Optical tracking of picosecond coherent phonon pulse focusing inside a sub-micron object

By means of an ultrafast optical technique, we track focused gigahertz coherent phonon pulses in objects down to sub-micron in size. Infrared light pulses illuminating the surface of a single metal-coated silica fibre generate longitudinal-phonon wave packets. Reflection of visible probe light pulses from the fibre surface allows the vibrational modes of the fibre to be detected, and Brillouin optical scattering of partially transmitted light pulses allows the acoustic wavefronts inside the transparent fibre to be continuously monitored. We thereby probe acoustic focusing in the time domain resulting from generation at the curved fibre surface. An analytical model, supported by three-dimensional simulations, suggests that we have followed the focusing of the acoustic beam down to a ~150-nm diameter waist inside the fibre. This work significantly narrows the lateral resolution for focusing of picosecond acoustic pulses, normally limited by the diffraction limit of focused optical pulses to ~1 μm, and thereby opens up a new range of possibilities including nanoscale acoustic microscopy and nanoscale computed tomography. Acousto-optics: optical tracking of sound waves in small objects Using an ultrafast optical technique, researchers in France, Japan and Austria have tracked focused phonon pulses in sub-micrometre objects. Confining light and sound waves in micro- and nanoscale structures promises enhanced interaction between light and sound, and hence superior control over light for various important applications. Until now, no one had succeeded in continuously probing acoustic waves in sub-micrometre objects. Oliver Wright of Hokkaido University in Japan and his co-workers have now achieved this by combining two techniques: a Brillouin-scattering technique and the generation of coherent acoustic waves on curved surfaces. They realized continuous optical tracking of focused gigahertz pulses of coherent phonons in silica fibres with diameters down to 800 nm. This method promises to facilitate the development of acoustic transducers for nanoscale imaging in areas such as nanotechnology and cellular biology.