Laser-based nonlinear surface acoustic waves: From solitary to bond- breaking shock waves

Peter Hess 1; Alexey M. Lomonosov 2; Victor V. Kozhushko 2
1University of Heidelberg, Physical Chemistry, Heidelberg, Germany; 2University of Heidelberg, Physical Chemistry, Heidelberg, Germany


Background and method: Nonlinear surface acoustic waves (SAWs) can be launched with pulsed lasers and detected with cw lasers in a pump-probe setup. With the absorption-layer method, strongly nonlinear SAW pulses can be excited. Due to elastic nonlinearity of materials SAW pulses with finite amplitude are modified during propagation and frequency-up conversion processes generate higher frequencies and shocks. This technique yields mechanical strains of ~0.01.
 
Results: By adding a dispersive layer, a length scale is introduced and the nonlinear behavior can be balanced by dispersion. Thus solitary surface waves could be generated and detected with the pump-probe setup. Strongly nonlinear SAW pulses with step shock fronts introduced fracture in single-crystal silicon. Nucleation started at the surface, with the highest transient strain. Tensile and shear stresses of ns SAW pulses with a bandwidth of 10 MHz to 1 GHz induce biaxial fracture. For low-index planes of Si the measured tensile strength varied between 2-7 GPa with dominant tensile forces. The results are compared with ab initio calculations of the ideal tensile strength of 22 GPa of the Si{111} cleavage plane and the low ideal shear strength of 6.8 GPa.

Surface-breaking microcracks, introduced by crack-generating nonlinear SAW pulses in fused silica, cause characteristic changes in the nonlinear SAW profile studied theoretically and experimentally in isotropic silica. The reflection and transmission of low-amplitude linear SAW pulses with a bandwidth£200 MHz by microcracks was probed optically and simulated by the finite difference method (FDM).

Conclusions: Solitary surface waves were realized in isotropic silica and anisotropic silicon. Intrinsic nucleation of microcracks without artificial pre-cracks was studied for defined planes and directions in anisotropic Si. Up to now mainly uniaxial fracture has been investigated in silicon, inducing nucleation at an artificial notch. Non-destructive evaluation (NDE) was extended to a spatial resolution of tens of micrometers, allowing the determination of the location, penetration depth, and nature of real microcracks.


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