Zone-folded Longitudinal Acoustic Phonons Research Articles

Low-frequency Raman spectrum of 2D layered perovskites: Local atomistic motion or superlattice modes?

We report the low-frequency Raman spectrum (ω = 10 cm-1-150 cm-1) of a wide variety of alkylammonium iodide based 2D lead halide perovskites (2D LHPs) as a function of A-site cation (MA = methylammonium and FA = formamidinium), octahedral layer thickness (n = 2-4), organic spacer chain length (butyl-, pentyl-, hexyl-), and sample temperature (T = 77 K-293 K). Using density functional theory calculations under the harmonic approximation for n = 2 BA:MAPbI, we assign several longitudinal/transverse optical phonon modes between 30 cm-1 and 100 cm-1, the eigendisplacements of which are analogous to that observed previously for octahedral twists/distortions in bulk MAPbI. Additionally, we propose an alternative assignment for low-frequency modes below this band (<30 cm-1) as zone-folded longitudinal acoustic phonons corresponding to the periodicity of the entire layered structure. We compare measured spectra to predictions of the Rytov elastic continuum model for zone-folded dispersion in layered structures. Our results are consistent across the various 2D LHPs studied herein, with energetic shifts of optical phonons corresponding to microscopic structural differences between materials and energetic shifts of acoustic phonons according to changes in the periodicity and elastic properties of the perovskite/organic subphases. This study highlights the importance of both the local atomic order and the superlattice structure on the vibrational properties of layered 2D materials.

Observation of hypersonic phononic crystal effects in porous silicon superlattices

Brillouin light scattering experiments were carried out on porous silicon superlattices with modulation wavelengths in the range 37–167 nm. Phonon frequencies deduced from the Brillouin spectra show good agreement with those obtained from an elastic continuum model for a system with one-dimensional periodicity. Evidence for the existence of a hypersonic phononic bandgap and zone-folded longitudinal acoustic phonons is reported.

Excitation mechanisms of coherent phonons unravelled by femtosecond X‐ray diffraction

AbstractCoherent zone‐folded longitudinal acoustic phonons (ZFLAPs) were measured nondestructively with sub‐picometer spatial and sub‐picosecond time resolution via ultrafast X‐ray diffraction (XRD). The femtosecond excitation of a GaAs/AlGaAs superlattice with 1.5 eV photons results in spatially periodic stress, which induces a coherent ZFLAP with a 3.5 ps period. The motion is directly monitored by femtosecond X‐ray pulses at a kilohertz repetition rate. The intensity change ΔR /R = 0.03 of the −1st order satellite peak of the (002) reflection of bulk AlGaAs is directly proportional to the induced lattice strain of Δa /a 0 = 1.5 × 10−4. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrates the displacive excitation of coherent phonons (DECP) as the dominant mechanism for strong excitation. We discuss different mechanisms which account for impulsive stress, and argue that the electronic stress due to the deformation potential dominates. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Control of Coherent Acoustic Phonons

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Ümit Özgür, Chang-Won Lee, and Henry O. Everitt, "Control of Coherent Acoustic Phonons," Optics & Photonics News 12(12), 66-66 (2001) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Temperature Dependence and Reflection of Coherent Acoustic Phonons in InGaN Multiple Quantum Wells

Sub-picosecond optical pump-probe techniques were used to generate coherent zone-folded longitudinal acoustic phonons (ZFLAPs) in an InGaN multiple quantum well structure. Differential transmission measurements revealed that carriers injected near the barrier band edge were quickly captured into the quantum wells and generated strong coherent ZFLAP oscillations. Differential reflection measurements were used to explore the acoustic phonon transport and reflection in the multiple quantum well structure.

Temperature Dependence and Reflection of Coherent Acoustic Phonons in InGaN Multiple Quantum Wells

Sub-picosecond optical pump–probe techniques were used to generate coherent zone-folded longitudinal acoustic phonons (ZFLAPs) in an InGaN multiple quantum well structure. Differential transmission measurements revealed that carriers injected near the barrier band edge were quickly captured into the quantum wells and generated strong coherent ZFLAP oscillations. Differential reflection measurements were used to explore the acoustic phonon transport and reflection in the multiple quantum well structure.

