Plasmonic microphone

ACS Photon. http://doi.org/brmn (2016)

Credit: AMERICAN CHEMICAL SOCIETY

Reducing the size of acoustic detectors is important for applications such as high-resolution ultrasound imaging. Although optic-fibre-based sound sensors are being explored it is not clear if they can be made any smaller than the micrometre scale since light confinement in such structures is limited by diffraction. Now, Yaoguang Ma and colleagues from the USA and China have created a nanoscale microphone with an optical readout. The key to the idea is detecting changes in scattered light from small metal nanoparticles in the optical near-field of a nanofibre while sound waves modulate the position of the particles and thus their scattering intensity. SnO₂ fibres with a refractive index of 2.1 were used to enable good optical confinement in a low-index liquid environment. A compressible polyelectrolyte layer was applied to the fibres and gold nanoparticles chemically attached. The fibres were excited by 325 nm light from a continuous-wave He–Cd laser to create broadband emission that propagates along the fibre and excites the plasmonic metal particles. Initial tests were conducted in water using an external speaker oscillating the nanoparticles, while later experiments, probing the limits of the system, used an electromagnet approach to mechanically drive the system. By monitoring the modulated scattered light from the particles, either in the far-field, or via transmission along the fibre, acoustic intensities of <10⁻⁸ W m⁻² were detected by the devices with an interaction area of <4 μm².