  Detection and controlling of acoustic (AC) phonon phase have been strenuous
tasks although such capability is crucial for further manipulating thermal
properties. Here, we present a versatile formalism for tracing AC nanowaves
with arbitrary strain compositions by incorporating the nonlinear
photoelasticity (PE) into ultrafast acoustics where broad AC spectrum
encompassing thermally important THz frequency range should be collected far
beyond Brillouin frequency. The initial AC phase upon displacive carrier
generation could be inherently varied depending on the bipolar AC compositions
by implementing externally biased piezoelectric diodes. The importance of
adopting nonlinear PE is then manifested from the transient phase shift either
abrupt at the point of diffuse surface scattering or gradual during
phonon-phonon or phonon-electron scattering events based on which the ratio of
nonlinear to linear PE coefficient is experimentally extracted as a function of
the detection probe energy, reaching 0.98 slightly below the bandgap. As the
probing energy is rather set away from the bandgap, AC phase is completely
invariant with any scattering events, exhibiting the conventional trend at
Brillouin frequency in linear regime. Under potent influence of nonlinear PE,
the AC dephasing time during the propagation are quantified as a function of AC
wavepacket size and further correlated with intrinsic and extrinsic AC
scattering mechanisms in electron reservoir.
