Have you ever wondered what a virus sounds like? Or what noise a bacterium makes when it moves between hosts? If the answer is yes, you may soon get your chance to find out, thanks to the development of the world's tiniest ear. The "nano-ear," a microscopic particle of gold trapped by a laser beam, can detect sound a million times fainter than the threshold for human hearing. Researchers suggest the work could open up a whole new field of "acoustic microscopy," in which organisms are studied using the sound they emit.
The concept of the nano-ear began with a 1986 invention known as optical tweezers. The tweezers use a laser beam focused to a point with a lens to grab hold of tiny particles and move them around. They've become a standard tool in molecular biology and nanotechnology, helping researchers inject DNA into cells and even manipulate it once inside. Optical tweezers can also be used to measure minuscule forces acting on microscopic particles; once you've grabbed hold of your particle with the laser beam, instead of moving it yourself, you simply use a microscope or other suitable monitoring apparatus to watch whether it moves of its own accord. That's where the nano-ear comes in.
Sound waves travel as a forward and backward displacement of the particles of the medium they pass through. So to detect sound, you need to measure this back-and-forth motion. Optical physicist Jochen Feldmann and colleagues in the Photonics and Optoelectronics Group at the University of Munich in Germany used a particle of gold 60 nanometers in diameter, immersed in water, and held in optical tweezers.
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