Finite-size effects on superlattice acoustic phonons

Interface Adhesion and Structural Characterization of Rolled-up GaAs/In0.2Ga0.8As Multilayer Tubes by Coherent Phonon Spectroscopy

We present a detailed experimental and theoretical study of the acoustic phonon modes in rolled-up multilayers with thickness of the layers in the nanometre and diameters in the micrometre range. We compare our results to planar, unrolled multilayers grown by molecular beam epitaxy. For the planar multilayers the experimentally obtained acoustic modes exhibit properties of a superlattice and match well to calculations obtained by the Rytov model. The rolled-up superlattice tubes show intriguing differences compared to the planar structures which can be attributed to the imperfect adhesion between individual tube windings. A transfer matrix method including a massless spring accounting for the imperfect adhesion between the layers yields good agreement between experiment and calculations for up to five windings. Areas with sufficient mechanical coupling between all windings can be distinguished by their acoustic mode spectrum from areas where individual windings are only partially in contact. This allows the spatially resolved characterization of individual tubes with micrometre spatial resolution where areas with varying interface adhesion can be identified.

Coherent terahertz sound amplification and spectral line narrowing in a stark ladder superlattice

The bias voltage applied to a weakly coupled n-doped GaAs/AlAs superlattice increases the amplitude of the coherent hypersound oscillations generated by a femtosecond optical pulse. This bias-induced amplitude increase and experimentally observed spectral narrowing of the superlattice phonon mode with a frequency 441 GHz provides the evidence for hypersound amplification by stimulated emission of phonons in a system where the inversion of the electron populations for phonon-assisted transitions exists.

RESONANCE BAND IN Ge/Si SUPERLATTICE STUDIED BY RAMAN SCATTERING

We have performed the low-frequency Raman measurement of MBE grown Ge/Si superlattice. Raman spectra were excited by He–Ne laser, and diode-pump YAG 532 nm laser, as well as several lines from Ar+ laser. Folded acoustic phonons of the Ge/Si superlattice were clearly found. The resonant effects were observed for the Ge/Si superlattices while the Raman spectra excited by the laser lines around 500 nm. A clearly resonance enhanced phonon signals are found for the spectrum excited by 532 nm laser line, and continuous emission can be clearly seen. By the continuous emission theory, we carried out a resonance band at 2.32 eV, which is closed to the E1 band of Germanium. Few weak resonant bands were also found near 2.32 eV, which may be related to confined band or the resonance band of interface layer between Si and Ge .