Nonlinear photoacoustics: Solitons, shocks, and bond-breaking in nano- and picosecond laser ultrasonics
Prof. Dr. Peter Hess
Physikalisch Chemisches Institut,Ruprecht-Karls-Universität Heidelberg
17:00 - 18:00 Tuesday 20 October 2009 KFU HS 05.01 The advent of lasers has revolutionized both optics and acoustics. This is especially true for the current development of nonlinear optics and nonlinear acoustics. Despite the fact that inherent nonlinear effects are much smaller in optics than in acoustics, currently the field of nonlinear optics is developing faster. However, the advent of confined laser radiation and of intense ultrashort laser pulses also initiated a strong impetus on the investigation of nonlinear photoacoustics. The efficient coupling between the acoustic and electromagnetic fields occurs via fluctuations in the dielectric constant with its dispersive and absorptive part, where transfer mainly occurs from the optic to the acoustic fields. Very large acoustic phonon populations and amplitudes can be realized giving access to extreme nonlinear effects including shock waves and breakdown. While in state-of-the-art experiments acoustic phonon pulses in the submicron and picosecond spatiotemporal scales can be realized coupling is efficient enough that already in the nanosecond time scale strongly nonlinear coherent acoustic pulses can be generated.
In picosecond ultrasonics nonlinear longitudinal bulk waves have been launched by thermoelastic emission, using a thin metal film transducer that strongly absorbs the laser radiation. The length of these acoustic pulses is in the picosecond range with a frequency bandwidth of several hundred gigahertz. At very low temperatures (<35 K), when attenuation essentially can be neglected, acoustic bulk solitons could be excited in crystalline materials [1]. With an amplified picosecond laser system, delivering millijoule laser pulses, strongly nonlinear strain pulses can be detected, even at room temperature, after propagation of a sufficient distance needed for the development of steep shock fronts [2]. The maximum strain reached with the thermolastic excitation process was in the range of 10-3. The steepest shock fronts had an estimated rise time of ~1 ps, corresponding to about ten lattice constants.
In nanosecond ultrasonics nonlinear effects are quite pronounced for guided surface acoustic waves (SAWs), propagating along the surface. In fact, compared to bulk acoustic waves, the confinement and directivity of nanosecond SAW pulses provide unique possibilities to study the elastic nonlinearity of solids up to the elastic limit. With the absorption-layer method SAW pulses with a strain of about 10-2 can be realized. SAWs in homogeneous solids are nondispersive, whereas layered systems are dispersive and the competing influence of nonlinearity, diffraction, dissipation, and dispersion has to be taken into account. If nonlinear and dispersive effects cancel each other solitary surface waves can be launched [3]. On the other hand, in SAWs the formation of steep shock fronts can be driven to the point where the material breaks. Thus nonlinear SAW pulses are an ideal diagnostic tool to study nonlinear mechanical properties, such as the critical fracture stress or strength of materials [4]. Furthermore, based on the directivity of plane elliptically polarized SAWs, the anisotropic fracture mechanics of crystals can be investigated in an unprecedented manner. The mode-resolved fracture strength of a defined failure geometry can be directly compared with ab initio calculations of the critical stress of ideal crystals. Furthermore, the results can be related to the nanoscale fracture mechanisms simulated by combining classical potentials and quantum mechanics.
[1] H.-Y. Hao and H. J. Maris, Phys. Rev. B 64, 064302 (2001).
[2] P. J. S. van Capel and J. I. Dijkhuis, Appl. Phys. Lett. 88, 151910 (2006).
[3] A. M. Lomonosov, P. Hess, and A. P. Mayer, Phys. Rev. Lett. 88, 076104 (2002).
[4] V. V. Kozhushko, A. M. Lomonosov, and P. Hess, Phys. Rev. Lett. 98, 195505 (2007).
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