Figure 2: Tailoring the phonon modes through nanostructure design.

In a symmetric nanostructure only a single phonon mode is observed (a), which is independent of the pump or probe polarization. This is in contrast to the 120 × 90 nm structure (Fig. 1), in which two acoustic eigenmodes, symmetric and antisymmetric, are excited and observed. The error bars are the s.d. of multiple pump–probe experiments. Time traces for parallel and orthogonal pump and probe polarizations are shown in Supplementary Fig. 11. The mechanism behind this effect is a zero overlap between the displacement fields generated by the laser pulse and those of the antisymmetric phonon mode. For a nanostructure small enough to be uniformly heated, this occurs when the nanostructure is symmetric (equal arm lengths). Under the assumption of isotropic heating, the initial amplitudes of the various phonon modes can be controlled through nanostructure design, as calculated for a nanostructure without a substrate in b. Fixing the length of the vertical arm (L_y), the excitation efficiency of the two phonon modes varies as a function of the horizontal arm length (L_x) (b). When the structure is symmetric (L_x=L_y), the antisymmetric mode is not excited, in agreement with experiments. Although not excited, the antisymmetric mode is still an eigenmode of the structure: it is ‘dark’ for this excitation mechanism. In contrast, a highly elongated structure (L_x>>L_y) more efficiently generates the antisymmetric mode.