Picosecond Ultrasonic Technique Research Articles

Hypersound tomography of graphitized layers buried into diamond matrix

Acoustic properties of buried graphitized layers in diamond formed by ion implantation followed by annealing were studied using the picosecond ultrasonic technique with spatial resolution. Two methods of elastic pulse generation were used: heating an aluminum film deposited on a diamond sample by femtosecond laser pulses and direct illumination of the graphitized layers by these pulses. We applied a multilayered model of the acousto-optical response to fit experimental results and estimate the distribution of the acoustical parameters (wave resistance, viscoelastic damping, and longitudinal sound speed) of the structures under study in depth. It was found that unique sets of spectral lines are present in the Fourier spectra of measured responses in regions with different internal structures. Mapping of the Fourier spectra made it possible to visualize regions with different internal structures. The combined use of depth profiling and mapping can serve as a tool for hypersound tomography.

Temperature-dependent elastic constants of thorium dioxide probed using time-domain Brillouin scattering

We report the adiabatic elastic constants of single-crystal thorium dioxide over a temperature range of 77–350 K. Time-domain Brillouin scattering, an all-optical, non-contact picosecond ultrasonic technique, is used to generate and detect coherent acoustic phonons that propagate in the bulk perpendicular to the surface of the crystal. These coherent acoustic lattice vibrations have been monitored in two hydrothermally grown single-crystal thorium dioxide samples along the (100) and (311) crystallographic directions. The three independent elastic constants of the cubic crystal (C11, C12, and C44) are determined from the measured bulk acoustic velocities. The longitudinal wave along the (100) orientation provided a direct measurement of C11. Measurement of C44 and C12 was achieved by enhancing the intensity of quasi-shear mode in a (311) oriented crystal by adjusting the polarization angle relative to the crystal axes. We find the magnitude of softening of the three elastic constants to be ∼2.5% over the measured temperature range. Good agreement is found between the measured elastic constants with previously reported values at room temperature, and between the measured temperature-dependent bulk modulus with calculated values. We find that semi-empirical models capturing lattice anharmonicity adequately reproduce the observed trend. We also determine the acoustic Grüneisen anharmonicity parameter from the experimentally derived temperature-dependent bulk modulus and previously reported temperature-dependent values of volumetric thermal expansion coefficient and heat capacity. This work presents measurements of the temperature-dependent elasticity in single-crystal thorium dioxide at cryogenic temperature and provides a basis for testing ab initio theoretical models and evaluating the impact of anharmonicity on thermophysical properties.

3D Hypersound Microscopy of van der Waals Heterostructures.

The mechanical properties of the layered crystals in the few layer limit are largely unexplored. We employ a picosecond ultrasonic technique to access the corresponding mechanical parameters. Temporal variation of the reflection coefficient of the Al film that covers hBN/WSe2/hBN (where hBN is hexagonal boron nitride) heterostructures on a sapphire substrate after the femtosecond laser pulse excitation is carefully measured using an interferometric technique with spatial resolution. The laser pulse generates a broadband sound wave packet propagating perpendicularly to the Al plane and partially reflecting from the heterostructural interfaces. The demonstrated technique allows one to resolve a WSe2 monolayer embedded in hBN. We apply a multilayered model of the optoacoustical response to evaluate the mechanical parameters, in particular, the rigidity of the interfaces. Mapping of the Fourier spectra of the response visualizes different composition regions and may serve as an acoustic tomography tool. Almost zero phonon dissipation below 150 GHz demonstrates the van der Waals heterostructures' potential for nanoacoustical applications.

Effect of nanostructuring on picosecond acoustics in a Zr film

Using the picosecond ultrasonic technique, we studied the effect of the formation of nanocrystallites on the surface of a metal transducer film. We found that the velocity of sound and the elastic constant are enhanced due to the nanocrystalline structures. The acoustic echo’s shape, resulting from the reflection of the acoustic pulse at the surface, is modified, and an echo with an asymmetric bipolar shape is obtained. The shape of the acoustic echo can be explained, taking into account the formation of nanocrystallites at the film’s surface.

