Optical beam induced conductivity

Imaging Modalities. Confocal contrast generation include reflectance, rescattering, and fluorescence. The first results in confocal imaging were obtained on unstained tissue in reflectance [78]. Reflectance is also the preferred imaging modality used in industrial inspection of semiconductors, often combined with methods such as optical beam induced conductivity (OBIC). 

As described above, recent advances in accelerator technology have enabled the production of very short electron pulses for the study of radiation-induced reaction kinetics. Typically, digitizer-based optical absorbance or conductivity methods are used to follow reactions by pulse radiolysis (Chap. 4). However, the time resolution afforded by picosecond accelerators exceeds the capability of real-time detection systems based on photodetectors (photomultiplier tubes, photodiodes, biplanar phototubes, etc.) and high-bandwidth oscilloscopes (Fig. 8). Faster experiments use streak cameras or various methods that use optical delay to encode high temporal resolution, taking advantage of the picosecond-synchronized laser beams that are available in photocathode accelerator installations. 

In this section, we discuss a mechanism for enhanced optical nonlinearity in which the photoinduced charges and fields occur within the liquid crystal. It is well known that dc field induced cmient flow in nematic liquid crystals, which possess anisotropic conductivities, could lead to nematic flows and director axis reorientation, and to the creation of a space-charge field. The charged carriers responsible for the electrical conduction come from impurities present in the otherwise purely dielectric nematic liquid crystal. If these impurities are photoionizable, an incident optical intensity [e g., an intensity grating created by the interference of two coherent optical beams (see Fig. 8.13)], it is possible, therefore, to create a space-charge... 

Similar spectral techniques as discussed for macroscopic tumour imaging can be employed for fluorescence microscopy. Confocal and two-photon-induced fluorescence microscopy [10.210], and imaging Fourier transform spectroscopy [10.211] are all valuable techniques for studies at the cellular level. Related to this field is the optical trapping of ceils with focused laser beams optical tweezers), which relies on gradient forces of the same kind as discussed in Sect. 9.8.5. Trapped cells and polymer strings can be manipulated in many ways to enable fundamental studies to be conducted [10.212]. 

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