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Integer, fractional and fractal Talbot effectsby: M. V. Berry, S. Klein
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AbstractSelf-images of a grating with period <i>a</i>, illuminated by light of wavelength &b.lambda;, are produced at distances <i>z</i> that are rational multiples <i>p/q</i> of the Talbot distance <i>z</i><sub>T</sub> = <i>a</i><sup>2</sup>/&b.lambda;; each unit cell of a Talbot image consists of <i>q</i> superposed images of the grating. The phases of these individual images depend on the Gauss sums studied in number theory and are given explicitly in closed form; this simplifies calculations of the Talbot images. In ‘transverse’ planes, perpendicular to the incident light, and with &b.zeta; = <i>z</i>/<i>z</i><sub>T</sub> irrational, the intensity in the Talbot images is a fractal whose graph has dimension <a name="ILM0001"> </a>. In ‘longitudinal’ planes, parallel to the incident light, and almost all oblique planes, the intensity is a fractal whose graph has dimension <a name="ILM0002"> </a>. In certain special diagonal planes, the fractal dimension is <a name="ILM0003"> </a>. Talbot images are sharp only in the paraxial approximation &b.lambda;/<i>a</i> → O and when the number <i>N</i> of illuminated slits tends to infinity. The universal form of the post-paraxial smoothing of the edge of the slit images is determined. An exact calculation gives the spatially averaged non-paraxial blurring within Talbot planes and defocusing between Talbot planes. Similar calculations are given for the blurring and defocusing produced by finite <i>N</i>. Experiments with a Ronchi grating confirm the existence of the longitudinal fractal, and the transverse Talbot fractal at the golden distance &b.zeta; = (3 - 5<sup>1/2</sup>)/2, within the expected resolutions.
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