Generation Of Coherent Acoustic Phonons Research Articles

Intermolecular electron transfer from intramolecular excitation and coherent acoustic phonon generation in a hydrogen-bonded charge-transfer solid.

Organic materials that produce coherent lattice phonon excitations in response to external stimuli may provide next generation solutions in a wide range of applications. However, for these materials to lead to functional devices in technology, a full understanding of the possible driving forces of coherent lattice phonon generation must be attained. To facilitate the achievement of this goal, we have undertaken an optical spectroscopic study of an organic charge-transfer material formed from the ubiquitous reduction-oxidation pair hydroquinone and p-benzoquinone. Upon pumping this material, known as quinhydrone, on its intermolecular charge transfer resonance as well as an intramolecular resonance of p-benzoquinone, we find sub-cm(-1) oscillations whose dispersion with probe energy resembles that of a coherent acoustic phonon that we argue is coherently excited following changes in the electron density of quinhydrone. Using the dynamical information from these ultrafast pump-probe measurements, we find that the fastest process we can resolve does not change whether we pump quinhydrone at either energy. Electron-phonon coupling from both ultrafast coherent vibrational and steady-state resonance Raman spectroscopies allows us to determine that intramolecular electronic excitation of p-benzoquinone also drives the electron transfer process in quinhydrone. These results demonstrate the wide range of electronic excitations of the parent of molecules found in many functional organic materials that can drive coherent lattice phonon excitations useful for applications in electronics, photonics, and information technology.

Ultrafast acousto-plasmonics in gold nanoparticle superlattices

We report the investigation of the generation and detection of GHz coherent acoustic phonons in plasmonic gold nanoparticles superlattices (NPS). The experiments have been performed from an optical femtosecond pump-probe scheme across the optical plasmon resonance of the superlattice. Our experiments allow to estimate the collective elastic response (sound velocity) of the NPS as well as an estimate of the nano-contact elastic stiffness. It appears that the light-induced coherent acoustic phonon pulse has a typical in-depth spatial extension of about 45 nm which is roughly 4 times the optical skin depth in gold. The modeling of the transient optical reflectivity indicates that the mechanism of phonon generation is achieved through ultrafast heating of the NPS assisted by light excitation of the volume plasmon. These results demonstrate how it is possible to map the photon-electron-phonon interaction in subwavelength nanostructures.

Three-dimensional imaging of acoustic phonons using an X-ray laser

"Key insights into the behaviour of materials can be gained by observing their structure when they undergo lattice distortion. Laser pulses on the femtosecond time scale can be used to induce disorder in a ``pump-probe"" experiment with the ensuing transients being probed stroboscopically using femtosecond pulses of visible light, X-rays or electrons (Fig. 1 A). Here we report two- and three-dimensional imaging of the generation and subsequent evolution of coherent acoustic phonons on the picosecond time scale within single gold nanocrystals using an X-ray free electron laser (Fig. 1 B). Our results allow comparisons between experiment and predictive models based on continuum elasticity theory and molecular dynamics simulations (Fig. 1 B). *Full author list - J.N. Clark L. Beitra, G. Xiong, A. Higginbotham, D. M. Fritz, H. T. Lemke, D. Zhu, M. Cholle, G. J. Williams, M. Messerschmidt, B. Abbey, R. J. Harder, A. M. Korsunsky, J. S. Wark, D. Reis, I. K. Robinson."

Generation and detection of gigahertz acoustic oscillations in thin membranes

Single crystalline membranes are a perfect model system for the study of coherent acoustic phonon generation and decay in the time domain. Coherent acoustical modes are excited and detected in thin single-crystalline silicon and gallium arsenide membranes with femtosecond pulses in the ultraviolet and infrared wavelength region using the asynchronous optical sampling technique. The measured acoustic spectra are compared with each other and are discussed in terms of different generation and detection mechanisms. A clear dependence of the generated spectra on the absorption length of the pump and probe pulses is observed. It is shown that a short absorption length for the pump pulse leads to the generation of coherent high frequency phonons up to several 100GHz frequencies. Membranes are demonstrated to be useful as broadband acoustic cavities and can help to disentangle details of high frequency phonon dynamics. Two-layer membrane systems offer additional insight into energy transfer in the GHz frequency range and adhesion properties.

Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action

In this review we address the microscopic mechanisms that are involved in the photogeneration processes of GHz–THz coherent acoustic phonons (CAP) induced by an ultrafast laser pulse. Understanding and describing the underlying physics is necessary indeed for improving the future sources of coherent acoustic phonons useful for the non-destructive testing optoacoustic techniques. Getting more physical insights on these processes also opens new perspectives for the emerging field of the opto-mechanics where lattice motions (surface and/or interfaces ultrafast displacements, nanostructures resonances) are controlled by light. We will then remind the basics of electron–phonon and photon-phonon couplings by discussing the deformation potential mechanism, the thermoelasticity, the inverse piezoelectric effect and the electrostriction in condensed matter. Metals, semiconductors and oxide materials will be discussed. The contribution of all these mechanisms in the photogeneration process of sound will be illustrated over several examples coming from the rich literature.

Dynamics of coherent acoustic phonons in thin films ofCoSb3and partially filledYbxCo4Sb12skutterudites

Skutterudites are considered as interesting material for thermoelectric applications. Filling foreign atoms into the cagelike structure of a ${\mathrm{CoSb}}_{3}$ skutterudite is beneficial to its thermoelectric properties by increasing phonon scattering while maintaining the electrical conductivity. In this paper we demonstrate the generation and detection of coherent acoustic phonons in thin films of ${\mathrm{CoSb}}_{3}$ and partially filled ${\mathrm{Yb}}_{x}{\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$ skutterudites using femtosecond pump-probe spectroscopy. By using a pulse-echo method, the longitudinal sound velocity of amorphous and polycrystalline ${\mathrm{CoSb}}_{3}$ thin films is obtained. For partially filled ${\mathrm{Yb}}_{x}{\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$ thin films, an obvious decrease of the longitudinal sound velocity is observed at high filling fraction. Concomitantly, the high frequency acoustic phonon modes are strongly damped as the $\mathrm{Yb}$ filling fraction increases, which gives direct evidence for acoustic phonon scattering processes. It is shown that the reduction of lattice thermal conductivity after Yb filling is mainly achieved by the strong scattering of acoustic phonons.

Coherent acoustic phonon generation and detection by femtosecond laser pulses in ZnTe single crystals

We report experiments of coherent acoustic phonon generations and detections using a two-color femtosecond pump-probe technique in [001] and [110] oriented ZnTe single crystals. The above-bandgap pump excitation initiates strain waves to yield coherent acoustic phonons. The generation and propagation of coherent acoustic waves not only modulate the intensity of the reflected probe light but also change its polarization periodically due to strain-induced birefringence. The nonuniform excitation due to Gaussian spatial distribution of pump pulses takes an important role in the polarimetric detection of coherent acoustic phonon oscillations.

Ultrafast Three-Dimensional Imaging of Lattice Dynamics in Individual Gold Nanocrystals

Key insights into the behavior of materials can be gained by observing their structure as they undergo lattice distortion. Laser pulses on the femtosecond time scale can be used to induce disorder in a "pump-probe" experiment with the ensuing transients being probed stroboscopically with femtosecond pulses of visible light, x-rays, or electrons. Here we report three-dimensional imaging of the generation and subsequent evolution of coherent acoustic phonons on the picosecond time scale within a single gold nanocrystal by means of an x-ray free-electron laser, providing insights into the physics of this phenomenon. Our results allow comparison and confirmation of predictive models based on continuum elasticity theory and molecular dynamics simulations.

Ultrafast coherent phonon dynamics in Ge2Sb2Te5 thin films

We investigated the coherent acoustic and optical phonon dynamics in a Ge2Sb2Te5 phase change material by using pump-probe spectroscopy. Oscillating reflectivity changes with oscillation period and de-phasing time on a timescale of tens of picoseconds were observed and could be consistently explained by coherent acoustic phonon generation in the form of a strain pulse propagating into substrate. The temperature-dependent frequency shift of the coherent optical A1 phonon mode was due to a three-phonon anharmonic decay while its de-phasing was dominated by temperatureindependent phonon-vacancy scattering at low temperature. A laser-fluence-dependent A1 phonon mode softening and a dephasing rate increase were observed and attributed to the high density of electrons and the accompanying lattice distortion, as well as the increase in the lattice temperature.

Strong Optical-Mechanical Coupling in a Vertical GaAs/AlAs Microcavity for Subterahertz Phonons and Near-Infrared Light

We show that distributed Bragg reflector GaAs/AlAs vertical cavities designed to confine photons are automatically optimal to confine phonons of the same wavelength, strongly enhancing their interaction. We study the impulsive generation of intense coherent and monochromatic acoustic phonons by following the time evolution of the elastic strain in picosecond-laser experiments. Efficient optical detection is assured by the strong phonon backaction on the high-Q optical cavity mode. Large optomechanical factors are reported (~THz/nm range). Pillar cavities based in these structures are predicted to display picogram effective masses, almost perfect sound extraction, and threshold powers for the stimulated emission of phonons in the range μW-mW, opening the way for the demonstration of phonon "lasing" by parametric instability in these devices.

