CAMBRIDGE, Mass. – 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
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
"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.