Unraveling Phononic, Optoacoustic, and Mechanical Properties of Metals with Light-Driven Hypersound

Hao Zhang, Alessandro Antoncecchi, Stephen Edward, Irwan Setija, Paul Planken, and Stefan Witte

Phys. Rev. Applied 13, 014010 – Published 8 January 2020

Abstract

A thorough understanding of the electron-phonon, thermo-optic, and acoustic properties of materials is of paramount importance for many applications in materials science and advanced applications adopting laser-induced sound waves. Even though metals are usually opaque to light, optical methods for materials characterization can still be developed, especially in the high-frequency regime, where phonon dynamics governs the thermal, acoustic, and even optical properties of metals. Ultrafast laser pulses incident on metals can lead to the generation of coherent phonon wave packets with frequencies in the gigahertz to terahertz range, providing a means to study the material properties in this otherwise inaccessible frequency range. While this principle has been known, the complex interplay of light and matter in both the generation and detection of such ultrafast hypersound pulses has limited its use mainly to geometrical effects. Here, we demonstrate the quantitative characterization of a range of different material properties using laser-driven hypersound. We use all-optical generation and detection of hypersound pulses to sensitively probe the bulk properties of various metals. We introduce an advanced two-dimensional numerical model that captures the generation, propagation, and detection of these hypersound waves in full detail. The combination of experiment and simulation allows us to unravel and elucidate various physical effects that appear over a wide range of different time scales. Through least-squares fitting of the data to the simulation results, we extract quantitative information about the electron-phonon, thermo-optic, and acoustic properties of metal films, establishing the ability to use light as a sensitive probe for the study of opaque materials.

Received 26 April 2019
Revised 21 November 2019

DOI:https://doi.org/10.1103/PhysRevApplied.13.014010

Physics Subject Headings (PhySH)

Acoustic phonons Electron-phonon coupling Mechanical & acoustical properties Ultrasonics

Metals Thin films

Ultrafast pump-probe spectroscopy Ultrasound techniques

General PhysicsAtomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Hao Zhang ^1,2,*, Alessandro Antoncecchi^1,2, Stephen Edward^1,3, Irwan Setija⁴, Paul Planken^1,3, and Stefan Witte^1,2,†

¹Advanced Research Center for Nanolithography, Science Park 106, 1098 XG Amsterdam, Netherlands
²Vrije Universiteit Amsterdam, De Boelelaan 1105, 1081 HV Amsterdam, Netherlands
³Universiteit van Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands
⁴ASML Research, De Run 6501, 5504 DR Veldhoven, Netherlands

^*h.zhang@arcnl.nl
^†S.Witte@arcnl.nl

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 13, Iss. 1 — January 2020

Subject Areas

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
A schematic of the laser-induced hypersound experiments. (a) An ultrafast laser pulse (here depicted as a transient grating, where two pulses interfere at the sample surface), heats the electrons in a thin layer at the surface of an opaque medium (here shown as partially transparent for viewing purposes). (b) The electron energy is transferred to the lattice, resulting in ultrahigh-frequency acoustic phonons that travel into the sample as longitudinal acoustic waves (LAWs) or along the surface as SAWs. A local temperature increase at the surface also results in thermal stress (TS). (c) The returning LAWs, as well as the SAWs, and the TS lead to surface displacement and refractive-index changes at the surface, which are detected with a delayed probe pulse through, e.g., induced diffraction.
Reuse & Permissions
Figure 2
(a) A schematic of the experimental setup. Two crossed pump beams are used to create interference fringes on the sample surface. (b) A typical diffraction-efficiency (DE) measurement on a 200-nm $Au$ film deposited on a glass substrate and a model calculation that reproduces the measured acoustic response well. The inset shows the calculated surface displacement (in the direction perpendicular to the surface) at the center of the grating line. The arrows mark the returning of echoes. Note that the experimental curve has been given an offset for clarity.
Reuse & Permissions
Figure 3
(a) The measured diffraction efficiency (dots) and corresponding model fit (solid line) on a 400-nm $Al$ free-standing membrane. The dotted line, dashed line, and dash-dotted line are diffraction efficiencies from each contribution: the total surface displacement, the surface displacement of thermal surface expansion (TSE) and Lamb waves, and the thermo-optic effect. The increase of the surface temperature at the center of the grating line is shown in (b). The calculated diffraction efficiency (normalized) due to the interference of Lamb waves is shown in (c) for an extended period of time, illustrating its periodic nature. The normal component of the displacement vector along the sample surface is shown in (d) for three time instants, which are marked by black arrows in (a). (e) The normal component of the surface displacement vector (blue dashed line) at the periodic boundary ( $y = 0$ ). The double-headed arrows mark the timing of the extra dips. The displacement vector distribution at four time instants [marked by red arrows in (a)], which illustrate (f) the generation of TSE and the LAW, (g) contraction of the surface due to the return of the longitudinal wave, (h) the recovery of the TSE, and (i) the appearance of Lamb waves at longer time delays. The color represents the increase in the lattice temperature. The blue arrows indicate the propagation direction of the longitudinal wave and the Lamb waves. Note that the displacement vectors are highly exaggerated in order to be visible for solely illustrative purposes. See also Videos 2 and 3 in the Supplemental Material [47].
Reuse & Permissions
Figure 4
The dispersion relation of Lamb waves in the aluminum membrane retrieved from the simulation (color map). The black and red dots are the solution of the analytical expression of the Lamb-wave dispersion relations. The black dots correspond to the asymmetric modes and the red dots to the symmetric modes. The vertical dashed line indicates the wave number of the transient grating used in the experiment, while the horizontal dashed line indicates the round-trip frequency of the longitudinal echoes.
Reuse & Permissions
Figure 5
The measured diffraction efficiency (dots) as a function of the pump-probe delay and the corresponding model fits (solid lines) on (a) a sample consisting of 500 nm of $Au$ on a glass substrate and (b) a 500-nm Au free-standing membrane. The dotted line in (b) is a calculation with no acoustic attenuation [ $α_{l} (ω) = 0$ ]. The insets are the calculated surface displacement at the center of the grating line. The dashed lines mark the level of zero surface displacement. (c) The frequency-dependent acoustic attenuation and (d) the phase-velocity dispersion calculated by Eqs. (9) and (10) using the best-fit parameters (solid lines). The dashed lines are lower and upper bounds obtained from those fits with a 3% $Δ χ^{2}$ criterion. The vertical dotted lines indicate the FWHM of the acoustic pulse bandwidth. See also Videos 4 and 5 in the Supplemental Material [47].
Reuse & Permissions
Figure 6
(a) The measured diffraction efficiency (magenta dots) and the corresponding model calculation (solid line) on the sample consisting of 500 nm of $Au$ on a glass substrate pumped from the $Au$ -air side and probed at the $Au$ -glass interface. The diffraction efficiencies (normalized by the maximum of the total contributions) calculated by only the displacement at the $Au$ -glass interface and by only the strain-optic effect in the glass substrate are shown as dashed and dotted lines. The total contributions are a coherence addition of these two effects. (b) The strain ( $s_{x}$ component, color scale) and the displacement-vector distribution (black arrows) at $t = 582 ps$ . Positive strain represents expansion while negative strain means compression. The filled circles indicate the location of the $Au$ -glass interface. The blue arrow shows the propagation direction of the strain wave in glass. The green arrows illustrate the reflection of the probe pulse from the propagating strain pulses and the gold rear surface. See also Video 7 in the Supplemental Material [47].
Reuse & Permissions

Physical Review Applied