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Physicists in Germany have developed a "nano-ear" of detecting sound on microscopic length scales. 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.
The researchers suspended gold nanoparticles in a drop of water. They trapped one sphere in a laser beam and then fired rapid pulses of light from a second laser at others a few micrometres away. The pulses heated the nanoparticles, which disturbed the water around them, generating pressure, or sound, waves. The device can optically trap gold nanoparticle and could be used to "listen" to biological micro-organisms as well as investigate the motion and vibrations in tiny machines. When laser light is focused at a point in space gold nanoparticles can be trapped in optical tweezers and an electric dipole moment is induced in the particle and drawn to the most intense part of the laser's electric field. The particle inside an optical trap can also be used as an extremely sensitive and minuscule sound detector. The trapped particle can be made to move from its equilibrium position by vibrations from nearby sound waves whose frequency can then be calculated by analysing how much the particle has been displaced.
The 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 an 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 estimated sensitivity is six orders of magnitude below the threshold of human hearing.
Sound detector
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 are six orders of magnitude lower than the threshold of a human ear.
The device could be used to analyze the sounds made by live micro-organisms, such as bacteria and viruses, 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 and to develop a new type of 'acoustic microscopy' because it is possible to bring very sensitive sound sensors in close vicinity to microscopic samples.

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