2022
I. Chaban, R. Deska, G. Privault, E. Trzop, M. Lorenc, S. E. Kooi, K. A. Nelson, M. Samoc, K. Matczyszyn, T. Pezeril
Nano Lett. 22, 4362 (2022).
Herein we describe a novel spinning pump–probe photoacoustic technique developed to study nonlinear absorption in thin films. As a test case, an organic polycrystalline thin film of quinacridone, a well-known pigment, with a thickness in the tens of nanometers range, is excited by a femtosecond laser pulse which generates a time-domain Brillouin scattering signal. This signal is directly related to the strain wave launched from the film into the substrate and can be used to quantitatively extract the nonlinear optical absorption properties of the film itself. Quinacridone exhibits both quadratic and cubic laser fluence dependence regimes which we show to correspond to two- and three-photon absorption processes. This technique can be broadly applied to materials that are difficult or impossible to characterize with conventional transmittance-based measurements including materials at the nanoscale, prone to laser damage, with very weak nonlinear properties, opaque, or highly scattering.
M.-C. Lee, N. Sirica, S. W. Teitelbaum, A. Maznev, T. Pezeril, R. Tutchton, V. Krapivin, G. A. de la Pena, Y. Huang, L. X. Zhao, G. F. Chen, B. Xu, R. Yang, J. Shi, J.-X. Zhu, D. A. Yarotski, X. G. Qiu, K. A. Nelson, M. Trigo, D. A. Reis, R. P. Prasankumar
Phys. Rev. Lett. 128, 155301 (2022).
Using femtosecond time-resolved x-ray diffraction, we investigated optically excited coherent acoustic phonons in the Weyl semimetal TaAs. The low symmetry of the (112) surface probed in our experiment enables the simultaneous excitation of longitudinal and shear acoustic modes, whose dispersion closely matches our simulations. We observed an asymmetry in the spectral line shape of the longitudinal mode that is notably absent from the shear mode, suggesting a time-dependent frequency chirp that is likely driven by photoinduced carrier diffusion. We argue on the basis of symmetry that these acoustic deformations can transiently alter the electronic structure near the Weyl points and support this with model calculations. Our study underscores the benefit of using off-axis crystal orientations when optically exciting acoustic deformations in topological semimetals, allowing one to transiently change their crystal and electronic structures.
2021
T. Parpiiev, A. Hillion, V. Vlasov, V. Gusev, K. Dumesnil, T. Hauet, S. Andrieu, A. Anane, T. Pezeril
Phys. Rev. B 104, 224426 (2021).
We experimentally and analytically investigate laser excitation of picosecond longitudinal and shear acoustic pulses in highly magnetostrictive terfenol-TbFe 2-epitaxial thin films. We report a magnetoelastic mechanism for picosecond longitudinal and shear acoustic strain excitation, so-called direct transient demagnetostriction, which denotes the ultrafast laser-induced demagnetization and release of static magnetostrictive built-in strains. From analytical considerations and numerical modeling, we demonstrate that the efficiency of ultrashort longitudinal and shear acoustic strains generation strongly depends on the magnetization orientation with respect to the lattice crystallographic directions. Overall, our study provides insight into ultrafast acoustic waves excitation and detection in all sorts of magnetic materials from laser-based mechanisms.
2020
O. Matsuda, K. Tsutsui, G. Vaudel , T. Pezeril, K. Fujita , V. Gusev
Phys. Rev. B 101, 224307 (2020).
Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches in the material both compression/dilatation (longitudinal) and shear coherent acoustic pulses in directions of different orders of acoustic diffraction. The propagation of the emitted acoustic pulses can be monitored by measuring the variation of the optical reflectivity of the time-delayed ultrashort probe laser pulses. The direction of probe light incidence and its polarization rela- tive to the sample surface as well as the orientation of the metallic grating should be specifically chosen for efficient Brillouin scattering of the probe light from shear phonons propagating in the elastically isotropic materials. As theoretically predicted, the obtained experimental data contain multiple frequency components which are due to a variety of possible Brillouin scattering angles for both shear and compression/dilatation coherent acoustic waves. All these different frequency components are explained through multiplexing the propagation directions of probe light and coherent sound by the metallic diffraction grating. Our experimental scheme of time-domain Brillouin scattering with metallic gratings operating in reflection mode provides access to monitoring the shear acoustic waves launched in the direction of the first diffraction order by backward Brillouin scattering process. Applications include simultaneous determination of several different acoustic mode velocities and optical refractive index and, potentially, measurements of the acoustic dispersion of samples with a single direction of possible optical access.
Y. Wang, D. Hurley, Z. Hua, T. Pezeril, S. Raetz, V. Gusev, V. Tournat, M. Khafizov
Nat. Comm. 11, 1597 (2020).
Characterization of microstructure, chemistry and function of advanced energy materials remains a challenge for instrumentation science with tremendous opportunity for new discoveries. This active area of research is starting to make considerable strides with examples ranging from methodologies that employ bright X-rays, electron microscopy and optical spectroscopy. However, further development of instruments, capable of multimodal measurements, is necessary to fully reveal complex microstructure evolution in realistic environments and dynamical systems that are far from equilibrium. In this regard, laser-based instruments have a unique advantage as multiple measurement methodologies are easily combined into a single instrument using standard optics. A new pump-probe method that uses optically generated acoustic phonons is expanding standard optical characterization by providing depth resolved information. Here we report on a noteworthy extension of this pump-probe method to image grain microstructure in ceria. Rich information regarding the orientation of individual crystallites is obtained by monitoring acoustic phonon velocities and noting how the polarization of the probe beam influences the detected signal amplitude. When paired with other optical microscopies, this methodology will provide new perspectives for optical characterization of ceramic materials.
I. Chaban, C. Klieber, R. Busselez, K. A. Nelson, T. Pezeril
J. Chem. Phys. 152, 014202 (2020).
We demonstrate that time-domain Brillouin scattering (TDBS), a technique based on an ultrafast pump-probe approach, is sensitive to phase transitions and apply it to the study of structural changes in 8CB liquid crystals at different temperatures across the isotropic, nematic, smectic, and crystalline phases. We investigate the viscoelastic properties of 8CB squeezed in a narrow gap, from the nanometer to submicrometer thickness range, and conclude on the long-range molecular structuring of the smectic phase. These TDBS results reveal that confinement effects favor structuring of the smectic phase into a crystallinelike phase that can be observed at wide distances far beyond the molecular dimensions.
