  We report first evidence for a 1.0 terahertz (THz) self-starting mirrorless
acoustic phonon parametric oscillator (MAPPO) produced from acousto-optic
phase-conjugate degenerate four-wave (D4WM) mixing in a THz laser-pumped
silicon doping superlattice (DSL). The DSL was grown by molecular beam epitaxy
on a (100) boron-doped silicon substrate. A superconducting NbTiN subwavelength
grating was used to couple the THz laser radiation into the DSL.
Superconducting granular aluminum bolometric detection, coupled with Si:B
piezophonon spectroscopy, revealed excitation of THz coherent compressional and
shear waves, along the <111> direction only. The Bragg scattering condition for
distributed feedback, and the energy conservation requirement for the D4WM
process, were both verified. A THz MAPPO could provide a testbed for studies of
non-classical acoustic phonon fields.
