Zone-folded Acoustic Phonons Research Articles

Coherent Oscillations in a SrRuO3/BiFeO3 Superlattice.

We investigated the ultrafast dynamics of a SrRuO3/BiFeO3 superlattice grown on a SrTiO3 substrate using a near infrared pump-probe technique at various temperatures. The superlattice exhibits a ferromagnetic order inherited from the SrRuO3 layer. The pump-induced changes in the reflectivity reveal periodic oscillations. We found that the oscillation frequency can be well explained by zone-folded acoustic phonon oscillations, whose dispersion depends on the sound velocity, density, and thickness within the supercell of each constituent layer. It is found that the observed oscillation frequency corresponds to the A1 mode, which suggests that oscillations are excited due to pump-induced expansion of the SrRuO3 layer that absorbs the pump photon. Temperature-dependent measurements reveal significant suppression of the oscillation amplitude in the ferromagnetic state. The suppressed amplitude is proportional to the square of the magnetization, M(T)2. This phenomenon can be attributed to a strong magnetostriction effect of SrRuO3 that suppresses lattice expansion upon pumping.

Atomistic Engineering of Phonons in Functional Oxide Heterostructures.

Engineering of phonons, that is, collective lattice vibrations in crystals, is essential for manipulating physical properties of materials such as thermal transport, electron‐phonon interaction, confinement of lattice vibration, and optical polarization. Most approaches to phonon‐engineering have been largely limited to the high‐quality heterostructures of III–V compound semiconductors. Yet, artificial engineering of phonons in a variety of materials with functional properties, such as complex oxides, will yield unprecedented applications of coherent tunable phonons in future quantum acoustic devices. In this study, artificial engineering of phonons in the atomic‐scale SrRuO3/SrTiO3 superlattices is demonstrated, wherein tunable phonon modes are observed via confocal Raman spectroscopy. In particular, the coherent superlattices led to the backfolding of acoustic phonon dispersion, resulting in zone‐folded acoustic phonons in the THz frequency domain. The frequencies can be largely tuned from 1 to 2 THz via atomic‐scale precision thickness control. In addition, a polar optical phonon originating from the local inversion symmetry breaking in the artificial oxide superlattices is observed, exhibiting emergent functionality. The approach of atomic‐scale heterostructuring of complex oxides will vastly expand material systems for quantum acoustic devices, especially with the viability of functionality integration.

Zone-Folded Longitudinal Acoustic Phonons Driving Self-Trapped State Emission in Colloidal CdSe Nanoplatelet Superlattices

Colloidal CdSe nanoplatelets (NPLs) have substantial potential in light-emitting applications because of their quantum-well-like characteristics. The self-trapped state (STS), originating from strong electron-phonon coupling (EPC), is promising in white light luminance because of its broadband emission. However, achieving STS in CdSe NPLs is extremely challenging because of their intrinsic weak EPC nature. Herein, we developed a strong STS emission in the spectral range of 450-600 nm by building superlattice (SL) structures with colloidal CdSe NPLs. We demonstrated that STS is generated via strong coupling of excitons and zone-folded longitudinal acoustic phonons with formation time of ∼450 fs and localization length of ∼0.56 nm. The Huang-Rhys factor, describing the EPC strength in SL structure, is estimated to be ∼19.9, which is much larger than that (∼0.1) of monodispersed CdSe NPLs. Our results provide an in-depth understanding of STS and a platform for generating and manipulating STS by designing SL structures.

Terahertz time-domain spectroscopy of zone-folded acoustic phonons in 4H and 6H silicon carbide.

We investigate the dielectric properties of the 4H and 6H polytypes of silicon carbide in the 0.1-19 THz range, below the fundamental transverse-optical phonons. Folding of the Brillouin zone due to the specific superlattice structure of the two polytypes leads to activation of acoustic phonon modes. We use a combination of ultrabroadband terahertz time-domain spectroscopy and simulations based on density-functional perturbation theory to observe and characterize these modes, including band splitting due to the dissimilar carbon and silicon sublattices of the structures, and an indirect measurement of the anisotropic sound velocities in the two polytypes.

Characterization of longitudinal acoustic phonons in InGaAsP multiple quantum wells by asynchronous optical sampling

We performed time-resolved reflectivity measurements and characterized the time-varying oscillations of longitudinal acoustic phonons in InGaAsP multiple quantum wells (MQWs) by asynchronous optical sampling with Er-doped fiber lasers. Oscillations of longitudinal acoustic phonons ranging from a few tens to a few hundred gigahertz were observed, which were induced by forward and backward Raman scattering in Brillouin and first zone-folded acoustic phonon modes. To investigate the dynamics of each phonon mode for various pump-probe time delays, we analyzed the experimental data with a continuous wavelet transformation (CWT) and a subsequent modified inverse CWT. For high-frequency oscillations, we observed the oscillatory profile with beats and identified the contribution of each phonon mode. For low-frequency oscillations, a series of dissipations of phonon wave packets from each QW to the InP substrate was deduced from the analyzed data. These results suggest that our analysis method is useful for investigating the dynamics of acoustic phonons.

