Fig. 11.1 — Rotating particles in Laguerre-Gaussian beams

Chapter 11 — Wavefront Engineering and Holographic Optical Tweezers*

The most basic optical tweezers set-up design, e.g., the one we built in Chapter 8, produces a single optical trap by focusing a single optical beam. By using more than one beam, or by splitting a single beam, it is possible to generate multiple traps. This procedure, however, leads to rather complex, and sometimes messy, set-ups. Additionally, in order to steer these traps it is necessary to move some mechanical components of the optical set-up. Finally, using non-Gaussian beams demands the use of specialised optical components, e.g., axicons to generate Bessel beams or holographic masks to generate higher-order Laguerre-Gaussian beams. In order to overcome these difficulties, it is possible to employ holographic optical tweezers (HOTs). HOTs use a computer-controlled diffractive optical element (DOE) to split a single collimated laser beam into several separate beams, each of which is focused into an optical tweezers. These optical traps can be made dynamic and displaced in three dimensions by projecting a sequence of holograms and, furthermore, non-Gaussian beam profiles can be straightforwardly encoded in the holographic mask, such as the Laguerre-Gaussian beams employed in the optical trapping experiment shown in Fig. 11.1. In this Chapter, we explain how to design and operate HOTs.

* This chapter was written together with Giuseppe Pesce.

11.1  Basic working principle

11.2  Computer-generated holograms
11.2.1  Single steerable trap
11.2.2  Random mask encoding
11.2.3  Superposition of gratings and lenses
11.2.4  Gerchberg–Saxton algorithm
11.2.5  Adaptive–additive algorithm
11.2.6  Direct search algorithms

11.3  Higher-order beams and orbital angular momentum

11.4  Continuous optical potentials

11.5  Set-up implementation
11.5.1  Spatial light modulators

11.6  Alternative approaches
11.6.1  Time-shared optical traps
11.6.2  Generalised phase contrast



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