Dynamics of a Kerr Nanoparticle in a Single Beam Optical Trap

Abstract

Single beam optical traps also known as optical tweezers, are versatile optical tools for controlling precisely the movement of optically-small particles. Single-beam trapping was first demonstrated with visible light (514 nm) in 1986 to capture and guide individual neutral (nonabsorbing) particles of various sizes (Ashkin et. al., 1986). Optical traps were later used to orient and manipulate irregularly shaped microscopic objects such as viruses, cells, algae, organelles, and cytoplasmic filaments without apparent damage using an infrared light (1060 nm) beam (Ashkin, 1990). They were later deployed in a number of exciting investigations in microbiological systems such as chromosome manipulation (Liang et.al., 1993), sperm guidance in all optical in vitro fertilization (Clement-Sengewald et.al.,1996) and force measurements in molecular motors such single kinesin molecules (Svoboda and Block, 1994) and nucleic acid motor enzymes (Yim et.al., 1995). More recently, optical tweezer has been used in single molecule diagnostics for DNA related experiments (Koch et.al., 2002). By impaling the beads onto the microscope slide and increasing the laser power, it was tested that the bead could be spot-welded to the slide, leaving the DNA in a stretched statea technique was used in preparing long strands of DNA for examination via optical microscopy. Researchers continue to search for ways to the capability of optical traps to carry out multidimensional manipulation of particles of various geometrical shapes and optical sizes (Grier, 2003; Neuman & Block, 2004). Efforts in optical beam engineering were pursued to generate trapping beams with intensity distributions other than the diffraction-limited beam spot e.g. doughnut beam (He et.al., 1995; Kuga et.al., 1997), helical beam (Friese et.al., 1998), Bessel beam (MacDonald et.al., 2002). Multiple beam traps and other complex forms of optical landscapes were produced from a single primary beam using computer generated holograms (Liesener et.al., 2000; Curtis et.al., 2002; Curtis et.al., 2003) and programmable spatial light modulators (Rodrigo et.al., 2005; Rodrigo et.al., 2005). Knowing the relationship between characteristics of the optical trapping force and the magnitude of optical nonlinearity is an interesting subject matter that has only been lightly investigated. A theory that accurately explains the influence of nonlinearity on the behavior of nonlinear particles in an optical trap would significantly broaden the applications of optical traps since most materials including many proteins and organic molecules, exhibit