Control of coherent acoustic phonon generation with external bias in InGaN/GaN multiple quantum wells

Control of acoustic phonon generation with external bias is demonstrated for an InGaN light-emitting diode by performing femtosecond pump–probe measurements. The evolution of both the carrier dynamics and the acoustic phonon strength with an external bias reveals that the acoustic wave is generated during the carrier sweeping process of the depletion field, where electric field screening accompanies stress of the lattice for a piezoelectric material.

Coherent control of sub-terahertz confined acoustic nanowaves: Theory and experiments

Recently it has been theoretically proposed that phonons could be used to store and process information. We report coherent control experiments on acoustic-phonon nanocavities in the sub-THz range and discuss the feasibility of using this system as a phononic memory. The nanocavities provide localized and monochromatic waves, two essential characteristics to work as memories and actuators. A writing-erasing-reading procedure based on the coherent generation, control, and detection of acoustic phonons using ultrashort light pulses is presented. The results are in agreement with numerical simulations using an implementation of a photoelastic model. Our results open the way to the practical realization of complex nano-optomechanical devices.

Theory of coherent generation and detection of THz acoustic phonons using optical microcavities

The coherent generation and detection of acoustic phonons in a superlattice embedded in an optical microcavity is theoretically analyzed. In this optical resonator, femtosecond light pulses can be spatially confined and amplified. We show that the acoustic phonon generation is enhanced as the intensity of the incident electromagnetic field is amplified in resonance with the optical microcavity. The detection process is also enhanced by the optical resonator. In the case of real photoelastic constants the maximum sensitivity occurs when the probe wavelength is tuned to where the derivative of the reflectivity has its maxima, at the optical cavity mode edges. We also analyze the role of the imaginary part of the photoelastic constants of the structure in the generation and detection processes. Finally, we study the enhancement efficiency of the microcavities when the coherent generation and detection are optimized simultaneously; we estimate phonon signals up to six orders of magnitude higher than the ones obtained with the superlattice without optical confinement.

Enhanced optical generation and detection of acoustic nanowaves in microcavities

The enhancement of the ultrafast coherent generation and detection of acoustic phonons in an optical microcavity is experimentally studied. We report pump-probe terahertz ultrasonic experiments in an optical microcavity as a function of laser energy and probe incidence angle. By tuning the laser wavelength to resonate with the microcavity mode at normal incidence and simultaneously varying the incidence angle of the probe beam we achieve a double optical enhancement. Under this condition both the coherent generation and detection of acoustic nanowaves are strongly enhanced. We demonstrate the use of optical microcavities as a promising tool to study acoustic phonons in reduced dimensions with increased sensitivity.

Ultrafast near-infrared spectroscopic study of coherent phonons in the phase-separated manganiteLa1/4Pr3/8Ca3/8MnO3

We report the generation of coherent optical and acoustic phonons in mixed valent manganite ${\text{La}}_{1/4}{\text{Pr}}_{3/8}{\text{Ca}}_{3/8}{\text{MnO}}_{3}$ using femtosecond infrared pump-probe spectroscopy. Temperature-dependent measurements of the time-resolved optical reflectance, obtained over a range of 5--300 K, revealed that the energy of the photoexcited electrons dissipated during relaxation to acoustic phonons, in the high-temperature paramagnetic phase, and to optical phonons, in the low-temperature charge-ordering phase. Analysis of the temperature-dependent behavior reveals that the modal amplitudes of the coherent phonons appear strongly correlated with the charge-ordering phase.

Coherent acoustic phonon generation in exciton self-trapping

The coupled electronic and vibrational dynamics of exciton self-trapping are studied in the quasi-one-dimensional material $[\text{Pt}{(\text{en})}_{2}][\text{Pt}{(\text{en})}_{2}{\text{Br}}_{2}]\ensuremath{\cdot}{({\text{PF}}_{6})}_{4}$ ($\text{en}=\text{ethylenediamine}$) using femtosecond impulsive excitation techniques. We report transient absorption measurements at 77 K that are modulated by a large amplitude, strongly damped oscillatory component at a frequency of $11\text{ }{\text{cm}}^{\ensuremath{-}1}$ in addition to the $110\text{ }{\text{cm}}^{\ensuremath{-}1}$ excited-state optical-phonon wave-packet oscillation previously observed at room temperature. We find that the characteristics of the low-frequency oscillatory response are consistent with the theoretically predicted generation of a propagating coherent acoustic wave accompanying the formation of the localized lattice deformation that stabilizes the self-trapped state. The observed low-frequency oscillation, interpreted in the context of theoretical models for polaron formation via coupling to acoustic phonons, provides an estimate of the spatial extent of the resulting localized state of $\ensuremath{\sim}5$ unit cells of the PtBr chain structure. This value is in good agreement with the localization length predicted by previous extended Peierls-Hubbard calculations.

