Developing a needle-like optical funnel: Numerical modelling

Non-diffracting optical quasi-Bessel beams provide an opportunity to construct optical fields of complex architecture. The constructed beams may have a bright central peak or zero intensity on the beam axis and have the beam size of only a few microns propagating over a long-defined distance, which is not possible with conventional Gaussian or high-order Laguerre- Gaussian beams. In this work we demonstrate the possibility of constructing a needle-like diverging optical funnel with zero intensity on the axis. The primary aim is to numerically construct and optimize the optical field, which could transversely compress and focus a stream of μm- and sub-μm size particles injected into vacuum or gaseous environment by applying light pressure and photophoretic forces pushing particles into the area with lower intensity. We present the results of numerical modelling of an "optical funnel" based on re-imaging a non-zero-order quasi-Bessel beam, formed by an axicon and a phase plate or using an SLM, with a collimator. The funnel geometry, namely, the μm-size of the beam cross-section, several-mm long propagation length and its divergence, all is controlled and optimized by changing the topological charge at a fixed collimation of the re-imaging optics, or/and by varying the collimation with fixed topological charge of the beam. The simulated profiles will have an application for optical guiding and focusing of aerosolised beam of particles, large biomolecules and viruses to the micron-size focus of x-ray Free Electron Lasers in order to increase the delivery efficiency of isolated single particles in coherent diffractive imaging experiments.