A Contact Lens for Lasers

A tiny lens integrated into semiconductor lasers promises better portable chemical sensors.

Researchers at Harvard University and Hamamatsu Photonics have created a highly directional semiconductor laser that should--by averting light loss with a nano-patterned metal coating akin to a contact lens--enable highly sensitive portable chemical sensors and cheaper, more efficient optical communications.

Laser luminary: Federico Capasso, a physicist at Harvard University, pictured with equipment used in testing a more efficient laser that doesn’t require lenses and mirrors. Credit: Harvard University

The new technology consists of a metal grating patterned on the light-emitting face of the laser. The grating decreases the divergence of the laser light by a factor of 25, eliminating the need for bulky, expensive mirrors and lenses. "It's a hell of a lot smaller" than traditional optics, says Alan Willner, who leads the Optical Communications Laboratory at the University of Southern California. "You can integrate it right on the [laser] structure," says Willner, who was not involved in the research. What's more, by adjusting the design of the metal grating, the researchers say, they can tune it to work with any kind of semiconductor laser.

Semiconductor lasers are widely used in everything from DVD players to telecommunication networks. In chemical-sensing systems, they're used to illuminate gaseous compounds whose identity and concentration can be determined by looking at the spectrum of light they emit.

As light travels out of a laser, it spreads out in a cone shape. Even with meticulously aligned lenses and mirrors, some light is wasted. These additional optics are expensive and make laser systems bulky. The Harvard researchers, led by physicist Federico Capasso, attacked the problem by depositing a layer of metal a few hundred nanometers thick on the facet of a laser, then etching it with slits and an aperture. Such metal gratings can be created using conventional fabrication techniques.

A large portion of light coming out the aperture bends 90 degrees and then propagates down the metal grating in a form of energy called surface plasmons. The grooves in the metal grating then scatter this energy into a narrow beam of light.

The researchers demonstrated the technique on a quantum-cascade laser, a type of laser developed by Capasso at Bell Labs in 1994. Quantum-cascade lasers can emit light over a broad spectrum--from the infrared to the visible. Light from the type of quantum-cascade laser used to test the gratings normally diverges at angles ranging from 40 to 80 degrees. In an article published online this week in the journal Nature Photonics, the Harvard and Hamamatsu researchers report that light from their laser diverges by only 2.5 degrees without significantly reducing the laser's power output.

This difference could make it possible to integrate quantum-cascade lasers into portable chemical sensors. Systems incorporating such lasers can detect a wide range of compounds with great sensitivity. However, due to divergence, these lasers are currently impractical for chemical sensors. The lasers aren't powerful enough over long distances to perform sensitive measurements. Bulky lenses improve their sensitivity but make it difficult to transport them.

Bruker Optics, of Billerica, MA, is collaborating with Capasso to develop chemical sensors incorporating compact lasers for the battlefield. "Quantum-cascade lasers would be immensely useful to monitor the output from power plants and automobiles and to detect chemical agents," says Tom Tague, chief scientist at Bruker Optics.

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"This is very exciting work," says Claire Gmachl, a professor of electrical engineering at Princeton University. "This is an important advance for quantum-cascade lasers, but also offers new prospects for other types of semiconductor lasers." The metal gratings "should be useful over a plethora of applications, for any optical signal processing," says Willner.