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.

Now, a team led by Jochen Feldmann and Andrey Lutich at the Ludwig-Maximilians University in Munich has shown that a particle inside an optical trap can also be used as an extremely sensitive and minuscule sound detector. The researchers have found that the trapped particle can be made to move from its equilibrium position by vibrations from nearby sound waves. The frequency of the sound can then be calculated by analysing how much the particle has been displaced.

Sound sources
The team's set-up consisted of two sound sources placed in a water-based medium. The first "loud" source is a tungsten needle glued on a loudspeaker that vibrates at a frequency of 300 Hz. The second, weaker source is made up of bunches of gold nanoparticles that are periodically heated by a second laser to create sound waves at a frequency of 20 Hz. The nano-ear is a 60 nm gold nanoparticle trapped in a 808 nm wavelength laser beam.

When either of the sound sources is turned on, the ensuing vibrations cause the trapped particle to move in the same direction as the propagating sound waves.

The researchers used a video camera to track the motion of the trapped particle. They then tested how sensitive their nano-ear was by analysing the recorded trajectories of the particle. The result is a frequency spectrum of the sound sources superimposed on the frequency spectrum of the trapped particle's Brownian motion.

Ultrasenstive detector of sound
The spectra reveal a clear, superimposed single peak at the frequency of the sound source. Further analysis reveals that the nano-ear can detect vibrations at a power level as low as –60 dB, which is six orders of magnitude lower than the threshold of a human ear.

According to the team, the device could be used to analyse the sounds made by live micro-organisms, such as bacteria and viruses. It might also be used to investigate artificial micro-objects that produce acoustic vibrations but that cannot be directly visualized in an optical microscope because of strong light absorption or scattering.

"We might even be able to develop a new type of 'acoustic microscopy' because it is possible to bring very sensitive sound sensors in close vicinity to microscopic samples," team member Alexander Ohlinger told nanotechweb.org.

The team, which is part of the research cluster Nanosystems Initiative Munich, published its work in Physical Review Letters.