Abstract
The possibility for the manipulation of many different samples using only the light from a laser beam opened the way to a variety of experiments. The technique, known as Optical Tweezers, is nowadays employed in a multitude of applications demonstrating its relevance. Since the pioneering work of Arthur Ashkin, where he used a single strongly focused laser beam, ever more complex experimental set-ups are required in order to perform novel and challenging experiments. Here we provide a comprehensive review of the theoretical background and experimental techniques. We start by giving an overview of the theory of optical forces: first, we consider optical forces in approximated regimes when the particles are much larger (ray optics) or much smaller (dipole approximation) than the light wavelength; then, we discuss the full electromagnetic theory of optical forces with a focus on T-matrix methods. Then, we describe the important aspect of Brownian motion in optical traps and its implementation in optical tweezers simulations. Finally, we provide a general description of typical experimental setups of optical tweezers and calibration techniques with particular emphasis on holographic optical tweezers.
Highlights
Since the pioneering work of Arthur Ashkin, where he used a single strongly focused laser beam, ever more complex experimental set-ups are required in order to perform novel and challenging experiments
Experiments to detect the mechanical effects of light were performed by Nichols and Hull [3] and Lebedev [4], who succeeded in detecting the radiation pressure acting on macroscopic objects using thermal light sources and a torsion balance
To calculate the multipole amplitudes Wi(,plm) of a focused beam, as is used in the case of an optical tweezers, we can exploit the expansion of the incoming beam into plane waves and its focusing in terms of the angular spectrum representation
Summary
The ability of light to exert a force on matter was recognised as early as 1619 by Kepler [1] who first described the deflection of comet tails by the rays of the sun. The DOE is a computer-controlled liquid-crystal spatial light modulator [36], since this affords the potential for dynamically changing the form of the DOE, permitting real-time control over number, intensity and positions of the optical traps. With this technique, it is possible to generate more complex trapping configuration, e.g. using Laguerre–Gaussian beams, which can produce torques by the transfer of orbital angular momentum [37–39]
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