2019
S. P. Zeuschner, T. Parpiiev, T. Pezeril, A. Hillion, K. Dumesnil, M. Anane, J. Pudell, L. Willig, M. Rossle, M. Herzog, A. von Reppert, M. Bargheer
Structural Dynamics 6, 024302 (2019).
We combine ultrafast X-ray diffraction (UXRD) and time-resolved Magneto-Optical Kerr Effect (MOKE) measurements to monitor the strain pulses in laser-excited TbFe2/Nb heterostructures. Spatial separation of the Nb detection layer from the laser excitation region allows for a background free characterization of the laser-generated strain pulses. We clearly observe symmetric bipolar strain pulses when the excited TbFe2 surface terminates the sample and a decomposition of the strain wavepacket into an asymmetric bipolar and a unipolar pulse, when a SiO2 glass capping layer covers the excited TbFe2 layer. The inverse magnetostriction of the temporally separated unipolar strain pulses in this sample leads to a MOKE signal that linearly depends on the strain pulse amplitude measured through UXRD. Linear chain model simulations accurately predict the timing and shape of UXRD and MOKE signals that are caused by the strain reflections from multiple interfaces in the heterostructure.
R. Salikhov, A. Alekhin, T. Parpiiev, T. Pezeril, D. Makarov, R. Abrudan, R. Meckenstock, F. Radu, M. Farle, H. Zabel, V. Temnov
Phys. Rev. B 99, 104412 (2019).
Engineering the magnetic properties (Gilbert damping, saturation magnetization, exchange stiff- ness and magnetic anisotropy) of multicomponent magnetic compounds plays a key role in funda- mental magnetism and its applications. Here we perform a systematic study of (Ni81Fe19)100−xGdx films with x = 0, 5, 9 and 13 % using ferromagnetic resonance (FMR), element-specific time-resolved X-ray magnetic resonance and femtosecond time-resolved magneto-optical pump-probe techniques. The comparative analysis of field and time domain FMR methods, with complimentary information extracted from the dynamics of high-frequency exchange magnons in ferromagnetic thin films, is used to investigate the dependence of Gilbert damping on the Gd concentration.
I. Chaban, H. Shin, C. Klieber, R. Busselez, V. Gusev, Keith A. Nelson, T. Pezeril
MRS Advances 4, 9 (2019).
We present results of time-domain Brillouin scattering (TDBS) to determine the local temperature of liquids. TDBS is based on an ultrafast pump-probe technique to determine the light scattering frequency shift caused by the propagation of coherent acoustic waves in a sample. Since the temperature influences the Brillouin scattering frequency shift, the TDBS signal probes the local temperature of the liquid. Results for the extracted Brillouin scattering frequencies recorded at different liquid temperatures and at different laser powers are shown to demonstrate the usefulness of TDBS as a temperature probe.
D. Veysset, S. E. Kooi, R. Haferssas, M. Hassani-Gangaraj, M. Islam, A. Maznev, Y. Chernukha, X. Zhao, K. Nakagawa, D. Martynowich, X. Zhang, A. Lomonosov, C. Schuh, R. Radovitzky, T. Pezeril, Keith A. Nelson
Scr. Mater. 158, 42 (2019).
Dynamic fracture of borosilicate glass through focusing of high-amplitude nanosecond surface acoustic waves (SAWs) at the micron scale is investigated in an all-optical experiment. SAWs are generated by an intense picosecond laser excitation pulse focused into a ring-shaped spot on the sample surface. Interferometric imaging of the sample surface with a femtosecond probe pulse allows quantitative measurement of the vertical surface displacements. Images taken at different pump-probe delays capture the SAW as it converges towards the center, focuses, and subsequently diverges. The amplitude of the SAW is varied by changing the laser excitation energy. Above a certain laser excitation energy threshold, a damage at the acoustic focal point is observed. A post-mortem investigation reveals a crater formed by the ejection of a piece of material. Numerical calculations matching the measured surface displacement profiles help us determine the time evolution of the stress distribution in the sample and at the acoustic focus. We find that the glass withstands a nanosecond local tensile stress of at least 6 GPa without visible fracture, which is well above the tensile limit observed in conventional shock spallation experiments.
2018
V. Juvé, G. Vaudel, Z. Ollmann, J. Hebling, V. Temnov, V. Gusev, T. Pezeril
Optics Letters 43, 5905 (2018).
Controlling light polarization is one of the most essential routines in modern optical technology. Since the demonstration of optical pulse shaping by spatial light modulators and its potential in controlling quantum reaction path, it paved the way to many applications as coherent control of photoionization process, space-time control of propagating phonons or as polarization shaping of Terahertz (THz) pulses. Here we evidenced efficient non- resonant and noncollinear χ(2)-type light-matter interaction in femtoseconds polarization sensitive time-resolved optical measurements. Such nonlinear optical interaction of visible light and ultra-short THz pulses leads to THz modulation of visible light polarization in bulk LiNbO3 crystal. Theoretical simulations based on the wave propagation equation capture the physical processes underlying this astonishing effect. Apart from the observed tunable polarization modulation at ultra-high frequencies of visible pulses, this new physical phenomenon can be envisaged in THz depth-profiling of materials.
D. Veysset, U. Gutiérez-Hernandez, L. Dresselhaus-Cooper, F. De Colle, S. Kooi, K. A. Nelson, P. A. Quinto-Su, T. Pezeril
Phys. Rev. E 97, 053112 (2018).
In this study a single laser pulse spatially shaped into a ring is focused into a thin water layer, creating an annular cavitation bubble and cylindrical shock waves: an outer shock that diverges away from the excitation laser ring and an inner shock that focuses towards the center. A few nanoseconds after the converging shock reaches the focus and diverges away from the center, a single bubble nucleates at the center. The inner diverging shock then reaches the surface of the annular laser-induced bubble and reflects at the boundary, initiating nucleation of a tertiary bubble cloud. In the present experiments, we have performed time-resolved imaging of shock propagation and bubble wall motion. Our experimental observations of single-bubble cavitation and collapse and appearance of ring-shaped bubble clouds are consistent with our numerical simulations that solve a one-dimensional Euler equation in cylindrical coordinates. The numerical results agree qualitatively with the experimental observations of the appearance and growth of large bubble clouds at the smallest laser excitation rings. Our technique of shock-driven bubble cavitation opens interesting perspectives for the investigation of shock-induced single-bubble or multibubble cavitation phenomena in thin liquids.
