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Introduction

Optical imaging and manipulation of cells, associated with computer-aided cell recognition algorithm, are essential measures to execute cell identification and classification in modern biological and medical engineering. In a bio-chip (lab-on-a-chip), throughput of cell processing is undoubtedly one of the most important characteristic parameters. Generally, implementation of physically classification of cells/bio- particles involve real-time recognition of cells upon acquisition of cellular image, followed up with optical cell manipulation such as branching (directly push off to another microfluidic channel) by an optical force using laser beams, or immediate trapping and translation by an optical tweezer to a designated position. Contactless and nondestructive nature of optical manipulation, where focused laser beams are used for trapping, manipulating, and arranging of biological particles, have created numerous applications in biotechnology, DNA nanotechnology, cell processing in microfluidics [1-3], and even provided potential medical treatments with cell tissue processes such as clearing obstructed blood vessels [4].

An advanced imaging technique of cells may further require rotation of a cell while being trapped so as to be able to view and access to different facets of a particular cell, providing appropriate orientation for bio-engineering processes such as cell nucleus replacement (CNR). Controlling cell orientation in imaging is more difficult than controlled translation of cell, which is largely due to small size of cells in micrometer scale, close to that of laser beam spot. In any of these measures to control cell orientation, trapping of a cell/particle is a pre-requisite before implementation of orientation change.

Current state-of-the art optical tweezers implementations have primarily used multi-beam manipulation based on fiber optics and on-chip waveguides [5] to achieve both rotational and translational degrees of freedom for particle manipulation. These manipulation techniques include using a pair of crossing optical fibers for rotation and levitation (Taguchi et al.) [2, 3], employing photonic linear momentum through synchronized rotation of rectangular apertures in the free-space optical beam path (O’Neil et al.) [6], absorption of angular momentum carried by a Laguerre-Gaussian (LG) beam with an annular intensity profile [7], or using a fiber cone fabricated by chemical corrosion to facilitate particle capture and translation (Hu et al.) [8-10]. With the exception of the LG-mode-based rotational technique, nearly all implementations have utilized focused beams in the TEM fundamental mode (Gaussian beam) for the optical trapping of biological cells. However, Gaussian beam distributions have limitations in application. The central spot in the beam profile raises the possibility of intensity-based damage to target particles. Further, a secondary focused beam is generally required to generate torque to enable target rotation, which adds complexity to the system with respect to both required space and additional optical elements. With the recent popularity of microfluidic and lab-on-a-chip analytical techniques, it becomes highly advantageous for a potential optical tweezers implementation to have a miniaturized probing tip while retaining a compact overall system size. An optical fiber tweezers system based on a multi-lobed beam to capture bio?logical particle or clusters for selective translation, rotation, and reorganization can allows for mobile applications for in-fleld biological or medical analyses.

We will discuss the manipulation of biological particles using a single beam in LP21 mode, a low-order fiber optic transmission mode [11]. With an intrinsic four- lobed intensity distribution and high coherence, we demonstrate that an LP21 mode beam can be focused to form an optical chuck, allowing the capture and reorganization of biological particles inside clusters, as well as both translation and rotation of the particle by simply rotating a segment of fiber in the optical train. The force that the optical chuck exerts on target bio-particles in the process of rotation and translation was analyzed using a theoretical model based on ray optics [12-14], with a good agreement between the simulated model and the experimental measurements.

 
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