Paper published in JOSA A

Our paper on the optical trapping forces of a poarization-structured beam focused by a fractal (Devil's) lens has been published as Zhirong Liu and P. H. Jones, 'Radiation forces acting upon a Rayleigh particle by highly focused alternate radially- and azimuthally-polarized beams modulated by a Devil's Lens', Journal of the Optical Society of America A 33 2501-2508 (2016).

From the abstract:  We propose and demonstrate a novel high numerical aperture (NA) focusing system composed of an annular beam with alternate radially and azimuthally polarized rings, focused by a devil’s lens (DL), and further investigate its radiation forces acting upon a Rayleigh particle both analytically and numerically. Strongly focused cylindrical vector beams produce either dark-centered or peak-centered intensity distributions depending on the state of polarization, whereas the DL produces a series of foci along the propagation direction. We exploit these focusing properties and show that by selecting an appropriate truncation parameter in front of the focusing lens, the proposed optical focusing system can selectively trap and manipulate dielectric micro-particles with low or high refractive indices by simply adjusting the radius of the pupil or the beam. Finally, the stability conditions for effectively trapping and manipulating Rayleigh particles are analyzed. The results obtained in this work are of interest in possible applications in optical confinement and manipulation, sorting micro-particles, and making use of a DL.