Study Of THz Wave Generation By Optical Rectification Using Two Pump Beams

Due to its strong penetrability, low photon energy, high resolution, wide spectralrange and other characteristics, terahertz(THz) wave is being used in a wide range offields, such as security, imaging and biomedicine. THz technology has attractedconsiderable attention as a forefront field of research. With the development ofhigh-power THz sources and high-sensitivity detection methods, THz wave will beapplied in more areas and stringent requirements are placed on THz sources:higher-power and more THz wave modulation techniques. This theses focuses onusing two-pump-beam-based optical rectification to modulate the generated THzwave and increase its power. The main contents are described as follows:First, an introduction to the history and development of THz technology, thespecial properties, application, generation and detection of THz wave is presented.Chapter two is devoted to the theory of optical rectification, where the nonlinearwave equation is derived from Maxwll equations under the slowly varing amplitudeassumption. The equation for the spectral amplitude of the THz field is then obtained.Based on this, the factors that affect THz wave generation efficiency are analyzed,and the relationship between the THz field and the polarization angle of the pumpfield with respect to the crystal axis of a (110)-cut GaP crystal is given.In Chapter three, we obtain the equation for the spectral amplitude of the THzfield for the case of two pump beams from that for a single pump beam. In theexperiment, two pump beams are used to excite a GaP crystal, with the resultsshowing that the coherent addition of the THz wave in the time domain leads tospectral modulation. The agreement between experiment and theory demonstrates thattwo-pump-beam-based optical rectification is a technique for THz wave modulation.Finally, based on the above results, we further investigate the possibility ofincreasing the THz wave power using the two pump beam technique. It is shown thatwhen the two pump beams overlap in time, the THz output power can be increased tothree times that for the case of a single pump beam. Hence, two-pump-beam-basedoptical rectification is an easy-to-implement way to further enhance THz wave power.