Page 217 - Organic Electronics in Sensors and Biotechnology

194 Cha pte r S i x

photodetectors, and we investigate how OPDs match up against other
technologies. Finally, we consider some promising application areas
where OPDs look set to find important uses. Other unexpected applica-
tions are bound to emerge in the coming years, and in writing this chap-
ter, we hope to stimulate exactly this outcome by encouraging further
research in this hugely important area.
The optical detector is one of the cornerstones of modern technology,
and it plays a critical role in numerous applications including imaging,
communications, data retrieval, proximity and motion detection, envi-
ronmental monitoring, and chemical analysis, to name but a few. In crude
terms, optical detection can be divided into two broad categories: data
communications (“data comms”), where light is used to carry an encoded
signal, and sensing, where it is the properties of the light itself that are of
interest. The frequency, intensity, and spectral characteristics of the optical
signal vary tremendously from one application to the next. In fiber-optic
communications systems, photodetectors are used to receive infrared (IR)
signals at rates of up to 100 GHz. In optical disc drives, they retrieve data
at up to 200 MHz by detecting visible laser light reflected from the pits of
a spinning disc. They are also used in lower bandwidth applications, such
as remote controls for electronic equipment, optical “trip switches” for
home security systems, proximity detectors for motor vehicles, and
orientation/position sensors in optical-mouse devices. In industry, pho-
todetectors play a critical role in process control where they are used to
ensure the correct positioning of components, to monitor product
throughput, and to provide real-time information for the feedback con-
trol of robotic systems. In chemical and biological analysis, they are used
to monitor changes in absorption, fluorescence, chemiluminescence, or
refractive index due to the presence of specific analytes. In environmental
monitoring, optical detectors are used to determine the concentration of
air- or waterborne particulates by monitoring the frequency of optical
scattering events—a technique that can also be used to determine particle
size via the autocorrelation of the scattering signal.
One-dimensional sensors based on arrays of CMOS photodiodes
or charge coupled devices (CCDs) are used for optical-scanning appli-
cations and for optical detection in spectrographs. Two-dimensional
image sensors based on the same technologies form the basis of digi-
tal cameras and play a critical role in quality-assured manufacturing
where (as the ‘‘eyes’’ of machine-vision systems) they are used to seek
out defective products. IR-sensitive image sensors are used in night
vision and thermographic imaging systems and even in on-board
missile guidance systems. Two-dimensional amorphous selenium
and silicon sensors, meanwhile, are rapidly replacing photographic
film as the preferred detectors in X-ray imaging systems due to the
immediacy of image replication, superior dynamic range, and easier
archiving and data retrieval.
The above examples cover just a small fraction of the uses for
photodetectors, and in choosing which detector to use for a given