A frog jumps into a pond, making ripples. A bat emits a high pitched squeak and thereby locates an insect. A woodpecker taps its beak against a tree, looking for grubs. Experiments mirroring these phenomena are in progress in our laboratory.
People have always romanticised at the beauty of ripples on water. The pleasing circles produced by a frog jumping into a still pond, for example. Now something new for wistful lovers to contemplate: the spreading of ripples on crystals.
Lord Rayleigh in 1885 predicted that ripples of sound on noncrystalline solids like glass should also spread in circular patterns. But in crystals the sound velocity is strongly dependent on the direction of sound travel, and the ripple patterns are more complicated.
We have extended the art of ripple-watching to crystal surfaces. We replace the frog in the pond with a brief laser pulse to stimulate high frequency sound waves, and we replace the water with a solid crystal surface. Meanwhile, the sensory apparatus, the human eye, is replaced by a separate laser pulse. In this way we can obtain animations of sound rippling over crystals and microscopic landscapes.
Just as a tree's rings can chronicle years, the ripples, often focused by the crystal or structure in certain directions, provide a record of prior disturbances. By reading the ripples the acoustic properties of the sample can be examined.
We hope that by watching ripples on crystals, in both natural and man-made 'phononic crystals', we can develop an experimental phonon 'optics' of surfaces that will become as familiar as conventional photon optics is today. In terms of crystal surfaces this research effectively gives our ears eyes for the first time.
(See Watching ripples on crystals, Watching whispering-gallery waves, Watching ripples on square phononic crystal lattices, Watching ripples on phononic crystals and Ripples between two crystals for the animations.)
Sound has always been essential to the survival of animals. The human ear can detect sounds up to about 20 kHz (20,000 cycles/second). Anything above this is termed ultrasonic.
Bats' vocal chords and ears can cope with frequencies up to 200 kHz. They sense their environment acoustically by emitting sound and listening for the reflected echo. We can mimic bats with man-made sound emitters and receivers, like those used in medical imaging or the sonar systems used when looking for submarines.
The same principle is adopted in our research but with vastly higher frequencies: the bats are put to shame by creating and picking up ultrasound as shrill as one terahertz or more (1 THz=one million million Hertz).
The detail we can visualize with sound depends on its wavelength, that is, the distance between adjacent crests of the wave. The higher the sound frequency the smaller is the wavelength. For bats this is as short as 2 millimetres, enough to locate an insect, whereas the human ear does well to manage a few centimetres.
With extremely brief laser pulses, lasting for only a picosecond or less (one million millionth part of a second), we can produce and detect terahertz ultrasound, which has a corresponding wavelength as short as a few nanometres (1 nanometre=10-9 m).
Just like a miniature sonar system we can use this 'ultrabat' technology to eavesdrop on sound passing through atomic cracks or ultrathin films. Since the whole measurement typically only takes a few tens of picoseconds, we are in the realm of picosecond ultrasonics.
We can also look at artificial nanoscale structures with these methods. When jolted with laser pulses such structures will vibrate at certain frequencies determined by their shape and make-up. These resonances can be detected inside semiconductor sandwich structures with layer thicknesses approaching atomic dimensions, for example.
Even nanobells — nicknamed, of course, Little Ben — in the form of gold nanorings are now tolling the virtues of the kingdom of the ultrasmall with ultrahigh-pitched sound in our laboratory.
(See Picosecond ultrasonics with ultrashort light pulses, Picosecond shearing, Picosecond ultrasonics in liquid mercury, Picosecond ultrasonics in ice and Vibrations of gold nanorings for further explanations.)
A woodpecker looks for grubs by tapping its beak against a tree, trying to sense the change in the elastic properties of the bark. We are also trying to imitate the woodpecker but using a tiny 'beak' only a few nanometres across.
We do this by tapping a microscopic tip mounted on a cantilever against a solid at ultrasonic frequencies and sensing how much the cantilever vibrates. This technique, called Ultrasonic Force Microscopy, can map the elastic properties of surfaces or nanostructures.
With our 'nanowoodpecker' we have been vibrating our tip at frequencies around 100 MHz to see the interior structure of tiny quantum dots and other nanostructures. Although the acoustic wavelength at this frequency is relatively large, about 0.1 mm, we can obtain a much better lateral resolution of about 1 nanometre thanks to the small size of the tip itself.
We want to make our nanowoodpecker peck faster and faster to investigate very high frequency elastic properties on nanometre length scales.
We are also training our woodpecker to use a modulated light source to excite the solid surface at megahertz frequencies, and thereby excite thermal waves in the sample. Using these thermal waves we have succeeded in locally probing the sub-surface structure of a sample.
(See Ultrasonic force microscopies for further explanations.)
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