Photoconductive semiconductors

Free-electron lasers have long enabled the generation of extremely intense, sub-picosecond TFlz pulses that have been used to characterize a wide variety of materials and ultrafast processes [43]. Due to their massive size and great expense, however, only a few research groups have been able to operate them. Other approaches to the generation of sub-picosecond TFlz pulses have therefore been sought, and one of the earliest and most successfid involved semiconducting materials. In a photoconductive semiconductor, carriers (for n-type material, electrons)... 

Mid- and near-infrared Nernst filament globar NaCl or KBr Grating interferometer Golay cell thermocouple bolometer pyroelectric photoconductive semiconductor... 

Zinc sulfate has been used as a mordant in calico printing for preserving wood and skins for the manufacture of other zinc salts for clarifying glue and in electrodeposition processes. The sulfide is used as a pigment and as a source of other pigments ZnS is used in X-ray and television screens and, with a trace of a radium or mesothorium salt, in luminous dials of watches. The telluride has been used as a photoconducting semiconductor. 

Detectors are similar in type to those for the far-infrared, namely thermocouples, bolometers, Golay cells or photoconductive semiconductors. 

NEA photocathodes are photoconductive semiconductors whose surface has been treated to obtain a state of negative electron affinity . This state is reached when the energy level of an electron at the bottom of the conduction band in the bulk of the semiconductor is greater than the zero energy level of an electron in the vacuum. Hence an electron excited to the conduction band within the bulk can, if it travels to the activated surface without first recombining, energetically fall out of the photocathode into free space. 

Recently a new type of photocathode has been developed which is based on photoconductive semiconductors whose surface has been treated to obtain a state of Negative Electron Affinity (NEA). In this state an electron at the bottom of the conduction band inside the semiconductor has a higher energy than the zero energy of a free electron in vacuum [4.63], When an electron is excited by absorption of a photon into such an energy level... 

Figure 6.12 shows the schematic constraction of a typical optically addressed liquid crystal spatial light modulator (OALCSLM) operating in the reflective mode. It consists of an aligned liquid crystal layer sandwiched between transparent electrodes and adjacent to a photoconducting semiconductor layer (a-Si H) sensitive to the writing beams and a dielectric mirror coated to block the reading beam. 

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