Fano resonance in picosecond ultrasonics

Abstract

Brillouin light scattering, the scattering of light by acoustic waves, is generally a Lorentzian type of resonance. In this thesis, we investigate the possibility to observe Fano resonance in the light scattering by acoustic waves thanks to the picosecond ultrasonic techniques. We analyze in detail a total of three scenarios, namely, the semi-infinite substrate, membrane, and film on a substrate, to reveal the physics of Fano resonance. We first give a brief overview of Fano resonances and present the thesis of this manuscript: the undetectable frequencies of acoustic phonons in picosecond ultrasonics are due to Fano resonance. We then introduce the fundamentals of picosecond ultrasonics and present our experimental setup, on which we performed pump-probe spectroscopy. We then present our findings. We first demonstrate the Fano resonance in a semi-infinite substrate. By considering the reflection of a strain pulse at a free surface, we find out that the Stokes and anti-Stokes Brillouin scattering can exist simultaneously and interfere, leading to the appearance of a Fano resonance in the scattering cross-section. Then we perform experiments and observe the Fano interference by using a thick Tungsten film and observe an excellent agreement with our theoretical predictions in the case of the semi-infinite substrate. We further analyze the Fano equation in our system and reveal its complex behaviour gives insights into energy dissipation. It leads to the applications of our work in non-destructively characterizing the surface roughness. In the membrane case, we present a model derived from the semi-infinite substrate that reveals the appearance of two Fano resonances resulting from the reflection at the front and the back interface with the Stokes and anti-Stokes Brillouin scattering mechanism. Finally, we discover another mechanism in realizing the Fano resonance with a film and substrate material, which comes from the interference of the resonance due to the lattice vibrations in the film layer and the Brillouin scattering in the substrate. Our work developed a profound framework about the Fano resonance in picosecond ultrasonics.