A long-range polarization-controlled optical tractor beam

Journal name:
Nature Photonics
Volume:
8,
Pages:
846–850
Year published:
DOI:
doi:10.1038/nphoton.2014.242
Received
Accepted
Published online

The laser beam has become an indispensable tool for the controllable manipulation and transport of microscopic objects in biology, physical chemistry and condensed matter physics. In particular, ‘tractor’ laser beams can draw matter towards a laser source and perform, for instance, all-optical remote sampling. Recent advances in lightwave technology have already led to small-scale experimental demonstrations of tractor beams1, 2, 3, 4. However, the realization of long-range tractor beams has not gone beyond the realm of theoretical investigations5, 6, 7, 8, 9. Here, we demonstrate the stable transfer of gold-coated hollow glass spheres against the power flow of a single inhomogeneously polarized laser beam over tens of centimetres. Additionally, by varying the polarization state of the beam we can stop the spheres or reverse the direction of their motion at will.

At a glance

Figures

  1. Concept of photophoretic light–particle interaction in gases.
    Figure 1: Concept of photophoretic light–particle interaction in gases.

    a, Interaction of an absorbing particle with a Gaussian beam. The photophoretic force F can drive the particle along the beam in the forward or backward direction, depending on the asymmetry vector ras. However, this motion is laterally unstable, as the transverse component of the photophoretic force FT is repulsive and therefore the particle is pushed away from the maximum of light intensity centred at the beam axis. b, Interaction of the same particle with a doughnut beam. In this case FT is restoring (that is, it is directed towards the beam axis) and does not allow the particle to escape from the beam, so the motion is laterally stable. c, A hollow glass sphere (external radius a, wall thickness d) coated with a thin Au film (film thickness h) exemplifies a semitransparent particle.

  2. Numerical modelling of reversible optical transport.
    Figure 2: Numerical modelling of reversible optical transport.

    a,b, Cross-sections of light intensity distribution near a partially absorbing hollow glass sphere (d = 300 nm; h = 10 nm) trapped at the axis of a radially (a) and azimuthally (b) polarized beam. The distributions are axially symmetric with respect to the z-axis. ce, Polarization-induced reversal of the J-factor of a hollow Au-coated glass sphere. In c and e, the pairs of points connected with arrows indicate when either a pushing (J < 0) or pulling (J > 0) photophoretic force can be exerted on the sphere depending on the incident beam polarization. In d, the shaded regions show when the reversible optical transport occurs for 7 nm (green) and 15 nm (yellow) Au coatings. In ae the spheres have the same external radius a ≈ 25 µm, and the radius of the maximum intensity contour at the beam waist (z = 0) is w0 = 35 µm. f, Illustration of polarization-controlled optical transport.

  3. Experiment.
    Figure 3: Experiment.

    a, Part of the experimental set-up with the beam convertor and particle dispenser. The half-wave plates used to change the polarization state of the beam are denoted by λ/2. b, Illustration of the longitudinal light intensity profile of the trapping beam. Each black square represents measured light intensity, and the solid line is an analytical fit based on λ02/(4π2 , where w0 is the radius of the maximum intensity contour at the beam waist (z = 0). The region of stable trapping is in yellow. The beam waist region is in red. c, Dependence of the velocity of the glass shells on their external diameter for azimuthal and radial polarization. Data pertaining to a particular shell are in one colour. In all the experiments w0 = 35 µm. d,e, Snapshots illustrating the motion of a shell of radius a = 25 µm and coating thickness h ≈ 15 nm trapped inside an azimuthally (d) and radially (e) polarized beam. In d, the shell moves against the beam propagation direction z (pulling force) at V = 0.8 mm s−1, and in e it moves along z (pushing force) at V = 0.4 mm s−1.

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Affiliations

  1. Laser Physics Centre, Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australia

    • Vladlen Shvedov,
    • Cyril Hnatovsky &
    • Wieslaw Krolikowski
  2. Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

    • Arthur R. Davoyan &
    • Nader Engheta
  3. Science Program, Texas A&M University at Qatar, Doha, Qatar

    • Wieslaw Krolikowski

Contributions

V.S. conceived the idea. V.S., C.H. and W.K. designed and conducted the experiments. A.R.D. and N.E. developed the theoretical description. A.R.D. performed the numerical simulations. C.H. and V.S. co-wrote the paper. W.K. supervised the project. All authors contributed to the discussion and data analysis.

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The authors declare no competing financial interests.

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