Nonlinear acoustics at GHz frequencies in a viscoelastic fragile glass former

Comprehensive characterization of thermal and mechanical properties in thin metal film-glass substrate system by ultrafast laser pump-probe method

Picosecond ultrasonics (PU), time-domain Brillouin scattering (TDBS), and time-domain thermo-reflectance (TDTR) are all in-situ, non-destructive, and non-contact experimental techniques based on the ultrafast laser pump-probe method, which can generate and detect coherent acoustic phonons (CAP) and thermal transport in thin metal film-glass substrate system. However, these techniques are generally considered different experimental methods to characterize the thermal or mechanical properties of metal nano-objects or transparent materials. Here we present a comprehensive characterization of the generation, propagation, and attenuation of high-frequency CAP and cross-plane thermal transport in the thin Cr film-glass substrate system by PU, TDBS, and TDTR. To investigate the key factors of characterizations, two kinds of thin Cr film-glass substrate systems were measured on the film side and substrate side. The measured thermal and mechanical properties show that boundary conditions and film thickness have significantly affected the characterization.

Optical measurement of the picosecond fluid mechanics in simple liquids generated by vibrating nanoparticles: a review

Standard continuum assumptions commonly used to describe the fluid mechanics of simple liquids have the potential to break down when considering flows at the nanometer scale. Two common assumptions for simple molecular liquids are that (1) they exhibit a Newtonian response, where the viscosity uniquely specifies the linear relationship between the stress and strain rate, and (2) the liquid moves in tandem with the solid at any solid-liquid interface, known as the no-slip condition. However, even simple molecular liquids can exhibit a non-Newtonian, viscoelastic response at the picosecond time scales that are characteristic of the motion of many nanoscale objects; this viscoelasticity arises because these time scales can be comparable to those of molecular relaxation in the liquid. In addition, even liquids that wet solid surfaces can exhibit nanometer-scale slip at those surfaces. It has recently become possible to interrogate the viscoelastic response of simple liquids and associated nanoscale slip using optical measurements of the mechanical vibrations of metal nanoparticles. Plasmon resonances in metal nanoparticles provide strong optical signals that can be accessed by several spectroscopies, most notably ultrafast transient-absorption spectroscopy. These spectroscopies have been used to measure the frequency and damping rate of acoustic oscillations in the nanoparticles, providing quantitative information about mechanical coupling and exchange of mechanical energy between the solid particle and its surrounding liquid. This information, in turn, has been used to elucidate the rheology of viscoelastic simple liquids at the nanoscale in terms of their constitutive relations, taking into account separate viscoelastic responses for both shear and compressible flows. The nanoparticle vibrations have also been used to provide quantitative measurements of slip lengths on the single-nanometer scale. Viscoelasticity has been shown to amplify nanoscale slip, illustrating the interplay between different aspects of the unconventional fluid dynamics of simple liquids at nanometer length scales and picosecond time scales.

Nonlinear Optical Absorption in Nanoscale Films Revealed through Ultrafast Acoustics

Herein we describe a novel spinning pump-probe photoacoustic technique developed to study nonlinear absorption in thin films. As a test case, an organic polycrystalline thin film of quinacridone, a well-known pigment, with a thickness in the tens of nanometers range, is excited by a femtosecond laser pulse which generates a time-domain Brillouin scattering signal. This signal is directly related to the strain wave launched from the film into the substrate and can be used to quantitatively extract the nonlinear optical absorption properties of the film itself. Quinacridone exhibits both quadratic and cubic laser fluence dependence regimes which we show to correspond to two- and three-photon absorption processes. This technique can be broadly applied to materials that are difficult or impossible to characterize with conventional transmittance-based measurements including materials at the nanoscale, prone to laser damage, with very weak nonlinear properties, opaque, or highly scattering.

