Generation Of Coherent Acoustic Phonons Research Articles

Capturing the non-equilibrium state in light–matter–free-electron interactions through ultrafast transmission electron microscopy

Ultrafast transmission electron microscope (UTEM) with the multimodality of time-resolved diffraction, imaging, and spectroscopy provides a unique platform to reveal the fundamental features associated with the interaction between free electrons and matter. In this review, we summarize the principles, instrumentation, and recent developments of the UTEM and its applications in capturing dynamic processes and non-equilibrium transient states. The combination of the transmission electron microscope with a femtosecond laser via the pump–probe method guarantees the high spatiotemporal resolution, allowing the investigation of the transient process in real, reciprocal and energy spaces. Ultrafast structural dynamics can be studied by diffraction and imaging methods, revealing the coherent acoustic phonon generation and photo-induced phase transition process. In the energy dimension, time-resolved electron energy-loss spectroscopy enables the examination of the intrinsic electronic dynamics of materials, while the photon-induced near-field electron microscopy extends the application of the UTEM to the imaging of optical near fields with high real-space resolution. It is noted that light–free-electron interactions have the ability to shape electron wave packets in both longitudinal and transverse directions, showing the potential application in the generation of attosecond electron pulses and vortex electron beams.

Ultrafast generation and detection of coherent acoustic phonons in SnS0.91Se0.09

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

Ultrafast Optical Probe of Coherent Acoustic Phonons in Dirac Semimetal Cd3As2 Film Epitaxied on GaAs(111)B Substrate.

Coherent longitudinal acoustic phonon (CAP) generation in epitaxial Dirac semimetal Cd3As2 films with different thicknesses was investigated by a time-resolved reflectance technique. The short-lived weak CAP oscillations can be observed only in the thicker Cd3As2 films, and their central frequency of 0.039 THz has no dependence on sample thickness, but is nearly inversely proportional to the probe wavelength. For the 20 nm thin film, the observed long-lived CAP with a central frequency of 0.049 THz is generated in the GaAs(111)B substrate underneath. A sound velocity of 3800 m/s for the Cd3As2 film and 5360 m/s for the GaAs(111)B substrate is thus deduced. In addition, the opposite CAP amplitude and lifetime dependence on temperature further confirms the electronic and thermal stress origination of CAP generated in GaAs(111)B and Cd3As2 film, respectively, based on the propagating strain pulse model. The central frequency of CAP is found to be stable with increasing pumping fluence and temperature, which makes Cd3As2 a potential material for thermoelectric device applications.

Launching Coherent Acoustic Phonon Wave Packets with Local Femtosecond Coulomb Forces.

Generation and manipulation of coherent acoustic phonons enables ultrafast control of solids and has been exploited for applications in various acoustic devices. We show that localized coherent acoustic phonon wave packets can be launched by ultrafast Coulomb forces in a scanning tunneling microscope (STM) using tip-enhanced terahertz electric fields. The wave packets propagate at the speed of the longitudinal acoustic phonon, creating standing waves up to 0.26THz for a 6.4nm thin Au film on mica. The ultrafast lattice displacement can be as large as 5pm and is precisely controlled by varying the tip-sample distance. This nonthermal femtosecond Coulomb-force-based excitation mechanism is applicable in nano-optomechanics for advanced terahertz engineering and opens new perspectives in exploiting coherent phonons at the atomic scale.

Ultrafast strain waves reconstruction from coherent acoustic phonons reflection

Picosecond ultrasonics, which studies laser-induced high-frequency strain waves, is a reliable and versatile method for nondestructive materials' characterization. Strain waves are generated through a light interaction with charges and their subsequent relaxation, and these waves conceal a wealth of information on the material. However, strain waves are detected through their convolution with a sensitivity function, which blurs much of this information. Here, we show that the reflection of strain waves at a free surface leads to the appearance of a Fano resonance in the reflectivity spectrum, accompanied by a drastic increase in the detection bandwidth. We take advantage of this feature to provide a method for the reconstruction of strain waves. We apply it to unambiguously highlight the exact origin of the generation of coherent acoustic phonons in Stranski–Krastanov grown quantum dots, revealing that both the wetting layer and quantum dots are responsible for the generation. Our results will offer the possibility to understand better the interaction of light with charges and their interactions with the lattice.

