Optically Controlled Particles for clean x-ray diffraction experiments

Optical Injector of Particles for X-ray Diffractive Imaging

Motivated by the need for new single particle sample delivery methods to be used with our coherent diffractive imaging program [1-4] we have developed an optical pipeline [5] to produce a highly collimated stream of particles in either gaseous or vacuum environments. We present first results of our efforts to guide particles with micron-size precision in gaseous and vacuum environments using a first order quasi-Bessel beam operating either independently or collinear with a particle laden gas stream produced by an aerodynamic lens. The centimeter long, low divergence, optical pipeline is formed by a first order Laguerre–Gaussian beam imaged trough an axicon. When used in conjunction with an aerodynamic lens the optical forces assist the migration of particles to the centerline of the gas flow. This produces a higher particle number density and therefore an easier target to hit when probing with a Free Electron Laser (FEL) or other pulsed source. We present estimated optical forces exerted on the particles and the stiffness of trapping in the transverse plane, both depending on the particle size, optical reflectance, laser power, and background-gas pressure.

A 532 nm cw vortex beam was formed by a helical phase plate, and passed through an axicon with a base angle of 0.5° to form a first order Bessel like beam propagating over a meter-long distance. The slowly diverging beam was then re-imaged with a ×5 microscope objective into a narrow pipeline with minimum ring diameter of 2.4 μm and aspect ratio of approximately ×1000, which we termed as ‘optical syringe’ (Fig.1). 5-μm size spherical particles of graphite and sapphire, and 5-μm and 1.9-μm polystyrene spheres were used in our experiments to evaluate the optical force and scale it down to sub-micron particle size. We demonstrate that the resulting optical forces exerted on micron-size particles, using 5-W of laser power, were able to deflect the particle jet by about ~80 μm at the distance of 67 mm from the output nozzle of the aerodynamic lens stack.

Optical pipeline setup

The experimental setup generating a micron-scale optical syringe based on an optical vortex combined with an axicon. (a-d) Images of the cross-sections of the beam before the phase plate (a), the 0.5° axicon (b), the microscope (c), and a magnified image of the beam near the nozzle of particle injector (d); the scale bar is 1 mm in (a-c) and 10 μm in (d). The inset illustrates the particle beam compression by optical forces. The graph on the right shows the beam radius vs. distance from the microscope.

  1. Spence, J. C. H., Weierstall, U. & Chapman, H. N. X-ray lasers for structural and dynamic biology. Rep. Prog. Phys. 75, 102601 (2012).
  2. Liu, P., et al., “Generating particle beams of controlled dimensions and divergence”. Aerosol Science and Technology 22, 293–313 (1995).
  3. Seibert, M. M. et al. “Single mimivirus particles intercepted and imaged with an X-ray laser”, Nature 470, 78–82 (2011).
  4. Loh, N. D. et al. “Fractal morphology, imaging and mass spectrometry of single aerosol particles in flight” Nature 486, 513–517 (2012).
  5. V. G. Shvedov, et al., “Giant optical manipulation” Phys. Rev. Lett. 105, 118103 (2010).

Research Team
Rick Kirian, Salah Awel, Henry N. Chapman, Jochen Küpper
Niko Eckerskorn, Andrei Rode (Australian National University, Canberra, Australia)

Acknowledgment


This work is supported by the excellence cluster “The Hamburg Centre for Ultrafast Imaging – Structure, Dynamics and Control of Matter at the Atomic Scale” of the Deutsche Forschungsgemeinschaft.