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 equation of motion for sound waves leads to a sound velocity strongly dependent on the direction of travel, that follows the symmetry of the crystal and its surface.
Our group in collaboration with theorists from Hokkaido and Le Mans in France now report the extension of the art of ripple-watching to crystal surfaces. We replace the frog in the pond with a brief laser pulse, thereby stimulating 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.
With a single point-source to excite the ripples on various crystal surfaces, instead of circles we observed exquisite square or star shaped ripple patterns. Fascinatingly, we also imaged ripples entering and leaving a tiny gold pyramid. Just as a tree's rings can chronicle years, the ripples provide a record of prior disturbances, and by reading the ripples the elastic properties of the solid itself can be examined. The channelling of the ripples' energy in certain directions is known as phonon focusing, and this can now be visualized dynamically.
We hope that by watching ripples on crystals we can develop a 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 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 greater ultrasonic 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 in a bulk medium 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 mm, enough to locate an insect, whereas the best human ear does well to manage a paltry 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, ultrathin films and minute nanostructures. Since the whole measurement typically only takes a few tens of picoseconds, we are in the realm of picosecond ultrasonics.
What is done is to focus a beam of light consisting of a periodic train of ultrashort pulses onto an opaque sample. This light is converted into bursts of sound, an effect first noted at lower frequencies in 1881 by Alexander Graham Bell when he shone a modulated beam of sunlight onto selenium. The sudden heating produced by the light expands (or contracts) the solid in the region of illumination, and, just like a small piston, the hot volume of solid near the surface drives a sound wave directly into the material. This sound, reflected from defects or interfaces in the interior, can be picked up as echoes on arrival at the surface. As with sonar or medical echography, we can then assess the sub-surface structure. Also, studying the pitch and harmonic content of the echoes, the 'vocal chords' of the set-up, tells us how the electrons behave on very short time scales.
Listening to such tiny sound waves at the surface requires an acutely sensitive detection combined with the same ultrashort time resolution as the sound production. The solution is to sense the sound with another beam of light pulses, delayed in time. This measurement beam can sense the tiny protrusion produced when the sound wave is reflected back to the surface of the material, very much like the bulge in a jelly that appears after being hit on the opposite side with a spoon. Vibrations of amplitude in the sub-nanometre range can be sensed. To help you imagine these minuscule vibrations better: if the illuminated region was the area of Australia, the scaled up displacement at the centre would only be about one metre. It is also possible to sense the sound by monitoring the ppm (parts per million) order changes in beam intensity.
As the pulses of high frequency sound travel backwards and forwards inside the material they change shape. By analysing this change we can investigate the elastic properties of the sample and how the sound velocity changes with ultrasonic frequency.
We can also look at artificial nanoscale structures with these methods. When jolted with light pulses such structures will vibrate at certain discrete frequencies determined by their shape and make-up. The resonances inside semiconductor sandwich structures with layer thicknesses approaching atomic dimensions is one example that we are investigating. Another is the ringing of microscopic semiconductor bridges. Even nanobells - nicknamed, of course, Little Ben - could soon be tolling the virtues of the kingdom of the ultrasmall with ultrahigh-pitched sound.
(See Picosecond sound pulse generation and detection with ultrashort light pulses 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 using a technique called Optical Heterodyne Force Microscopy.
(See Ultrasonic force microscopies for further explanations.)