Parametric resonance of acoustic and optical phonons in GaAs/GaAsAl quantum well in the presence of laser field

In this paper, we analytically investigated the possibility of parametric resonance of acoustic and optical phonons. We obtained a general dispersion equation for parametric amplification and transformation of phonons. The dispersions of the resonant acoustic phonon modes and the threshold amplitude of the field for acoustic phonon parametric amplification are obtained. The parametric amplification for acoustic phonons in a GaAs/GaAsAl quantum well can occur under the condition that the amplitude of the external electromagnetic field is higher than the threshold amplitude. Full Text: PDF ReferencesB. A. Glavin, V. A. Kochelap, T. L. Linnik, P. Walker, A. J. Kent and M. Henini, "Monochromatic terahertz acoustic phonon emission from piezoelectric superlattices", Journal of Physics: Conference Series, Vol 92, (2007). CrossRef O. A. C. Nunes, "Piezoelectric surface acoustical phonon amplification in graphene on a GaAs substrate", Journal of Applied Physics 115, 233715 (2014). CrossRef Yu. E. Lozovik, S. P. Merkulova, I. V. Ovchinnikov, "Sasers: resonant transitions in narrow-gap semiconductors and in exciton system in coupled quantum wells", Phys. Lett. A 282, 407-414, (2001). CrossRef R.P. Beardsley, A.V. Akimov, M. Henini and A.J. Kent, "Coherent Terahertz Sound Amplification and Spectral Line Narrowing in a Stark Ladder Superlattice", PRL 104, 085501, (2010). CrossRef Pascal Ruello, Vitalyi E. Gusev, "Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action", Ultrasonics 56, 21-35, (2015). CrossRef L. Esaki, in Proc. 17th Int. Conf. Phys. Semiconductors, San Francisco, CA, Aug, J.D. Chadi and W.A. Harrison, Eds, Berlin: Springer- Verlag, 473, (1984). DirectLink W. Xu, F. M. Peeters and J. T. Devreese, "Electrophonon resonances in a quasi-two-dimensional electron system", Phys. Rev. B 48, 1562 (1993). CrossRef Tran Cong Phong, Nguyen Quang Bau, "Parametric resonance of acoustic and optical phonons in a quantum well", Journal of the Korean Physical Society, Vol. 42, No. 5, 647-651, (2003). DirectLink

Fiber-integrated microcavities for efficient generation of coherent acoustic phonons

Coherent phonon generation by optical pump-probe experiments has enabled the study of acoustic properties at the nanoscale in planar heterostructures, plasmonic resonators, micropillars, and nanowires. Focalizing both the pump and the probe on the same spot of the sample is a critical part of pump-probe experiments. This is particularly relevant in the case of small objects. The main practical challenges for the actual implementation of this technique are stability of the spatiotemporal overlap, reproducibility of the focalization, and optical mode matching conditions. In this work, we solve these three challenges for the case of planar and micropillar optophononic cavities. We integrate the studied samples to single mode fibers lifting the need for focusing optics to excite and detect coherent acoustic phonons. The resulting reflectivity contrast of at least 66% achieved in our samples allows us to observe stable coherent phonon signals over at least a full day and signals at an extremely low excitation power of 1 μW. The monolithic sample structure is transportable and could provide a means to perform reproducible plug-and-play experiments.

High-resolution analogue of time-domain phonon spectroscopy in the transmission electron microscope.

Femtosecond photoexcitation of semiconducting materials leads to the generation of coherent acoustic phonons (CAPs), the behaviours of which are linked to intrinsic and engineered electronic, optical and structural properties. While often studied with pump-probe spectroscopic techniques, the influence of nanoscale structure and morphology on CAP dynamics can be challenging to resolve with these all-optical methods. Here, we used ultrafast electron microscopy (UEM) to resolve variations in CAP dynamics caused by differences in the degree of crystallinity in as-prepared and annealed GaAs lamellae. Following in situ femtosecond photoexcitation, we directly imaged the generation and propagation dynamics of hypersonic CAPs in a mostly amorphous and, following an in situ photothermal anneal, a mostly crystalline lamella. Subtle differences in both the initial hypersonic velocities and the asymptotic relaxation behaviours were resolved via construction of space-time contour plots from phonon wavefronts. Comparison to bulk sound velocities in crystalline and amorphous GaAs reveals the influence of the mixed amorphous-crystalline morphology on CAP dispersion behaviours. Further, an increase in the asymptotic velocity following annealing establishes the sensitivity of quantitative UEM imaging to both structural and compositional variations through differences in bonding and elasticity. Implications of extending the methods and results reported here to elucidating correlated electronic, optical and structural behaviours in semiconducting materials are discussed. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

