Full text loading...
(a) TEM imaging of the studied QD crystal without the Si cap layer. The inset shows a unit cell of this nanocrystal. (b) The band structure image by combining FFT power spectra of laterally-averaged displacements in the QD crystal under an impulsive excitation, indicating the dispersion relation of longitudinal acoustic phonons traveling along the surface normal . Cross sections of the displacement fields of two modes are shown in the insets. (c) Strain field on the plane of the QD layer inside a unit cell at the frequency of the first zone-edge band gap. The dashed circle presents the position of the QD.
Differential optical reflection changes as a function of time delay. Differentiation of the data is helpful to eliminate slowly varied background signal and thus clarifies the acoustic dynamics. The inset is the magnified data for observing the features of the acoustic echos clearly, where the line is a typical fitting curve.
(a) Fourier transform power spectra of the experimental results obtained in the QD crystal and the randomly distributed QDs. Two results are normalized according to the oscillatory signal at the first 20 ps. The inset shows the complete spectrum measured in the QD crystal. The spectrum for the data at the first 20 ps is also shown to illustrate the broad bandwidth of the generated acoustic phonon pulse. Both spectra are normalized. (b) The dashed line is the calculated reflectivity spectrum of the QD crystal, and the solid line is the simulated acoustic signal with considerations of the whole sample structure and the photoelastic effect of the Al film.
Article metrics loading...