Elsevier

Optics Communications

Volume 185, Issues 1–3, 1 November 2000, Pages 77-82
Optics Communications

Multi-functional optical tweezers using computer-generated holograms

https://doi.org/10.1016/S0030-4018(00)00990-1Get rights and content

Abstract

Optical tweezers are capable of trapping microscopic particles by photon momentum transfer. The use of dynamic computer-generated holograms for beam shaping allows a high flexibility in terms of trap characteristics and features. We use a liquid crystal display (LCD) to display the holograms. Efficiency losses caused by the periodic electrode structure of the LCD have been clearly reduced by use of an optically addressed spatial light modulator. We realized multiple traps, which can hold and move at least seven silica spheres independently in real time. We also demonstrate the controllability of trapped particles in three dimensions without the need for mechanical elements in the setup.

Introduction

The ability to manipulate micrometer-sized particles with laser beams has led to applications, mainly in biological fields. Early works of Ashkin et al. in 1970 [1] and in the 1980s [2] served as seeds for this technique now known as optical tweezers. Therein ray optics and photon momentum considerations were used to show that, roughly speaking, a high-index particle (nparticle>nmedium) is attracted by the beam focus if the parameters are chosen appropriately.

In experimental works of several authors [3], [4], [5], [6], [7], [8] different setups have been realized showing the usability of optical tweezers for manipulation and investigation purposes. Normally a trapped particle is moved by motion of a microscope stage. Alternatively adjustable mirrors or accousto-optical modulators can be used to steer the beam and thus the trapped particle [9]. Such setups become quite complicated if three-dimensional steering or multiple trapping is desired [10].

In Ref. [11] we have presented a holographic tweezer setup in which computer-generated holograms written on a liquid crystal display (LCD) have been used to control the number, positions, and shapes of optical traps in two dimensions. The basic setup is briefly explained in Section 2. We extend this method to the full three-dimensional manipulation of multiple objects as shown in Section 3. In Section 4 we present a modified setup with an optically and an electrically addressed LCD in order to improve the diffraction efficiency considerably and hence much more laser power is available for the hologram reconstruction.

Section snippets

Basic tweezer setup

The basic experimental setup of the tweezer is shown in Fig. 1. Its central element is the LCD which is controlled by a personal computer. It displays the Fourier holograms which are read out by a collimated 1 W Ar+-laser (Spectra Physics 165) at a wavelength of 488 nm. The beam is then coupled into the microscope setup by a dichroic mirror (DM). The beam diameter is reduced to fit the 2 mm aperture of the water immersion microscope objective, MO (Zeiss Achroplan 100×/1.0 W). The hologram

Three-dimensional trapping

By no means is one limited to a manipulation in just one plane. By adding a lens term to the blazed gratings the beam focus can be shifted up and down parallel to the optical axis:Φ(x,y)=2πΛxx+2πΛyy+Γ(x2+y2)mod2πΓ controls the axial position of the trap. The appearance of a typical hologram is depicted in Fig. 3. It is a hologram for three laterally and axially displaced traps which fills the entire LCD panel.

Fig. 4 shows an experiment that demonstrates the controllability in three dimensions.

Improved setup using an optically addressed liquid crystal display

For the practical use of optical tweezers and holographic applications in general it is often desirable to have maximum laser power available in the hologram reconstruction. The setup of Fig. 1, however, leads to some loss of laser power caused by the LCD structure.

The loss can be traced back to two major limiting factors: First, about 56% of the laser power is blocked by the mask structure surrounding each pixel which also defines the fill factor of 44%. Second, the periodic structure of the

Conclusion

The versatility of optical tweezers using computer-generated holograms displayed on an LCD has been demonstrated. The technique qualifies for a wide range of applications. We demonstrated the ability to trap and manipulate multiple particles independently in three dimensions without mechanical elements. In terms of hologram reconstruction efficiency a clear improvement has been accomplished by use of an additional, OALCD. The efficiency was raised by a factor of almost 6 compared to a single

Acknowledgments

We thank the Deutsche Forschungsgemeinschaft for the financial support.

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