O. Matsuda, T. Pezeril, I. Chaban, K. Fulita, V. Gusev
Phys. Rev. B 97, 064301 (2018).
Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches coherent compression/dilatation acoustic pulses in directions of different orders of acoustic diffraction. Their propagation is detected by delayed laser pulses, which are also diffracted by the metallic grating, through the measurement of the transient intensity change of the first order diffracted light. The obtained data contain multiple frequency components which are interpreted by considering all possible angles for the Brillouin scattering of light achieved through multiplexing of the propagation directions of light and coherent sound by the metallic grating. The emitted acoustic field can be equivalently presented as a superposition of plane inhomogeneous acoustic waves, which constitute an acoustic diffraction grating for the probe light. Thus, the obtained results can also be interpreted as a consequence of probe light diffraction by both metallic and acoustic gratings. The realized scheme of time-domain Brillouin scattering with metallic gratings operating in reflection mode provides access to wide range of acoustic frequencies from minimal to maximal possible values in a single experimental optical configuration for the directions of probe light incidence and scattered light detection. This is achieved by monitoring the backward and forward Brillouin scattering processes in parallel. Potential applications include measurements of the acoustic dispersion, simultaneous determination of sound velocity and optical refractive index, and evaluation of samples with a single direction of possible optical access.
2017
Ievgeniia Chaban
The phenomenon of liquid structuring near interfaces is related to the liq- uid/interface interaction forces at distances of some molecular dimensions. Despite the fact that this universal structuring effect plays a key role in various fields such as heat transport, particle transport through biological membranes, nanofluidics, micro- biology and nanorheology, the experimental investigation of liquid structuring remains challenging.
The aim of this PhD thesis is the experimental study of the structuring/ordering of liquids at nanoscale distances from their interfaces with solids. In this context, we have adapted the experimental technique of picosecond laser ultrasonics to inves- tigate high-frequency longitudinal acoustic properties of ultrathin liquids (glycerol, OMCTS, liquid crystals) confined between solid surfaces of different types. At first, we will present results of time-domain Brillouin scattering (TDBS) used to determine the temperature distribution profile in the investigated liquid volume which can be extrapolated to nanometer dimensions. Results for the evolution of the extracted Brillouin scattering frequencies and attenuation rates recorded at different laser pow- ers give insight to the intrinsic relationship between thermal and mechanical properties of liquids. Second, we will describe our results for the measurements of mechanical properties of ultrathin liquids with a nanometric resolution. Fourier analysis of the recorded TDBS signals for different liquid thicknesses yield the value of the longitu- dinal speed of sound and attenuation at GHz frequencies. These are compared to the results of separate measurements of bulk liquids at ambient temperature. This novel TDBS experimental scheme is a first step towards the understanding of confined liquids measured by GHz ultrasonic probing.
Tymur Parpiiev
The advent of femtosecond lasers has been at the origin of many discoveries in solid state physics. In particular, in the field of magnetism it became possible to measure how femtosecond optical demagnetization can probe the exchange interaction in ferromag- netic metals. Laser-induced demagnetization of materials with strong magneto-elastic coupling should lead to the release of its build-in strains, thus to the generation of both longitudinal and shear acoustic waves. The physical mechanism is known as direct magnetostriction, which is the property of ferromagnetic materials to change their shape or dimensions during the process of magnetization/demagnetization. The inverse phe- nomenon appears when an applied external stress modifies statically or dynamically the magnetization configuration of a ferromagnet. A number of new physical phenomena are expected to arise when an ultrafast acoustic pulse, exited by absorption of a femtosecond laser pulse in photoacoustic transducer, is injected to a ferromagnetic sample.
In this thesis, generation of shear picosecond acoustic pulses in strongly magne- tostrictive materials such as Terfenol is processed analytically and shown experimentally. In case of Terfenol with strong magneto-crystalline anisotropy, laser-induced demagne- tostriction is responsible for shear waves excitation.
First, the phenomenological model of direct magnetostriction in a Terfenol monocrys- talline film is developed. Transversal strain generation eciency strongly depends on the orientation of the film magnetization. Time-resolved linear MOKE pump-probe ex- periments show that transient laser-induced release of the magnetoelastic strains in the films lead to the excitation of GHz longitudinal and shear acoustic waves. These results are the first experimental observation of picosecond shear acoustic wave excitation by laser-induced demagnetostriction mechanism.
Second, the interaction of an optically generated longitudinal acoustic wave with the magnetization of a Terfenol thin film is reported. Arrival of the ps strain wave alters a change of its magnetization and leads to the acoustic mode conversion. Which is an- other pathway of shear acoustic wave generation. This e↵ect highlight a strong coupling of elastic and spin subsystems in Terfenol. Hence, there are two laser-induced mecha- nisms of the shear wave excitation in materials with strong magneto-elastic coupling: (i) transient demagnetostriction and (ii) acoustic mode conversion. The Frequency band- width of the generated acoustic pulses matches the demagnetization timescale and lies in the range of several hundreds of GHz, close to 1 THz.
These experimental and theoretical results could have large impacts in several fields of applications: Brillouin spectroscopy at GHz-THz frequencies with both longitudinal and shear acoustic polarizations, magnetic recording with ultrafast longitudinal and shear acoustic waves.
T. Parpiiev, M. Servol, M. Lorenc, I. Chaban, R. Lefort, E. Collet, H. Cailleau, P. Ruello, N. Daro, G. Chastanet, T. Pezeril
Appl. Phys. Lett. 111, 151901 (2017).
We report GHz longitudinal as well as shear acoustic phonons photoexcitation and photodetec- tion using femtosecond laser pulses in a spin-crossover molecular crystal. From our experimental observation of time domain Brillouin scattering triggered by the photoexcitation of acoustic waves across the low-spin (LS) to high-spin (HS) thermal crossover, we reveal a link between molecular spin state and photoexcitation of coherent GHz acoustic phonons. In particular, we experimentally evidence a non-thermal pathway for the laser excitation of GHz phonons. We also provide experi- mental insights to the optical and mechanical parameters evolving across the LS/HS spin crossover temperature range.
