Selective acoustic phonon mode excitation of multi-mode silver nanoprisms

Nanoprisms are known to exhibit two major planar phonon modes in ultrafast experiments, namely, a stronger mode related to the bisector height and a weaker mode related to half of the edge length. In this work we demonstrated photoacoustic excitation of a specific acoustic phonon mode in silver nanoprisms using two properly timed pump pulses. Using this technique, we could selectively excite only the weak totally symmetric mode while suppressing the dominant breathing mode. Via direct observation of such a mode, which is hardly detectable directly with the conventional single pump pulse technique, we could elucidate the roles of hot electrons in photoinduced ultrafast structural dynamics in laser-heated nanoparticles.