# Optical tweezer arrays and optical substrates created with diffractive optics

Original Entry: Tom Dimiduk APPHY 225 Fall 2010

## Keywords

Optical Tweezers, Colloids, Templated Self-Assembly, Directed diffusion

## Summary

Figure 1: Optical setup for holographic optical tweezers.
Schematic of operation of the diffractive optical element which yields the sixteen beams used in this paper.
Figure 2: "FIG. 2. 4x4 Optical tweezer array created from a single laser beam using a holographic array generator. (a) The tweezer array illuminates the 70x70 $\mu m^2$ field of view with light backscattered from trapped silica particles. The scale bar represents 10 $\mu m$. (b) The particle array 1/30 s after being released. (c) The same field of view 3.1 s later. (d) Trajectories of the particles in the field of view after being released. Dark traces indicate particles initially trapped in the array. Shorter tracks indicate particles which diffused out of the $\pm 200 nm$ depth of focus."

The authors demonstrate a way to create a large number of optical tweezers within a sample. They use a diffractive optical element to split a single 100 mW laser beam into 16 diverging beams. When these beams are focused within a sample (by the optics in Figure 1), they form a pattern of 16 trap foci, each of which can trap a particle (Figure 2). The optics is relatively standard for an optical tweezer setup, adding beam telescopes to produce additional conjugate planes in which do do diffraction, filtering, and steering.

They demonstrate the utility of this technique by trapping 16 colloids (500 nm silica spheres), demonstrating that they can hold them for several minutes, and then releasing them to observe their diffusion. The authors believe this technique can be naturally extended to trap higher index mismatch particles by using optical vorticies which are straightforward to obtain with diffractive optical elements.

## Soft Matter Discussion

The authors are able to estimate the potential well depths of their traps to be at least 6 kT based on the > 100s residence time of particles within the traps.

The authors propose that this technique could be useful for templating self assembly. An array of tweezers could be used to hold particles in a starting nucleus from which a structure could grow. This kind of growth is potentially interesting for growing photonic structures.

Combined with temporal modulation, this technique could be used to sort particles by directed diffusion. Their diffusion from 16 point lattice could serve as a prototype experiment for studying the breakdown of colloidal structures.