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.