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Manipulation and filtration of low index particles with holographic Laguerre-Gaussian optical trap arrays

P. A. Prentice, M. P. MacDonald, T. G. Frank, A. Cuschieri, G. C. Spalding, W. Sibbett, P. A. Campbell, and K. Dholakia

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Abstract

Multiple low index particles (micrometer-sized ultrasound contrast agent), have been optically trapped using a 4×4 Laguerre-Gaussian trap array. The trapping efficiency of the Laguerre-Gaussian arrangement was measured using a Stokes’ flow approach whereby the critical relative fluid velocity required to remove particles from the optical trap was measured. The dependence of trapping efficiency on beam power was also explored and the optimum beam parameters were identified. Finally, the utility of the array as a selective filter was demonstrated by tweezing multiple low-index particles from a population exhibiting an inherent distribution in size. This procedure represents a unique remote non-contact process that may have significant applicability throughout the fields of biophysics and biotechnology.

©2004 Optical Society of America

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Figures (5)
Fig. 1.
Fig. 1. Schematic of tweezing set-up. L – lens, M – Mirror, H1 – LG hologram, BG – IR filter, H2 – 4×4 hologram.

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Fig. 2.
Fig. 2. (a) The intensity distribution at the focal plane of the objective on illuminating the hexadeca DOE with a standard Gaussian beam. (b) The intensity distribution on illuminating the DOE with an LG l=3 beam (i.e. after insertion of H1 into the optical path. Each annulus has the potential to trap a low index particle of suitable diameter. (c) A binary level version of the pattern used to produce the 4×4 spot intensity distribution of (a) and (b). The black regions shift the phase by π radians.

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Fig. 3.
Fig. 3. Logarithm of the critical velocity required to displace both high and low index particles from the optical trap versus the laser power. The effect of location is also apparent: in both instances, there is a marked increase in critical velocity when the particles are trapped away from a solid surface.

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Fig. 4.
Fig. 4. Graph of critical velocity required to laterally displace the CA particle from the optical trap, versus the laser power (and equivalent pumping current). There is no enhancement in trapping efficiency beyond laser powers of around 30mW as the effects of scattering push the particles further below the trap plane.

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Fig. 5.
Fig. 5. Photomicrographs demonstrating optical manipulation of 8 micro-bubbles simultaneously out of the plane of the top of the sample. In the left-most image, the trap array is established within a population of the contrast agent micro-bubbles, trapping 8 particles in the process. In the subsequent images, the array plane is physically moved in the z-direction so that trapped particles are selectively and simultaneously removed away from the rest of the population.

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Tables (1)

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Table 1. Average values of Q-lateral and Q-axial for CA micro-bubbles optically trapped in a Laguerre-Gaussian l=3 beam, as calculated using equations (4) and (5).

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Equations (5)

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r max = ω l 2
F trap = Q n o P c
F drag = 3 π η v d
Q lateral = c n o 3 π η v c d P
F min = π 6 ( ρ p − ρ m ) d 3 g + 2 k T d
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