July 28, 2008 Applied scientists at Harvard University in collaboration with researchers from Hamamatsu Photonics in Hamamatsu City, Japan, have demonstrated, for the first time, highly directional semiconductor lasers with a much smaller beam divergence than conventional ones. The innovation opens the door to a wide range of applications in photonics and communications. Harvard University has also filed a broad patent on the invention.
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 28th issue of Nature Photonics and will appear in the September print issue.
"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 widely used in everyday products such as communication devices, optical recording technologies, and laser printers, they suffer from poor directionality. Divergent beams from semiconductor lasers are focused or collimated with lenses that typically require meticulous optical alignment—and in some cases bulky optics.
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 sub-wavelength grooves, directly on the facet of a quantum cascade laser emitting at a wavelength of ten 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 twenty-five 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 Dr. 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—both from Hamamatsu Photonics.
The research was partially supported by Air Force Office of Scientific Research. The Harvard authors also acknowledge the support of Harvard's Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN).
Lasers are often considered to be highly directional light sources: their beams are able to propagate over long distances without substantial spreading. This, however, is not always the case. Semiconductor lasers, the most commonly used among all lasers, suffer from a large beam divergence. Such divergence is governed by the principle of diffraction, which predicts bending and spreading of light around small obstacles or apertures. Light beams endure strong diffraction when emerging from the small light-emitting regions of semiconductor lasers (the dimensions of which are comparable to the laser wavelength). This leads to a beam divergence angle of tens of degrees for most semiconductor lasers.
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. Beam collimation is usually achieved using lenses or other bulky optical devices that typically require meticulous alignment.
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. 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. Stated slightly differently, grooves in the plasmonic collimator act essentially as an array of coherent light sources that interfere constructively so that optical energy is projected into the far field in a single direction perpendicular to the laser facet with small divergence. 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.
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