Optical gradient trap

Fig. 17. The optical gradient trap of Ashkin et al. (1986). Reprinted with permission from Ashkin, A., Dziedzic, J. M., Bjorkholm, J. E., and Chu, S., Optics Letters 11,288-290, Copyright 1986, The Optical Society of America.

Optical traps have also been used to retain cells [176,866-868,1174]. For instance, manipulation of polystyrene beads by optical gradient force (attractive) and scattering force (repulsive) was achieved in a PDMS chip (see Figure 8.26). A polystyrene bead was first retained by an optical trap. Then the bead was moved and released so that it flowed to a desired channel downstream [1174]. 

We are currently working on two types of laser traps an alternating light beam trap [17], with modifications first mentioned by DaUbard and Phillips [19], and a six-beam gradient trap [16] in the optical molasses geometry. 

Ashkin and Dziedzic (1987) gave an impressive demonstration of the optical trapping of dielectric particies in experiments with a biological particle (tobacco mosaic virus) in water by means of a single-beam gradient trap formed by an argon laser 0.1-0.3 W in power. The rodlike tobacco mosaic virus is 3000 nm long and 200 nm in diameter and has a refractive index of 1.57. In this experiment, Ashkin and Dziedzic... 

J. E. and Chu, S. (1986) Observation of a single-beam gradient force optical trap for dielectric particles. Opt. Lett., 11, 288-290. 

Hopkins, R. J. Mitchem, L. Ward, A. D. Reid, J. P., Control and characterisation of a single aerosol droplet in a single beam gradient force optical trap, Phys. Chem. Chem. Phys. 2004, 6, 4924 4927... 

Ashkin, A., Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime, Biophys.., 61, 1992, 569-582. 

The LS theory was applied to the localization of a Brownian particle in a three-dimensional optical trap [89] a transparent dielectric spherical silica particle of diameter 0.6 pm suspended in a liquid [88]. The particle moves at random within the potential well created with a gradient three-dimensional optical trap—a technique widely used in biophysical studies. The potential was modulated by a biharmonic force. By changing the phase shift between the two harmonics it was possible to localize the particle in one of the wells in very good quantitative agreement with the predictions based on the LS. 

The optical trapping method uses a highly focused laser beam to trap and manipulate particles of interest in a medium (illustrated in Figure 3). The laser is focused on a dielectric particle (e.g., a silica microscopic bead), the refractive index of which is higher than the suspension medium. This produces a light pressure (or gradient force), which moves the particle towards the focal point of the beam, that is, the beam waist (Lim et al., 2006). 

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