SC Laser Beam Sharpened

A highly directional semiconductor (SC) laser has been developed that could be used for long range chemical sensing in the atmosphere, including homeland security and environmental monitoring, without requiring bulky collimating optics.

Applied scientists at Harvard University, in collaboration with researchers from Hamamatsu Photonics in Hamamatsu City, Japan, are the first to demonstrate highly directional semiconductor lasers with a much smaller beam divergence than conventional ones, opening the door to a wide range of applications in photonics and communications.

Spearheaded by graduate student Nanfang Yu and Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, all of Harvard's School of Engineering and Applied Sciences (SEAS), and by a team at Hamamatsu Photonics headed by Dr. Hirofumi Kan, general manager of the Laser Group, the findings were published online in the July 28 issue of Nature Photonics and will appear in the September print issue.

Harvard School of Engineering and Applied Sciences' researchers Federico Capasso (right) and Nanfang Yu working at the far-field measurement setup. (Photo: Eliza Grinnell, Harvard School of Engineering and Applied Sciences)

"Our innovation is applicable to edge-emitting as well as surface-emitting semiconductor lasers operating at any wavelength -- all the way from visible to telecom ones and beyond," said Capasso. "It is an important first step towards beam engineering of lasers with unprecedented flexibility, tailored for specific applications. In the future, we envision being able to achieve total control of the spatial emission pattern of semiconductor lasers such as a fully collimated beam, small divergence beams in multiple directions, and beams that can be steered over a wide angle."

While semiconductor lasers are the most commonly used of all lasers, used as light sources in everyday products such as communication devices, optical recording technologies and laser printers, the beams they produce spread out, or diverge, over long distances, like the light from a flashlight. Divergent beams from semiconductor lasers have to be focused or collimated with lenses that typically require meticulous optical alignment, and bulky optics in some cases.

Laser beams with small divergence angles are important for many applications such as free-space communication, remote sensing, and pointing. High directionality is desirable for efficiently coupling laser power into waveguides and optical fibers without the need for lenses.

To create semiconductor lasers with highly directional output, the researchers incorporated a properly tailored metallic structure, named a plasmonic collimator, directly onto the laser facet. The plasmonic collimator consists of an aperture centered on the laser active region and a periodic array of grooves nearby, as shown in the figure. The aperture couples part of the emitted light into surface electromagnetic waves (so-called surface plasmons) on the laser facet.

An illustration showing a quantum cascade laser patterned with a plasmonic collimator which greatly reduces the divergence in the vertical direction. (Image courtesy Capasso Lab, Harvard School of Engineering and Applied Sciences)

As the surface waves propagate on the facet, they are progressively scattered by the grooves and are reemitted into the far field. These beams are in phase when they arrive at the same position in the far field, so that the optical energy is concentrated into a small solid angle.

The collimation effect in the innovative laser resembles that of the phased antenna array (an array of antennas emitting in phase), which has already been widely used in applications such as directional broadcasting and space communication.

In the present work, low beam divergence has been achieved in the vertical direction, parallel to the direction of the polarization of the laser. By replacing the metallic structure with a series of concentric grooves of circular shape one can achieve also a very small divergence in the horizontal direction. This will result in full beam collimation. Preliminary results have shown that this scheme works very well: a divergence of a few degrees in the horizontal and vertical planes has been achieved in a quantum cascade laser, in accordance with simulations.

To get around such conventional limitations, the researchers sculpted a metallic structure, dubbed a plasmonic collimator, consisting of an aperture and a periodic pattern of subwavelength grooves, directly on the facet of a quantum cascade laser emitting at a wavelength of 10 microns, in the invisible part of the spectrum known as the mid-infrared where the atmosphere is transparent.

In so doing, the team was able to dramatically reduce the divergence angle of the beam emerging from the laser from a factor of 25 down to just a few degrees in the vertical direction. The laser maintained a high output optical power and could be used for long range chemical sensing in the atmosphere, including homeland security and environmental monitoring, without requiring bulky collimating optics.

"Such an advance could also lead to a wide range of applications at the shorter wavelengths used for optical communications. A very narrow angular spread of the laser beam can greatly reduce the complexity and cost of optical systems by eliminating the need for the lenses to couple light into optical fibers and waveguides," said Kan.

The team's other authors are graduate student Jonathan Fan, postdoctoral researchers Qijie Wang and Christian Pflügl, research associate Laurent Diehl -- all from Harvard University -- and researchers Tadataka Edamura and Masamichi Yamanishi of Hamamatsu Photonics.

The research was partially supported by Air Force Office of Scientific Research.

Harvard University has filed a patent on the invention.

For more information, visit: www.seas.harvard.edu