A very interesting (acoustical) observation !!
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
sound energy. 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; dbmurray@mail.silk.net;
lucien.saviot@u-bourgogne.fr)
