Researchers demonstrate highly directional
terahertz semiconductor laser
A collaborative team of applied scientists from Harvard
University and the University of Leeds has demonstrated
a new terahertz (THz) semiconductor laser that emits beams
with a much smaller divergence than conventional THz laser
sources. The advance, published in the August 8th issue
of Nature Materials, opens the door to a wide range of applications
in terahertz science and technology. Harvard has filed a
broad patent on the invention.
The finding was spearheaded by postdoctoral fellow Nanfang
Yu and Federico Capasso, Robert L. Wallace Professor of
Applied Physics and Vinton Hayes Senior Research Fellow
in Electrical Engineering, both of Harvard's School of Engineering
and Applied Sciences (SEAS), and by a team led by Edmund
Linfield at the School of Electronic and Electrical Engineering,
University of Leeds.
Terahertz rays (T-rays) can penetrate efficiently through
paper, clothing, plastic, and many other materials, making
them ideal for detecting concealed weapons and biological
agents, imaging tumors without harmful side effects, and
spotting defects, such as cracks, within materials. THz
radiation is also used for high-sensitivity detection of
tiny concentrations of interstellar chemicals.
"Unfortunately, present THz semiconductor lasers are
not suitable for many of these applications because their
beam is widely divergent-similar to how light is emitted
from a lamp" says Capasso. "By creating an artificial
optical structure on the facet of the laser, we were able
to generate highly collimated (i.e., tightly bound) rays
from the device. This leads to the efficient collection
and high concentration of power without the need for conventional,
expensive, and bulky lenses."
Specifically, to get around the conventional limitations,
the researchers sculpted an array of sub-wavelength-wide
grooves, dubbed a metamaterial, directly on the facet of
quantum cascade lasers. The devices emit at a frequency
of 3 THz (or a wavelength of one hundred microns), in the
invisible part of the spectrum known as the far-infrared.
"Our team was able to reduce the divergence angle
of the beam emerging from these semiconductor lasers dramatically,
whilst maintaining the high output optical power of identical
unpatterned devices," says Linfield. "This type
of laser could be used by customs officials to detect illicit
substances and by pharmaceutical manufacturers to check
the quality of drugs being produced and stored."
The use of metamaterials, artificial materials engineered
to provide properties, which may not be readily available
in Nature, was critical to the researchers' successful demonstration.
While metamaterials have potential use in novel applications
such as cloaking, negative refraction and high resolution
imaging, their use in semiconductor devices has been very
limited to date.
"In our case, the metamaterial serves a dual function:
strongly confining the THz light emerging from the device
to the laser facet and collimating the beam," explains
Yu. "The ability of metamaterials to confine strongly
THz waves to surfaces makes it possible to manipulate them
efficiently for applications such as sensing and THz optical
circuits."
Additional co-authors of the study included Qi Jie Wang,
formerly of Harvard University and now with the Nanyang
Technological University in Singapore; graduate student
Mikhail A. Kats and postdoctoral fellow Jonathan A. Fan,
both of Harvard University; and postdoctoral fellows Suraj
P. Khanna and Lianhe Li and faculty member A. Giles Davies,
all from the University of Leeds.
The research was partially supported by the Air Force Office
of Scientific Research. The Harvard-based authors also acknowledge
the support of the Center for Nanoscale Systems (CNS) at
Harvard University, a member of the National Nanotechnology
Infrastructure Network (NNIN). The Leeds-based authors acknowledge
support from the UK's Engineering and Physical Sciences
Research Council.
Quantum Cascade Lasers were first invented and demonstrated
by Federico Capasso and his team at Bell Labs in 1994. At
the shorter wavelengths of the mid-infrared spectrum these
compact millimeter length semiconductor lasers operate routinely
at room temperature with high optical powers and are a rapidly
growing commercial sector for a wide range of military and
civilian applications including infrared countermeasures
and chemical sensing. They are made by stacking ultra-thin
atomic layers of semiconductor materials on top of each
other. By varying the thickness of the layers one can design
the energy levels in the structure to create an artificial
laser medium.