Investigation of laser-induced acoustic phonons in poly(p-phenylenevinylene) uniaxially stretched films

Elastic moduli of a 10:1 uniaxially stretched film of poly-p-phenylene vinylene are determined using the technique of laser-induced acoustic phonon in a degenerate four-wave mixing phase conjugate geometry. The four-wave mixing signal obtained as a function of film rotation provides acoustic speed as a function of orientation. In order to explain the experimental observation, a theoretical description is presented to describe the acoustic wave generated by laser-induced transient grating for an arbitrary propagation direction in a medium with any symmetry. This theoretical description also provides the dependence of the acoustic wave on the polarization, the propagation direction, and the pulse width of the laser pulses. The theoretical analysis of our experimental result, using the general Christoffel equation, yields the complete elastic modulus tensor, including both the longitudinal and the shear components. The value of the elastic modulus along the draw direction is 46.6 GPa. The analysis also shows a novel feature of mode jump, not reported previously. We observe that within a certain range of angles between the acoustic propagation direction and the draw direction one kind of acoustic mode (quasi-longitudinal) is generated but for another set of angles the mode jumps to become quasi-transverse in nature. The theoretical description presented here also explains this mode-jump behavior.