Christoph Klieber

Mechanical spectra of glass-forming liquids. II. Gigahertz-frequency longitudinal and shear acoustic dynamics in glycerol and DC704 studied by time-domain Brillouin scattering

This paper presents and discusses the temperature and frequency dependence of the longitudinal and shear viscoelastic response at MHz and GHz frequencies of the intermediate glass former glycerol and the fragile glass former tetramethyl-tetraphenyl-trisiloxane (DC704). Measurements were performed using the recently developed time-domain Brillouin scattering technique, in which acoustic waves are generated optically, propagated through nm thin liquid layers of different thicknesses, and detected optically after transmission into a transparent detection substrate. This allows for a determination of the frequency dependence of the speed of sound and the sound-wave attenuation. When the data are converted into mechanical moduli, a linear relationship between longitudinal and shear acoustic moduli is revealed, which is consistent with the generalized Cauchy relation. In glycerol, the temperature dependence of the shear acoustic relaxation time agrees well with literature data for dielectric measurements. In DC704, combining the new data with data from measurements obtained previously by piezo-ceramic transducers yields figures showing the longitudinal and shear sound velocities at frequencies from mHz to GHz over an extended range of temperatures. The shoving models prediction for the relaxation times temperature dependence is fairly well obeyed for both liquids as demonstrated from a plot with no adjustable parameters. Finally, we show that for both liquids the instantaneous shear modulus follows an exponential temperature dependence to a good approximation, as predicted by Granatos interstitialcy model.

Narrow-band acoustic attenuation measurements in vitreous silica at frequencies between 20 and 400 GHz

Acoustic attenuation rates in vitreous silica in the 20–400 GHz frequency range have been measured using a multiple-pulse optical technique for generation of tunable multicycle acoustic waves that are detected interferometrically after traversal of the sample. The results connect the frequency ranges of several measurement methods, yielding a consistent description of the acoustic behavior.

Optical generation and detection of gigahertz-frequency longitudinal and shear acoustic waves in liquids: Theory and experiment

We describe an adaptation of picosecond laser ultrasonics tailored for study of GHz-frequency longitudinal and shear acoustic waves in liquids. Time-domain coherent Brillouin scattering is used to detect multicycle acoustic waves after their propagation through variable thickness liquid layers into a solid substrate. A specialized optical pulse shaping method is used to generate sequences of pulses whose repetition rate determines the acoustic frequency. The measurements reveal the viscoelastic liquid properties and also include signatures of the optical and acoustic cavities formed by the multilayer sample assembly. Modeling of the signals allows their features to be distinguished so that liquid properties can be extracted reliably. Longitudinal and shear acoustic wave data from glycerol and from the silicon oil DC704 are presented.

Measurement of shorter-than-skin-depth acoustic pulses in a metal film via transient reflectivity

The detection of ultrashort laser-generated acoustic pulses at a metal surface and the reconstruction of the acoustic strain profile are investigated. A 2 ps-long acoustic pulse generated in an SrRuO3 layer propagates through an adjacent gold layer and is detected at its surface by a reflected probe pulse. We show that the intricate shape of the transient reflectivity waveform and the ability to resolve acoustic pulses shorter than the optical skin depth are controlled by a single parameter, which is determined by the ratio of the real and imaginary parts of the photoelastic constant of the material. We also demonstrate a Fourier transform-based algorithm that can be used to extract acoustic strain profiles from transient reflectivity measurements.

Coherent Brillouin spectroscopy in a strongly scattering liquid by picosecond ultrasonics

In a modification of a picosecond ultrasonic technique, a short acoustic pulse is launched into a liquid sample by a laser pulse absorbed in a semitransparent transducer film and is detected via coherent Brillouin scattering of a time-delayed probe pulse. With both excitation and probing performed from the transducer side, the arrangement is suitable for in vivo study of biological tissues. The signal is collected from a micrometer-thick layer next to the transducer and is not affected by the diffuse scattering of probe light deeper in the sample. The setup, utilizing a 33 nm thick single crystal SrRuO(3) transducer film, is tested on a full fat milk sample, with 11 GHz acoustic frequency recorded.