I. Chaban, H. Shin, C. Klieber, R. Busselez, V. Gusev, Keith A. Nelson, T. Pezeril
Rev. Sci. Instrum. 88, 074904 (2017).
We present an optical technique based on ultrafast photoacoustics to determine the local temperature distribution profile in liquid samples in contact with a laser heated optical transducer. This ultra- fast pump-probe experiment uses time-domain Brillouin scattering (TDBS) to locally determine the light scattering frequency shift. As the temperature influences the Brillouin scattering frequency, the TDBS signal probes the local laser-induced temperature distribution in the liquid. We demonstrate the relevance and the sensitivity of this technique for the measurement of the absolute laser-induced temperature gradient of a glass forming liquid prototype, glycerol, at different laser pump powers— i.e., different steady state background temperatures. Complementarily, our experiments illustrate how this TDBS technique can be applied to measure thermal diffusion in complex multilayer systems in contact with a surrounding liquid.
D. Veysset, A. Maznev, István A. Veres, T. Pezeril, S. Kooi, Alexey M. Lomonosov, Keith A. Nelson
Appl. Phys. Lett. 111, 031901 (2017).
Focusing of high-amplitude surface acoustic waves leading to material damage is visualized in an all-optical experiment. The optical setup includes a lens and an axicon that focuses an intense picosecond excitation pulse into a ring-shaped pattern at the surface of a gold-coated glass substrate. Optical excitation induces a surface acoustic wave (SAW) that propagates in the plane of the sample and converges toward the center. The evolution of the SAW profile is monitored using interferometry with a femtosecond probe pulse at variable time delays. The quantitative analysis of the full-field images provides direct information about the surface displacement profiles, which are compared to calculations. The high stress at the focal point leads to the removal of the gold coating and, at higher excitation energies, to damage of the glass substrate. The results open the prospect for testing material strength on the microscale using laser-generated SAWs.
2016
D. Veysset, A. Maznev, T. Pezeril, S. Kooi, Keith A. Nelson
Scientific Reports 6, 24 (2016).
Shock waves in condensed matter are of great importance for many areas of science and technology ranging from inertially confined fusion to planetary science and medicine. In laboratory studies of shock waves, there is a need in developing diagnostic techniques capable of measuring parameters of materials under shock with high spatial resolution. Here, time-resolved interferometric imaging is used to study laser-driven focusing shock waves in a thin liquid layer in an all-optical experiment. Shock waves are generated in a 10 µm-thick layer of water by focusing intense picosecond laser pulses into a ring of 95 µm radius. Using a Mach-Zehnder interferometer and time-delayed femtosecond laser pulses, we obtain a series of images tracing the shock wave as it converges at the center of the ring before reemerging as a diverging shock, resulting in the formation of a cavitation bubble. Through quantitative analysis of the interferograms, density profiles of shocked samples are extracted. The experimental geometry used in our study opens prospects for spatially resolved spectroscopic studies of materials under shock compression.
M. Kouyaté, T. Pezeril, V. Gusev, O. Matsuda
J. Opt. Soc. Am. B 33, 2634 (2016).
A theoretical formalism describing the heterodyne detection of plane inhomogeneous shear acoustic waves by probe laser-induced gratings is developed. An inhomogeneous plane shear acoustic wave is a purely shear wave with a plane phase front and mechanical displacement vector, which is sinusoidally spatially modulated in magnitude. It could be generated by laser-induced gratings and could be useful for the acoustic testing of sub-micrometer thick membranes and coatings in the gigahertz frequency range. The theory reveals the potential advantages and disadvantages of this wave application in non-destructive testing of materials and fundamental research from the point of view of the feasibility of their experimental detection.
V. Temnov, I. Razdolski, T. Pezeril, D. Makarov, D. Seletskiy, A. Melnikov, Keith A Nelson
Journal of Optics 18, 093002 (2016).
We review the recent progress in experimental and theoretical research of interactions between the acoustic, magnetic and plasmonic transients in hybrid metal-ferromagnet multilayer structures excited by ultrashort laser pulses. The main focus is on understanding the nonlinear aspects of the acoustic dynamics in materials as well as the peculiarities in the nonlinear optical and magneto-optical response. For example, the nonlinear optical detection is illustrated in detail by probing the static magneto-optical second harmonic generation in gold–cobalt–silver trilayer structures in Kretschmann geometry. Furthermore, we show experimentally how the nonlinear reshaping of giant ultrashort acoustic pulses propagating in gold can be quantified by time-resolved plasmonic interferometry and how these ultrashort optical pulses dynamically modulate the optical nonlinearities. An effective medium approximation for the optical properties of hybrid multilayers enables the understanding of novel optical detection techniques. In the discussion we also highlight recent works on the nonlinear magneto-elastic interactions, and strain-induced effects in semiconductor quantum dots.
T. Pezeril
Opt. & Laser Tech. 83, 177 (2016).
The aim of this article is to provide an overview of the up-to-date findings related to ultrafast shear acoustic waves. Recent progress obtained for the laser generation and detection of picosecond shear acoustic waves in solids and liquids is reviewed. Examples in which the transverse isotropic symmetry of the sample structure is broken in order to permit shear acoustic wave generation through sudden laser heating are described in detail. Alternative photo-induced mechanisms for ultrafast shear acoustic generation in metals, semiconductors, insulators, magnetostrictive, piezoelectric and electrostrictive materials are reviewed as well. With reference to key experiments, an all-optical technique employed to probe longitudinal and shear structural dynamics in the GHz frequency range in ultra-thin liquid films is described. This technique, based on specific ultrafast shear acoustic transducers, has opened new perspectives that will be discussed for ultrafast shear acoustic probing of viscoelastic liquids at the nanometer scale.
M. Lejman, G. Vaudel, I. C. Infante, I. Chaban, T. Pezeril, M. Edely, G. F. Nataf, M. Guennou, J. Kreisel, V. E. Gusev, B. Dkhil, P. Ruello
Nature Communications 7, 12345 (2016).
The ability to generate efficient giga–terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse, we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO3 and LiNbO3. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics.