Direct Observation of Coherent Longitudinal and Shear Acoustic Phonons in TaAs Using Ultrafast X-Ray Diffraction

Using femtosecond time-resolved x-ray diffraction, we investigated optically excited coherent acoustic phonons in the Weyl semimetal TaAs. The low symmetry of the (112) surface probed in our experiment enables the simultaneous excitation of longitudinal and shear acoustic modes, whose dispersion closely matches our simulations. We observed an asymmetry in the spectral line shape of the longitudinal mode that is notably absent from the shear mode, suggesting a time-dependent frequency chirp that is likely driven by photoinduced carrier diffusion. We argue on the basis of symmetry that these acoustic deformations can transiently alter the electronic structure near the Weyl points and support this with model calculations. Our study underscores the benefit of using off-axis crystal orientations when optically exciting acoustic deformations in topological semimetals, allowing one to transiently change their crystal and electronic structures.

Glass transition of nanometric polymer films probed by picosecond ultrasonics

The possibility to measure the glass transition temperature in poly(methyl methacrylate) (PMMA) films by picosecond ultrasonics with thicknesses ranging from 458 nm to 32 nm is demonstrated. A shift of the longitudinal acoustic eigenmodes towards lower frequencies with temperature is observed accompanied by a change in the temperature-frequency slopes at the glass transition temperature. The contributions to the frequency shift from changes in film thickness and sound velocity are discussed and the latter is extracted below the glass transition temperature. Finally, the advantages and disadvantages of the current approach in a comparison to other methods based on acoustic measurements in the GHz regime are reviewed.

Revisiting impulsive stimulated thermal scattering in supercooled liquids: Relaxation of specific heat and thermal expansion

Impulsive stimulated thermal scattering (ISTS) allows one to access the structural relaxation dynamics in supercooled molecular liquids on a time scale ranging from nanoseconds to milliseconds. Till now, a heuristic semi-empirical model has been commonly adopted to account for the ISTS signals. This model implicitly assumes that the relaxation of specific heat, C, and thermal expansion coefficient, γ, occur on the same time scale and accounts for them via a single stretched exponential. This work proposes two models that assume disentangled relaxations, respectively, based on the Debye and Havriliak-Negami assumptions for the relaxation spectrum and explicitly accounting for the relaxation of C and γ separately in the ISTS response. A theoretical analysis was conducted to test and compare the disentangled relaxation models against the stretched exponential. The former models were applied to rationalize the experimental ISTS signals acquired on supercooled glycerol. This allows us to simultaneously retrieve the frequency-dependent specific heat and thermal expansion up to the sub-100 MHz frequency range and further to compare the fragility and time scale probed by thermal, mechanical, and dielectric susceptibilities.

Highly Spherical Nanoparticles Probe Gigahertz Viscoelastic Flows of Simple Liquids Without the No-Slip Condition

Simple liquids are conventionally described by Newtonian fluid mechanics, based on the assumption that relaxation processes in the flow occur much faster than the rate at which the fluid is driven. Nanoscale solids, however, have characteristic mechanical response times on the picosecond scale, which are comparable to mechanical relaxation times in simple liquids; as a result, viscoelastic effects in the liquid must be considered. These effects have been observed using time-resolved optical measurements of vibrating nanoparticles, but interpretation has often been complicated by finite velocity slip at the liquid-solid interface. Here, we use highly spherical gold nanoparticles to drive flows that are theoretically modeled without the use of the no-slip boundary condition at the particle surface. We obtain excellent agreement with this analytical theory that considers both the compression and shear relaxation properties of the liquid.

Viscoelasticity Enhances Nanometer-Scale Slip in Gigahertz-Frequency Liquid Flows

The interaction between flowing liquids and solid surfaces underpins many physical phenomena and technologies, such as the ability of an airfoil to generate lift and the mixing of liquids for industrial applications. These phenomena are often described using the Navier-Stokes equations and the no-slip boundary condition: the assumption that the liquid immediately adjacent to a solid surface does not move relative to the surface. Herein, we observe violation of the no-slip condition with strong enhancement of slip due to intrinsic viscoelasticity of the bulk liquid. This is achieved by measuring the 20 GHz acoustic vibrations of gold nanoparticles in glycerol/water mixtures, for which the underlying physics is explored using rigorous, theoretical models. The reported enhancement of slip revises current understanding of ultrafast liquid flows, with implications for technologies ranging from membrane filtration to nanofluidic devices and biomolecular sensing.

