Optical tweezers1 are commonly used for manipulating microscopic particles, with applications in cell manipulation2, colloid research3,4,5, manipulation of micromachines6 and studies of the properties of light beams7. Such tweezers work by the transfer of momentum from a tightly focused laser to the particle, which refracts and scatters the light and distorts the profile of the beam. The forces produced by this process cause the particle to be trapped near the beam focus. Conventional tweezers use gaussian light beams, which cannot trap particles in multiple locations more than a few micrometres apart in the axial direction, because of beam distortion by the particle and subsequent strong divergence from the focal plane. Bessel beams8,9, however, do not diverge and, furthermore, if part of the beam is obstructed or distorted the beam reconstructs itself after a characteristic propagation distance10. Here we show how this reconstructive property may be utilized within optical tweezers to trap particles in multiple, spatially separated sample cells with a single beam. Owing to the diffractionless nature of the Bessel beam, secondary trapped particles can reside in a second sample cell far removed (∼3 mm) from the first cell. Such tweezers could be used for the simultaneous study of identically prepared ensembles of colloids and biological matter, and potentially offer enhanced control of ‘lab-on-a-chip’ and optically driven microstructures.

Simultaneous micromanipulation in multiple planes using a self-reconstructing light beam

Optical forces can be used to manipulate biological and colloidal material in a non-contact manner. This forms the foundation of a wealth of exciting science, particularly in the fields of physics, biology and soft condensed matter. Although the standard Gaussian single-beam trap remains a very powerful tool, shaping the phase and amplitude of a light field provides unusual light patterns that add a major new dimension to research into particle manipulation. This Review summarizes the impact and emerging applications of shaped light in the field of optical manipulation.

Shaping the future of manipulation

We investigate both theoretically and experimentally the self-healing properties of accelerating Airy beams. We show that this class of waves tends to reform during propagation in spite of the severity of the imposed perturbations. In all occasions the reconstruction of these beams is interpreted through their internal transverse power flow. The robustness of these optical beams in scattering and turbulent environments is also studied experimentally. Our observations are in excellent agreement with numerical simulations.

/pdf/self-healing-properties-of-optical-airy-beams-1fpzb2knrt.pdf

Self-healing properties of optical Airy beams

A prototype microscope built with self-reconstructing Bessel beams is shown to be able to reduce scattering artifacts as well as increase image quality and penetration depth in three-dimensional inhomogeneous opaque media.

/pdf/microscopy-with-self-reconstructing-beams-5721v7dbio.pdf

Microscopy with self-reconstructing beams

Based on the separability of the Helmholtz equation into elliptical cylindrical coordinates, we present another class of invariant optical fields that may have a highly localized distribution along one of the transverse directions and a sharply peaked quasi-periodic structure along the other. These fields are described by the radial and angular Mathieu functions. We identify the corresponding function in the McCutchen sphere that produces this kind of beam and propose an experimental setup for the realization of an invariant optical field.

Alternative formulation for invariant optical fields: Mathieu beams

Self-reconstruction of a distorted nondiffracting beam

Self-reconstruction effect in free propagation of wavefield

Experimental realization of self-reconstruction of the 2D aperiodic objects

The nondiffractive propagation of the ideal zero-order Bessel beam is explained in terms of both the geometrical imaging and the interference of spherical waves emitted from the ring source. The source shift with respect to the focal plane of the imaging system is simply related to the divergence of the produced beam. The intensity distribution of the ideal Bessel beam in the focal region is examined for an aberration-free lens of finite aperture. Both the resolution gain at the focal plane and the position of the axial intensity maximum are investigated in dependence on the truncation of the input Bessel beam.

Bessel beams in the focal region

Some properties of the exact propagation-invariant solutions of the Maxwell equations are pointed out. Attention is focused on the transversality, energy flow and phase singularities of the stationary propagation-invariant electromagnetic fields.

J. Wagner

Papers

Self-reconstruction of a distorted nondiffracting beam

Self-reconstruction effect in free propagation of wavefield

Experimental realization of self-reconstruction of the 2D aperiodic objects

Bessel beams in the focal region

Propagation-Invariant Electromagnetic Fields.