Laser-induced effects in carbon suspensions and diffraction by volume gratings in liquids

Two projects are covered in this thesis. The first project is an investigation of acoustic and chemical effects generated by high power laser pulses in carbon suspensions. Carbon particles absorb energy from the laser pulses and are heated to a few thousand degrees C. The high temperatures initiate reactions between carbon and the surrounding water generating permanent gases. Hydrogen, carbon monoxide, carbon dioxide, and several hydrocarbons have been identified as product gases. With respect to sound wave generation the change in volume of the material owing to thermal and chemical expansion is discussed. A thermodynamic theory governing the generation of the photoacoustic waves from these two mechanism is developed. A comparison between photoacoustic effects caused mainly by the chemical mechanism and those generated by the thermal mechanism is given. The chemical mechanism gives an acoustic signal 2,000 times greater in magnitude than would be generated by purely thermal mechanism (normalized to absorption coefficient). Structural changes of the carbon particle are reported. The originally solid particles first become large hollow particles and then disappear according to electron microscopy.; The second project deals with diffraction by volume gratings. A rigorous theory governing non-attentuated planar volume gratings developed by Gaylord and Moharam is utilized. A generalized theory incorporating attentuation along the depth for planar volume gratings with TE incident probe beams is developed. Experiments have been carried out to investigate the diffraction of volume gratings generated by two coherent nanosecond laser pulses in methanol. The magnitude of the change in index of refraction is extracted. Criteria for delineation of different diffraction regimes are discussed. Approximate solutions to the first order coupled-wave equations are given substantiating the criteria.