Gold and diamonds, accouterments at many weddings, have another curious
affinity. They have almost the same acoustic impedance, a fact which
two physicists are hoping to exploit in order to get nanoparticles,
embedded in a crystalline network, to ring with a pure tone, which in
turn should help in the development of various nanotechnology devices.
The acoustic impedance, the acoustic analogue of a material's optical
index of refraction, is defined as the density times the velocity of
sound in that material. Gold has a high density but a moderate sound
speed (3330 m/sec), while diamond has a low density but a very high
speed of sound; indeed, at a speed of 18,190 m/sec, sound waves in diamond
travel twice as fast as the Space Shuttle in Earth orbit. Thus, these
two materials are very different in many respects but alike in their
impedance to sound, which is to say their propensity to take up or dissipate
Now, one would expect that for two materials with similar acoustic
impedance sound would move all too easily from the one to the other.
(Optical analog: a piece of glass becomes almost invisible in a bath
of water since the indices of refraction for glass and water are almost
the same.) But the research turned this expectation on its head. A gold
nanoparticle, once set vibrating in a diamond matrix, should actually
keep vibrating, the new studies show. In other words, the particle's
sound energy, the energy of its vibrating in place, does not leak out
into the surrounding crystal.
According to Lucien Saviot at the Universite de Bourgogne (Dijon, France)
and Daniel Murray of Okanagan University College (Kelowna, British Columbia,
Canada), the resolution of this apparent paradox is that people had
for many years been using the wrong formula for acoustic impedance.
The correct formula, they argue, is more complicated. It's not just
density times speed of sound, but involves also the radius of curvature
of the interface and also the sound frequency.
The authors of the new study have not yet implanted gold nanoparticles
inside diamonds but they have studied the case of how gold particles
ring while ensconced in silica and sapphire. Their surprising result
is that the particle keeps ringing. The particles are set in motion
by a pulse of laser light, shining in through the crystal, and its ringing
can also be monitored by laser light; the vibrations show up as the
amount of energy sapped from the probe laser beam. (Saviot
and Murray, Physical Review Letters, 30 July 2004; email@example.com;