Laser-ultrasonics and imaging through metal layers
The combination of light and sound can be used to detect and image structures under opaque layers. In this thesis, we present the result of our studies on imaging and metrology through metal layers using laser-induced ultrasound. Ultrafast lasers can generate acoustic waves at high frequencies through the thermoelastic effect. The absorption of the energy of the laser pulse is followed by an increase in temperature and a subsequent expansion of the metal lattice. Stress is generated in the lattice, launching an acoustic wave inside the metal layer. The acoustic waves travel through the layers that compose the samples, reflecting off different interfaces and returning to the surface. When the reflected acoustic pulse comes back to the surface, it causes a change in the optical constants, as well as a physical displacement of the surface. A second laser pulse can be used to detect these changes in optical properties of the surface caused by the acoustic wave. In our research, we performed pump-probe experiments to study the generation, propagation and detection of ultrasound waves in metal layers.