Magnetic field enhanced detection of coherent phonons in a GaMnAs/GaAs film

Using a two-color time-resolved pump-probe spectroscopy scheme, we studied the generation and detection of longitudinal coherent acoustic phonons, generated by ultrafast laser pulses in a semiconductor heterostructure. Our structure was a $p$-doped, 100-nm-thick ferromagnetic GaMnAs layer grown on a GaAs substrate. By probing the transient reflectivity in the time domain, we observed a strong dependence of the coherent phonon's amplitude on the external magnetic field. Our theoretical model relates this dependence to the increase in the detectability of coherent phonons in the presence of external magnetic fields. This enhancement comes from the formation of Landau levels in the absorption spectrum and leads to large changes in the real and imaginary parts of the dielectric function with strain. When the probe laser energy is close to an allowed Landau-level transition, the detectability of the coherent phonons can be significantly enhanced. Our results suggest that not only can one increase the detection of coherent phonons with magnetic fields, one can also enhance the generation by tuning the wavelength of the pump laser pulse to coincide with a Landau-level resonance.

Coherent acoustic phonons generated by ultrashort terahertz pulses in nanofilms of metals and topological insulators

We report the generation of coherent acoustic phonons in materials with terahertz ultrashort pulses. This is demonstrated in metals and topological insulators by exciting an acoustic eigenmode in nanometric-sized thin films. The efficiency of the coupling is quadratic in the terahertz electric field strength within the range of investigation. Owing to a quantitative comparison between terahertz and near-infrared ultrashort pulse excitations, we show that the process of acoustic phonon generation by terahertz radiation is mainly driven by thermoelastic stress. While for the near-infrared light excitation the lattice temperature increase comes from a rapid energy transfer from the hot carriers to the phonon bath during carrier intraband relaxation, the thermoelastic stress induced by the terahertz electric field is linked to the scattering of the accelerated electrons leading to an ultrafast Joule effect.

Coherent phonon generation in laser-heated gold nanofilm.

We have studied the mechanism of coherent acoustic phonon generation in gold nanofilm induced by ultrafast laser-heating. Under the non-equilibrium condition when the lattice heating time is much longer than the film vibration period, we clearly identified the contribution of electronic thermal stress to drive the lattice motion and successfully measured the electronic Grüneisen parameter γe to be 1.6 ± 0.3. We also found that lattice heating via the electron-phonon coupling process lagged behind the coherent lattice motion, which we attributed to the prolonged thermalization process of the laser-excited non-thermal electrons under high pumping conditions. By taking such a process into account, the improved model fit our experimental data much better, and the extracted γe of gold was still around 1.6.

Anisotropic Charge Carrier and Coherent Acoustic Phonon Dynamics of Black Phosphorus Studied by Transient Absorption Microscopy

Due to its corrugated hexagonal lattice structure, Black phosphorus (BP) has unique anisotropic physical properties, which provides an additional freedom for designing devices. Many body interactions, including interactions with phonon, is crucial for heat dissipation and charge carrier mobility in device. However, the rich properties of the coherent acoustic phonon, including anisotropy, propagation and generation were not fully interrogated. In this paper, the polarization-resolved transient absorption microscopy was conducted on BP flakes to study the dynamics of photoexcited charge carriers and coherent acoustic phonon. Polarization-resolved transient absorption images and traces were recorded and showed anisotropic and thickness-dependent charge carriers decay dynamics. The damping of the coherent acoustic phonon oscillation was found to be anisotropic, which was attributed to the polarization-dependent absorption length of the probe pulse. From the analysis of initial oscillation amplitude and phase of coherent acoustic phonon oscillation, we proposed that the direct deformation potential mechanism dominated the generation of coherent acoustic phonons in our experiment. Besides, we obtained the sound velocity of the coherent acoustic phonon from the oscillation frequency and the acoustic echo, respectively, which agreed well with each other. These findings provide significant insights into the rich acoustic phonon properties of BP, and promise important application for BP in polarization-sensitive optical and optoelectronic devices.

