Abstract
We report what is to our knowledge the first experimental demonstration and theoretical analysis of an optical laser trap that uses a pair of mutually phase-conjugate beams. A primary trapping beam derived from an argon laser (514.5 nm) together with its counterpropagating phase-conjugate beam creates a self-aligned dual-beam laser trap that provides stable three-dimensional confinement for micrometer-sized dielectric particles. The transverse trapping efficiency, experimentally measured for low-numerical-aperture (N.A. 0.40–0.85) objective lenses, is found to be comparable with that produced by a single-beam gradient force trap. A theoretical analysis, which compares the performance of the self-aligned dual-beam trap against that of single-beam gradient force and conventional counterpropagating dual-beam laser traps, shows that phase-conjugate trapping provides a slight improvement in axial trapping efficiency over the other trapping geometries. The advantages of combining laser trapping with photorefractive optical phase conjugation for simultaneous sample micromanipulation and optical image processing are discussed.
Highlights
In 1969 Ashkin demonstrated that two counterpropagating mildly focused laser beams form a stable threedimensional trap for small dielectric spheres1 [Fig. 1(a)]
In optical phase conjugation (OPC),[6] a nonlinear medium and an input laser beam are used to generate a reflected beam that precisely reverses the direction of propagation and recovers the overall phase or wave front of the input beam in a time-reversed manner
In this paper we report what are to our knowledge the first experimental demonstration and theoretical study of a two-beam optical trap that uses a pair of self-aligned, mutually phase-conjugate beams
Summary
In 1969 Ashkin demonstrated that two counterpropagating mildly focused laser beams form a stable threedimensional trap for small dielectric spheres1 [Fig. 1(a)]. In optical phase conjugation (OPC),[6] a nonlinear medium and an input laser beam are used to generate a reflected (phaseconjugate) beam that precisely reverses the direction of propagation and recovers the overall phase or wave front of the input beam in a time-reversed manner. This process is most accomplished by use of the photorefractive effect[7,8] in nonlinear crystals and in spectral regions (e.g., 500–1500 nm) that are commensurate with those frequently used for optical laser trapping. When a phase-conjugate beam is used as one of the beams in a double-beam trapping geometry, a laser trap can be created that combines features of both optical trapping and OPC, including specimen manipulation, tensiometric measurement, and optical image processing functions such as aberration correction,[5,6] novelty filtering,[5,9] and contrast reversal.[5,10] Such a trap should prove to be a useful tool in the fields of biology and biomedicine in which the handling, manipulation, and image processing of biological specimens are routinely performed
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