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Résumé:

A powerful imaging technique based on femtosecond time-resolved measurements with a high dynamic range, commercial CCD camera is presented. Ultrafast phenomena induced by a femtosecond laser pump are visualized through the lock-in type acquisition of images recorded by a femtosecond laser probe. This technique allows time-resolved measurements of laser excited phenomena at multiple probe wavelengths (spectrometer mode) or conventional imaging of the sample surface (imaging mode). Two examples based on time-resolved imaging of the sample surface are presented. First, we follow the non-linear reshaping of an acoustic strain pulse in a hybrid gold/cobalt layer at ambient temperature in a sequence of images. Second, we examine time-resolved images of transient thermal heating in graphene samples where we observe heat dissipation as fast as one picosecond which neither depends on the number of layers nor the shape of the graphene sample. A wealth of adaptations of the presented technique can be considered such as imaging of ultrasonic echoes in biological samples (ultrasonic microscopy with resolution down to tens of nanometer).