OPTICAL ALIGNMENT AND SPINNING OF LASER-TRAPPED MICROSCOPIC PARTICLES

Light-induced rotation of absorbing microscopic particles by transfer of angular mometum from light ao the material raises the possibility o f optically driven microsmachines. The phenomenon has been observed us ing elliptically polarized laser beams(1) or beams with helical phase structure(2,3). But it is difficult to develop high power in such expe riments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in princip le be overcome by using transparent particles, transferring angular mo mentum by a mechanism first observed by Beth in 1936(4), when he repor ted a tiny torque developed in a quartz 'wave-plate' owing to the chan ge in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers(5). Depending on the polarization of the inc ident beam, the particles either become aligned with the plane of pola rization (and thus can be rotated through specified angles) or spin wi th constant rotation frequency. Because these microscopic particles ar e transparent, they can be held, in three-dimensional optical traps at very high power without beating, leading to rotation rates of over 35 0 Hz.