Photoelasticity of VO2 nanolayers in insulating and metallic phases studied by picosecond ultrasonics

We use a picosecond ultrasonic technique to evaluate the photoelastic parameters at the wavelength of 1028 nm in epitaxial vanadium dioxide $({\mathrm{VO}}_{2})$ nanolayers grown on c-cut sapphire substrates. In the experiments, we monitor the picosecond evolution of the reflectivity of ${\mathrm{VO}}_{2}$ in insulating and metallic phases under the impact of a picosecond longitudinal strain pulse injected into the nanolayer from the side of the substrate. We show that in a 145-nm-thick granular nanolayer, the temporal features of the reflectivity are clearly dependent on the phase state of ${\mathrm{VO}}_{2}$, showing the change of the photoelastic parameters upon the insulator-metal transition. Analytical consideration and numerical simulations of the optical response to the picosecond strain pulse show that the temporal evolution of the reflectivity strongly depends on the complex photoelastic parameter. The analysis enables us to obtain the values of the photoelastic parameters in the studied nanolayer in both insulating and metallic phases. We find that for a 145-nm film of ${\mathrm{VO}}_{2}$ in an insulating state the imaginary part of the photoelastic constant is negligible. This means that in the insulating phase the strain does not affect the optical absorption of ${\mathrm{VO}}_{2}$. In the metallic phase, the photoelastic parameter of ${\mathrm{VO}}_{2}$ is found to be similar to that typical for metals with positive real and negative imaginary parts. We further show that the optical response to the strain pulse in the layer consisting of disconnected ${\mathrm{VO}}_{2}$ nanohillocks with an average height of 70 nm is governed by their morphology and is different from what is predicted in the plane ${\mathrm{VO}}_{2}$ films.

Coherent acoustic phonons in van der Waals nanolayers and heterostructures

Terahertz (THz) and sub-THz coherent acoustic phonons have been successfully used as probes of various quantum systems. Since their wavelength is in the nanometer range, they can probe nanostructures buried below a surface with nanometer resolution and enable control of electrical and optical properties on a picosecond time scale. However, coherent acoustic phonons have not yet been widely used to study van der Waals (vdW) two-dimensional (2D) materials and heterostructures. This class of 2D systems features strong covalent bonding of atoms in the layer planes and weak van der Waals attraction between the layers. The dynamical properties of the interface between the layers or between a layer and its supporting substrate are often omitted as they are difficult to probe. On the other hand, these play a crucial role in interpreting experiments and/or designing new device structures. Here we use picosecond ultrasonic techniques to investigate phonon transport in vdW InSe nanolayers and InSe/hBN heterostructures. Coherent acoustic phonons are generated and detected in these 2D systems and allow us to probe elastic parameters of different layers and their interfaces. In particular, our study of the elastic properties of the interface between vdW layers reveals a strong coupling over a wide range of frequencies up to 0.1 THz, offering prospects for high-frequency electronics and technologies that require control over the charge and phonon transport across an interface. In contrast, we reveal a weak coupling between the InSe nanolayers and sapphire substrates, relevant in thermoelectrics and sensing applications, which can require quasi-suspended layers.

Thermomechanical characterization of a nanoscale copper thin-film using picosecond ultrasonics

Measurements were performed using the picosecond ultrasonic technique to evaluate thermal and elastic properties of a nanoscale copper (Cu) thin-film in the present study. A 50-nm thick Cu film was deposited using an e-beam evaporator, and the reflectance changes induced by thermal diffusion and acoustic wave propagation in the film upon illumination of femtosecond laser pulses were recorded. A numerical analysis was conducted using the finite difference method in order to extract the thermal conductivity and Young’s modulus of the material. Results showed that the film’s thermal conductivity decreased compared to the bulk property while its elastic modulus increased due to the scale effect originating from changes in microstructural contributions to the resultant characteristics. Results were compared to literature values and validated. The present study demonstrates an advanced measurement technique that can effectively and simultaneously analyze thermomechanical characteristics of nanoscale materials in a nondestructive way.

Nonlinear acoustics at GHz frequencies in a viscoelastic fragile glass former.

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.

Picosecond ultrasonic measurements using an optical mask

In this paper we describe results obtained using a variation of the picosecond ultrasonics technique. We place a transparent optical mask very close to the surface of the sample. The lower surface of the mask has a series of grooves to produce a variation of the intensity of the pump and probe light pulses across the surface of the sample. Because the light intensity varies with position, the application of the pump light pulse can generate surface acoustic waves with a wavelength equal to the period of the mask. We report results obtained in this way and discuss the possible practical applications of this new approach.

