Devices pyroelectric

The results obtained may contribute to the development of more efficient pyroelectric devices on the basis of thin-film techniques. 

Sources and detectors Specific discussions of sources and detectors have been covered elsewhere in this article. The issues here are more service and performance related. Most sources have a finite lifetime, and are service replaceable items. They also generate heat, which must be successfully dissipated to prevent localized heating problems. Detectors are of similar concern. For most applications, where the interferometer is operated at low speeds, without any undesirable vibrational/mechanical problems, the traditional lithium tantalate or DTGS detectors are used. These pyroelectric devices operate nominally at room temperature and do not require supplemental cooling to function, and are linear over three or four decades. 

Attempts to produce pyroelectric devices have been less numerous than those aimed at producing second harmonic generators. Clearly a pyroelectric device must be non-centrosymmetric in character and must thus be formed either from an alternate layer structure or from a Z layer structure. 

The electrical characterization of polar media is crucial to investigate their suitability for ferroelectric memories, piezo- or pyroelectric devices and many other ferroelectric applications (see Chapter 3). Optical characterization of polar media is fundamental to investigate their ser-vicability for electro-optic devices or applications in the field of nonlinear optics (see Chapter 4). Additionally there are intentions to characterize polar media with a combination of both, electrical and optical methods, such as to understand ferroelectric phenomena that are influenced by the action of light. 

There are a wide variety of measurement systems available that will determine capacitance and loss. These include capacitance bridges (manual and auto-balance), impedance analysers, network analysers etc. It is important for the user to consider the frequency range over which the pyroelectric devices are to be used, as this will largely determine the selection of the instrument to use. As most pyroelectric detectors are used in the range 0.1 to 100 Hz, the instrument should ideally permit measurement over this frequency range. There are very few commercial capacitance and impedance analysers that will work below 20 Hz, and many low cost units... 

The use of bulk ferroelectrics in pyroelectric devices inevitably leads to a situation where the material must be cut, lapped and polished to make a thin, thermally-sensitive layer. If an array of detectors is required for thermal imaging, this must be metallized on both faces,... 

Pyroelectric infrared detectors are inferior in detectivity by one or two orders of magnitude compared with photoconductors such as cadmium mercury telluride, as shown in Fig. 7.15. However, such materials require temperatures of 200 K for efficient operation and generally respond to rather narrow bands at the infrared wavelengths. Pyroelectric devices can discriminate temperature differences of 0.1 K but find many useful applications in which the discrimination is limited to about 0.5 K. They have the great practical advantage of operating at normal ambient temperatures. 

The use of bulk p)Toelectiics in pyroelectric devices inevitably leads to a situation where the material must be cut, lapped, and polished to make a thin, thermally sensitive layer. If an array of detectors is required for a thermal goal, this must be metallized on both faces, processed photolithographically, and bonded to a silicon read-out circuit to yield a complete hybrid array. Clearly, it would be desirable if the material could be deposited as a thin film, to remove the requirement for lapping and polishing, if possible directly onto a complete wafer of chips, where it could be processed to yield an array of thin. 

There are two commonly used detectors employed for the mid-infrared region. The normal detector for routine use is a pyroelectric device incorporating deuterium tryglycine sulfate (DTGS) in a temperature-resistant alkali halide window. 

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