The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer's high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate.

The influence of femtosecond laser pulse chirp on laser-generated longitudinal acoustic strains in Si (100) monocrystal substrates is studied. Degenerate femtosecond pump-probe transient reflectivity measurements are performed using a layered structure of thin Ti transducer film on an Si substrate. Experimental results show that acoustic strains, manifested as strong Brillouin oscillations, are more effectively induced when negatively chirped femtosecond laser pulses pump the transducer. These results are theoretically supported by a modified thermo-mechanical model based on the combination of a revised two-temperature model and elasticity theory that takes into account the instantaneous frequency of the chirped femtosecond laser pump pulses.

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The generation of high frequency acoustic waves by picosecond laser pulses in the ultraviolet region and their detection by optical interferometric schemes, is presented. The two main acoustical modes, longitudinal and shear are clearly apparent in the time resolved spectra of solid materials, for various absorbing energies, extending from the thermoelastic to the ablative regime. The ultraviolet light is strongly absorbed by insulator materials like Pyrex and thus strong elastic waves are produced. From the time separation of the longitudinal waves we have deduced values for the speed of sound in various materials and of different thickness, that are in very good agreement with those reported in the literature. Also the time bandwidth of the sound waves is measured and significant differences, originating from different sample thickness, are apparent.