B. Perrin

Nondestructive evaluation of micrometric diamond films with an interferometric picosecond ultrasonics technique

Synthetic diamond thin or thick films are suitable materials for many applications because of their weak friction coefficient and their large hardness, Young modulus, and thermal conductivity. A series of diamond thin films of variable quality obtained by plasma-assisted chemical vapor deposition lying on titanium alloy substrates has been studied by picosecond ultrasonics. Femtosecond laser pulses allow the generation and detection of picosecond acoustic pulses that can be used to study the elastic properties of thin micrometric and submicrometric films. Acoustic fields generated in the substrate and propagating in the transparent diamond film are detected by an interferometric technique; they give rise to oscillations and abrupt step-like changes in the transient reflectivity variations. An analysis of these different features allows the determination of various characteristics of these films, such as longitudinal elastic constant, thickness, and roughness in terms of the diamond quality.

Equation of state, stability, anisotropy and nonlinear elasticity of diamond-cubic (ZB) silicon by phonon imaging at high pressure

Experimental phonon imaging in diamond anvils cell is demonstrated to be an adequate tool to extract the complete set of elastic constants of single-crystalline silicon up to the ZB ->beta-Sn transition (10 GPa). Contrary to what was commonly admitted, we demonstrate that the development of the strain-energy density in terms of strains cannot be stopped, for silicon, after the terms containing the third-order elastic constants. Nonlinear elasticity, degree of anisotropy and pressure-induced mechanical stability of the cubic silicon structure are thus revisited and investigated in more detail.

Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P)

The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of (Ga,Mn)As codoped with phosphorus was probed using magneto-optical Kerr effect (MOKE). A transient MOKE signal followed by low-amplitude oscillations was evidenced, with a strong dependence on applied magnetic field, temperature, and ps-ASP amplitude. Careful interferometric measurement of the layers thickness variation induced by the ps-ASP allowed us to model very accurately the resulting signal, and interpret it as the strain modulated reflectivity (differing for

Liquid mercury sound velocity measurements under high pressure and high temperature by picosecond acoustics in a diamond anvils cell

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Evidence for the alteration of an organic/metal interface resulting from the formation of a broad interfacial layer

probe polarizations), independently from dynamic magnetization effects.

Elasticity of ultrathin copper-phthalocyanine Langmuir–Blodgett films by picosecond ultrasonics

Recent improvements to measure ultrasonic sound velocities of liquids under extreme conditions are described. Principle and feasibility of picosecond acoustics in liquids embedded in a diamond anvils cell are given. To illustrate the capability of these advances in the sound velocity measurement technique, original high pressure and high temperature results on the sound velocity of liquid mercury up to 5 GPa and 575 K are given. This high pressure technique will certainly be useful in several fundamental and applied problems in physics and many other fields such as geophysics, nonlinear acoustics, underwater sound, petrology or physical acoustics.

Mirage effect: A theoretical and experimental study of anisotropic media in rear configuration

The alteration of the organic/electrode interface is partially responsible for the deterioration of the electrical efficiency of some molecular electronic devices. We used the picosecond ultrasonics to investigate the changes that occur at the interface copper-phthalocyanine/M (M=Al or Au) after the electrode deposition: the absorption of an ultrashort laser pulse sets the heterostructure into vibration and the nature of the interface is deduced from the photoelastic response of the samples. We show that a broad interfacial layer is formed, the thickness of which is estimated. We show also that the slow migration of metallic particles within the organic layer leads, after a few weeks, to a broadening of the interfacial region.

Photothermal Study of Anisotropic Media

The picosecond ultrasonic technique is used to investigate the elasticity of Langmuir–Blodgett submicronic films of copper (II) octabutoxyphthalocyanine. The amplitude of the acoustic echoes is measured against the number of monolayers (1–24, i.e., from a pure bidimensional behavior to a tridimensional one). The longitudinal sound velocity and the ultrasonic attenuation for frequencies up to about 40 GHz are deduced from this behavior. For the thinnest samples, the acoustic echoes are observed with a delay whose origin is analyzed. We show that the accuracy of the method is only limited by the precise knowledge of the density of the material.

Ultrasonic attenuation in a quasicrystal studied by picosecond acoustics as a function of temperature and frequency

In this article, we describe a method based on mirage effect in rear configuration, i.e., a modulated pump beam illuminates one side of the sample and the temperature gradient is probed on the opposite side. We show that a detailed investigation of the amplitude and phase of the photothermal deflection angle, versus the modulation frequency, connected with a numerical analysis, provides a determination of the thermal conductivity coefficient in the normal direction to the sample surface. The method, suitable for thin slabs of low thermal diffusivity materials, is calibrated with a series of known samples and then applied to a polydiacetylene single crystal.

Lattice instability and elastic response of metastable Mo 1 − x Si x thin films

Photothermal techniques are based on the illumination of the sample under study by a modulated light beam (pump beam). In absorbing media, light is converted into heat and induces thermal waves within the sample. This heat flow produces a refractive index gradient in the adjacent air medium, close to the sample surface. In the probe beam deflection technique [1], a second laser beam is directed through the heated region and is deflected by the mirage effect. The analysis of this deflection gives the temperature distribution in the sample and information about its thermal properties. The 3d photothermal deflection has been extensively reported in the literature for isotropic media [2] but only a few results are related to anisotropic ones [3].