Sound velocity of iron up to 152 GPa by picosecond acoustics in diamond anvil cell

AbstractHigh‐pressure method combining diamond anvil cell with picosecond ultrasonics technique is demonstrated to be a very suitable tool to measure the acoustic properties of iron up to 152 GPa. Such innovative approach allows to measure directly the longitudinal sound velocity under pressure of hundreds of GPa in laboratory, overcoming most of the drawbacks of traditional techniques. The very high accuracy, comparable to piezoacoustics technique, allows to observe the kink in elastic properties at the body‐centered cubic–hexagonal close packed transition and to show with a good confidence that the Birch's law still stands up to 1.5 Mbar and ambient temperature. The linear extrapolation of the measured sound velocities versus densities of hcp iron is out of the preliminary reference Earth model, arguing for alloying effects or anharmonic high‐temperature effects. A comparison between our measurements and shock wave experiments allowed us to quantify temperature corrections at constant pressure in ~−0.35 and ~−0.30 m s−1/K at 100 and 150 GPa, respectively. More in general, the here‐presented technique allows detailed elastic and viscoelastic studies under extreme thermodynamic conditions on a wide variety of systems as liquids, crystalline, or polycrystalline solids, metallic or not, with very broad applications in Earth and planetary science.

Thermal Boundary Resistance between GaN and Cubic Ice and THz Acoustic Attenuation Spectrum of Cubic Ice from Complex Acoustic Impedance Measurements

A phonon nanoscopy method, based on the picosecond ultrasonics technique, capable of studying the complex acoustic reflection coefficient at frequency up to 1 THz is proposed and demonstrated. By measuring the reflection coefficient at the same surface location at the interface between GaN and air, and between GaN and the material to characterize, we get access to the THz amplitude and phase spectra of the acoustic phonon reflection. The retrieval of both these pieces of information then allows the calculation of the attenuation in a wide range of frequency and gives new insight into the Kapitza anomaly. This method is then applied to cubic ice, and the measurements of the elastic properties, the phonon anharmonic decay spectrum up to 1 THz, as well as the measurements of the thermal phonon lifetime at 150 K are all achieved.

Complete elastic characterization of lithium phosphorous oxynitride films using picosecond ultrasonics

We report on a complete elastic characterization by picosecond ultrasonics technique of lithium phosphorous oxynitride (LiPON) thin films, which are nowadays, considered as promising solid-state electrolytes for microbatteries technology. Longitudinal sound velocity is deduced from Brillouin oscillation detection at variable incidence angles. The density is estimated through an interfacial energy analysis related to the echo amplitude decay. Transverse wave velocity is obtained across the dispersion curve of Rayleigh surface waves. Combining these different approaches, Young's modulus and Poisson's ratio of LiPON are finally found to be 79GPa and 0.27, respectively.

Competition between inverse piezoelectric effect and deformation potential mechanism in undoped GaAs revealed by ultrafast acoustics

By using the picosecond ultrasonics technique, piezoelectric effect in GaAs undoped sample at both faces (A(111) and B(-1-1-1)) is experimentally studied. We demonstrate that piezoelectric generation of sound can dominate in GaAs material over the deformation potential mechanism even in the absence of static externally applied or built-in electric field in the semiconductor material. In that case, the Dember field, caused by the separation of photo-generated electrons and holes in the process of supersonic diffusion, is sufficient for the dominance of the piezoelectric mechanism during the optoacoustic excitation. The experimental results on the sample at both faces reveal that in one case (A face), the two mechanisms, piezoelectric effect and deformation potential, can compensate each other leading to a large decrease of the measured Brillouin oscillation magnitude.

Evaluation of mechanical properties of fixed bone cells with sub-micrometer thickness by picosecond ultrasonics

The picosecond ultrasonics technique is used to investigate the viscoelastic properties of nucleus of fixed single osteoblast progenitor cells adhering on a titanium alloy substrate. A two-color probing picosecond ultrasonics and a fluorescence visualization setups were developed and combined to allow to distinguish subcomponents inside the cell under investigation. It opens the way for quantitative measurements of the viscoelastic properties of single cells and of their sub-micrometer thickness. It is shown that a blue probe, λ = 400 nm, is preferable to a red probe, λ = 800 nm, to perform these measurements with fixed sub-micrometer bone cells. 26 GHz acoustic frequencies are detected in cells as thin as 135 nm. A 1D analytical model of the acoustic generation and of the optical detection is used to describe the experimental results. The nucleus longitudinal elastic moduli (13–16 GPa) and dynamic viscosities (13–30 cP) are measured at high frequencies (GHz) from a time-frequency analysis of the experimental data of fixed single cells.

