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Light measures the minute motion of nanoscale vocal cords

Researchers at FOM Institute AMOLF and Ecole Polytechnique Fédérale de Lausanne (EPFL) have developed a new way to measure tiny displacements with light.

Schematic representation of structure used

Schematic representation of structure used. Light is confined in the 20 nanometer wide gap between the two gold-coated silicon nitride bridges.

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June 13, 2013

By confining light in a resonant cavity that is only twenty nanometers wide, they could precisely measure mechanical motion smaller than the size of an atom. Their research is published on 11 June 2013 in the journal Nano Letters.

To measure the tiny displacements, the researchers made a structure reminiscent of human vocal cords, but scaled down a thousand times. They created two long 'bridges' of gold-coated silicon nitride. The bridges have a length of 20 micrometers and a thickness of 120 nanometers. They are separated by a narrow gap of only 20 nanometers wide. Light is confined in this gap.    

Where vibrating vocal cords generate a vibration in the air between them, these nanoscale vocal cords generate a modulation in the intensity of the light trapped between the bridges. By measuring the modulations in the light, the researchers are able to determine mechanical vibrations of the nano-bridges with very high sensitivity.  

Light waves
Usually, light cannot be confined to length scales smaller than approximately 200 nanometers. The researchers avoided this problem by converting the light to surface plasmons: a special type of light waves that are bound to the surface of a metal.    

The key feature of this work is that the researcher can use these plasmons to register the mechanical motion of the nanostructure. As the plasmons are confined to the nanoscale, they are efficiently affected by even the smallest motions of the tiny vocal cords. That allows the physicists to determine the amplitude of intrinsic thermal motions of the cavity. This amplitude is only 10 picometers – smaller than an atom. With this result, the researchers show for the first time coupling between plasmons and mechanical motion. They also show that the mechanical motion can itself be influenced by changing the light intensity. "This system can be applied in ultrasensitive sensors', says prof.dr. Albert Polman, workgroup leader and director of AMOLF. " The extremely small light cavities allow these sensors to detect vibrations with very high frequencies, up to GHz range. We can use the sensors for determining the mass of molecules, for example."  

Scanning Electron Microscope
Image of the structure made with a scanning electron microscope.  

Reference
Rutger Thijssen, Ewold Verhagen, Tobias J. Kippenberg, and Albert Polman Plasmon nanomechanical coupling for nanoscale transduction

Nano Letters (2013), DOI: 10.1021/nl4015028