Studies Of The Reflection 4f Coherent Imaging Technique Used To Measure Nonlinear Optical Characteristics Of Materials' Surfaces

It has been 60 years past since the nonlinear optical phenomena were discovered. With the developing of the nonlinear optics, people begin to focus on materials of law-dimension such as thin films,the surface of the material…. People are also interested in researching the techniques to measure the nonlinear optical characteristic of materials'surface. In the past, for some opaque materials of interesting, the reflection Z-scan (RZ-scan) technique applied for the definition of the nonlinear optical parameters. This method is convenient but it may damage the sample, so in this paper, we will present a single-laser-shot reflection imaging technique, which bases on the reflection 4f coherent imaging system.The design and theory of this method are simple. The principles are based on the analysis of the phase and amplitude characteristics of the beam reflected from the interface. The nonlinear sample, titled at a fixed angle, is located in the focal plane of 4f system. The CCD, which is located at the exit plane of the system, would catch the image which is formed by one pulse reflected from the surface of the sample. The intensity of the image show the spatial modification of the pulse reflected from a single interface with intensity-dependent refractive index. The change of the reflected beam is mainly due to a surface-effect (the reflection coefficient change). Compared with RZ-scan used in lots of experiments, other advantages of this method include no displacement of the nonlinear medium and lower sensitivity to statistical fluctuations of the laser beam.With the help of computer and MATLAB, we have done some theoretical analysis to validate our approach. Some parameters will affect the results of the experiment, for example the linear refraction, the linear absorption, the incident intensity and the angle of incidence. We have found some conclusions. The closer the incident angle approaches the Brewster's angle, the higher the sensitivity of the experiment will be. It's a great value to us.