(A) When the sample has transverse symmetry, only longitudinal L acoustic waves are laser excited.
(B) In case the sample has transverse broken symmetry, then the laser excites longitudinal L and shear S acoustic waves.
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After careful analysis of the time domain Brillouin scattering signals, differences in Brillouin signal amplitudes from acoustic waves that propagated through different liquid thicknesses yielded acoustic attenuation lengths, and differences in acoustic transit times across liquid layers with thickness differences that were smaller than the 20–50 nm acoustic wavelengths in the liquid were observed as phase shifts in the time-dependent Brillouin signal oscillations, and yielded the acoustic speed of the liquid. Thus by extracting the amplitude and the phase of the time domain Brillouin scattering signals, we can obtain the value of the longitudinal and the shear acoustic velocities and damping coefficients. This procedure can be performed at different scattering angles or different probe wavelengths in order to probe different Brillouin scattering frequencies, or at different temperatures in order to probe different relaxation pathways for any given liquid.
Frequency dependent measurements and temperature dependent measurements for distinct glass former liquids can be found in the references below. We believe that at shorter acoustic wavelengths, i.e., higher Brillouin frequencies- the approach used here may also be applied to study extremely thin liquid layers, down to a monolayer, to explore confinement and interfacial effects for example.
(A) Time derivative of two distinct measured shear signals for two distinct positions X1 and X2 of the sample corresponding to two distinct liquid layer thicknesses, d1 and d2 > d1. The excitation pulse sequence is visible between 0 and 280 psec due to the electronic response that is induced in the iron film, which is detected by the probe light. This signal is followed by oscillations due to coherent shear waves in the glass substrate. (B) Acoustic amplitude spectra of both acoustic signals, FFT1 and FFT2, and (C) the corresponding phases. The phase and amplitude differences yield the acoustic speed and attenuation length, respectively, in the liquid at the specified Brillouin frequency. Note that in (C), only the phase at frequencies around the Brillouin peak at 24.6 GHz contains relevant information.
In the front-back optical pump-probe type measurement that we used, see figure below, longitudinal and shear acoustic waves were optically generated in a "broken symmetry" iron semitransparent thin film upon ultrafast laser irradiation. The liquid is squeezed between a curved lens and a flat substrate. In this case, the liquid thickness depends on the laser spot position onto the sample. After crossing the liquid layer of a define thickness, the longitudinal and the shear acoustic waves were detected in a transparent substrate by a time delayed probe. The coherently scattered probe field, whose optical phase varies depending on the acoustic wave position inside the transparent substrate, superposes with the reflected probe field, resulting in signal intensity that shows time-dependent oscillations at a specific frequency that depends on the substrate material and on the optical configuration. This phenomenon is known as Brillouin scattering. In the particular case of pump-probe type of experiments, we specify "time domain Brillouin scattering". In order to improve the signal brightness at the monochromatic Brillouin frequency, we used an excitation pulse sequence whose timing matches the Brillouin scattering frequency.
We used this approach for probing picosecond shear as well as longitudinal acoustic waves in liquids in the GHz-frequency range. We have designed a sample and optical configuration that permit the measurements to be conducted in viscoelastic liquids, whose GHz-frequency acoustic responses are of particular interest in connection with complex structural relaxation dynamics.
First, above all the difficulties related to picosecond shear waves, the main difficulty is how to generate them ? The detection of shear waves is rather sophisticated too, but it is not an issue at all. The key idea to overcome the problem of excitation of picosecond shear waves is to break the symmetry. Because of the transverse symmetry of the photoacoustic interaction, only longitudinal acoustic wavepackets are generated by usual samples - for example a metal layer deposited on a substrate. In this case the mechanism of generation is thermoelastic, the laser creates a transient heating of the metal that launches plane longitudinal acoustic waves. By breaking the transverse symmetry of the sample, the generation of plane shear acoustic waves becomes possible. Some recent examples have emerged where metallic crystals have been cute obliquely such that the laser heating that creates a stress in the direction normal to the surface sample will create a longitudinal strain as well as a shear strain.
Picosecond shear acoustic waves
Picosecond ultrasonics is a type of ultrasonics that uses ultra-high frequency ultrasound generated by ultrashort light pulses. It is a non-destructive technique in which picosecond acoustic pulses penetrate into thin films or nanostructures to reveal internal features such as film thickness or rigidity, as well as cracks, delaminations and voids. It can also be used to probe liquids.
However, one of the main challenges involving shear waves in Picosecond ultrasonics is how to be able to probe shear waves in liquids. It is sometimes believed that shear waves can’t propagate in liquids, at the contrary with longitudinal or pressure waves. Some everyday life example is for instance the fact that shear waves don't propagate in the inner core of earth that is assumed to be liquid like. The fact that shear waves don't propagate in liquids is partially true, at low frequency only. Actually, if the shear transducer can reach a threshold frequency where the liquid become solid-like - i.e. the molecules have no time to rearrange, like frozen- then it is predicted that shear waves can propagate. It has already been proven at MHz frequencies with viscous liquids, like glycerol that is 1000 times more viscous than water but at least 1000 times less viscous than honey.