J. Janušonis, T. Jansma, C. L. Chang, Q. Liu, A. Gatilova, A. M. Lomonosov, V. Shalagatskyi, T. Pezeril, V. V. Temnov, R. I. Tobey
Scientific Reports 6, 29143 (2016).
Surface magnetoelastic waves are coupled elastic and magnetic excitations that propagate along the surface of a magnetic material. Ultrafast optical techniques allow for a non-contact excitation and detection scheme while providing the ability to measure both elastic and magnetic components individually. Here we describe a simple setup suitable for excitation and time resolved measurements of high frequency magnetoelastic waves, which is based on the transient grating technique. The elastic dynamics are measured by diffracting a probe laser pulse from the long-wavelength spatially periodic structural deformation. Simultaneously, a magnetooptical measurement, either Faraday or Kerr effect, is sensitive to the out-of-plane magnetization component. The correspondence in the response of the two channels probes the resonant interaction between the two degrees of freedom and reveals their intimate coupling. Unraveling the observed dynamics requires a detailed understanding of the spatio-temporal evolution of temperature, magnetization and thermo-elastic strain in the ferromagnet. Numerical solution of thermal diffusion in two dimensions provides the basis on which to understand the sensitivity in the magnetooptic detection.
S. Volz, J. Ordonez-Miranda, A. Shchepetov, M. Prunnila, J. Ahopelto, T. Pezeril and 17 more
Eur. Phys. J. B 89, 15 (2016).
Understanding and controlling vibrations in condensed matter is emerging as an essential necessity both at fundamental level and for the development of a broad variety of technological applications. Intelligent design of the band structure and transport properties of phonons at the nanoscale and of their interactions with electrons and photons impact the efficiency of nanoelectronic systems and thermoelectric materials, permit the exploration of quantum phenomena with micro- and nanoscale resonators, and provide new tools for spectroscopy and imaging. In this colloquium we assess the state of the art of nanophononics, describing the recent achievements and the open challenges in nanoscale heat transport, coherent phonon generation and exploitation, and in nano- and optomechanics. We also underline the links among the diverse communities involved in the study of nanoscale phonons, pointing out the common goals and opportunities.
2015
P. Ruello, A. Ayouch, G. Vaudel, T. Pezeril, N. Delorme, S. Sato, K. Kimura, and V. E. Gusev
Phys. Rev. B 92, 174304 (2015).
We report the investigation of the generation and detection of GHz coherent acoustic phonons in plasmonic gold nanoparticle superlattices (NPSs). The experiments have been performed with an optical femtosecond pump-probe scheme across the optical plasmon resonance of the superlattice. Our experiments allow us to estimate first the fundamental mechanical parameters such as the collective elastic response (sound velocity) of the NPS and the nanocontact elastic stiffness. Furthermore, it appears that the light-induced coherent acoustic-phonon pulse has a typical in-depth spatial extension of about 45 nm which is roughly four times the optical skin depth in gold. The modeling of the transient optical reflectivity indicates that the mechanism of phonons generation is achieved through ultrafast heating of the NPS assisted by light excitation of the volume plasmon polariton. Based on these results, we demonstrate that it is possible to map the photon-electron-phonon interaction in subwavelength nanostructures which, in particular, provides insights on the fundamental properties of these nanometamaterials.
D. Veysset, T. Pezeril, S. Kooi, A. Bulou, Keith A. Nelson
Appl. Phys. Lett. 106, 161902 (2015).
We demonstrate that in-plane 2D propagation and focusing of a laser-induced shock wave result in enhanced nano-crystallization of highly ordered pyrolytic graphite. Throughout the 2D shock focusing technique, which enables to clearly distinguish between the laser-induced and the shock-induced transformation/transition, our findings establish the role of the shock wave during the transformation/transition process. This configuration could open the way to an alternative path for laser shock fabrication of graphitic compounds and would give access to real time investigation of shock waves mediated phase transitions.
C. Klieber, V. Gusev, T. Pezeril, Keith A. Nelson
Phys. Rev. Lett. 114, 065701 (2015).
Using a picosecond pump-probe ultrasonic technique, we study the propagation of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the viscoelastic fragile glass former DC704. We observe an increase of almost 10% in acoustic pulse propagation speed at the highest optical pump fluence which is a result of the supersonic nature of nonlinear propagation in the viscous medium. From our measurement, we deduce the nonlinear acoustic parameter of the glass former in the gigahertz frequency range across the glass transition temperature.
2014
G. Vaudel, T. Pezeril, A. Lomonosov, M. Lejman, P. Ruello, V. Gusev
Phys. Rev. B 90, 014302 (2014).
We experimentally demonstrate the optical generation of hypersound in a piezoelectric GaAs semiconductor through laser excitation of the THz photo-Dember electric field. Such an ultrashort transient Dember electric field is linked to the spatial separation of photoexcited electrons and holes right after above-band-gap femtosecond laser excitation. Through time-domain coherent Brillouin scattering we demonstrate that photoinduced piezoelectric generation of hypersound can dominate even in the absence of preexisting built-in fields and we observe, with increasing laser fluence, a nonlinear optoacoustic excitation process. These results reveal the onset of the THz photo-Dember electric field and highlight the transition from non-ambipolar flow to ambipolar diffusion of the photoexcited electron-hole plasma.
in glycerol glass-former
R. Busselez, T. Pezeril, V. Gusev
J. Chem. Phys. 140, 234505 (2014).
By means of large scale molecular dynamics simulations, we explore mesoscopic properties of pro- totypical glycerol glass-former above and below the glass transition. The model used, in excellent agreement with various experimental techniques, permits to carefully study the structure and the vi- brational dynamics. We find that a medium range order is present in glycerol glass-former and arises from hydrogen bond network extension. The characteristic size of the structural heterogeneities is re- lated to the anomalous properties of acoustic vibrations (Rayleigh scattering, “mode softening,” and Boson Peak) in the glassy state. Finally the characteristic size of these heterogeneities, nearly con- stant in temperature, is also connected to the cross-over between structural relaxation and diffusion in liquid glycerol.
T. Pezeril, C. Klieber, V. Shalagatskyi, G. Vaudel, V. Temnov, O. G. Schmidt, D. Makarov
Opt. Express 22, 4590 (2014).
