Quasi-Bessel hollow beam as optical guide for micro-particles

We have developed a high-aspect ratio optical pipeline aiming to produce a highly collimated stream of micron-size particles in either gaseous or vacuum environments. A hollow, first-order quasi-Bessel beam with variable divergence was generated with a phase-only spatial light modulator (SLM), by superimposing the quadratic phase of a lens and an axicon with a 0.5° base angle. The beam was further re-imaged to form a centimetre-long funnel beam with ~5μm diameter and up to 2000 length-to-diameter aspect ratio. The divergence of the central core of the Bessel beam was controlled by varying the effective lens in the hologram. The SLM-based optical beam was compared to a similar beam composed using a physical axicon. The experimental tests were conducted with 2-μm size polystyrene spherical particles to evaluate the optical force. We present estimated optical forces exerted on the particles in the transverse plane, both depending on the particle size, laser power, and background-gas pressure.