Toshimitsu Asakura’s research while affiliated with Hokkaido University and other places

The observable quantities in an optical experiment are influenced by the fractal dimension and the average cluster size of mutually self-similar gels. We study the scaling properties of the scattering and the transport mean free path in order to use them for the microscopical characterization of fractal gels.

The zero-crossing number of speckle intensity fluctuations produced in the diffraction field by a diffuse object moving in a plane and integrated spatially by a finite-size aperture having the directional forms of, for example, the ellipse and the rectangle is studied theoretically and experimentally. The theoretical and experimental results indicate that the magnitude and direction of the moving object's velocity can be simultaneously measured by counting the zero-crossings of the spatially integrated speckle intensity fluctuations.

It was commonly believed that the spherical wavefront aberration of an optical beam always leads to a lower maximum focused intensity along the optical axis. It was shown recently both theoretically and experimentally that this was only true for high Fresnel number systems. In low Fresnel number focusing systems a properly chosen spherical wavefront aberration can increase the maximum focused intensity. In this paper, we call this 'super focusing'. To investigate the characteristics of the super focusing, an improved axial intensity formula has been newly derived for Gaussian beams with spherical aberrations. Based on this formula, several examples of axial intensity distributions are shown. The cause of the super focusing is considered based on these intensity distributions and the derived formulae.

I Theory of Reflectometry.- 1 Demands on Measurement of Optical Constants of Liquids in Science and Industry.- 2 Liquids.- 3 Theory of Optical Constants.- 4 Theory of Reflectance.- 5 Probe Window Contamination and Reflectance.- 6 Wavelength Spectra Analysis.- II Practical Reflectometry.- 7 Introduction.- 8 Definitions of Optical Instrumentation and Measurement.- 9 Exploring the Insides of a Spectrophotometer.- 10 Understanding Your Signal.- 11 Before Measurement.- 12 From Theory to Measurement.- Appendices.- A Intensity Law for Nonlinear Absorption.- B Complex Angle of Refraction.- C Cauchy-Riemann Equations.- D Kramers-Kronig Integrals and Symmetry Properties.- References.

Kramers-Kronig relations and sum rules for effective linear permittivity of the Maxwell Garnett liquid-nanosphere system were obtained using complex analysis and general expression of the effective permittivity. When reflectance from optically linear and nonlinear Maxwell Garnett nanoparticles was calculated it was observed that the reflectance, in the case of attenuated total reflection, depends on the fill fraction of the nanospheres and their nonlinear susceptibility. According to simulations a good sensitivity of the nonlinear contribution on the reflectance can be obtained by using a probe wavelength corresponding to the resonance frequency of the nanosphere system. In Kretchmann#x0027;s configuration it was observed that the surface-plasmon-resonance angle depends on the fill fraction and on the intensity of the incident light. By using reflectance, it is possible to detect optically nonlinear nanospheres in liquids.

The zero-crossing problem is studied for the purpose of investigating the dynamic properties of speckle produced in the diffraction field by a moving diffuse object under illumination of coherent light. The zero-crossing method is applied to the time-differentiated speckle intensity fluctuations and studied in some detail with respect to the influence of noise and the low-pass filter used for suppressing the noise. From the theoretical and experimental studies, the velocity of the moving object is found to be accurately determined by measuring the number of zero-crossings per second of the time-differentiated speckle intensity fluctuations when both the optical configuration used for producing speckles and the characteristic of the low-pass filter for suppressing the noise are known beforehand.

The probability density function of the partially developed non-Gaussian speckle intensity is experimentally investigated and distinctive differences are confirmed to exist between the probability density functions of partially developed Gaussian and non-Gaussian speckle intensities. To characterise these differences, a new skewness parameter is introduced which describes the asymmetry of the probability density distribution of speckles, and some theoretical considerations are also presented. Finally it is shown that the skewness parameter expresses both the average contrast and the dependence of speckle statistics on both the random phase fluctuation of diffusers and the effective number of scatters contributing to the observation point.

A method consisting of the median filtering and the differential Fourier analysis is proposed to analyze the speckle correlation fringes. In the method, these two operations act effectively as a band-pass filter since the speckle noise with high frequencies is suppressed by the median filtering and the pedestal variation with low frequency is removed through the differentiation of fringe signals. In computer simulations, the performance of the method is compared with that based on the ordinary Fourier analysis and examined with respect to the window size of median filters used. It is revealed that speckled fringes with the low visibility can be successfully analyzed with high accuracy. The validity of the method is also experimentally shown by applying it to the analysis of speckle correlation fringes obtained in speckle photography.

The signal visibility characteristics of spatial filtering velocimetry using a transmission grating have been investigated theoretically and experimentally. It is found that the visibility of output signals is given as a function of the ratio of both the size and the separation of particles to the grating period in a certain restricted case. In general, it also depends on both the line number of the grating and its geometrical shape. In order to make the signal visibility always usable for the sizing of individual particles, the effect of the spatial particle density on the visibility must be minimized.

Use of an image fiber bundle is proposed in the two-dimensional vector velocimeter based on the time-varying spatial filter method using two rotating transmission gratings. The spatial filtering characteristics of both the gratings and the image fiber bundle are theoretically analyzed by using power spectral density functions. Two methods are investigated theoretically and experimentally to eliminate the signal generated by the image fiber bundle. An experiment was performed to measure the flow velocity components and their spatial distribution in a small glass tube. The experimental results show that the image fiber bundle gives the high flexibility of velocity measurements in the vector velocimeter using two rotating transmission gratings.