  The paper reports on experimental observation of sub-terahertz sound wave
generation and detection by a long Josephson junction. This effect was
discovered at spectral measurements of sub-terahertz electromagnetic emission
from a flux-flow oscillator (FFO) deposited on optically polished Si substrate.
The back action of the acoustic waves generated by the FFO and reflected by the
bottom surface of the Si substrate results in the appearance of resonant steps
in the FFO IVCs with spacing as small as 29 nV for 0.3 mm substrate thickness;
these steps manifest themselves in a pronounced resonant structure in the
emission spectra with spacing of about 14 MHz precisely according to the
Josephson relation. The mechanism of acoustic wave generation and detection by
the FFO is discussed; a possibility for employing the discovered effect for FFO
frequency stabilization has been demonstrated. A simple and reliable way to
suppress the superfine resonant structure has been developed and proven; this
invention allows for continuous frequency tuning and FFO phase locking at any
desired frequency, all of which is vitally important for most applications.
