Longitudinal acoustic phonons with few-GHz frequency were detected in three-dimensional supracrystals [1] (with face-center cubic lattice) of 7-nm cobalt nanocrystal spheres [2]. In analogy with usual atomic crystals, where longitudinal acoustic phonons propagate with the speed of sound through coherent movements of atoms of the lattice out of their equilibrium positions [3], in these supracrystals atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (alkyl chains) which act like stiff mechanical springs holding together the nanocrystals. We induced a strain wave (i.e. a travelling phonon wavepacket) using a femtosecond pump pulse absorbed by a metal transducer and we followed its propagation inside the supracrystal monitoring the modulations in the transient pump-probe signal in reflection geometry, as in a picosecond acoustics experiment. Broadband femtosecond white-light pulses coupled to a spectrometer enable us to verify the expected dependence of the observed modulation period on the probe wavelength and the incidence angle of the laser. We could extract the speed of sound inside the supracrystal, which is 1100±100 m/s at room temperature. Very interestingly, the speed of sound is found to be strongly dependent on the sample temperature: its value increases from 760 m/s at 410K to 3140 m/s at 110K, with a change in speed/temperature slope at ≈250K. Such change can be associated to a temperature transition and is attributed to a transformation of the alkyl chain configuration from ordered to disordered.

© 2013 IEEE

PDF Article
More Like This
Visualizing Coherent Phonon Propagation in the 100 GHz Range: a Broadband Picosecond Acoustics Approach

C. Ferrante, E. Pontecorvo, D. Polli, G. Cerullo, and T. Scopigno
QTh4H.5 Quantum Electronics and Laser Science Conference (QELS) 2012

Generation of ultrashort acoustic pulses in cobalt by ballistic electrons in gold

Oleksandr Kovalenko, Viktor Shalagatskyi, Thomas Pezeril, Vitalyi Gusev, Vasily V. Temnov, Denys Makarov, Luyang Han, and Oliver G. Schmidt
QTh3E.5 CLEO: QELS_Fundamental Science (CLEO_QELS) 2013


You do not have subscription access to this journal. Citation lists with outbound citation links are available to subscribers only. You may subscribe either as an Optica member, or as an authorized user of your institution.

Contact your librarian or system administrator
Login to access Optica Member Subscription