Credit: © 2008 NPG

Light can exert a mechanical force on an object in two ways — through radiation pressure or through a force that arises from gradients in the light beam. The force exerted by radiation pressure has been observed in experiments on microcavities, but this approach cannot be extended down to the nanoscale. The gradient force has now been measured in a nanomechanical system for the first time by Hong Tang and co-workers at Yale University and the University of Washington1.

Building on previous theoretical work by a group from Stanford, MIT and Harvard, Tang and co-workers suspended a silicon waveguide, measuring 10 µm long, 500 nm wide and 110 nm thick, above a substrate, and passed a laser beam through it. The beam in the waveguide coupled to the substrate, leading to an asymmetry in the electromagnetic field that resulted in a gradient force acting on the waveguide. Tang and co-workers used a second laser beam to measure the displacement of the waveguide.

The Yale–Washington team achieved a sensitivity of 18 femtometres per root Hertz at room temperature, which is comparable to the best results obtained at cryogenic temperatures. This new approach — which is compatible with CMOS technology — could have applications in all-optical switching and optomechanical signal processing.