that’s totally surrounded by low-index material” – in this case, air.

The thinner mirror also could make it easier and faster to “tune” lasers to certain wavelengths of light.

“It’s sort of like an organ pipe,” said Kevin Lear, an electrical and computer engineering professor at Colorado State University at Fort Collins. “You’d vary the size of pipe according to the note you want to make…. This mirror potentially makes it possible to tune the laser faster and over a wider range of wavelengths.”

Tunable lasers could be used to get more information on a single fiber-optic wire, for example, or for gas-specific sensors. “You could move a laser wavelength around to tune it into diff. spectroscopic lines,” Mr. Lear explained. Ms. Chang-Hasnain is the founder of Bandwidth9, a Fremont, California startup that made tunable lasers

Among potential applications of the patent-pending, ultra-thin, high-reflection mirror include photovoltaic cells, light-emitting diodes (LEDs), sensors, and computer chips.