Nanometer-displacement detection of optically trapped metallic particles based on critical angle method for small force detection

We have developed a method of measuring the displacement of optically trapp ed metallic particles (10 mu m in diameter) in water with nanometer resolut ion to detect small forces. Metallic particles were optically trapped in tw o dimensions by focusing a laser beam below the particle using an objective lens with a numerical aperture of 0.9. Displacement of a trapped metallic particle was detected using the light reflected from the particle based on the critical angle method. The lateral spring constant was estimated from t he equipartition theorem to be on the order of 10(-6) N/m and found to incr ease as the incident laser power increased. Consequently, a trapped metalli c particle can be used to detect small forces (10(-13) N). (C) 1999 America n Institute of Physics. [S0034-6748(99)01607-X].