Study On The Contrast Of Image Speckles Produced By Strong Scattering-object And The Femtosecond Laser Pulses' Near-field And Far-field Speckles

Great attention has been paid to the study of the statistical properties of random surfaces, and it is of great importance in many science and technology fields such as the growth of thin films, the fine machining, the manufacture of optical devices, etc. Near-field optics is a new interdisciplinary subject which studies the optical phenomena within one wavelength, and with the rapid development of near-field optical microscopy, considerable advancement has been achieved in the studies of near-field optics. Femtosecond laser pulse is the laser pulse that consists of a coherent superposition of many frequency components. This dissertation is concentrated on theoretical and simulational studies on the contrast of image speckles and the scattering and the diffraction of femtosecond laser pulse from rough random surfaces in the near-field and the Fresnel regions. The whole dissertation is divided into four chapters.In chapter 1, we give a summary and an overall review for the statistical properties and measurements of random surfaces, the fundamental theories of light scattering, and the properties of near-field optics and femtosecond laser pulse.In chapter 2, using the generic expression of the complex amplitude of speckle and the approximation of the double exponential function and the rotational transformation of the real and imaginary parts of the complex amplitude of speckle, we obtain the expression for the contrast of the image speckles produced by strong scattering-objects in the 4/ optical imaging system, which is explicitly related to the statistical parameters of random surface and to the parameters of the imaging system. Based on the obtained results, we can see that the contrast of the image speckles produced by strong scattering-objects is related to roughness w, lateral correlation length , and aperture length R when R is littleenough. Our results are an obvious improvement on the literature, where the relations including such implicit quantities as the average size of the scattering grains of the random surface and the number of scattering grains are usually used. The results would be of great significance for the characterization of random surface by speckle contrast.In chapter 3, we propose the method for the numerical calculations of the femtosecond laser pulse propagation through a subwavelength aperture. The time-dependent laser pulse is first decomposed into the superposition of series of monochromatic simple harmonic waves. For the light field of the harmonic wave with a single frequency, the numerical calculation is made on the basis of solution of the Green's integral equation set of electromagnetic waves. Such numerical solution is iterated for all the waves with different frequencies, and all the numerical solutions are transformed into the light fields in the time domain by inverse Fourier transform. The light intensity distributions transmitted from the subwavelength aperture are calculated and the results show the propagation of the light field along the direction of the medium interface. The method and the results would be important in understanding the process of the construction and the propagation of the evanescent waves in the area of near-field optics.Chapter 4 is concentrated on theoretical and simulational studies on the statistical properties of the speckle field in the Fresnel diffraction region produced by the scattering of femtosecond laser pulse from random surfaces. At first, we obtain the auto-correlation function of speckles by using the property of femtosecond laser pulse and the ^-correlation model of the light filed of the random surfaces, and then we obtain the auto-correlation function of light intensities. The simulational results give some statistical properties of the speckle fields, which are different from those with monochromatic light illuminations. We find that the speckle granules become smaller and speckle fields become spatially periodically oscillated with the time past. Reasonable theoretical explanation for these properties are given.