Optical orbital angular momentum (OAM) has found many applications in imaging, lithography and quantum information. However, most of the existing OAM generators are wavelength dependent and therefore are not compatible with broadband operation. This limitation has now been overcome by Frédéric Bouchard and colleagues at the University of Ottawa in Canada and the University of Rochester in the USA, who have experimentally demonstrated an achromatic spin-to-orbital angular momentum convertor with high conversion efficiency. The team used a specially designed system of 'two-Fresnel-rhomb' reflectors (each formed from two glued truncated hollow axicons) to flip the helicity of an incoming circularly polarized beam by inducing a π phase shift. Importantly, this induced phase shift is largely independent of the beam's wavelength, thus allowing broadband operation. The team reports an OAM value of ±2 and an average conversion efficiency of 95% for red, green and blue colours. They say that higher OAM values can be obtained by cascading these devices with appropriate waveplates. The broadband operation of the device makes it attractive for a range of imaging applications from microscopic to astronomical systems — such as optical vortex coronagraph, where a white vortex beam is required.
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Won, R. Broadband generation. Nature Photon 9, 75 (2015). https://doi.org/10.1038/nphoton.2015.14