Engineers at Harvard's School of Engineering and Applied Sciences have demonstrated a highly versatile, compact and
portable Quantum Cascade Laser sensor for the fast detection of a large
number of chemicals, ranging from infinitesimal traces of gases to
liquids, by broad tuning of the emission wavelength. The potential
range of applications is huge, including homeland security, medical
diagnostics such as breadth analysis, pollution monitoring, and
environmental sensing of the greenhouse gases responsible for global
warming.
The team, which reported its findings in the Dec. 3 issue of Applied Physics Letters, was headed by Federico Capasso,
the Robert L. Wallace Professor of Applied Physics and Vinton Hayes
Senior Research Fellow in Electrical Engineering, and includes graduate
student Benjamin Lee, researchers Mikhail Belkin
and Jim MacArthur, and undergraduate Ross Audet, all of Harvard's
School of Engineering and Applied Sciences. The researchers have also
filed for U.S. patents covering this new class of laser chips.
The broad emission spectrum of the Quantum Cascade Laser
material, grown by a commercial reactor used for the mass production of
semiconductor lasers, is designed using state-of-the-art nanotechnology
by controlling the size of nanometric thin quantum wells in the active
region. An array of 32 lasers, each designed to emit at a specific
wavelength, is then fabricated on a single chip by standard
semiconductor processing techniques to have a size of less than
one-fourth of a dime. A microcomputer individually fires up and tunes
each laser in the array in any desired sequence. This generates a broad
and continuously tunable wavelength spectrum that can be used to detect
a large number of chemical compounds.
"Our versatile laser spectrometer currently emits any wavelengths
between 8.7 and 9.4 microns, in the so-called 'molecular fingerprint
region' where most molecules have their telltale absorption features
which uniquely identify them," Belkin says. "This ability to design a
broad laser spectrum anywhere in the fingerprint region holds the
promise of replacing the bulky and large infrared spectrometers
currently used for chemical analysis and sensing."
The tunability of the laser chip can be extended up to 10-fold and
several widely spaced absorption features can be targeted with the same
chip, which will enable the detection in parallel of an extremely large
number of trace gases in concentrations of parts per billion in volume.
A portable compact spectrometer with this capability would
revolutionize chemical sensing.
"These millimeter-size laser chips exploit the inherent enormous
wavelength agility of state-of-the-art Quantum Cascade Lasers," says
Capasso, who co-invented them in 1994 at Bell Labs. "As a first
application we have shown that these widely tunable and extremely
compact sensors can measure the spectrum of liquids with the same
accuracy and reproducibility of state-of-the-art infrared
spectrometers, but with inherently greater spectral resolution."
The team's co-authors are research associates Laurent Diehl and
Christian Pflügl of Harvard's School of Engineering and Applied
Sciences; Doug Oakley, David Chapman, and Antonio Napoleone of MIT
Lincoln Laboratory; David Bour, Scott Corzine, and Gloria Höfler, all
formerly with Agilent Technologies; and Jérôme Faist of ETH Zurich. The
research was supported by DARPA's Optofluidics Center. The authors also
acknowledge the support of Harvard's Center for Nanoscale Systems, a
member of the National Nanotechnology Infrastructure.