Laser-induced coherent acoustical phonons mechanisms in the metal-insulator transition compoundNdNiO3: Thermal and nonthermal processes

Coherent acoustic phonon generation processes on the picosecond time scale were investigated in the metal-insulator transition compound ${\text{NdNiO}}_{3}$. Time-resolved optical pump-probe methods were employed. While at high temperature thermoelastic process drives the photogeneration of acoustic phonons, an additional contribution can be identified when the transition to the insulating state at $T<{T}_{\text{MI}}$ takes place. The latter additional photogenerated stress is possibly of electronic stress nature (deformation-potential mechanism). We show that it competes with thermoelastic stress, leading to reduction in the magnitude of a photogenerated acoustic phonon pulse. In that insulating state, we suggest that the electron-phonon deformation-potential coupling in ${\text{NdNiO}}_{3}$ leads to a laser-induced lattice cell volume contraction. Furthermore, in the temperature range where a first-order phase transition takes place and where both metallic and insulating metastable phases coexist, the photogenerated ultrashort picosecond acoustic pulse magnitude cannot be described by a classical thermoelastic process. We suggest that such a phenomenon is connected to unusual thermoelasticity in a metastable state. In particular, we discuss the definition of the thermal-expansion coefficient of solids in a metastable state subjected to ultrafast optical excitation.

Excitation of longitudinal and transverse coherent acoustic phonons in nanometer free-standing films of (001) Si

Transmission electron diffraction is naturally sensitive to the detection of shear-type deformations in single-crystalline structures due to the effective tilting of the lattice planes characteristic of shear, but in general is insensitive to longitudinal phonon propagation. Here, we report on the generation and detection of both transverse and longitudinal coherent acoustic phonons in 33 nm free-standing (001)-oriented single crystalline Si films using femtosecond electron diffraction (FED) to monitor these laser-induced atomic displacements. The mechanism for excitation of the shear mode that leads to coupling to the longitudinal phonon is attributed to the inhomogeneous lateral profile of the optical-pump pulse and the periodic boundary condition imposed by the supporting grid structure. In this application, the constructive interference in the diffraction process makes FED particularly sensitive to the detection of coherent phonon modes and offers an atomic perspective of the dynamics involving collective motions.

Femtosecond electron diffraction: Direct probe of ultrafast structural dynamics in metal films

Femtosecond electron diffraction is a rapidly advancing technique that holds a great promise for studying ultrafast structural dynamics in phase transitions, chemical reactions, and function of biological molecules at the atomic time and length scales. In this paper, we summarize our development of a tabletop femtosecond electron diffractometer. Using a delicate instrument design and careful experimental configurations, we demonstrate the unprecedented capability of detecting submilli-ångström lattice spacing change on a subpicosecond timescale with this new technique. We have conducted an in-depth investigation of ultrafast coherent phonon dynamics induced by an impulsive optical excitation in thin-film metals. By probing both coherent acoustic and random thermal lattice motions simultaneously in real time, we have provided the first and unambiguous experimental evidence that the pressure of hot electrons contributes significantly to the generation of coherent acoustic phonons under nonequilibrium conditions when electrons and phonons are not thermalized. Based on these observations, we also propose an innovative approach to measure the electronic Grüneisen parameter in magnetic materials at and above room temperature, which provides a way to gain new insights into electronic thermal expansion in ferromagnetic transition metals.

Influence of doping profiles on coherent acoustic phonon detection and generation in semiconductors

The doping profile in different n-doped GaAs homoepitaxial structures grown by molecular beam epitaxy is investigated in the time domain by employing a laser based picosecond ultrasound technique in a contactless and noninvasive way. Experiments based on asynchronous optical sampling employ two femtosecond lasers, which allow us to detect changes in the optical reflectivity over a 1 ns time delay with a signal-to-noise ratio of 107 and 100 fs time resolution in <1 min of acquisition time. We show that the doping profile with doping densities of the order of 1018 cm−3 can be detected with picosecond ultrasound, although there is no difference in the acoustic properties of the doped and undoped region. The detection mechanism is based on a different sensitivity function for a coherent strain pulse in the doped and undoped regions. These results are corroborated by experiments at room temperature and 10 K.