We have developed a high-sensitivity, low-noise femtosecond imaging technique based on pump-probe time-resolved measurements with a standard CCD camera. The approach used in the experiment is based on lock-in acquisitions of images generated by a femtosecond laser probe synchronized to modulation of a femtosecond laser pump at the same rate. This technique allows time-resolved imaging of laser-excited phenomena with femtosecond time resolution. We illustrate the technique by time-resolved imaging of the nonlinear reshaping of a laser-excited picosecond acoustic pulse after propagation through a thin gold layer. Image analysis reveals the direct 2D visualization of the nonlinear acoustic propagation of the picosecond acoustic pulse. Many ultrafast pump-probe investigations can profit from this technique because of the wealth of information it provides over a typical single diode and lock-in amplifier setup, for example it can be used to image ultrasonic echoes in biological samples.
M. Lejman, V. Shalagatskyi, O. Kovalenko, T. Pezeril, V. V. Temnov, P. Ruello
J. Opt. Soc. Am. B 31, 282 (2014).
Ultrafast laser-excited hot electrons in metals can transport energy supersonically far from the region where they are initially produced. We show that this ultrafast energy transport is responsible for the emission of coherent acoustic phonons deep beneath the free surface of a weak electron–phonon coupling copper metal sample. Special attention is taken to investigate the interaction between superdiffusive hot electrons at the bi-metallic buried interface (Cu–Ti). To discuss the underlying physics and the ultrafast transient optical properties, several con- figurations developed in the frame of ultrafast optical pump–probe technique have been used. In particular, we have performed backward and forward detection of both coherent acoustic phonons and superdiffusive hot electrons. From an original probe wavelength dependence study of the optical detection process, we clearly establish the signature of superdiffusive hot transport within the copper film and the link with the acoustic phonon emission. A comparison with a strong electron–phonon coupling metal, like titanium, where there is no super- diffusive transport is also provided. These results and observations are important to quantify the role of super- diffusive carriers in ultrafast energy transport, which is involved in different processes in solid state physics or femtochemistry.
2013
O. Kovalenko, T. Pezeril, V. Temnov
Phys. Rev. Lett. 110, 266602 (2013).
It is shown theoretically that a single acoustic pulse, a few picoseconds long, can reverse magnetization in a magnetostrictive material Terfenol-D. Following giant magnetoelastic changes of free energy density, the magnetization vector is ejected from a local in-plane energy minimum and decays into another minimum. For an acoustic pulse duration significantly shorter than magnetization precession period Tac<<Tprec, the switching threshold is determined by the acoustic pulse area, i.e., pulse integral in the time domain, similar to coherent phenomena in optics. Simulation results are summarized in a magneto- acoustic switching diagram and discussed in the context of all-optical magnetization switching by circularly polarized light pulses.
C. Klieber, T. Hecksher, T. Pezeril, D. Torchinsky, J. Dyre, Keith A. Nelson
J. Chem. Phys. 138, 12A544 (2013).
This paper presents and discusses the temperature and frequency dependence of the longitudinal and shear viscoelastic response at MHz and GHz frequencies of the intermediate glass former glycerol and the fragile glass former tetramethyl-tetraphenyl-trisiloxane (DC704). Measurements were performed using the recently developed time-domain Brillouin scattering technique, in which acoustic waves are generated optically, propagated through nm thin liquid layers of different thicknesses, and detected optically after transmission into a transparent detection substrate. This allows for a determination of the frequency dependence of the speed of sound and the sound-wave attenuation. When the data are converted into mechanical moduli, a linear relationship between longitudinal and shear acoustic moduli is revealed, which is consistent with the generalized Cauchy relation. In glycerol, the temperature dependence of the shear acoustic relaxation time agrees well with literature data for dielectric measurements. In DC704, combining the new data with data from measurements obtained previously by piezo-ceramic transducers yields figures showing the longitudinal and shear sound velocities at frequencies from mHz to GHz over an extended range of temperatures. The shoving model's prediction for the relaxation time's temperature dependence is fairly well obeyed for both liquids as demonstrated from a plot with no adjustable parameters. Finally, we show that for both liquids the instantaneous shear modulus follows an exponential temperature dependence to a good approximation, as predicted by Granato's interstitialcy model.
2012
J.-H. Lee, D. Veysset, J. P. Singer, M. Retsch, G. Saini, T. Pezeril, K. A. Nelson, E. L. Thomas
Nature Communications 3, 1164 (2012).
Insight into the mechanical behaviour of nanomaterials under the extreme condition of very high deformation rates and to very large strains is needed to provide improved understanding for the development of new protective materials. Applications include protection against bullets for body armour, micrometeorites for satellites, and high-speed particle impact for jet engine turbine blades. Here we use a microscopic ballistic test to report the responses of periodic glassy-rubbery layered block-copolymer nanostructures to impact from hypervelocity micron-sized silica spheres. Entire deformation fields are experimentally visualized at an exceptionally high resolution (below 10 nm) and we discover how the microstructure dissipates the impact energy via layer kinking, layer compression, extreme chain conformational flattening, domain fragmentation and segmental mixing to form a liquid phase. Orientation-dependent experiments show that the dissipation can be enhanced by 30% by proper orientation of the layers.
C. Klieber, T. Pezeril, S. Andrieu, and Keith A. Nelson
J. Appl. Phys. 112, 013502 (2012).
We describe an adaptation of picosecond laser ultrasonics tailored for study of GHz-frequency longitudinal and shear acoustic waves in liquids. Time-domain coherent Brillouin scattering is used to detect multicycle acoustic waves after their propagation through variable thickness liquid layers into a solid substrate. A specialized optical pulse shaping method is used to generate sequences of pulses whose repetition rate determines the acoustic frequency. The measurements reveal the viscoelastic liquid properties and also include signatures of the optical and acoustic cavities formed by the multilayer sample assembly. Modeling of the signals allows their features to be distinguished so that liquid properties can be extracted reliably. Longitudinal and shear acoustic wave data from glycerol and from the silicon oil DC704 are presented.
P. Ruello, T. Pezeril, S. Avanesyan, G. Vaudel, V. Gusev, I.C. Infante, B. Dkhil
Appl. Phys. Lett. 100, 212906 (2012).