Control of coherent acoustic phonons in semiconductor quantum wells.

Using subpicosecond optical pump-probe techniques, coherent zone-folded longitudinal acoustic phonons (ZFLAPs) were investigated in an InGaN multiple quantum well structure. A two-pump differential transmission technique was used to generate and control coherent ZFLAP oscillations through the relative timing and amplitude of the two pump pulses. Enhancement and suppression of ZFLAP oscillations were demonstrated, including complete cancellation of generated acoustic phonons for the first time in any material system. Coherent control was used to demonstrate that ZFLAPs are generated differently in InGaN multiple quantum wells than in GaAs/AlAs superlattices.

Coherent Zone-Folded Longitudinal Acoustic Phonons in Semiconductor Superlattices: Excitation and Detection

Coherent zone-folded acoustic phonons are excited in GaAsyAlAs superlattices by femtosecond laser pulses via resonant impulsive stimulated Raman scattering in both forward and backward scattering directions. The relative amplitudes of three distinct modes of first and second backfolded order match well with scattering intensities calculated within an elastic continuum model. The detection of the coherent acoustic modes is based on the modulation of the interband transitions via the acoustic deformation potential and exhibits a strong enhancement at interband transitions. [S0031-9007(98)08288-X]

Real time-space dynamics of zone-folded phonons in [formula omitted] superlattices

Using time delay modulation technique, we have studied the zone-folded longitudinal acoustic phonons in GaAs AlAs superlattices. Up to seven phonon modes are clearly observed and their wave vectors correspond to those expected in forward and backward Raman scattering experiments. This technique leads to the observation of coherent phonon signal of the lowest frequency mode of 1.5 cm −1 and characteristic abrupt changes of its amplitude and phase in the time domain. They are well explained by the back and forth propagation of the single phonon wave packet in the GaAs AlAs superlattices. These experimental findings show quite new aspects of the dynamics of coherent phonons.

Coherent dynamics of zone-folded acoustic phonons in GaAs/AlAs superlattices

The zone-folded longitudinal acoustic phonons in GaAs/AlAs superlattices are studied by the time-resolved pump-probe technique. Up to seven phonon modes are clearly observed and they are classified into standing and propagating modes according to their wave vectors. The standing modes are generated by the selective excitation of the well layers and the propagation modes are generated as a phonon wave packet at the substrate interface,

Optical Study of MBE Grown Undoped Si-Si1-xGex/Si Superlattices

Raman spectroscopy and spectroscopic ellipsometry have been used to characterize Si/Si0.78 Ge0.22 superlattices grown by molecular beam epitaxy on (001)Si at different substrate temperatures. The results are interpreted to give information on material and interface quality, layer thicknesses, and state of strain. The observed frequencies of zone-folded longitudinal acoustic phonons in a high quality sample agree well with those calculated using Rytov's theory of acoustic vibrations in layered media.

Phonon Raman scattering in InSb/In1-xAlxSb strained-layer superlattices.

Raman scattering has been used to study a variety of InSb/${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$Sb strained-layer superlattices grown by magnetron sputter epitaxy. The observed frequencies of zone-folded longitudinal acoustic phonons agree well with those calculated using Rytov's theory of acoustic vibrations in layered media. The intensities of these phonons do not coincide with those calculated within the regime of the photo- elastic mechanism for light scattering because the exciting light energy is close to resonance with superlattice electronic transitions. The longitudinal optic phonons in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$Sb layers exhibit two-mode behavior and their shift due to the intralayer strain is discussed.

Raman scattering study of folded acoustic phonons in GaAs/InxGa1−xAs strained-layer superlattices

The excitation of zone-folded longitudinal acoustic phonons in GaAs/InxGa1−xAs strained-layer superlattices is observed by Raman scattering. The Raman data agree well with theory and are consistent with high indium concentrations achieved in the fabrication of these metastable superlattices of good crystalline quality.