Acoustic phonons reach the nanoscale

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PostPosted: Tue Aug 19, 2008 12:04 pm    Post subject: Acoustic phonons reach the nanoscale Reply with quote

Acoustic phonons reach the nanoscale

Belle Dumé

Nanoacoustic waves with the smallest ever spot sizes have been optically generated by researchers in Taiwan. The waves could be used to make a non-destructive subsurface nanoimaging tool – something that is still lacking today.

To generate coherent nanoacoustic waves – or acoustic phonons – researchers illuminate semiconductor quantum well nanostructures using a femtosecond laser pulse (see figure). The laser pulse produces photocarriers (electrons and holes) in the nanostructure and nanoacoustic waves are generated with a wavelength that depends on the periodicity of the nanostructure. The phonons typically have wavelengths of tens of nanometres, which makes them useful for applications such as non-invasive ultrasound imaging and amplifying sound.

The problem is that optical diffraction effects mean that the nanoacoustic waves are produced with spot sizes that are microns across. Although near-field optical techniques can generate waves with smaller spot sizes, they only work close to surfaces.

Now, Chi-Kuang Sun of the National Taiwan University and colleagues have shown that a far-field optical technique can be used to manipulate how the phonons are generated so that nanoacoustic waves with spot sizes much smaller than the laser wavelength are emitted. These spots are between just 10 and 100nm across.

The researchers achieved their feat by manipulating the driving force used to produce the phonons. They did this by using a different laser pulse to pre-inject a non-uniform distribution of photocarriers into semiconductor quantum-well nanostructures. These carriers screen the built-in piezoelectric field inside the structures and nanoacoustic waves are efficiently generated. "Through manipulation, the photocarrier distribution and spatial saturation of the piezoelectric force can be exploited to reduce the generated acoustic spot sizes," explained Sun.
Coherent acoustic phonon generation in piezoelectric semiconductor nanostructures.
When a femtosecond optical pulse (blue cone) illuminate a quantum-well structure, it can generate coherent acoustic phonons (also known as nanoacoustic waves; white arrows) with wavelengths of 10 nm or less. Credit: Nature Nanotechnology.

"These nanoacoustic waves provide a perfect source for 'nano-ultrasonic imaging', which is the nano version of medical ultrasound," he said.

The team would now like to further improve its technique to realize a true 3D nanoacoustic wave. "An ideal non-destructive subsurface nanoimaging tool should then be close," said Sun. The researchers will begin work on this next step by further reducing the effective area of the optical piezoelectric transducer.

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