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TECHNOLOGY NEWS
Terahertz laser at
room temperature
Engineers demonstrate first room temperature electrically pumped
semiconductor laser source of T-rays. Breakthrough could greatly
enhance applications ranging from security screening to chemical
sensing.
A
pplied physicists and engineers lasers has been a challenge.
from Harvard University have These devices require
demonstrated the first room cryogenic cooling, limiting
temperature electrically pumped semiconductor everyday application.
laser source of Terahertz (THz) radiation, also “By contrast, this device
known as T-rays. The breakthrough in laser emits T-rays with several
technology has the potential to become a standard hundreds of nanowatts of
Terahertz source to support applications ranging power at room temperature
from security screening to chemical sensing. and microwatts of power at
Spearheaded by research associate Mikhail temperatures easily
Belkin and Federico Capasso, Robert L. Wallace achievable with
Professor of Applied Physics and Vinton Hayes commercially available
Senior Research Fellow in Electrical Engineering, thermoelectric coolers,”
both of Harvard’s School of Engineering and says Belkin. “Further, there is the potential of A photograph of a
Applied Sciences (SEAS). increasing the terahertz output to milliwatt levels bar with 10 terahertz
“Terahertz imaging and sensing is a very by optimising the semiconductor nanostructure of laser sources
promising but relatively new technology that the active region and by improving the extraction developed by the
requires compact, portable and tuneable sources to efficiency of the terahertz radiation.” Harvard University
achieve widespread penetration. Our devices are an To achieve the breakthrough and overcome the engineers. One of the
important first step in this direction,” said temperature limitations of current laser designs, lasers is connected
Capasso. “We believe our laser has great the researchers engineered a room temperature to the contact pad
development potential because the nanoscale infrared Quantum Cascade Laser (QCL) that emits (seen on the left) by
material used was grown by Molecular Beam light at two frequencies simultaneously. The two thin gold wires. A
Epitaxy, a commercial and widely used thin film generation of T-rays occurs at room temperature 2mm-diameter
growth technique which ‘spray paints’ atoms on a inside the laser material via the process of Silicon hyper-
surface one layer at a time.” difference-frequency generation. The frequency of hemispherical lens is
The ability of Terahertz rays to penetrate the emitted radiation is 5 THz (equal to the attached to the facet
efficiently through paper, clothing, cardboard, difference of the two infrared QCL frequencies). of the device to
plastic and many other materials makes them ideal QCLs were invented and demonstrated by collimate the
for use in many applications. For example, a device Capasso and his team at Bell Labs in 1994. The terahertz output. The
emitting T-rays could be used to detect chemical compact millimetre length semiconductor lasers emission frequency is
and biological agents through sealed packages, see operate routinely at room temperature with high 5 THz, corresponding
tumours without causing any harmful side effects, optical powers and are increasingly used in the to a wavelength of 60
and spot defects within materials such as cracks in commercial sector for wide range of applications in microns. (Credit:
the Space Shuttle’s foam insulation. chemical sensing and trace gas analysis. The Courtesy of the
Using lasers in the Terahertz spectral range, devices, made by stacking ultra thin atomic layers Capasso Lab,
which covers wavelengths from 30 to 300µm, has of semiconductor materials on top of each other, Harvard School of
long presented a hurdle to engineers. In particular, are variable and tuneable, allowing an engineer to Engineering and
making electrically pumped room temperature and adjust the energy levels in the structure to create Applied Sciences)
thermoelectrically cooled Terahertz semiconductor artificial laser medium.
www.euroasiasemiconductor.com August 2008
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