A gigahertz (GHz) ultrasonic interferometer for the diamond-anvil cell
A few pictures from the elasticity laboratory I am
building in the Department of Earth and Planetary Sciences,
For details, see:
Photo (left) and schematic (right) of the GHz-ultrasonic diamond-anvil cell (DAC).
P-waves are produced by a thin-film ZnO transducer, sputtered onto the side of a crystal buffer rod. The buffer rod delivers the elastic waves to the sample through one of the diamond anvils. GHz shear-waves are made by P-to-S conversion inside the buffer rod (pictured here).
Short acoustic tone bursts (usually about 100 nanoseconds wide) are introduced into the diamond-cell. By overlapping the returning echoes from the diamond-sample interface (the diamond echo) and echoes from the far-end of the sample in contact with a pressure medium (the sample echo) a complex tone burst is made. The amplitude of the complex tone burst returning from the cell is measured at a position in time where there is first-order interference between the two echoes. The frequency is scanned, and an interference pattern is produced, from which the P- or S-wave travel-time through the sample is calculated (usually only about 10-30 nanoseconds!).
The radio-frequency source produces a highly-stable signal that is triggered in synch with the pulse generator, ensuring phase coherence throughout the measurement – this is akin to using a laser in optical interferometry – but remember, these are sound waves!
Photo looking down on the ultrasonic DAC.
In the window of the 20GHz mainframe oscilloscope, you can see one of the complex tone bursts returning from the diamond-culet and sample, held at about 10 GPa (100 kbars) inside the cell. The width of the tone burst you see is 100 ns. There are about 100 cycles of the 1GHz sine-wave signal inside this tone burst (the dots result from aliasing). The travel-time through the sample is only about 10 ns. Going about 10-20% of the way into the tone burst, a positive step can be seen – this is constructive interference between the diamond and sample echoes at this frequency!
Photo of the ultrasonic DAC
on a four-circle X-ray diffractometer (in
Once the acoustic travel-time has been measured in the ultrasonic lab, I can carry the cell to other labs in the building and measure the sample volume or crystal structure with X-rays, or I can measure vibrational properties with spectroscopy – and all under the same conditions of pressure. The pressure is normally determined spectroscopically using the ruby-fluorescence scale.
In the near future, I aim to introduce in-situ Raman and heating capabilities to the GHz-ultrasonic DAC for complete equation-of-state experiments.