Using femtosecond laser pulses, coherent GHz acoustic phonons are efficiently photogenerated and photodetected in BiFeO3 (BFO) multiferroic single crystal. Due to the crystal lattice symmetry, longitudinal as well as two transverse acoustic modes are generated and detected, and the corresponding sound velocities are determined. This provides the opportunity to experimentally evaluate the elastic coefficients of the multiferroic compound BiFeO3 that have been estimated so far only through ab initio calculations. The knowledge of the elastic properties of BFO is highly desired for BFO integration in nanoelectronic devices. Moreover, our findings highlight also that BFO may be a good candidate for light-controlled coherent acoustic phonons sources.
M. Kouyate, T. Pezeril, D. Mounier, V. Gusev
J. Appl. Phys. 110, 123526 (2012).
The detailed theoretical description of how picosecond plane shear acoustic fronts can be excited by ultrafast lasers at the interface of two isotropic media, a transparent medium and an opaque medium, is presented. The processes leading to the emission of inhomogeneous plane bulk shear acoustic modes from the interaction at the interface of plane inhomogeneous compression/dilatation modes thermoelastically generated by laser interference gratings are analyzed. The theory describes the basic features of the spectral transformation function of the laser light conversion into shear modes and predicts an interval of frequencies where it is possible to achieve the emission into the transparent medium of propagating shear inhomogeneous modes only, while the compression/dilatation inhomogeneous modes will be evanescent and will be localized at the interface. The guidelines for optimal choice of the materials, with the goal of improving the amplitude of the photoexcited picosecond shear acoustic fronts are proposed. All-optical monitoring, i.e., excitation and detection, by fs-ps laser pulses of picosecond plane inhomogeneous shear acoustic fronts propagating in thin films and substrates can be applied for the noncontact determination of shear rigidity of materials.
2011
T. Pezeril, G. Saini, D. Veysset, S. Kooi, P. Fidkowski, R. Radovitzky, and Keith A. Nelson
Phys. Rev. Lett. 106, 214503 (2011).
Direct real-time visualization and measurement of laser-driven shock generation, propagation, and 2D focusing in a sample are demonstrated. A substantial increase of the pressure at the convergence of the cylindrical acoustic shock front is observed experimentally and simulated numerically. Single-shot acquisitions using a streak camera reveal that at the convergence of the shock wave in water the supersonic speed reaches Mach 6, corresponding to the multiple gigapascal pressure range 30 GPa.
C. Klieber, E. Peronne, K. Katayama, J. Choi, M. Yamaguchi, T. Pezeril, and K. A. Nelson
Appl. Phys. Lett. 98, 211908 (2011).
Acoustic attenuation rates in vitreous silica in the 20-400 GHz frequency range have been measured using a multiple-pulse optical technique for generation of tunable multicycle acoustic waves that are detected interferometrically after traversal of the sample. The results connect the frequency ranges of several measurement methods, yielding a consistent description of the acoustic behavior.
P. Babilotte, P. Ruello, T. Pezeril, G. Vaudel, D. Mounier, J.-M. Breteau, and V. Gusev
J. Appl. Phys. 109, 064909 (2011).
Both experiments with deeply penetrating femtosecond laser pulses and theoretical analysis demonstrate that at low laser fluences on (111) and (-1-1-1) surfaces of n-doped GaAs semiconductors the hypersound generation mechanism is the inverse piezoelectric effect. The transient electric field causing the inverse piezoelectric effect is due to the spatial separation in the built-in near-surface electric field of the electrons and holes photoexcited directly in the depletion region and also of those photoexcited outside the depletion region and diffusing toward it. However, with increasing laser fluence the amplitude of the acoustic signal generated by laser-induced transient electric fields saturates and the hypersound generation through electron–hole–phonon deformation potential mechanism becomes predominant. The peculiar dependencies of the hypersound amplitude and phase on pump laser fluence reveal the transition between the two physical mechanisms of optoacoustic conversion. The phase of the acoustic signal contains information on the temporal dynamics of the screening of the built-in electric field.
2010
P. Babilotte, P. Ruello, G. Vaudel, T. Pezeril, D. Mounier, J.-M. Breteau, and V. Gusev
Appl. Phys. Lett. 97, 174103 (2010).
We demonstrate by experiment and theoretical analysis that the presence of built-in electric fields
near the (111) and (-1 -1 -1) surfaces of p-doped GaAs causes efficient generation of acoustic waves due to the laser-induced inverse piezoelectric effect. At the same time, the generation efficiency from the electron-hole-phonon deformation potential is shown to be reduced. The polarity of the acoustic
pulse is inverted when changing the laser irradiated surface from (111) to (-1 -1 -1). The results have ramifications for optically controlled piezoelectric ultrasound transducers.
P. Babilotte, P. Ruello, D. Mounier, T. Pezeril, G. Vaudel, M. Edely, J-M. Breteau, V. Gusev, K. Blary
Phys. Rev. B 81, 245207 (2010).
The experimental detection of short picosecond photoacoustic response induced by femtosecond laser pump radiation deeply penetrating in GaAs and generating long acoustic strain pulse is reported. It is demonstrated that it is possible, in this case, to achieve high-frequency coherent acoustic phonon monitoring in semiconductors as efficiently as in the case of metals where the penetration depth of pump radiation is shorter and the generated acoustic strain pulse is short itself. The physical origin of such monitoring of high-frequency acoustic phonons is discussed thanks to a detailed analysis of the spectral transformation functions of both the generation process -opto-acoustic transformation- and the detection process -acousto-optic transformation. We show that it is possible to tune the detection process from a narrowband to a broadband spectrum detection process. In particular, the broadband detection process is achieved with a proper choice of the probe wavelength permitting efficient coupling between the acoustic field and the probe electromagnetic wave only in a nanometric thin region near semiconductor surface. Efficient broadband acousto-optic detection results in the short pulsed photoacoustic responses in transient optical reflectivity even when long acoustic pulses are generated, because of sufficient sensitivity to high-frequency coherent phonons even weakly contributing to the total acoustic field. These results indicate the opportunity to broaden the range of the substrate materials and pump laser frequencies that can be used in high-frequency coherent phonon spectroscopy of the materials and nanostructures.
D. Mounier, P. Picart, P. Babilotte, P. Ruello, J.-M. Breteau, T. Pézeril, G. Vaudel, M. Kouyaté, V. Gusev
Optics Express 18, 6767 (2010).
