Nonlinear Photoelasticity to Explicate Acoustic Dephasing Dynamics

TL;DR

This paper introduces a nonlinear photoelasticity-based formalism for tracking acoustic phonon phase dynamics across broad spectra, revealing how nonlinear effects influence dephasing and scattering mechanisms in ultrafast acoustics.

Contribution

It presents a novel approach integrating nonlinear photoelasticity into ultrafast acoustics to analyze phase shifts and dephasing in acoustic phonons over a wide frequency range.

Findings

Nonlinear photoelasticity significantly affects acoustic phase shifts.

Dephasing times depend on phonon wavepacket size and scattering mechanisms.

Experimental extraction of nonlinear to linear PE ratio reaches 0.98 near bandgap.

Abstract

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.