Picosecond Ultrasonic Experiments Research Articles

Probing gigahertz coherent acoustic phonons in TiO2 mesoporous thin films

Ultrahigh-frequency acoustic-phonon resonators usually require atomically flat interfaces to avoid phonon scattering and dephasing, leading to expensive fabrication processes, such as molecular beam epitaxy. Mesoporous thin films are based on inexpensive wet chemical fabrication techniques that lead to relatively flat interfaces regardless the presence of nanopores. Here, we report mesoporous titanium dioxide-based acoustic resonators with resonances up to 90 GHz, and quality factors from 3 to 7. Numerical simulations show a good agreement with the picosecond ultrasonics experiments. We also numerically study the effect of changes in the speed of sound on the performance of the resonator. This change could be induced by liquid infiltration into the mesopores. Our findings constitute the first step towards the engineering of building blocks based on mesoporous thin films for reconfigurable optoacoustic sensors.

Multilayer transducer for highly efficient initiation of time-resolved Brillouin scattering

Structures made of a metallic film deposited on a substrate are conventionally used as opto-acoustic transducers for picosecond ultrasonic experiments where detection in the time domain of the Brillouin scattering in a transparent sample is sought. In this paper, we substitute the metallic film for a periodic stack of nanometric layers made of gold and lithium fluoride to increase the amplitude, at the Brillouin frequency shift, of the strain generated by the photo-thermal effect. A model is used to analyze the generated strain amplification with the volume fraction and with the total thickness of this structure and to evaluate the gain in terms of sample dynamic reflectivity changes. Amplification by a factor of 20 is measured when using the composite structure with respect to signals detected with a transducer made of a single gold layer.

Nondestructive Picosecond Ultrasonic Probing of Intralayer and van der Waals Interlayer Bonding in α‐ and β‐In2Se3

AbstractThe interplay between the strong intralayer covalent‐ionic bonds and the weak interlayer van der Waals (vdW) forces between the neighboring layers of vdW crystals gives rise to unique physical and chemical properties. Here, the intralayer and interlayer bondings in α and β polytypes of In2Se3 are studied, a vdW material with potential applications in advanced electronic and optical devices. Picosecond ultrasonic experiments are conducted to probe the sound velocity in the direction perpendicular to the vdW layers. The measured sound velocities are different in α‐ and β‐In2Se3, suggesting a significant difference in their elastic properties. Density functional theory and an effective spring model are used to calculate the elastic stiffness of the layer and vdW gap in α‐ and β‐In2Se3. The calculated elastic moduli show good agreement with experimental values and reveal the dominant contribution of interlayer atomic bonding to the different elastic properties of the two polytypes. The findings show the power of picosecond ultrasonics for probing the fundamental elastic properties of vdW materials. The data and analysis also provide a reliable description of the intra‐ and interlayer forces in complex crystal structures, such as the polytype phases of In2Se3.

Semiconductor superlattices: A tool for terahertz acoustics

The properties of optical to acoustic transduction of semiconductor superlattices have been explored for several years in the sub terahertz frequency range. Using femtosecond laser pulses focused on these structures, acoustic modes are excited with a frequency related to the periodicity of the structure stacking. We have shown that these acoustic waves can be extracted and can propagate in the underlying substrate. We study superlattices ability to be quasi monochromatic generators. On the other hand, superlattices have been found to be very sensitive and selective detectors. We present a set of experimental results concerning the generation, propagation over large distances and detection of acoustic waves at high frequencies, up to the challenging 1THz by picosecond ultrasonics experiments in transmission geometry.

Dispersion of an acoustic pulse passing through a large-grained polycrystalline film

Propagation of a short acoustic pulse through a polycrystalline film comprised of large randomly oriented elastically anisotropic grains is analyzed theoretically. For average grain size much larger than the film thickness, a short acoustic pulse launched normally into the film will traverse each grain in a time determined by the acoustic slowness in the direction normal to the film, which will depend on the local grain orientation. A typical measurement averages over a large number of grains resulting in the broadening of the composite output pulse. The resulting pulse shape is characterized by distinct features related to stationary values of the directionally dependent acoustic slowness of the crystalline material. Maxima and minima in the slowness yield discontinuities in the pulse shape, while saddle points yield logarithmic singularities. For cubic and hexagonal crystals, power law singularities result from cones of directions in which the slowness is a maximum or minimum. Numerical results, taking into account Gaussian broadening of the input pulse, are presented for thin film materials commonly encountered in picosecond ultrasonic experiments, such as copper, gold, and aluminum.

