Guillaume Dodane defended his PhD thesis on october 14th, 2015

High resolution femtosecond instrumentation platform (in French)

We present in this work an investigation of thermal and acoustic properties of samples by thermoreflectance with high spatial resolution and by acoustic picosecond. This thesis is part of the ANR project PHEMTO (PHEnomènes et Métrologie Thermiques femtOsecondes), where a femtosecond heterodyne pump-probe platform was developed.

A theoretical model, the thermal quadrupole method, is used to extract the thermal properties of the studied sample. Some parameters have to to be fixed and the free parameters (thermal conductivity of the different layers and of the substrate as well as the thermal interfaces resistances) are obtained by adjusting the theoretical curves with the experimental ones. The fixed parameters are the thickness of the thin layers, measured by acoustic picosecond and the optical beams sizes. These sizes are measured by a method using the spatial convolution between the Gaussian profile of the probe beam and the variation of reflectivity induced by the pump beam.

Thereafter, to increase the spatial resolution of the pump-probe system, a solid immersion lens (SIL) has been designed and manufactured. The increase of the spatial resolution was first demonstrated using a continuous laser then the lens was integrated into the femtosecond platform. The position of the SIL, aligned with the axis of the objective, on the sample is the critical point of this integration. To manage this positioning, a deflectometry method and a reflected power measurement method were first used. To improve the coupling between the lens and the sample, interference fringes were used. However, the use of integrated solid immersion lens into the femtosecond pump-probe platform has not yet resulted in an improvement in spatial resolution.

Thermal and acoustic characterizations were carried out on different samples. A study of the thermoreflectance coefficient of various metals was performed at the wavelength of 515 nm. The speed of propagation of longitudinal and Rayleigh waves were measured and compared with bulk values. Thermal and acoustic characterizations were also performed on nanostructured films by GLAD technique (glanced Angle Deposition). The originality of these samples is that they can in particular be anisotropic. This in plane anisotropy has indeed been highlighted particularly in the measurement of elastic wave velocities.

Keywords : Pump-probe, Thermoreflectance, Femtosecond, Picosecond acoustics, Solid Immersion Lens (SIL), Glanced Angle Deposition (GLAD)