Scientists demonstrate highly directional semiconductor lasers

Applied scientists at Harvard collaborating with researchers at 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).