Laser trapping of microscopic particles for undergraduate experiments

We present detailed instructions for constructing and operating an optical trap using a hollow core fiber and two-laser beams. The trap is stable, confining 100-nm to 10-mum particles of a variety of materials for hours of leisurely observation. The trap operates at room temperature and atmospheric pressure, costs less than $1400, and requires no machining. The hollow fiber provides automatic relative beam alignment and shields trapped particles from the ambient convective flow. An experiment and analysis of the Mie scattering from the trapped particle is outlined. A list of other interesting experiments based on the trap is provided. (C) 2000 American Association of Physics Teachers.