Nonlinear Acoustics at GHz Frequencies in a Viscoelastic Fragile Glass Former

Using a picosecond pump-probe ultrasonic technique, we study the propagation of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the viscoelastic fragile glass former DC704. We observe an increase of almost 10% in acoustic pulse propagation speed at the highest optical pump fluence which is a result of the supersonic nature of nonlinear propagation in the viscous medium. From our measurement, we deduce the nonlinear acoustic parameter of the glass former in the gigahertz frequency range across the glass transition temperature.

Femtosecond imaging of nonlinear acoustics in gold

We have developed a high-sensitivity, low-noise femtosecond imaging technique based on pump-probe time-resolved measurements with a standard CCD camera. The approach used in the experiment is based on lock-in acquisitions of images generated by a femtosecond laser probe synchronized to modulation of a femtosecond laser pump at the same rate. This technique allows time-resolved imaging of laser-excited phenomena with femtosecond time resolution. We illustrate the technique by time-resolved imaging of the nonlinear reshaping of a laser-excited picosecond acoustic pulse after propagation through a thin gold layer. Image analysis reveals the direct 2D visualization of the nonlinear acoustic propagation of the picosecond acoustic pulse. Many ultrafast pump-probe investigations can profit from this technique because of the wealth of information it provides over a typical single diode and lock-in amplifier setup, for example it can be used to image ultrasonic echoes in biological samples.

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.

Toward broadband mechanical spectroscopy

Significance Viscous liquids of all kinds, from honey to glycerol to common polymers, display remarkably similar dynamical properties upon cooling from high temperatures at which local structural relaxation and flow occur quickly, to moderate temperatures at which key components of the dynamics slow down dramatically, to cold temperatures at which a glassy solid is formed. The similarities suggest a common theoretical framework, but comprehensive measurements of liquids’ mechanical properties covering the extraordinary range of time scales spanned have been elusive. Using seven experimental methods covering 13 decades with few gaps, we demonstrate that broadband mechanical spectra are now within reach. Data obtained for the molecular glass former studied, a commercial silicone oil, follow key scaling laws predicted long ago. Diverse material classes exhibit qualitatively similar behavior when made viscous upon cooling toward the glass transition, suggesting a common theoretical basis. We used seven different measurement methods to determine the mechanical relaxation kinetics of a prototype molecular glass former over a temporal range of 13 decades and over a temperature range spanning liquid to glassy states. The data conform to time–temperature superposition for the main (alpha) process and to a scaling relation of schematic mode-coupling theory. The broadband mechanical measurements demonstrated have fundamental and practical applications in polymer science, geophysics, multifunctional materials, and other areas.T. Hecksher, a) D. H. Torchinsky, a) C. Klieber, a) J. A. Johnson, a) J. C. Dyre, and Keith A. Nelson Glass and Time, IMFUFA, Dept. of Science and Environment, Roskilde University, DK-4000 Roskilde, Denmark Department of Physics, MIT, Cambridge, MA 02139, USA Department of Chemistry, MIT, Cambridge, MA 02139, USA DNRF Centre Glass and Time, IMFUFA, Dept. of Sciences, Roskilde University, DK-4000 Roskilde, Denmark

Effect of microannulus on ultrasonic pulse-echo resonance, flexural, and extensional Lamb-wave cement-evaluation measurements

Subterranean wells are usually constructed by cementing steel tubes, called casings, inside the borehole. The cement quality is typically verified through ultrasonic measurements deployed from inside the casing. Environmental effects such as cement shrinkage or changes in static pressure can alter the bonding properties between casing and cement with significant effects on the acoustic measurement response. The cement may detach from the casing, opening a gap, called a microannulus. This microannulus is sized from submicrometer to hundreds of micrometers and filled with either gas or liquid. The subwavelength nature of the microannuli does not allow a direct, unambiguous characterization through an ultrasonic measurement. We studied the measurement signature of ultrasonic-pulse-echo resonance, and flexural and extensional Lamb waves for air- and liquid-filled microannuli for various annulus materials and steel-casing thicknesses. This characterization allows statistically linking measured results to micro...