A theoretical analysis of the transient optical reflectivity of a sample by a normalized Jones matrix is presented. The off-diagonal components of the normalized matrix are identified with the complex rotation of the polarization ellipse. Transient optical polarimetry is a relevant technique to detect shear acoustic strain pulses propagating normally to the surface of an optically isotropic sample. Moreover, polarimetry has a selective sensitivity to shear waves, as this technique cannot detect longitudinal waves that propagate normally to the sample surface.
2009
T. Pezeril, C. Klieber, S. Andrieu, and K. A. Nelson
Phys. Rev. Lett. 102, 107402 (2009).
Picosecond laser ultrasonic techniques for acoustic wave generation and detection have been employed to probe shear acoustic waves in liquid glycerol at gigahertz frequencies. The experimental approach uses a unique laser pulse shaping technique and a crystallographically canted metal layer to generate frequency-tunable transverse acoustic waves, and uses time-domain coherent Brillouin scattering to detect the waves after they propagate through a liquid layer and into a solid substrate. A linear frequency dependence is found for both the shear speed and attenuation rate in glycerol.
2008
T. Pezeril, F. Leon, D. Chateigner, S. Kooi, Keith A. Nelson
Appl. Phys. Lett. 92, 061908 (2008).
The laser photoacoustic technique is used to generate and detect picosecond coherent acoustic vibrations in gold film media deposited on Si substrates. As a consequence of the gold crystallites’ canted orientation, the pump-probe picosecond transient reflectivity shows oscillations at the fundamental shear mode frequency. The shear character of the mode is suggested by its dispersion, by the dependence of the signal on the probe laser wavelength, and by x-ray texture analysis.
2007
T. Pezeril, P. Ruello, S. Gougeon, N. Chigarev, D. Mounier, J.-M. Breteau, P. Picart,
V. Gusev
Phys. Rev. B 75, 174307 (2007).
Hypersound generation and detection by laser pulses incident on the interface of an opaque anisotropic crystal are theoretically and experimentally investigated in the case where the symmetry is broken by a tilt of its axis of symmetry relative to the interface normal. A nonlocal volumetric mechanism of plane shear sound excitation is revealed and a modification of the temporal shape of the reflectivity signal with variation in probe light polarization is observed, both attributed to asynchronous propagation of the acoustic eigenmodes. Experiments and theory demonstrate the possibility of using polycrystalline materials with an arbitrary distribution of grain orientations for the generation and the detection of picosecond shear ultrasound.
G. Saini, T. Pezeril, D.H. Torchinsky, J. Yoon, S.E. Kooi, E.L. thomas, K.A. Nelson
J. Mat. Research 22, 719 (2007).
We investigated the acoustic properties in the sub-GHz frequency regime of a multilayer system comprising alternating 100-nm scale TiO2/poly(methyl methacrylate) (PMMA) layers through a laser photoacoustic method, impulsive stimulated thermal scattering (ISTS). The acoustic dispersion curves were determined, and the mechanical properties were extracted from the experimental results.
2006
T. Pezeril, N. Chigarev, P. Ruello, S. Gougeon, D. Mounier, J.-M. Breteau, P. Picart,
V. Gusev
Phys. Rev. B 73, 132301 (2006).
Two electromagnetic modes, ordinary and extraordinary, simultaneously propagating in a single crystal excite, via the optoacoustic effect, all three acoustic eigenmodes of the crystal, and, at a delayed time, detect these three modes via the acousto-optic effect. As a result of asynchronism in the propagation of the different acoustic modes, a volumetric, although a nonlocal excitation of shear sound is possible, even by spherical -isotropic- laser-induced stresses. The temporal profile of optical reflectivity, modified by sound, strongly depends on probe light polarization. It is shown that shear hypersound beams can be monitored by lasers, not only in single crystals in geometries with broken symmetry, but also in polycrystalline materials.
2005
P. Ruello, B. Perrin, T. Pezeril, V.E. Gusev, S. Gougeon, N. Chigarev, P. Laffez, P. Picart,
D. Mounier, J.M. Breteau
Physica B 363, 43 (2005).
A pump–probe optical spectroscopy technique was used to study the time-resolved reflectivity in the range 0–160ps of the metal–insulator transition compound NdNiO3. It enables, for the first time, the direct real-time observation of the non-equilibrium electron dynamics in the insulating and in the metallic state of that compound and the acoustical response. Strong change of the transient reflectivity is observed as a function of temperature. First of all, the magnitude of the fast component of transient reflectivity increases drastically when T<Tmi. Moreover, the relaxation time of the fast-component increases when the insulating state is reached. Vanishing of the acoustic echo magnitude with decreasing temperature is also reported. All these singular properties could be connected to the gap opening mechanism below MIT.
T. Pezeril, V. Gusev, D. Mounier, N. Chigarev, P. Ruello
J. Phys. D: Appl. Phys. 38, 1421 (2005).
Analytical solutions for the acoustic wave equations obtained by temporal Fourier and spatial Laplace transformations directly provide a description of the motion of the crystal surface caused by the spatially distributed laser heating of a semi-infinite crystal. Evaluation of the acoustic field in the bulk of the material is not needed here. In general, all three acoustic modes are excited due to the laser-induced thermoelastic effect and contribute to each of the three components of the transient surface displacement. Numerical simulations of the surface displacement as a function of time and crystal surface orientation are performed with the use of the analytical formulae derived in the case of a hexagonal crystal, for which only two modes are excited. The formulae obtained make it possible to optimize the orientation of the surface of the crystal in order to improve the efficiency of the excitation of the in-plane motion of the surface.
P. Laffez, X.Y. Chen, G. Banerjee, T. Pezeril, M.D. Rossell, G. Van Tendeloo, P. Lacorre, J.M. Liu, Z.G. Liu
Thin Solids Films 22, 21068 (2005).
Synthesis conditions of La2Mo2O9 thin film by radio frequency (RF) sputtering technique on Al2O3 ceramic substrates are studied. It is found that the deposition temperature and oxygen partial pressure are the most important factors for obtaining pure La2Mo2O9 films. Varying both parameters, Mo-rich, stoichiometric, and Mo-deficient films are obtained. With increasing the La:Mo ratio, films become denser. A crust layer is observed on top of the Mo-rich and the Mo-deficient films. The formation of the La2Mo2O9 phase is discussed with respect to the sputtering mechanism.