Ultrafast optical control of multiple coherent phonons in silicon carbide using a pulse-shaping technique

We demonstrated ultrafast optical control of multiple coherent phonons in silicon carbide (SiC) using a pulse-shaping technique combined with sub-10-fs optical pulses and a two-dimensional spatial light modulator. Because our technique produces a pulse train with the desired number of pulses and time intervals, precise manipulation of the amplitudes of the zone-folded optical and acoustic phonon modes could be achieved simultaneously and was well reproduced by a model calculation assuming the two modes are excited independently by each pulse. These results show that our scheme can provide high controllability of multiple phonon modes in SiC.

Two-colour high-speed asynchronous optical sampling based on offset-stabilized Yb:KYW and Ti:sapphire oscillators.

We present a high-speed asynchronous optical sampling system, based on two different Kerr-lens mode-locked lasers with a GHz repetition rate: An Yb:KYW oscillator and a Ti:sapphire oscillator are synchronized in a master-slave configuration at a repetition rate offset of a few kHz. This system enables two-colour pump-probe measurements with resulting noise floors below 10⁻⁶ at a data aquisition time of 5 seconds. The measured temporal resolution within the 1 ns time window is below 350 fs, including a timing jitter of less than 50 fs. The system is applied to investigate zone-folded coherent acoustic phonons in two different semiconductor superlattices in transmission geometry at a probe wavelength far below the bandgap of the superlattice constituents. The lifetime of the phonon modes with a zero wave vector and frequencies in the range from 100 GHz to 500 GHz are measured at room temperature and compared with previous work.

Influence of nonmagnetic Zn substitution on the lattice and magnetoelectric dynamical properties of the multiferroic material CuO

Dynamic magnetoelectric coupling in the improper ferroelectric ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}\mathrm{O} (x=0,x=0.05)$ was investigated using terahertz time-domain spectroscopy to probe electromagnon and magnon modes. Zinc substitution was found to reduce the antiferromagnetic ordering temperature and widen the multiferroic phase, under the dual influences of spin dilution and a reduction in unit-cell volume. The impact of Zn substitution on lattice dynamics was elucidated by Raman and Fourier-transform spectroscopy, and shell-model calculations. Pronounced softenings of the ${A}_{u}$ phonons, active along the direction of ferroelectric polarization, occur in the multiferroic state of ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}\mathrm{O}$, and indicate strong spin-phonon coupling. The commensurate antiferromagnetic phase also exhibits spin-phonon coupling, as evidenced by a Raman-active zone-folded acoustic phonon, and spin dilution reduces the spin-phonon coupling coefficient. While the phonon and magnon modes broaden and shift as a result of alloy-induced disorder, the electromagnon is relatively insensitive to Zn substitution, increasing in energy without widening. The results demonstrate that electromagnons and dynamic magnetoelectric coupling can be maintained even in disordered spin systems.

Selective excitation of zone-folded phonon modes within one triplet in a semiconductor superlattice

Coherent control of phonon modes by use of multipulse setups has been studied in various semiconductor systems ranging from bulk materials to multiple-quantum-well super­ lattice structures 1-6 as well as in other solid-state systems 7.8 and for molecular vibrationsY The control of first- and second-order back-folded acoustic modes in a superlattice has been demonstrated. lo In contrast to previous work, we present selective excitation of phonon modes within a single triplet of zone-folded modes where the frequency spacing is only a few tens of GHz. The evolution of the modes is followed throughout different control pulse delays to map out the continuous tuning of their amplitude ratios. The principle is based on the use of a second pump or control pulse for phonon generation with a defined phase with respect to the first excitation. The coherent superposition leads to a modification of the phonon amplitudes within the triplet structure of the first branch of zone-folded acoustic phonons. This results in an enhancement or suppression of the phonon oscillations. The manipulation and selective excitation of the phonon modes within a zone-folded triplet are possible by taking advantage of the high signal-to-noise ratio and the fast data acquisition rate of a high-speed asynchronous optical sampling (ASOPS) system. II The demonstrated principle could be extended to prepare well-defined phonon states for phonon spectroscopy and phonon amplification schemes 12 and to study phonon­ phonon interactions.

Coherent phonon dynamics at the martensitic phase transition of Ni2MnGa

We use time-resolved optical reflectivity to study the laser stimulated dynamics in the magnetic shape memory alloy Ni2MnGa. We observe two coherent optical phonons, at 1.2 THz in the martensite phase and at 0.7 THz in the pre-martensite phase, which we interpret as a zone-folded acoustic phonon and a heavily damped amplitudon, respectively. In the martensite phase the martensitic phase transition can be induced by a fs laser pulse on a timescale of a few ps.

