Creating λ /3 focal holes with a Mach–Zehnder interferometer
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- Engel, E., Huse, N., Klar, T. et al. Appl. Phys. B (2003) 77: 11. doi:10.1007/s00340-003-1239-y
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We report the formation of doughnut-shaped focal intensity distributions with hole diameters of λ/3.3=232 nm full-width-at-half-maximum. The doughnut shape is created by illuminating a high-numerical-aperture lens with the output of a Mach–Zehnder interferometer, in which half of the wavefront in each arm is phase retarded by π. The focal intensities are probed with a point-like scatterer and compared with the predictions of a vectorial focusing theory. The orientation of the phase-discontinuity line with respect to the electric field determines whether a strong longitudinal or a vanishing electric field is produced at the focal point. Conditions are given for creating high-contrast focal holes at the sub-micron scale.