Elastic stiffness of single-crystalline FeSe measured by picosecond ultrasonics

We report investigations on the elasticity of superconducting FeSe using picosecond ultrasonic technique. The tetragonal (001) FeSe thin film, deposited on a processed SrTiO3 substrate by the pulsed laser deposition, exhibits distinct c-axis preferred orientation and single-crystalline features as a result of the x-ray diffraction. The high-quality crystallinity thus enables quantitative examinations of anisotropic stiffness coefficients (C33) of FeSe, correlating to the interatomic interaction in the simplest iron-based superconductor. Our experiment indicates a room-temperature C33 of 40.9 ± 0.4 GPa and material stiffening of 4.3% at low temperatures, which can be explained by the weakening of anharmonic phonon–phonon interactions.

Coherent Brillouin spectroscopy in a strongly scattering liquid by picosecond ultrasonics

In a modification of a picosecond ultrasonic technique, a short acoustic pulse is launched into a liquid sample by a laser pulse absorbed in a semitransparent transducer film and is detected via coherent Brillouin scattering of a time-delayed probe pulse. With both excitation and probing performed from the transducer side, the arrangement is suitable for in vivo study of biological tissues. The signal is collected from a micrometer-thick layer next to the transducer and is not affected by the diffuse scattering of probe light deeper in the sample. The setup, utilizing a 33 nm thick single crystal SrRuO(3) transducer film, is tested on a full fat milk sample, with 11 GHz acoustic frequency recorded.

Mechanical characterization of temperature-sensitive objects using picosecond ultrasonics

Biological objects are exquisitely sensitive to temperature variations and their mechanical characterization is often a challenge when using the picosecond ultrasonics technique. To reduce the laser-induced temperature rise, we place single biological cells on a thin metal transducer and we focus the laser beam that generates the acoustic waves at frequencies ≤ 150 GHz on the rear side of the transducer. The acoustic waves propagate through the transducer and are partially transmitted to the cell to create the so-called Brillouin oscillations. The frequency of these oscillations provides a direct measurement of the sound velocity. The simultaneous measurement of the acoustic reflection coefficient at the transducer/cell interface allows the determination of both the density and the compressibility of the cell.

Negative refraction of surface acoustic waves in the subgigahertz range

We used the picosecond ultrasonic technique to experimentally demonstrate the negative refraction of surface acoustic waves in a two-dimensional phononic crystal. The sample is made of a square lattice of circular voids drilled at the surface of a thick silica substrate. The lattice parameter is of a few micrometers. Broad band surface acoustic waves with Fourier components in the gigahertz range were excited. The negative refraction is observed for Rayleigh waves with frequencies in the second band and propagating along the direction $\ensuremath{\Gamma}\text{M}$ in the reduced Brillouin zone. We then used a plane-wave expansion method to calculate the dispersion of the waves in our sample and to analyze our experimental data.

Picosecond ultrasonic experiments with water and its application to the measurement of nanostructures

The propagation of ultrashort sound pulses in water has been studied by using the picosecond ultrasonic technique and a pulse time-of-flight technique for measuring the depths of deep channels in Si-based nanostructure was demonstrated. The sound pulses were generated when light was absorbed in a metal transducer film and detected by a time-delayed probe light pulse. First, the attenuation and velocity of sound of frequency 4.8 GHz in water were measured through an analysis of the Brillouin frequency oscillations in the reflectivity of the probe light. Measurements at frequencies up to about 11 GHz were made by sending a sound pulse across a thin layer of water and measuring the change in shape of the returning echo due to the attenuation of the different Fourier components. Second, we also report on proof-of-concept ultrasonic experiments to acquire spatial profile information from nanostructures, where sound pulses propagate down narrow channels in patterned nanostructures. We have been able to detect acoustic echoes for sound propagating along a channel as narrow as 35 nm.

High frequency longitudinal and shear acoustic waves in glass-forming liquids

Three different picosecond ultrasonic techniques for longitudinal and transverse acoustic pulse generation have been combined with Impulsive Stimulated Thermal Scattering (ISTS) to probe structural relaxation dynamics in glycerol and DC704 (tetramethyl tetraphenyl trisiloxane) at megahertz and gigahertz frequencies (~ 50 MHz – 100 GHz) from below their respective glass transition temperatures up to 370 K.