Optical tweezers  exploit optical gradient forces to trap dielectric particles in three dimensions. The electric dipole moment induced in a dielectric particle by an impinging beam of light induces an electric dipole moment in a dielectric particle. This dipole senses gradients in the field intensity and is drawn to the brightest region. Tightly focusing a laser beam with a high numerical aperture lens creates a local intensity maximum which can attract a particle strongly enough to overcome both radiation pressure and thermal forces. The same high numerical aperture objective lens used to image particles also can be used to form optical tweezers. Since the focal point can be arranged to lie in the microscope's focal plane, trapped particles are guaranteed to be in focus for the imaging system.
Our dual optical tweezer system is powered by a Spectra Diode Labs 100 mW diode laser operating at 780 nm. We introduce laser light into the objective lens with a dichroic beam splitter which facilitates simultaneous trapping and imaging. Varying the angle at which the collimated beam enters the back aperture of the objective lens moves the trapping point across the focal plane. This allows precise placement of one or more optical tweezers in the field of view. Projecting the conjugate plane to the objective lens' back aperture out of the microscope with a Keplerian telescope allows us to steer each trap with an external gimbal mounted mirror. Moving optical tweezers have been used in video microscopy studies of DNA and other polymers tethered to dielectric spheres . They also have been used to mimic a temporally varying spatially anisotropic potential for imaging studies of directed diffusion .
We perform dynamical measurements by locating our tweezers conveniently in the field of view and chopping them several times a second. Particles are free to diffuse while the laser is interrupted and are drawn back to the starting point in three dimensions when the traps reform. By collecting data only when the traps are off, we readily amass a large sample of dynamical data for isolated freely moving particles without having to chase them or refocus the microscope. We also can ensure that particles never wander close to walls or to other particles and thereby control systematic effects in our measurements. Synchronizing the chopper to the video sync signal from our camera ensures that the traps are either completely on or completely off in each video frame. The diffusion data in Fig. 6 was collected in this manner in less than two hours.
Using optical tweezers to manipulate colloidal particles would not be a useful adjunct to dynamical measurements if the laser illumination altered the system's behavior. Simple calculations show, however, that the time scale for both thermal and viscous relaxation in water on micron length scales is much shorter than a millisecond. Thus, any trap-induced thermal or hydrodynamic perturbation has already died out by the time we start collecting data. It should be noted that for some systems with slow relaxation, such as spheres in polymer solutions, this criterion might not be so easily met.