Laser Picosecond Acoustics Research Articles

Thermodynamic properties of liquid gallium from picosecond acoustic velocity measurements

Due to discrepancies in the literature data the thermodynamic properties of liquid gallium are still in debate. Accurate measurements of adiabatic sound velocities as a function of pressure and temperature have been obtained by the combination of laser picosecond acoustics and surface imaging on sample loaded in diamond anvil cell. From these results the thermodynamic parameters of gallium have been extracted by a numerical procedure up to 10 GPa and 570 K. It is demonstrated that a Murnaghan equation of state accounts well for the whole data set since the isothermal bulk modulus BT has been shown to vary linearly with pressure in the whole temperature range. No evidence for a previously reported liquid–liquid transition has been found in the whole pressure and temperature range explored.

Optical detection of longitudinal and shear acoustic waves with laser picosecond acoustics

The absorption of picosecond light pulses in a medium can generate sub-THz acoustic waves. These cause a transient optical reflectance change that can be monitored by delayed probe light pulses. This technique, termed laser picosecond acoustics, can be used for the nondestructive evaluation of the physical properties of thin films and substrates. This paper describes a general method for quantitative analysis of such reflectance changes. It is applicable to multiple anisotropic layers that may be opaque or transparent. Longitudinal or shear acoustic waves propagating along the stacking direction of the multilayers modulate the dielectric permittivity anisotropically and inhomogeneously through the photoelastic effect, through local rotation, or through the surface and interface displacements. We describe how the optical reflectance for obliquely incident probe light can be calculated for the modulated medium. We then demonstrate the method with reference to experimental results for a sample consisting of a silica film on a zinc substrate in which both longitudinal and shear acoustic waves are generated and detected. The analysis yields the film thickness, sound velocity, and photoelastic tensor components, for example. The method is also applicable to various light scattering problems involving the inhomogeneous modulation of optical properties such as in photothermal experiments.

Theory of Detection of Shear Strain Pulses with Laser Picosecond Acoustics

Theory of Detection of Shear Strain Pulses with Laser Picosecond Acoustics

In situ monitoring of the growth of ice films by laser picosecond acoustics

Ultrashort optical pulses are used to excite and probe picosecond acoustic pulses in a sample consisting of an opaque material upon which ice is continuously deposited from the vapor phase at ∼110K. By analysis of the ultrasonic propagation and reflection inside the submicron ice film and taking into account the scattering of the probe light by the acoustic waves, the thickness, sound velocity, refractive index, ultrasonic attenuation, and photoelastic constant of the ice film are derived. This method should find applications for the in situ monitoring of thin transparent films during growth.

Direct measurement of ultrafast surface displacement in laser picosecond acoustics

Picosecond acoustic pulses generated in solids with ultrashort optical pulses can be detected through the reflectance or phase changes of optical probe pulses. The detection relies on acousto-optic coupling resulting from a combination of the photoelastic effect and transient surface displacements. The acoustic strain pulse shape gives access to the ultrafast dynamics of the electrons that influence the generation process. This pulse shape is directly related to the contribution to the probe light modulation resulting from surface displacement. Here we present a method that allows the discrimination between the photoelastic and surface displacement contributions using a novel oblique optical incidence geometry, allowing the strain pulse shape to be directly obtained in a single measurement.

Laser picosecond acoustics in a two-layer structure with oblique probe light incidence

A theory for the analysis of experiments involving laser picosecond acoustics with obliquely incident probe light in a two-layer structure is outlined. The reflectance and phase changes of the reflected light are calculated with a theory that takes into account the effects of multiple optical reflections. The sample consists of a single partially transparent layer on a substrate, both with arbitrary optical constants. We discuss the conditions in which one may discriminate between components of the optical modulation of a probe beam arising from the photoelastic effect and from the displacement of the sample interfaces induced by the acoustic strain.

Laser picosecond acoustics with oblique probe light incidence

Laser picosecond acoustics involves the excitation and detection of picosecond acoustic strain pulses in thin films with ultrashort light pulses. The use of oblique probe light incidence permits a greater degree of freedom in choosing the optical probing conditions and thereby should allow the extraction of more information about the profile of the propagating acoustic strain pulses. Here, we present a theory for the modulation of light reflected at oblique incidence from a solid containing an acoustic strain distribution. The theory can account for the real and imaginary parts of the reflectance variation, and involves both the effect of the transient surface motion and of the photoelastically modulated dielectric constants in the solid. We show, in particular, how the theory can be applied to extract the contribution to the reflectance change arising from the surface motion in an opaque isotropic solid, thereby allowing direct access to the shape of the acoustic strain pulse propagating therein.

Laser picosecond acoustics in multilayer structures

The field of laser picosecond acoustics has thrived owing to the ease of detection of propagating picosecond acoustic pulses through changes in optical reflectance. Reflectance changes are caused by the inhomogeneous modulation of the refractive index by the propagating elastic strain through the photoelastic effect and also by the associated induced motion of the surface and interfaces. In this paper we present a general formula for calculating the reflectance change based on a rigorous one-dimensional treatment of the perturbation in optical properties of arbitrary multilayer structures. The theory is applied to the quantitative analysis of data obtained by laser picosecond acoustics for a SiO 2–Cr double-layer film on a fused silica substrate. The analysis allows the discrimination of the photoelastic contribution and the surface or interface motion contribution to the experimental reflectance variation.

Laser picosecond acoustics in double-layer transparent films

By the optical pump-and-probe technique, picosecond acoustic pulses are excited and detected in thin transparent double-layer films of silica and silicon nitride upon opaque chromium substrates. By taking both acoustic and optical multiple reflections into account, one can successfully model the main features of the reflectance variation. The film thicknesses, sound velocities, and photoelastic constants are derived by use of known values of the refractive indices.

Thickness and sound velocity measurement in thin transparent films with laser picosecond acoustics

A new thin-film optical testing method is described for both thickness and sound velocity of transparent films on opaque substrates with laser picosecond acoustics using the optical pump-probe technique. The theory of excitation and detection of ultrasonic stress pulses for this geometry is presented in detail together with experimental results for sputtered thin films of silica of thickness 200 nm–2 μm on amorphous germanium substrates. Reflectance variations, measured as a function of pump-probe delay time, are characterized by echoes and beating oscillations superimposed on periodic steplike changes. These effects are modeled as a sum of an echo contribution from the stress-induced modulation of the substrate reflectance, an interference contribution from the light reflected by the moving stress pulse in the transparent film, a contribution from the modulation of the light on transmission through this stress pulse, and a contribution from the stress-induced ultrafast vibrations of the film interfaces of order 10−3 nm. The latter contribution arises from thin-film interference effects that represent a novel detection mechanism for surface vibrations in the picosecond regime. Sound velocity and thickness are derived from the data to an accuracy of a few percent, and the photoelastic constant of the transparent film is determined.

Laser Picosecond Acoustics in Various Types of Thin Film

We describe the excitation and detection of picosecond stress pulses in a variety of thin films using an ultra fast laser pump-probe technique to measure the film reflectance variation. An opaque thin film of amorphous diamond-like carbon of known thickness is first investigated and the longitudinal sound velocity derived. Two methods of testing transparent films on opaque substrates are then considered, one a conventional technique in which an opaque transducer layer is deposited on the transparent film, and one a new technique in which the transparent film is uncoated. For the latter technique it is shown how both the sound velocity and the film thickness can be determined.