Crystalline-like ordering of 8CB liquid crystals revealed by time-domain Brillouin scattering

We demonstrate that time-domain Brillouin scattering (TDBS), a technique based on an ultrafast pump-probe approach, is sensitive to phase transitions and apply it to the study of structural changes in 8CB liquid crystals at different temperatures across the isotropic, nematic, smectic, and crystalline phases. We investigate the viscoelastic properties of 8CB squeezed in a narrow gap, from the nanometer to submicrometer thickness range, and conclude on the long-range molecular structuring of the smectic phase. These TDBS results reveal that confinement effects favor structuring of the smectic phase into a crystallinelike phase that can be observed at wide distances far beyond the molecular dimensions.

Tracking picosecond strain pulses in heterostructures that exhibit giant magnetostriction

We combine ultrafast X-ray diffraction (UXRD) and time-resolved Magneto-Optical Kerr Effect (MOKE) measurements to monitor the strain pulses in laser-excited TbFe₂/Nb heterostructures. Spatial separation of the Nb detection layer from the laser excitation region allows for a background-free characterization of the laser-generated strain pulses. We clearly observe symmetric bipolar strain pulses if the excited TbFe₂ surface terminates the sample and a decomposition of the strain wavepacket into an asymmetric bipolar and a unipolar pulse, if a SiO₂ glass capping layer covers the excited TbFe₂ layer. The inverse magnetostriction of the temporally separated unipolar strain pulses in this sample leads to a MOKE signal that linearly depends on the strain pulse amplitude measured through UXRD. Linear chain model simulations accurately predict the timing and shape of UXRD and MOKE signals that are caused by the strain reflections from multiple interfaces in the heterostructure.

Time-domain Brillouin scattering for the determination of laser-induced temperature gradients in liquids

We present an optical technique based on ultrafast photoacoustics to determine the local temperature distribution profile in liquid samples in contact with a laser heated optical transducer. This ultrafast pump-probe experiment uses time-domain Brillouin scattering (TDBS) to locally determine the light scattering frequency shift. As the temperature influences the Brillouin scattering frequency, the TDBS signal probes the local laser-induced temperature distribution in the liquid. We demonstrate the relevance and the sensitivity of this technique for the measurement of the absolute laser-induced temperature gradient of a glass forming liquid prototype, glycerol, at different laser pump powers-i.e., different steady state background temperatures. Complementarily, our experiments illustrate how this TDBS technique can be applied to measure thermal diffusion in complex multilayer systems in contact with a surrounding liquid.

Interferometric analysis of laser-driven cylindrically focusing shock waves in a thin liquid layer

Shock waves in condensed matter are of great importance for many areas of science and technology ranging from inertially confined fusion to planetary science and medicine. In laboratory studies of shock waves, there is a need in developing diagnostic techniques capable of measuring parameters of materials under shock with high spatial resolution. Here, time-resolved interferometric imaging is used to study laser-driven focusing shock waves in a thin liquid layer in an all-optical experiment. Shock waves are generated in a 10 µm-thick layer of water by focusing intense picosecond laser pulses into a ring of 95 µm radius. Using a Mach-Zehnder interferometer and time-delayed femtosecond laser pulses, we obtain a series of images tracing the shock wave as it converges at the center of the ring before reemerging as a diverging shock, resulting in the formation of a cavitation bubble. Through quantitative analysis of the interferograms, density profiles of shocked samples are extracted. The experimental geometry used in our study opens prospects for spatially resolved spectroscopic studies of materials under shock compression.

Ultrafast acousto-optic mode conversion in optically birefringent ferroelectrics

The ability to generate efficient giga–terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse, we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO₃ and LiNbO₃. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics.

Electrically driven acousto-optic light modulators are limited to frequencies of a few hundred megahertz and are typically no smaller than a few micrometres. Here, the authors demonstrate gigahertz acousto-optic conversion of light polarization in a region of a few nanometres using pulsed laser stimulation of a ferroelectric.