Terahertz coherent acoustic experiments with semiconductor superlattices

We demonstrate the generation, propagation, and detection of coherent acoustic waves at 1 THz. We performed picosecond ultrasonics experiments in an acoustic transmission geometry, which allows a total decoupling of the phonon generation and detection processes, occurring in two different superlattices grown on opposite sides of a substrate, respectively. We show that a 1 ns burst of monochromatic terahertz coherent acoustic waves could be generated on one side of a substrate, propagated over a large distance and detected on the opposite side. We also measured the lattice dispersion owing to the time delay between the terahertz burst and much lower frequency acoustic waves.

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.

Photogeneration of coherent shear phonons in orientated wurtzite semiconductors by piezoelectric coupling

Using piezoelectric wurtzite semiconductor with accurate control of the crystal cut, we investigate the photogeneration of coherent shear acoustic phonons through anisotropic piezoelectric coupling. Theoretical study suggests the dominant contribution of the piezoelectric effect to the shear phonon generation in $a$-plane wurtzite GaN, which is also confirmed by the picosecond ultrasonic experiments with different dopant and excitation conditions. Piezoelectric transduction thus explains the observed remarkable coherent shear phonon signal. The influence of the ultrafast screening of surface electric field by photocarriers on the intensity and waveform of the generated shear acoustic pulse are discussed.

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

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

Acoustic waves generated by a laser point pulse in a micrometric fiber

Having emerged in the 1980s, the laser ultrasonics technique with its non-contact generation and detection process overpasses the difficulties of coupling piezoelectric transducers with curved surfaces. To date, the authors [1] have been interested in acoustic generation for cylinders opaque at a given laser wavelength and for the acoustic source located at the cylinder surface. In this presentation, assuming point focusing of the laser pulses, we propose a three-dimensional (3D) semi-analytical model for acoustic waves generation and propagation in a partly transparent isotropic cylinder. First, the radial displacement at any position on the free surface is derived, in a 3D Fourier domain, for an inner point source. The response to a volume-source distribution along a radius is obtained as a convolution of the above Green function with the corresponding source distribution caused by optical absorption. Three inverse transforms are then applied to obtain the radial displacement at the cylinder surface. Picosecond ultrasonics experiments are performed on different micrometric fibers and compared with calculated waveforms for different optical absorptive properties.

Imaging of 3D acoustic wave-fronts by means of picosecond laser ultrasonics in GaAs substrate

Picosecond ultrasonic experiments are widely used as pump-probe technique using longitudinal acoustic pulses and measuring time dependence of the signal. We present an experiment which combines an interferometer with the control of the pump-probe spatial overlap. Such set-up can perform a new kind of bulk phonon imaging based on ultrafast optical generation and detection scheme. When ultrafast acoustic longitudinal wave propagate in crystals they generate coherent transverse acoustic wave by diffraction. Transverse waves are known to give rise to phonon focusing patterns. The imaging of such patterns in time and space allowed us to partially reconstruct the wave front surface of bulk acoustic wave in crystals. The data may be viewed as 2D slices of the 3D acoustic wavefront revealing the basic anisotropy of the solid with unprecedented micronic lateral and in-depth resolution. Measurements are carried out in a 356 μm-thick GaAs sample as an example, but many different experimental conditions are compatible with the present set-up. This technique provides a new way to perform phonon energy and phonon polarisation dependent experiment with high frequency phonons.

Monochromatic high frequency coherent phonons propagation with superlattice transducers

The availability of efficient and compact phonons transducers in the THz range would be very interesting for phonons spectroscopy, acoustic microscopy and study of vibrational and electronic properties of nanostructures. Thanks to epitaxial growth of semiconductors multilayers, high quality phononic nanostructures with standard semiconductors, such as superlattices (SL) and nanocavities can be obtained for the GHz and THz transduction. Picosecond ultrasonics experiments have been performed in transmission geometry with pump and probe incident on opposite sides of the substrate, allowing discoupling acoustic generation and detection processes. By these means, we have shown independently that SL are very efficient high frequency monochromatic phonon generators and detectors. We report on experiments where two superlattices have been grown on the opposite sides of a substrate: a first SL with uniform layer thickness over the whole surface sample is used as a generator; the other one, used as the detector, presents a thickness gradient and the location of the detection is chosen in order to have the best matching with the emitted frequency. This setup is used to study the propagation of monochromatic high frequency coherent phonons as a function of temperature.

