Tech Briefs Magazine - June 2022 - 29

Optics
High-Power Terahertz Laser for Sensing and Imaging
A tunable design could be used for chemical detection in outer space, medical imaging,
and sensing.
Massachusetts Institute of Technology, Cambridge, MA
A
terahertz laser was developed with
high constant power, tight beam pattern,
and broad electric frequency tuning
for a wide range of applications in
chemical sensing and imaging. The optimized
laser can be used to detect interstellar
elements in an upcoming NASA
mission that aims to learn more about
the galaxy's origins. The photonic wire
laser could also be used for improved
skin and breast cancer imaging, detecting
drugs and explosives, and more.
The laser's design pairs multiple
semiconductor-based, efficient wire lasers
and forces them to " phase lock, " or
sync oscillations. Combining the output
of the pairs along the array produces
a single, high-power beam with minimal
beam divergence. Adjustments to
the individual coupled lasers allow for
broad frequency tuning to improve resolution
and fidelity in the measurements.
Achieving all three performance
metrics means less noise and
higher resolution for more reliable and
cost-effective chemical detection and
medical imaging.
Terahertz lasers can send coherent
radiation into a material to extract the
material's spectral " fingerprint. " Different
materials absorb terahertz radiation
to different degrees, meaning each
has a unique fingerprint that appears
as a spectral line. This is especially valuable
in the 1-5 terahertz range; for example,
for contraband detection, heroin's
signature is seen around 1.42 and
3.94 terahertz, and cocaine's at around
1.54 terahertz.
Photonic wire lasers are bidirectional,
meaning they emit light in opposite directions,
which makes them less powerful.
In traditional lasers, that issue is easily
remedied with carefully positioned
mirrors inside the laser's body. But it's
very difficult to fix in terahertz lasers, because
terahertz radiation is so long, and
the laser so small, that most of the light
travels outside the laser's body.
To achieve frequency tuning, tiny
" knobs " were used to change the curTech
Briefs, June 2022
A tiny terahertz laser achieves three key performance goals at once: high power, tight beam, and
broad frequency tuning. (Image: Courtesy of the researchers)
rent of each wire laser, which slightly
changes how light travels through the
laser - called the refractive index.
That refractive index change, when applied
to coupled lasers, creates a continuous
frequency shift to the pair's
center frequency.
The researchers are also building a
system for imaging with high dynamic
range - greater than 110 decibels -
that can be used in applications such as
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skin cancer imaging. Skin cancer cells
absorb terahertz waves more strongly
than healthy cells, so terahertz lasers
could potentially detect them. The lasers
previously used for the task, however, are
massive and inefficient, and not frequency-tunable.
The new chip-sized device
matches or outstrips those lasers in output
power and offers tuning capabilities.
For more information, contact Abby
Abazorius at abbya@mit.edu; 617-253-2709.
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Tech Briefs Magazine - June 2022

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