  This paper reports experimental evidence for phononic low-order to high-order
harmonic conversion leading to high harmonic generation. Phononic high harmonic
generation is mediated by a threshold dependent instability of a driven phonon
mode. Once the threshold for instability is met, a cascade of harmonic
generation processes is triggered. Firstly, the up-conversion of first harmonic
phonons into second harmonic phonons is established. Subsequently, the
down-conversion of second harmonic phonons into first harmonic phonons and
conversion of first and second harmonic phonons into third harmonic phonons
occur. On the similar lines, an eventual conversion of third harmonic phonons
to high orders is also observed to commence. This physical pathway for phononic
low-order to high-order harmonic conversion may find general relevance to other
physical systems.