Coherent generation and detection of acoustic phonons in topological nanocavities

Inspired by concepts developed for fermionic systems in the framework of condensed matter physics, topology and topological states are recently being explored also in bosonic systems. Recently, some of these concepts have been successfully applied to acoustic phonons in nanoscale multilayered systems. The reported demonstration of confined topological phononic modes was based on Raman scattering spectroscopy [M. Esmann et al., Phys. Rev. B 97, 155422 (2018)], yet the resolution did not suffice to determine lifetimes and to identify other acoustic modes in the system. Here, we use time-resolved pump-probe measurements using an asynchronous optical sampling (ASOPS) technique to overcome these resolution limitations. By means of one-dimensional GaAs/AlAs distributed Bragg reflectors (DBRs) used as building blocks, we engineer high frequency (∼200 GHz) topological acoustic interface states. We are able to clearly distinguish confined topological states from stationary band edge modes. The generation/detection scheme reflects the symmetry of the modes directly through the selection rules, evidencing the topological nature of the measured confined state. These experiments enable a new tool in the study of the more complex topology-driven phonon dynamics such as phonon nonlinearities and optomechanical systems with simultaneous confinement of light and sound.

Cross-plane coherent acoustic phonons in two-dimensional organic-inorganic hybrid perovskites

Two-dimensional Ruddlesden–Popper organic–inorganic hybrid layered perovskites (2D RPs) are solution-grown semiconductors with prospective applications in next-generation optoelectronics. The heat-carrying, low-energy acoustic phonons, which are important for heat management of 2D RP-based devices, have remained unexplored. Here we report on the generation and propagation of coherent longitudinal acoustic phonons along the cross-plane direction of 2D RPs, following separate characterizations of below-bandgap refractive indices. Through experiments on single crystals of systematically varied perovskite layer thickness, we demonstrate significant reduction in both group velocity and propagation length of acoustic phonons in 2D RPs as compared to the three-dimensional methylammonium lead iodide counterpart. As borne out by a minimal coarse-grained model, these vibrational properties arise from a large acoustic impedance mismatch between the alternating layers of perovskite sheets and bulky organic cations. Our results inform on thermal transport in highly impedance-mismatched crystal sub-lattices and provide insights towards design of materials that exhibit highly anisotropic thermal dissipation properties.

Ultrafast switching of valence and generation of coherent acoustic phonons in semiconducting rare-earth monosulfides

We report on the ultrafast carrier dynamics and generation of coherent acoustic phonons in YbS, a semiconducting rare-earth monochalcogenide, using two-color pump–probe reflectivity. Compared to the carrier relaxation processes and lifetimes of conventional semiconductors, recombination of photoexcited electrons with holes in localized f orbitals is found to take place rapidly with a density-independent time constant of <500 fs in YbS. Such carrier annihilation signifies the unique and ultrafast nature of valence restoration of ytterbium ions after femtosecond photoexcitation switching. Following transfer of the absorbed energy to the lattice, coherent acoustic phonons emerge on the picosecond timescale as a result of the thermal strain in the photoexcited region. By analyzing the electronic and structural dynamics, we obtain the physical properties of YbS including its two-photon absorption and thermooptic coefficients, the period and decay time of the coherent oscillation, and the sound velocity.

Pump-Power Dependence of Coherent Acoustic Phonon Frequencies in Colloidal CdSe/CdS Core/Shell Nanoplatelets.

Femtosecond optical pump-probe spectroscopy resolves hitherto unobserved coherent acoustic phonons in colloidal CdSe/CdS core/shell nanoplatelets (NPLs). With increasing pump fluence, the frequency of the in-plane acoustic mode increases from 5.2 to 10.7 cm-1, whereas the frequency of the out-of-plane mode remains at ∼20 cm-1. Analysis of the oscillation phases suggests that the coherent acoustic phonon generation mechanism transitions from displacive excitation to subpicosecond Auger hole trapping with increasing pump fluence. The measurements yield Huang-Rhys parameters of ∼10-2 for both acoustic modes. The weak electron-phonon coupling strengths favor the application of NPLs in optoelectronics.

