Abstract
We present a method of both polarization and amplitude modulations on an incident beam to
obtain a longitudinally polarized subwavelength-sized optical needle. A 4-f system with a spatial
light modulator is used to generate experimentally a two-mode alternate cylindrical vector beam by
polarization modulation, enhancing performance and facilitating implementation. We optimize the beam
focusing properties after passing the beam through an annular aperture to obtain amplitude
modulation by the simulated annealing algorithm. Numerical results indicate that a sharp focal spot
(0.417λ) with a long focal depth (8λ) and a strong longitudinally polarized field can be
easily achieved.
© 2013 Chinese Optics Letters
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