Coherent acoustic phonons are generated at terahertz frequencies when semiconductor quantum-well nanostructures are illuminated by femtosecond laser pulses. These phonons-also known as nanoacoustic waves-typically have wavelengths of tens of nanometres, which could prove useful in applications such as non-invasive ultrasonic imaging and sound amplification by the stimulated emission of radiation. However, optical diffraction effects mean that the nanoacoustic waves are produced with spot sizes on the micrometre scale. Near-field optical techniques can produce waves with smaller spot sizes, but they only work near surfaces. Here, we show that a far-field optical technique--which suffers no such restrictions--can be used to spatially manipulate the phonon generation process so that nanoacoustic waves are emitted with lateral dimensions that are much smaller than the laser wavelength. We demonstrate that nanoacoustic waves with wavelengths and spot sizes of the order of 10 nm and 100 nm, respectively, can be generated and detected.

Articles you may be interested inUltrafast dynamics of the dielectric functions of ZnO and BaTiO3 thin films after intense femtosecond laser excitation Free exciton and above-band-gap free carrier dynamics in ZnO nanorods have been investigated at room temperature with a femtosecond transient transmission measurement. Following the photoexcitation of above-band-gap free carriers, an extremely fast external thermalization time on the order of 200 fs can be observed. Under high excitation, hot phonon effects were found to delay the carrier cooling process. While the photoexcitation energy was tuned to match the free exciton transition, stable exciton formation can be uncovered while no evident exciton ionization process can be found unless the photoexcited exciton density exceeded the Mott density.

This letter examines the anharmonic decay in GaN of subthermal ͑ប Ӷ k B T͒ coherent longitudinal acoustic ͑LA͒ phonons with a frequency in the subterahertz range. In a collisionless regime ͑ Ͼ 1͒, the anharmonic decay rate of subterahertz coherent LA phonons in GaN shows square dependence on the phonon frequency and cubic dependence on the crystal temperature. As confirmed by the authors experiments, this behavior agrees with a model based on Herring scattering, indicating its importance for the estimation of acoustic attenuation in the subterahertz range.

Two-dimensional ultrasonic imaging is demonstrated by using acoustic nanowaves. With a 14 nm acoustic wavelength, both axial and transverse resolutions of a few tens of nanometers are thus achieved. This ultrasonic-based nondestructive technique not only images but also reconstructs the subsurface nanostructures including the depth positions of the buried interfaces. By demonstrating two-dimensional nanoultrasonic scans in depth and transverse ͑or z-x͒ axes, we show that acoustic nanowaves can be a promising tool for future subsurface three-dimensional noninvasive imaging with nanometer resolutions.