Microscopic particles with varying optical properties may be induced to move in different ways when placed on a sculpted optical potential due to differences in shape, size or polarisability. The separation of red blood cells (erythrocytes) and white blood cells (lymphocytes) is achieved in a non-invasive manner and in the absence of any microfluidic systems using a 'non-diffracting' circularly symmetric Bessel beam. The Bessel beam, which consists of a series of concentric rings, each of equal power and of 3.2μm thickness with a spacing of 2μm around a central maximum of 5μm diameter (and is akin to a rod of light as its propagation distance is 3mm), is directed upward into a sample chamber containing blood. Fluctuations in Brownian motion cause cells to escape from individual rings of the Bessel beam and travel towards the beam centre, where the intensity of the rings increases. However, these cells must be able to overcome the potential barrier of each ring which gets larger toward the central maximum. Lymphocytes - spherical in shape and 7μm in diameter (therefore overlapping two rings) - are transported, due to the gradient force of the optical field, to the beam centre where they are guided upwards and form a vertical stack, whereas erythrocytes re-align on their sides in the outer rings and are then guided upwards, because once aligned they cannot escape the potential barrier and 'lock-in' to that ring. The optical power required for optimal sorting in this static sorter which requires no fluid flow is investigated.
In the optical domain, the gradient force may be exploited in optical tweezers to confine high-index particles to points of maximum light intensity [1]. This methodology has enabled key advances in biology enabling a deeper understanding of molecular motors and the properties of DNA. Optical traps have also enabled a wide range of studies in optical angular momentum, colloid science and microfluidics. Recent work has shown that extended, optically tailored landscapes can offer a mechanism by which to arrange and accumulate microparticles in pre-described arrays [2]). The ability to sculpt and reconfigure the optical potential energy landscape external to the sample is a key component of such studies. We
may add a tilt to the potential or, more generally, break symmetry, enabling unprecedented control over directed transport of particles [3] Three dimensional optical lattices may be used for sorting and fractionation of biological material in a microfluidic flow [4]. However it would be advantageous to be able to separate and even accumulate both biological and colloidal matter in the absence of any flow within any sample chamber. This would allow true compatibility of sorting and separation without the need to implement flows and microfluidic systems. We exploit the varying affinity of mesoscopic objects to a circularly symmetric optical landscape to demonstrate this effect and demonstrate separation of cells and chromosomes. The differing Kramers residence time in each part of the light pattern
leads to a thermally activated method for sorting based on their hopping probabilities within the rings of the Bessel beam. Whilst we employ a Bessel beam to elucidate and demonstrate the dynamics of the sorting other tailored landscapes can also be used.
Optical micro-manipulation has seen a resurgence of interest in recent years which has been due in part to new application areas and the use of tailored forms of light beam. In this paper, experimental observations of fluctuation-driven transport of silica microspheres within a two-dimensional optical potential of circular symmetry are observed. The potential is created by a Bessel light beam. The optical field is tailored to break the symmetry and create a static tilted periodic (washboard) potential. Transitions between locked and running modes may be observed. The running mode manifests itself by rapid accumulation of particles at the beam centre. We discuss what happens with mixtures of particles in such an optical potential.
Optical micro-manipulation has seen a resurgence of interest in recent years, which has been due in part to new application areas and the use of tailored forms of light beam. In this paper, experimental observations of fluctuation-driven transport of silica microspheres within a two-dimensional optical potential of circular symmetry are observed. The potential is created by a Bessel light beam. The optical field is tailored to break the symmetry and create a static tilted periodic (washboard) potential. Transitions between locked and running modes may be observed. The running mode manifests itself by rapid accumulation of particles at the beam centre. We discuss what happens with mixtures of particles in such an optical potential.
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