Physicists in Germany have developed the first-ever “nano-ear” capable of detecting sound on microscopic length scales with an estimated sensitivity that is six orders of magnitude below the threshold of human hearing. The device is based on an optically trapped gold nanoparticle, and its inventors claim that it could be used to “listen” to biological micro-organisms as well as investigate the motion and vibrations in tiny machines.
Particles can be trapped in “optical tweezers”, which are formed when laser light is focused at a point in space. An electric dipole moment is induced in the particle and it is drawn to the most intense part of the laser’s electric field. The technique was discovered in the 1980s and is used routinely in research labs around the world. It is particularly useful for manipulating biological objects, since the optical field used to make the trap is non-destructive.