Diagram of a Laser Tweezers

    'Laser tweezers', or 'Optical tweezers', are nothing more than a laser beam, focused down to a diffraction-limited spot.  This provides an electric field (the laser beam) with a gradient in each direction, so that there is one point of maximum field strength.  Dielectric objects (i.e. objects in which and electric field can induce a dipole) feel a force proportional to the magnitude of the gradient, that pulls them into the the region of maximum field strength. Thus, for small dielectric objects (like silica or polystyrene beads, or membrane-bound vesicles), the focused laser beam is sort of a tractor-beam: a free particle gets sucked into the beam, and has a preferred location in the beam, so that when the beam moves, the particle moves with it. Thus, the beam  can be used to apply a force to such a particle. Because the applied forces can be calibrated, this is an excellent tool for measuring the forces generated by molecular motors as they move cargos along microtubules or actin filaments (simply grab the cargo, and measure the force required to stop the motion). See my Cell paper (in publications) for one such usage, also see e.g. the work of Dr. Jim Spudich (Stanford), Dr. Steven Block (Stanford), and Dr. Steven Chu (Stanford). No, you don't have to be at Stanford to use or build one!

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