The Nano-ear: ‘Hearing’ using optical tweezers
Steven Block from Stanford University is one of the pioneers in optical manipulation, and his work is particularly famous for pushing the technique to its sensitivity limits. His work, which explores the behaviour of molecular motors has shown that optical tweezers technique can be used to measure molecular motion down to an Angstrom, or in other words, you can use a bead about a millionth of a meter in diameter to observe the motion of a molecule moving a distance equivalent to the width of a hydrogen atom. This is pretty staggering in my view! One of Block’s party tricks at conferences was to show the sensitivity of his system by playing a recording of the signal picked up by a laser that scattered off the trapped bead. If the radio was turned on in the room next door then the bead was able to pick up the vibrations of the sound waves, and playing back the light scattering signal gave you a hissy version of, say, “Good Vibrations”.
This is very neat, and demonstrates quite clearly how it might be possible to use an optical trap as some form of acoustic pick up. Now researchers from the LMU in Munich, and published in Physical Review Letters has taken this idea a stage further and produced a ultrasensitive tweezers based pick-up using a trapped 60nm gold nanoparticle. The experimental set up is shown below – a straightforward optical tweezers setup combined with a loudspeaker with a needle attached to introduce sound waves into the chamber. The authors also use heated nanoparticles to create sound waves as well – offering an optothermal method of doing the same thing as the vibrating needle.
By monitoring the particle trajectories the influence of a sound wave of a particular frequency can then be detected. This is achieved by taking what is known as the Fourier transform of the position data. The Fourier transform takes data in position (real) space and translates them into frequency space, so we can see the behaviour of the particle as a function of frequency – this is a very powerful method in a wide range of physics and engineering devices and technical approaches. There are clear signatures in the frequency data at the applied sound frequencies.
The key point of the paper is the sensitivity of the technique – the measured power levels are as low as -60dB. The decibel (dB) is a logarithmic scale which measures the ratio of the measured value to some reference, which for sound is usually set at the limit of human hearing (0dB). This means the detected power by the tweezers technique is a million times lower than can be detected by the human ear!
One thing that is worth noting is that the experiment makes use of only very low frequencies (10-20Hz). This is due to the low sampling rate of the camera used to track the particle trajectories, so it will be interesting to try the same experiment with a much faster experiment and see the response at higher frequencies.
What then can we use the ear to listen too? Well I have seen it suggested before, and the paper here also mentions the idea, that such a device could be used to listen to biological processes in objects such as cells and viruses. I’m not 100% what these processes are, but it may enable very sensitive measurements of the motion of such objects, or even subcellular components. It may also be possible to set up arrays of the nano-ears to create new forms of acoustic microscopy.
This is a well written (and very easy to read) paper looking at pushing the sensitivity of optical tweezers to a new limit, and it will be interesting to see how it progresses – and as the technique seems not too difficult to set up it may find widespread use, and I’m sure there will be as yet unforeseen applications.
The APS Physics online magazine also has an overview of this paper.
Ohlinger, A., Deak, A., Lutich, A., & Feldmann, J. (2012). Optically Trapped Gold Nanoparticle Enables Listening at the Microscale Physical Review Letters, 108 (1) DOI: 10.1103/PhysRevLett.108.018101