Coupling of intersubband transitions to zone-folded acoustic phonons in a GaN/AlN superlattice

Spectrally resolved femtosecond pump-probe measurements of intersubband transitions in a strongly polar GaN/AlN superlattice are presented. A Fourier transform of the data reveals backfolded longitudinal acoustic phonons. The observed phonons couple strongly to the intersubband transitions and modulate the spectral width and position of the intersubband absorption. The coupling is caused by piezoelectric effects and by a phonon-induced modulation of the well width.

Intra-acceptor hole relaxation in Be δ-doped GaAs/AlAs multiple quantum wells

This paper studies the dynamics of intra-acceptor hole relaxation in Be δ-doped GaAs/AlAs multiple quantum wells (MQW) with doping at the centre by time-resolved pump-probe spectroscopy using a picosecond free electron laser for infrared experiments. Low temperature far-infrared absorption measurements clearly show three principal absorption lines due to transitions of the Be acceptor from the ground state to the first three odd-parity excited states respectively. The pump-probe experiments are performed at different temperatures and different pump pulse wavelengths. The hole relaxation time from 2p excited state to 1s ground state in MQW is found to be much shorter than that in bulk GaAs, and shown to be independent of temperature but strongly dependent on wavelength. The zone-folded acoustic phonon emission and slower decay of the wavefunctions of impurity states are suggested to account for the reduction of the 2p excited state lifetime in MQW. The wavelength dependence of the 2p lifetime is attributed to the diffusion of the Be atom δ-layer in quantum wells.

Investigating Coherent Zone-Folded Acoustic Phonons in Si/SiGe Superlattices by Transient Thermoreflectance Technique

AbstractIn this paper, we present a systematic study of coherent phonons in Si/SiGe superlattices with two different periods. The superlattice periodicity affects the acoustic properties of the structure. Transient thermoreflectance (TTR) technique is used to perform picosecond ultrasonics experiments and then, investigate coherent zone-folded acoustic phonons in different Si/SiGe superlattice structures. Several classes of coherent phonons are produced in the superlattice, whose generation mechanisms are different: Brillouin oscillations, coherent longitudinal-acoustic phonon Bragg reflection and impulsive stimulated Raman scattering (ISRS).

Coherent Atomic Motions in a Nanostructure Studied by Femtosecond X-ray Diffraction

Reversible structural changes of a nanostructure were measured nondestructively with subpicometer spatial and subpicosecond temporal resolution via x-ray diffraction (XRD). The spatially periodic femtosecond excitation of a gallium arsenide/aluminum gallium arsenide superlattice results in coherent lattice motions with a 3.5-picosecond period, which was directly monitored by femtosecond x-ray pulses at a 1-kilohertz repetition rate. Small changes (DeltaR/R = 0.01) of weak Bragg reflexes (R = 0.005) were detected. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrate that displacive excitation of the zone-folded acoustic phonons is the dominant mechanism for strong excitation.

Observation of coherent zone-folded acoustic phonons generated by Raman scattering in a superlattice

We have used pulse time-of-flight techniques to examine the phonon emission from an optically excited GaAs/AlAs superlattice structure. For laser excitation wavelengths shorter than 767 nm (the energy of E1HH1 transition), we detect a significant longitudinal acoustic phonon component directed in a narrow beam normal to the structure. Under identical excitation conditions, generation of coherent longitudinal acoustic phonons has previously been observed in this structure. We suggest that the excitation wavelength and angular characteristics of the longitudinal acoustic emission is consistent with those of propagating modes produced as coherent phonons “leak” from the superlattice structure.

Coherent acoustic phonons in GaAs/AlAs superlattices

Coherent zone-folded acoustic phonons are generated in GaAs/AlAs superlattices by excitation with femtosecond laser pulses. Three modes of first and second backfolded order are driven by resonant impulsive stimulated Raman scattering in forward and backward directions. The coherent vibrations are analyzed in a time resolved detection measurement and compared with Raman scattering cross sections.

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]

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,

Coherent oscillation of zone-folded phonon modes in GaAs-AlAs superlattices.

We have observed a coherent oscillation of zone-folded acoustic phonons in GaAs-AlAs superlattices for the first time by means of a femtosecond time-resolved pump-probe technique. The oscillatory component in the time-resolved reflection signal corresponds to the upper branch (${B}_{2}$ symmetry) of the first-order doublet mode. Carriers were selectively excited in well layers and the ${B}_{2}$-symmetry phonon mode was selectively generated. The dephasing time of the ${B}_{2}$ mode was measured to be 70 ps from our experiments.

Coherent oscillation of zone-folded phonons in GaAs-AlAs superlattices

We have observed a coherent oscillation of zone-folded acoustic phonons in GaAs-AlAs superlattices for the first time by means of a femtosecond time-resolved pump and probe technique. The oscillatory component in the time-resolved reflection signal corresponds to the upper branch of the first-order doublet mode.