Helico-conical beams for generating optical twisters: [invited]

We describe a diffracting beam with orbital angular momentum (DAM) but with a helical profile in both phase and amplitude of the beam. This is different from Laguerre-Gaussian (LG) beams where only the phase component has a helical profile. The beam we describe here is initially characterized with an apodized helical phase front at the outskirts and linearly scaled towards no phase singularity at the centre of the beam. At the focal volume, we show that our beam fonms an intensity distribution that can be accurately described as an "optical twister" as it propagates along the optical axis. Unlike LG beams, an optical twister can have minimal changes in radius but with a scalable DAM. Furthenmore, we characterize the DAM in tenms of its capacity to introduce spiral motion on particles trapped along its orbit. We also show that our "optical twister" maintains a high concentration of photons at the focus even as the topological charge is increased. Such beams can be applied to fundamental studies of light and atoms such as in quantum entanglement of the DAM, toroidal traps for cold atoms and for optical manipulation of microscopic particles. OPTICAL TWISTERS An optical vortex or light beam with helical phase introduces a component of the Poynting vector along the azimuthal angle. When such a beam undergoes strong focusing, the azimuthal components destructively interfere giving rise to a dark centre surrounded by a high intensity ring of light via constructive interference. As the topological charge is increased, the area where destructive interference at the centre increases thereby enlarging the light ring. The propagation along the optical axis follows a conical ray of light where the concentration of high intensities is maintained at the outskirts of the conical beam where constructive interference takes place. Here, we describe a diffracting beam with a spiral profile on both the amplitUde and phase of the beam. The spiral beam is a special case of a general set of Helico-Conical beams described in our previous work [1,21. This family of beams is initially characterized with an apodized helical phase front at the outskirts and linearly scaled towards no phase singularity at the center of the beam. At the focus, Vie show that diffracting spiral beams have minimal changes in radius but with a scalable DAM. Further, we characterize an increasing DAM in terms of its capacity to introduce spiral motion on particles trapped along its orbit and show that spiral beams maintain a high concentration of photons at the focus even as the topological charge is increased.