Innovation opens the door to a wide range of applications in photonics and communications.

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
fibres and wave guides,” 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 Centre for Nanoscale Systems
(CNS), a member of the National Nanotechnology Infrastructure Network
(NNIN).