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A Bessel beam is a field of electromagnetic, acoustic or even gravitational radiation whose amplitude is described by a Bessel function of the first kind.〔D. McGloin, K. Dholakia, Bessel beams: diffraction in a new light, Contemporary Physics 46 (2005) 15-28〕 A true Bessel beam is non-diffractive. This means that as it propagates, it does not diffract and spread out; this is in contrast to the usual behavior of light (or sound), which spreads out after being focussed down to a small spot. Bessel beams are also ''self-healing'', meaning that the beam can be partially obstructed at one point, but will re-form at a point further down the beam axis. As with a plane wave a true Bessel beam cannot be created, as it is unbounded and would require an infinite amount of energy. Reasonably good approximations can be made, however, and these are important in many optical applications because they exhibit little or no diffraction over a limited distance. Approximations to Bessel beams are made in practice by focusing a Gaussian beam with an axicon lens to generate a Bessel-Gauss beam, by axisymmetric diffraction gratings, or by placing a narrow annular aperture in the far field. ==Properties== The properties of Bessel beams〔F. O. Fahrbach, P. Simon, A. Rohrbach, Microscopy with self-reconstructing beams, Nature Photonics 4 (2010) 780–785〕〔F. G. Mitri, Arbitrary scattering of an electromagnetic zero-order Bessel beam by a dielectric sphere. Optics Letters 36 (2011) 766-768〕 make them extremely useful for optical tweezing, as a narrow Bessel beam will maintain its required property of tight focus over a relatively long section of beam and even when partially occluded by the dielectric particles being tweezed. Similarly, particle manipulation with acoustical tweezers may be feasible with a Bessel beam that scatters〔P. L. Marston, Scattering of a Bessel beam by a sphere, J. Acoust. Soc. Am. 121 (2007) 753-758〕〔G. T. Silva, Off-axis scattering of an ultrasound bessel beam by a sphere. IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 58 (2011) 298-304〕〔F. G. Mitri, G. T. Silva, Off-axial acoustic scattering of a high-order Bessel vortex beam by a rigid sphere, Wave Motion 48 (2011) 392-400〕 and produces a radiation force resulting from the exchange of acoustic momentum between the wave-field and a particle placed along its path.〔F. G. Mitri, Acoustic radiation force on a sphere in standing and quasi-standing zero-order Bessel beam tweezers, Annals of Physics 323 (2008) 1604-1620〕〔F. G. Mitri, Z. E. A. Fellah, Theory of the acoustic radiation force exerted on a sphere by a standing and quasi-standing zero-order Bessel beam tweezers of variable half-cone angles, IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 55 (2008) 2469-2478〕〔F. G. Mitri, Langevin acoustic radiation force of a high-order Bessel beam on a rigid sphere, IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 56 (2009) 1059-1064〕〔F. G. Mitri, Acoustic radiation force on an air bubble and soft fluid spheres in ideal liquids: Example of a high-order Bessel beam of quasi-standing waves, European Physical Journal E 28 (2009) 469-478〕〔F. G. Mitri, Negative Axial Radiation Force on a Fluid and Elastic Spheres Illuminated by a High-Order Bessel Beam of Progressive Waves, Journal of Physics A - Mathematical and Theoretical 42 (2009) 245202〕〔F. G. Mitri, Acoustic scattering of a high-order Bessel beam by an elastic sphere, Annals of Physics 323 (2008) 2840-2850〕〔F. G. Mitri, Equivalence of expressions for the acoustic scattering of a progressive high-order Bessel beam by an elastic sphere, IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 23 56 (2009) 1100-1103〕〔P. L. Marston, Axial radiation force of a bessel beam on a sphere and direction reversal of the force, J. Acoust. Soc. Am. 120 (2006) 3518-3524〕〔P. L. Marston, Radiation force of a helicoidal Bessel beam on a sphere, J. Acoust. Soc. Am. 125 (2009) 3539-3547〕 The mathematical function which describes a Bessel beam is a solution of Bessel's differential equation, which itself arises from separable solutions to Laplace's equation and the Helmholtz equation in cylindrical coordinates. The fundamental zero-order Bessel beam has an amplitude maximum at the origin, while a high-order Bessel beam (HOBB) has an axial phase singularity along the beam axis; the amplitude is zero there. HOBBs can be of vortex (helicoidal) or non-vortex types.〔F. G. Mitri, Linear axial scattering of an acoustical high-order Bessel trigonometric beam by compressible soft fluid spheres, J. Appl. Phys. 109 (2011) 014916〕 X-waves are special superpositions of Bessel beams which travel at constant velocity. Mathieu beams and parabolic (Weber) beams 〔(M.A. Bandres, J.C. Gutiérrez-Vega, and S. Chávez-Cerda, "Parabolic nondiffracting optical wave fields," Opt. Lett. 29, 44-46 (2004). )〕 are other types of non-diffractive beams that have the same non-diffractive and self-healing properties of Bessel beams but different transverse structures. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bessel beam」の詳細全文を読む スポンサード リンク
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