07/30/2014
With
applications in astronomy, sensing, defense and security, and communications, terahertz
detectors are in high demand. To date, terahertz detectors made
from antenna-coupled, bundled, and individual metallic single-walled carbon
nanotubes (CNTs) or even graphene-based devices exhibit very small collection
areas and typically require coupling of the terahertz radiation with antennas.
Recognizing that narrowband, cryogenic,
small-area terahertz detectors serve only limited terahertz-detection
applications, researchers at Rice University (Houston, TX), Tokyo Institute of
Technology (Tokyo Tech; Japan), and Sandia National Laboratories (Livermore,
CA) have successfully collaborated in the development of a large-area,
broadband, polarization-dependent, and room-temperature-operable terahertz
detector that uses an ensemble of single-wall CNTs with mixed chirality.1
Broadband operation
Single-wall CNTs with mixed chirality can absorb radiation at essentially any wavelength in the electromagnetic spectrum via intraband (free-carrier) and interband (excitonic) absorption, a property shared with graphene.
Single-wall CNTs with mixed chirality can absorb radiation at essentially any wavelength in the electromagnetic spectrum via intraband (free-carrier) and interband (excitonic) absorption, a property shared with graphene.
The solid-state CNT-based terahertz detector
begins with aligned ultralong CNTs grown vertically on a silicon substrate
through chemical vapor deposition. They are then transferred to a Teflon
substrate to form a highly flexible film of horizontally aligned CNTs. This
film is strongly polarization-dependent due to the aligned nature of the
nanotubes and collective antenna effects. The n-type region is made
by doping the as-grown p-type CNT film with benzyl viologen; two
gold electrodes complete the p-n junction detector
device (see figure).
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A schematic (a) shows the carbon nanotube (CNT)-based p-n junction
terahertz detector. The current-voltage characteristics (b) are shown for the
device when illuminated by a 2.52 THz beam (red) and when not illuminated
(black). (Courtesy:
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When illuminated with terahertz light at 3.11,
2.52, and 1.39 THz, responsivity values are 2.5, 2.4, and 1.7 V/W,
respectively. The noise-equivalent-power, a figure of merit for photodetectors,
is 20 nW/Hz1/2 at room temperature—a value already
approaching that of existing uncooled terahertz detectors. Through a series of
experiments the research team demonstrated the photothermoelectric nature of
the response, which clears the path towards improving the
noise-equivalent-power by correspondingly improving the device design and the
thermoelectric properties of the CNT thin film.
“This work opens up a new avenue for developing
terahertz detectors that are low cost and flexible,” says François Léonard,
distinguished member of technical staff at Sandia. “We are looking for partners
to further develop the technology and realize its full promise.”
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