Optical needles, i.e., laterally highly confined optical fields with ultra-high depth-of-field, are considered as promising tools in different optics disciplines, ranging from optical manipulation, trapping, and guiding of microparticles to nanophotonics and imaging. The experimental characterization is a sophisticated task, owing to their extreme physical dimensions. In this article, L. Turquet and co-workers generate and experimentally measure an ultra-long optical needle resulting from the tight focusing of a specially prepared input beam. The needles reach a length of approximately 30 wavelengths at a transverse diameter of only half a wavelength, combined with an extraordinary homogeneity with respect to their intensity along the optical axis. By utilizing a technique introduced earlier by the authors, they experimentally verify the successful and high-quality creation of these intriguing light fields. Their method is based on second-harmonic generation in gallium-arsenide nanowires, which are dominantly sensitive to the longitudinal field.
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