The study is based on the success in obtaining the first experimental evidence for the direct excitation of coherent nanosecond-pulsed high-frequency acoustic phonons in semiconducting doping superstructures by electromagnetic fields of the same frequency. Acoustic phonons are detected by a superconducting bolometer, with nanosecond resolution, at the appropriate time-of-flight across a (100) silicon substrate for ballistic longitudinal phonons when a silicon delta-doped doping superlattice is illuminated with grating-coupled nanosecond-pulsed 246-GHz laser radiation with an approximate power density of 1 kW/mm2. The absorbed phonon power density in the bolometer detector is estimated to be 10 μW/mm2, in agreement with theory. The absence of any detected transverse acoustic phonon signal by the superconducting bolometer is particularly striking – implying that coherent THz longitudinal acoustic phonons can be generated in silicon doping superlattices with negligible associated heat pulse generation. As a first application of this novel phonon source but at higher frequency (due to enhanced scattering), team propose to undertake a study of 1 THz acoustic phonon transmission through thin layers of plasma thruster wall materials, in an attempt to quantify the level of defect generation resulting from plasma exposure. A key life-limiting mechanism for most plasma thrusters is their internal erosion through nonequilibrium plasma-material interactions mainly in the form of energetic ion impact. It is well known that lattice defects can strongly scatter acoustic phonons.
Wilson TE, Boyd ED. "Progress on: Coherent terahertz acoustic phonon scattering: Novel diagnostic for erosion in plasma thruster discharge chamber walls" Air Force Office of Scientific Research (AFOSR) 2012 Space Propulsion and Power Contractors Meeting. Arlington, Virginia. 10 Sep 2012.