Intrinsic coherent acoustic phonons in the indirect band gap semiconductors Si and GaP

We report on the intrinsic optical generation and detection of coherent acoustic phonons at (001)-oriented bulk Si and GaP without metallic phonon transducer structures. Photoexcitation by a 3.1-eV laser pulse generates a normal strain pulse within the $\ensuremath{\sim}100$-nm penetration depth in both semiconductors. The subsequent propagation of the strain pulse into the bulk is detected with a delayed optical probe as a periodic modulation of the optical reflectivity. Our theoretical model explains quantitatively the generation of the acoustic pulse via the deformation potential electron-phonon coupling and detection in terms of the spatially and temporally dependent photoelastic effect for both semiconductors. Comparison with our theoretical model reveals that the experimental strain pulses have finite buildup times of 1.2 and 0.4 ps for GaP and Si, which are comparable with the time required for the photoexcited electrons to transfer to the lowest X valley through intervalley scattering. The deformation potential coupling related to the acoustic pulse generation for GaP is estimated to be twice as strong as that for Si from our experiments, in agreement with a previous theoretical prediction.

Ultrafast optical probe of coherent acoustic phonons in Heusler film**Project supported by the National Basic Research Program of China (Grant No. 2013CB922303) and the National Natural Science Foundation of China (Grant No. 61334006).

In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co2MnAl films which are epitaxially grown on GaAs substrates are observed and investigated as a function of film thickness, probe wavelength, external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of (3.85 ± 0.1)×103 m/s at 800 nm for the epitaxial Co2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.

Integrating Ultrafast Coherent Diffractive Imaging and Atomistic Simulations to Investigate Externally Stimulated Metallic Nanocrystals

Despite the increasing role of nanomaterials in technology, the mechanical and dynamical properties of single nanoparticles at picosecond timescales are not well understood. In particular, core-shell nanocrystals are an important class of nanoparticles for catalysis, biomedical and energy applications. The effect of the nanoscale interface between the core and shell material on coherent lattice vibrations (phonons) is yet to be investigated in detail. Understanding the temporal behavior of such nanomaterials under conditions of external thermal stimulation is crucially important for energy research. In addition, observing ultrafast lattice distortions can provide key insights into the behavior of nanomaterials and nanoscale interfaces. Recently, experimental techniques have evolved to conduct time-dependent lattice dynamics measurements in nanomaterials. In particular, Bragg Coherent Diffraction Imaging (BCDI) in conjunction with optical pump-probe experiments via ultrafast x-ray lasers has been used to directly image the generation and subsequent evolution of coherent acoustic phonons within nanocrystals. In particular, experiments on bimetal (Au/Al) core-shell nanocrystals have revealed inhomogeneous effects in the lattice breathing upon heating with femtosecond x-ray lasers. Conventional theoretical models cannot be used to explain the physics of the phenomena in such non-equilibrium environments that involve extreme heat fluxes and temperature gradients. Atomistic molecular dynamics (MD) simulation is an appropriate technique to investigate lattice dynamics in such environments, particularly in core-shell structures where interfacial effects can play an important role in phonon scattering. With the convergence of time and length scales accessible by both experiments and simulations, we are now able to integrate experimental observations with multi-million atom MD simulations to enhance the fundamental understanding of materials behavior under external stimulation. In this talk, we demonstrate the workflow to integrate ultrafast time-resolved coherent diffractive imaging with large-scale atomistic molecular dynamics simulations to investigate lattice dynamics in externally stimulated core-shell bimetallic nanocrystals. We will focus, primarily, on the MD simulations performed on core-shell bimetallic nanocrystals under the influence of extreme heat fluxes. We explore the vibrational modes that dominate lattice breathing and look into the effect the nanoparticle size and shape have on the same. In addition, we attempt to identify the origin of the inhomogeneous effects, observed experimentally, in the lattice breathing of core-shell structures. Figure 1

Ultrafast Anisotropic Optical Response and Coherent Acoustic Phonon Generation in Polycrystalline BaTiO3-BiFeO3

Abstract We optically study the as-yet little explored multiferroic material, BaTiO3-BiFeO3 (BTO-BFO), that has demonstrated enhanced magnetic properties, a higher DC resistance in comparison to BFO, and improved magnetoelectric coupling. Our studies include: ultrafast time resolved differential reflection, optically induced birefringence, and second-harmonic nano-imaging of the ferroic order. We observe a strong sensitivity to pump/probe polarizations, photo-induced ferroelectric poling on a picosecond timescale, as well as the generation of photo-induced coherent acoustic phonons with a frequency of ∼ 11 $ \sim 11$ GHz. The second-harmonic generation nano-imaging reveals disordered but distinct ferroelectric domain order, percolating even across grain boundaries in the poly-crystalline thin film.