Holographic line trap

Figure 5 shows the intensity, force and torque distribution experienced by a micrometer-diameter silica sphere in a holographic line trap (20). This is another generalization of an optical tweezer whose focal point is extended along a line segment in the focal plane, as shown in Fig. 5(a). Holographic line traps also come to a diffraction-limited focus in the axial direction, as Fig. 5(d) shows, and therefore can trap objects stably in three dimensions, as shown in Fig. 5(e), with a trapping efficiency of $Q_{\text {max}} = 0.05$ . The addition of an appropriate phase profile then facilitates creating a tailored force profile along the line's length (7), even when its intensity is uniform. The effect of a confining phase profile is demonstrated in Fig. 5(b), with superimposed trajectories converging on a region of mechanical equilibrium (7). A uniform phase profile eliminates the inward force along the line, and would allow a particle to diffuse freely in the $\hat{\boldsymbol{y}}$ direction (7). Switching the sign of the phase profile would drive particles to the ends of the line (7).

The computed torque distributions in Figs. 5(c) and (f) show that an illuminated particle again would tend to spin in the proximity of the strongly focused trap. The torque distribution for a holographic line trap is simpler than that for a holographic ring trap, with the particle's rotation axis varying little along the line and its direction flipping as the particle crosses the line. The torque efficiencies are comparably large, however.

Figure 5: (a) Intensity of a uniformly bright holographic line trap in the $(x,y)$ plane. (b) In-plane force distribution, showing the influence of a parabolic phase profile. Superimposed trajectories converge at stable equilibrium point. Hue indicates direction of the force according to the color wheel, and saturation indicates magnitude. Full saturation corresponds to $Q_{\text {max}} = 0.06$ . (c) In-plane component of the torque with $\tau _{\text {max}} = 2 \times 10^{-5}$ . (d) Intensity in the $(x,z)$ plane showing diffraction-limited focusing. (e) Associated axial force distribution with superimposed trajectories demonstrating stable three-dimensional trapping. $Q_{\text {max}} = 0.05$ (f) Axial torque distribution. $\tau _{\text {max}} = 6 \times 10^{-7}$ .