Thickness dependence of the acoustical response of ultra-thin metallic films studied by Colored Picosecond Ultrasonics

We have previously demonstrated [1,2] that a connection exists between Colored Picosecond Ultrasonic (CPU) experiments and electronic structure of metallic thin films. Indeed, a strong change of the detected acoustic echoes is observed when the laser is tuned around an interband transition wavelength. This connection suggests that CPU can be an useful tool for measuring interband transitions in thin metallic films. Surprisingly, by doing such a measurement on a series of ultra-thin Aluminium films, we observed a significant shift of the transition (from 880 to 970 nm) as the film thickness is reduced (from 400 to 120 A). We will discuss the origin of the phenomenon and propose some applications to the characterization of ultra-thin metallic films. [1] A. Devos and C. Lerouge, Phys. Rev. Lett. 86, 2669 (2001) [2] A. Devos and A. Le Louarn, Phys. Rev. B 68, 045405 (2003)

Compositional dependence of longitudinal sound velocities of piezoelectric (111) InxGa(1−x)As measured by picosecond ultrasonics

Picosecond ultrasonic experiments were performed to generate and detect acoustic pulses in piezoelectric (111) epilayers of InxGa(1−x)As, and the longitudinal sound velocities in the [111] direction of InxGa(1−x)As were thus directly determined by controlling the indium content x. We found that the sound velocity exhibits an obvious bowing with different alloy compositions and can be described as a quadratic function of VL(x)=458x2−1451x+5414. This result supports a nonlinear compositional dependence of the elastic constants of InxGa(1−x)As and indicates that the linear-interpolation approximation is inapplicable for estimating sound velocity in this specific case.

Generation and detection of incoherent phonons in picosecond ultrasonics

In picosecond ultrasonics experiments the absorption of a femtosecond laser pulse in a thin metallic transducer is used to generate very short acoustic pulses. These pulses are made of coherent longitudinal waves with a frequency spectrum that can reach 100–200 GHz. The laser pulse absorption gives rise to a heating of the film of a few Kelvin within a typical time of 1 ps. Later on, the heat goes in the substrate through an interface thermal resistance and is diffused by thermal conduction. At very low temperature and in pure crystals the thermal phonons emitted by the heated metallic film can propagate ballistically over large distances and produce a so-called heat pulse. We report on the experimental evidence of the coexistence of the coherent acoustic pulse and the incoherent heat pulse generated and detected by laser ultrasonics.

Refractive index, sound velocity and thickness of thin transparent films from multiple angles picosecond ultrasonics

We present a method for refractive indices and longitudinal sound velocity measurements from picosecond ultrasonic experiments made at different probe incidence angles. For transparent or semitransparent materials such as dielectrics or semiconductors, picosecond ultrasonic experiments can lead to oscillations in the reflectivity curves whose frequency depends on the refractive indices, the sound velocity and the experiments angle. From these data we establish a simple method for the calculation of the refractive indices and verify it on a GaAs sample. We show on fluorinated silica glass and aluminum nitride practical applications of this method on thin films. From two experiments we measure the refraction index and the sound velocity of these materials, with no assumption on the materials properties or on the sample layers’ thicknesses. Here the materials are buried under a thin aluminum film. It illustrates the fact that the method can be applied to multilayers. From the same experiments we then derive the thickness of the layers. It shows that this method can render picosecond ultrasonic experiments independent from other characterization means.

Picosecond ultrasonic experiments to study phonon solitons

We describe experiments to study the propagation of phonon solitons in crystalline solids. A picosecond light pulse is used to generate a short strain pulse. As this pulse propagates, its shape changes as a result of phonon dispersion and elastic non-linearity. After traveling a sufficient distance, the pulse breaks up into a number of solitons. The amplitude and number of these solitons are in excellent agreement with theoretical expectations.

Strong picosecond ultrasonic responses of semiconductors probed close to interband transitions

We present experimental and numerical results of picosecond ultrasonic experiments performed in different semiconductors at different laser wavelengths. We first come back on the strong oscillations which appear in the reflectivity curves of silicon as the probe pulse is tuned in the blue range. We show that they come from an effect of the electronic transitions on the piezo-optic couplings. We then present similar results obtained in germanium which displays three interband transitions which fall in our laser source tuning range: 3.11, 2.11 and 0.8 eV (399, 587 and 1540 nm). Here again we have noticed strong responses in the reflectivity curves. We then make a comparison between silicon and germanium in respect with the laser wavelength effects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong oscillations detected by picosecond ultrasonics in silicon: Evidence for an electronic-structure effect

We present the results of picosecond ultrasonic experiments performed at different laser wavelengths in different samples all grown on silicon (100) substrates. In the blue range we report on strong oscillations in the reflectivity curves. We present experimental and numerical results that identify the origin of these oscillations in a strong acousto-optic interaction in the silicon substrate. Then we propose to explain the high amplitude of these oscillations by a relation with direct interband transitions in silicon which fall in the same range. This explanation is supported by two other experimental evidences: First, sudden changes in the phase of the detected signal are observed as the laser wavelength is tuned around the interband transitions. Second, we report on a shift of this phase change as the substrate is heated. This study shows that the effect of interband transitions on picosecond ultrasonic studies we had first demonstrated in thin metallic films can be extended to semiconductors.