In this Rapid Communication we report the first time-resolved measurements of confined acoustic phonon modes in free-standing Si membranes excited by fs laser pulses. Pump-probe experiments using asynchronous optical sampling reveal the impulsive excitation of discrete acoustic modes up to the 19th harmonic order for membranes of two different thicknesses. The modulation of the membrane thickness is measured with fm resolution. The experimental results are compared with a theoretical model including the electronic deformation potential and thermal stress for the generation mechanism. The detection is modeled by the photoelastic effect and the thickness modulation of the membrane, which is shown to dominate the detection process. The lifetime of the acoustic modes is found to be at least a factor of 4 larger than that expected for bulk Si

We present a method of selective epitaxial growth of few layers graphene (FLG) on a "prepatterned" silicon carbide (SiC) substrate. The methods involves, successively, the sputtering of a thin aluminium nitride (AlN) layer on top of a monocrystalline SiC substrate and, then, patterning it with e-beam lithography and wet etching. The sublimation of few atomic layers of Si from the SiC substrate occurs only through the selectively etched AlN layer. The presence of the Raman G-band at similar to 1582 cm(-1) in the AlN-free areas is used to validate the concept. It gives absolute evidence of selective FLG growth. (C) 2008 American Institute of Physics.

An investigation of the early stage formation of graphene on the C face of 6H-silicon carbide (SiC) is presented. We show that the sublimation of few atomic layers of Si out of the SiC substrate is not homogeneous. In good agreement with the results of theoretical calculations it starts from defective sites, mainly dislocations that define nearly circular graphene layers, which have a pyramidal, volcanolike shape with a center chimney where the original defect was located. At higher temperatures, complete conversion occurs but, again, it is not homogeneous. Within the sample surface, the intensity of the Raman bands evidences inhomogeneous thickness.

We theoretically study the propagation of sound waves in GaAs/AlAs superlattices focussing on periodic modes in the vicinity of the band gaps. Based on analytical and numerical calculations, we show that these modes are the product of a quickly oscillating function times a slowly varying envelope function. We carefully study the phase of the envelope function compared to the surface of a semi-infinite superlattice. Especially, the dephasing of the superlattice compared to its surface is a key parameter. We exhibit two kind of modes: Surface Avoiding and Surface Loving Modes whose envelope functions have their minima and respectively maxima in the vicinity of the surface. We finally consider the observability of such modes. While Surface avoiding modes have experimentally been observed (Phys. Rev. Lett. 97, 1224301 (2006)), we show that Surface Loving Modes are likely to be observable and we discuss the achievement of such experiments. The proposed approach could be easily transposed to other types of wave propagation in unidimensional semi-infinite periodic structures as photonic Bragg mirror.

Calculations of the electron-acoustic phonon interaction, and Raman scattering efficiency, in matrix embedded Ge quantum dots (QDs) are presented. The work is focused on the understanding of the inelastic light scattering process excited close to resonance with the confined E-1 transitions. Due to the large joint density of states at the E-1 point, many intermediate electronic states contribute to the overall scattering efficiency. This particular situation leads to quantum interference effects between different scattering paths and has, therefore, a strong impact on the Raman line shapes and intensities. Quantum confinement of the electron and hole states is treated within the envelope wave function approximation. The QD/matrix acoustic vibrations are deduced from elasticity theory. Deformation-potential interaction between the electrons (and holes) and acoustic vibrations is assumed. The resonant Raman spectra are calculated using third order perturbation quantum theory. A Raman-Brillouin electronic density is constructed as a linear combination of the electronic states involved in the inelastic light scattering. It allows one to plot, for each excitation energy, the spatial distribution of the electronic density that gives rise to the Raman (or Brillouin) signal. It is calculated for both diagonal and off-diagonal transitions between the confined electronic states. The dependence of the spectral line shapes and intensities on homogeneous broadening of the E-1 transitions, QD size, surface boundary conditions is discussed in details. The calculated spectra are then compared to those measured for different QD size distributions.