Researchers demonstrate highly directional terahertz laser rays

A collaborative team of scientists at Harvard and the University of Leeds have demonstrated a new terahertz (THz) semiconductor laser that emits beams with a much smaller divergence than conventional THz laser sources.  The advance, published in NatureMaterials, 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, the 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.