A nanoear to listen into the silence
The new method realized by the Munich physicists opens a new world to scientists: for the first time, otherwise imperceptibly weak motions – minuscule sound waves – can be visualized. The scientists developed the nanoear in two stages. “First, we validated the basic principle using a relatively strong sound source” group leader Andrey Lutich explains. “In the second step we were able to detect significantly weaker acoustic excitations.” The main element in both cases is a gold nanoparticle, 60 nm in diameter, which is kept in levitation by a so-called optical trap using a red laser. Each of the experiments was done in a small water drop on a cover slide.
In the first case, a needle serves as a sound source. It is glued onto a loudspeaker membrane and emits sound waves towards the trapped gold particle. The scientists successfully detected the oscillations of the trapped particle optically using a dark-field microscope and an ordinary digital camera. The recorded videos, each 30 seconds in length, clearly showed the particle oscillating parallel to the propagation direction of the sound waves.
In a second step, the physicists used the so-called nanoprinting method to fix a small number of gold particles on the cover slide. These particles are heated periodically using a green laser. As a result they emit very weak sound waves towards the single levitating gold nanoparticle. The interaction between the sound waves and the trapped particle is very weak. Therefore, the displacement of the particle cannot be detected directly with available optical methods. The scientists used the mathematical Fourier Transformation to obtain the frequency spectrum of the particle's motion. The physicists could show that the frequency of the sound source is clearly enhanced in this spectrum. Control experiments in which the sound source is driven at varying frequencies confirmed this observation and the high sensitivity of the nanoear.
“With our nanoear, we have developed a nanomicrophone that allows us to get closer than ever to microscopic objects” Alexander Ohlinger, first author of the publication, explains. “By observing the oscillations of a single gold nanoparticle, tiny movements can be detected.” In this way, the nanoear could yield important information on the minute motions of cells, cell organelles or artificial microscopic objects. Additionally, no high-end devices are necessary as only well-established methods are used. (NIM)
Optically Trapped Gold Nanoparticle Enables Listening at the Microscale.
Alexander Ohlinger, Andras Deak, Andrey A. Lutich, and Jochen Feldmann.
Phys. Rev. Lett. 108, 018101 (January 2012)
Prof. Dr. Jochen Feldmann
Chair of Photonics and Optoelectronics
Faculty of Physics
Phone: +49 89-2180-3359