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Scattering of a Focused Shifted LaserBeam by an Elongated Large-Size Spheroidal ParticleElsayed Esam M. Khaled, and Hany Louka1Electrical Engineering Department, Faculty of Engineering, Assiut University, Assiut, Egypt.1Telecom Egypt, Qina, Egypt.esamk54_2000@hotmail.com, hany_louka@yahoo.comAbstract :Scattered and internal intensities of an elongated, large size parameter spheroidal particle illuminated with an arbitrary laser beam are calculated. A combination of the T-matrix and the plane wave spectrum methods is used. © 2008 Optical Society of America OCIS codes: (290-5850) Scattering, particles 1. IntroductionInternal and scattered intensities of a spheroidal particle (either a prolate or an oblate) illuminated with an arbitrarily laser beam are important in areas such as particles characterization, and modeling nonlinear optics in droplets. To calculate the intensities we use the T-matrix and the plane wave spectrum methods. The particle can be large size parameter, elongated, and illuminated with a focused, shifted and polarized laser beam. We consider a homogeneous spheroid of a size parameter x which is calculated using an equivalent spherical volume concept. The spheroid is centered at the origin of a Cartesian coordinate system (x,y,z). The radii of the spheroid are b and a along the x- and z-axis, respectively. The axial ratio is ρ=a/b. The spheroid is illuminated with a Gaussian beam of a wavelength λ and polarized in the xz-plane. The beam is propagating in the z-direction with a spot size wo. The detailed analysis of the beam modeling and the T-matrix method can be found in Refs. [1,2]. Here we built computer codes to compute the internal and scattered intensities for a spheroid illuminated with an on-axis or shifted laser beam that focused with respect to the wavelength and the radii of the spheroid. 2. Results :To confirm that the technique is accurate we compare our results with the published cases[3]. No differences were noticed. For more illustration we calculated the scattered and internal intensity distribution in the xz-plane for a spheroidal particle of a refractive index m=1.36, and a size parameter x=47.3094299. The axial ratio of the prolate is ρ=1.4 and for the oblate is ρ=0.7. The particle is illuminated with an off-axis Gaussian beam of λ=1.06μm and wo=2μm (wo/b=0.3495 for the prolate and wo/b=0.1748 for the oblate). The shift of the beam is xo=b along the x-axis for both cases. The contour plots of the scattered and internal intensities for the prolate and the oblate are shown in Figs. 1(a), and 1(b) respectively. The results show that more energy distribution inside the oblate than that of the prolate.
(1) (2) Fig 1. Contour plots of the scattered and internal intensities distributions in the x-z plane for a spheroidal particle of x = 47.3094299, m=1.36 illuminated with a Gaussian beam of λ=1.064μm, and wo=2μm. The beam is shifted with xo=b, yo=zo=0. (a) a prolate of ρ=1.4 and wo=0.3495b (b) an oblate of ρ=0.7 and wo=0.1748b. 3. References[1] E. E. M. Khaled, S. C. Hill, and P. W. Barber, "Scattered and internal intensity of a sphere illuminated with a Gaussian beam," IEEE Trans., Antennas Propag. AP-41, 295-303 (1993). [2] P. W. Barber and S. C. Hill, Light scattering by particles: computational method (Singapore: World Scientific, 1990), Chap. 3. [3] J. P. Barton, "Internal and near-surface electromagnetic fields for a spheroidal particles with arbitrary illumination," Appl. Opt. 34, 5542-5551 (1995).
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Last Update 12-5-2007 |
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