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Figure 2.4 Two nanotubes grown on the silicon substrate and covered with aluminium
electrodes. Source : http://www.springerlink.com/content/8k4503l258h54387/fulltext.pdf
Photo detector
Photo detectors are based on semiconductor materials and detect IR signals more quickly
than bolometers. Mercury cadmium telluride (HgCdTe), which works at both medium and
long IR wavelengths, is one of the most common materials used in photo detectors;
however its limitation is the need to use a cryostat to manage heat generated in the
device.
Researchers at Northwestern University have developed a less expensive hand-held
infrared imaging device, that does not require cooling. Using an InAs/GaSB type II
superlattice (an atomic assembly of layers of only a few nanometres thick) they have
produced a non-cooled 256x256 pixel camera. The InAs/GaSb type-II strained layer
superlattice (SLS) is of great interest for both mid- and long-wave infrared detection. As
photonic detection is faster than thermal detection it can be used for operations in which
speed is a necessity, for example missile detection.
Another possibility for the enhancement of IR photo detectors is the use of quantum
dots. InGaAs quantum dots, grown by self-assembly on an InGaP matrix show several
advantages for middle wavelength infrared detection, including: absorption of normally
incident light, due to the three dimensional confinement of electrons; higher
responsiveness due to the longer lifetime of excited electrons; and higher operation
temperature due to the low dark current 2 (J. Jiand et al., 2004). Vertical aligned
superlattices of multiple self-assembled Ge island layers, separated by Si spacer layers
on Si substrates also show improved IR photodetection (W. Minsheng et al., 2004).
Optical antenna
An optical antenna is a dipole antenna coupled to a transducer. The size of an optical
antenna is in the range of the detected wavelengths and involves fabrication techniques
with nanoscale spatial resolution. Due to their optical, electrical, and thermal properties
carbon nanotubes have been studied as replacements for traditional antenna materials.
Metallic rods of 50 nm in diameter and 200-1000 nm in length composed of multi walled
carbon nanotubes (MWCNT) can interact with electromagnetic radiation like a dipole
antenna, demonstrating both the polarization and the length antenna effect. The first
effect is characterized by a suppression of the reflected signal when the electric field of
the incoming radiation is polarized perpendicular to the nanotube axis. The second, the
antenna length effect, maximizes the response when the antenna length is a proper
multiple of the half-wavelength of the radiation. These effects can be used in a variety of
optoelectronic devices including IR detectors (Y. Wang et al., 2004).
2 dark current is background electrical noise produced by a photodetector even in the absence of light. It is
normally compensated for by decreasing the operating temperature.
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