Gold and Diamonds

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.)