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07 July 2005 – An
Anglo-French team composed of TeraView Ltd., Cambridge
University, the University of Paris 7, and Thales
R&T, has reported the operation of quantum cascade
lasers (QCLs) emitting at 2.8 Terahertz (THz) with
a threshold current of 19 mA at 4K (see S. Dhillon
et al., Applied Physics Letters, August 1, 2005 issue),
the lowest threshold ever measured without the help
of intense magnetic field. 
Fig. 1 Under a bias voltage of 5 V, these
devices yield a dissipated power at a threshold of
just 100 mW, thus offering a concrete solution for
operation with lightweighted and portable closed
cycle Stirling cryocoolers. According to the researchers
this represents an important technological breakthrough,
which is foreseen to positively impact on the commercialization
of THz QCLs. Indeed, thanks to their high power,
compactness and low cost, these far infrared semiconductor
lasers could become the next generation of commercial
THz sources. Promising applications include security
screening, namely the detection of plastic explosives
and other chemical and biological agents, and medical
imaging. TeraView Ltd., the world’s first company
solely devoted to the commercial exploitation of
THz light, has led pioneering research in these fields
developing state of the art imaging systems based
on photoconductive THz generation. Ultra-low threshold current operation
was obtained by embedding the active region of a
recently demonstrated QCL (S. Barbieri et al., Applied
Physics Letters, vol. 85, p. 1674, 2004) into a novel
THz buried waveguide scheme. Such a scheme is based
on surface plasmon waves, formed at the interface
between a metal and a semiconductor. By exploiting
this concept the research team has recently demonstrated
(J. Alton et al., Applied Physics Letters, vol. 86,
p. 71109, 2005) that two-dimensional optical confinement,
i.e. in the directions parallel and perpendicular
to the surface, can be obtained via the simple evaporation
of a metal stripe on top of a semiconductor (see
the left panel of Fig. 1, the shaded area represents
the computed optical mode intensity). This technique
allows for the realization of ridge-free, buried
waveguide-cavities, where the active region is completely
surrounded by the semiconductor material. This improves
the device thermal conductivity, increasing the maximum
operating temperature in continuous wave. In the right panel of Fig. 1,
a schematic layout of the buried waveguide realized
by the authors is reported. It is based on a double
plasmonic confinement obtained by sandwiching the
11.5 microns thick active region between two metal
layers, also providing the device top and bottom
electrical contacts. This so-called “double-metal” geometry
was initially applied to THz QCLs by Williams and
co-workers in 2003 (B. S. Williams et al., Applied
Physics Letters, vol. 83, p. 5142, 2003). 
Fig. 2 Compared to the use of a single
surface metal layer, in this case the optical mode
is bound at two metal-semiconductor interfaces. This
yields a stronger lateral optical confinement, allowing
the fabrication of devices with a width smaller to
the wavelength in the material without reducing the
overlap factor with the electrically pumped active
region. As shown in Fig. 1 the authors have implemented
this double-metal scheme into a buried, 37 micron-wide
waveguide. In order to confine the electrical current,
a final crucial fabrication step consisted in the
realization, via proton-implantation, of high-resistivity
regions on both sides of the top-contact. In Fig. 2 the 4K Voltage/Current
and Light/Current characteristics of a 37 micron-wide,
500 micron-long device are shown. The threshold current
density is of 127 A/cm2, corresponding to a current
density of 19 mA. This represents the lowest value
of any Fabry-Perot cavity QCL reported to date. The
inset of Fig. 2 shows a typical CW spectrum, recorded
at 4K. The emission wavelength is centred at ~ 106 µm
(2.8 TH), and the laser remains mono-mode over the
entire current operating range, with a side mode
suppression ratio of 30 dB. The threshold current
density as a function of temperature is plotted in
Fig. 3 for pulsed and CW operation: maximum temperatures
are 70K and 65K respectively. Such small difference
reflects the device high thermal conductivity, a
result of the buried cavity design. 
Fig. 3 Presently the team of researchers
is concentrating on improving the wafer bonding technique
used to position the active region on top of the
host n+ substrate (see Fig. 1). Recently, by using
a gold-gold bonding, the operating voltage dropped
from 5 to 2V, reducing the dissipated power at threshold
by more than a factor of two. For further information, please
contact:-
Dr Stefano Barbieri
TeraView Limited
Platinum Building
St John’s Innovation Park
CAMBRIDGE
CB4 0WS
UK stefano.barbieri@TeraView.com
Dr Carlo Sitori
Thales Research & Technology
Domaine de Corbeville
91404 Orsay Cedex
FRANCE
carlo.sitori@thalesgroup.com
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