Most of the experimental techniques we use are rather new and in a state of development. Therefore we spend considerable effort on understanding and improving the methods.

	Optical tweezers are a means to three-dimensionally trap microscopic particles using momentum transfer from scattered light. We use near-infrared laser light, focussed with a high-numerical-aperture objective into a sample, where we can trap particles of sizes between tens on nm to many micrometers. Maximal forces are on the order of 100 pN. We use photodiodes to detect the motions of the trapped particles and the forces exerted on them by the light.
	The optical trapping of refractile particles can be understood considering the interaction of the induced dipole moment in the dielectric particles with the illuminating field. A gradient of force develops, pointing towards the focus of the laser beam.
	The detection of the motion of trapped objects with Angstrom-resolution can be done with various methods, all more or less based on interference between the illuminating light and the light scattered by the trapped object. By placing a position sensitive photodetector, such as a quadrant photodiode in the back-focal plane of the microscope condenser, the momentum transfer on the light and the particle can be measured directly.

Current topics of interest are various interferometric detection methods to monitor motion in optical traps with Angstrom-resolution, noise limitations on the experiments, focussing of high numerical aperture laser beams and optical aberrations, interesting side effects of optical trapping such as 'optical binding forces', dissipative effects etc.

Collaborators

Frederick Gittes, Dept. Physics, Vanderbilt University, Nashville, TN, USA