Time-resolved images of transient thermal heating in graphene samples. The heat dissipation is as fast as one picosecond and does not depend on the number of layers or the shape of the graphene sample.
Transient heating of graphene
This sequence of images shows the arrival of a relatively strong acoustic pulse at the gold free surface. Since the acoustic amplitude follows the gaussian profile of the focused pump beam, because of acoustic non-linearities in gold at high pressure, the center part of the acoustic pulse, with higher pressure, travels faster than the edge part with lower pressure. Thus the center part, in red, arrives before the edge part, in black. The difference in arrival time of about 2 picoseconds is in agreement with the non-linear parmeters of gold. This imaging technique can be used in order to extract non-linear acoustic parameters of any given material.
Time-domain Brillouin scattering in a silicon substrate. The semi-transparent Ti layer partially absorbes the pump light and launches an acoustic wavepacket that back-scatters the probe light inside the Silicon substrate. The light scattering by the acoustic waves appears as a time dependent modulation of the probe light intensity at the Brillouin frequency of Si of about 200 GHz.
Time-domain Brillouin scattering in a glass substrate. Once the acoustic waves, excited at the aluminium layer, are transmitted inside the SiO2 transparent substrate, the probe light intensity undergoes a time dependent modulation at the Brillouin frequency of SiO2 -under the specific optical configuration of a 400 nm probe light this frequency is about 40 GHz.
Time resolved Brillouin scattering
Brillouin scattering occurs when light is scattered by propagating acoustic waves in a medium. The light scattered field, whose optical phase varies depending on the acoustic wave peak and null positions, superposes with the reflected probe field. This results in signal intensity that shows time-dependent oscillations at a monochromatic frequency, named the Brillouin frequency, that depends on the material properties and on the optical configuration. The movies below show the modulation of the probe light intensity at the Brillouin frequency for two different materials.
Sketch of the pump-probe imaging experimental set up. The microscope objective is adjusted to image the sample surface on the CCD camera with high magnification. The pump beam that creates the phenomena being observed is modulated at the same rate than the image acquisition. After image processing, we obtain sequences of images showing time resolved evolution of the photoexcited phenomena.
Ultrafast Imaging of Laser-induced Phenomena
We have developed a technique based on femtosecond time-resolved measurements with a standard CCD camera with high dynamic range. The scheme used for the experiment is based on lock-in acquisitions of images from a femtosecond laser probe coupled with modulation of a femtosecond laser pump at the same rate. This technique allows time-resolved measurements of laser excited phenomenon at multiple probe wavelengths or traditional imaging of the surface sample. The technique could go far beyond the examples shown here, for instance it could potentially be used to image ultrasonic echoes in biological samples. Several examples based on time-resolved imaging of the sample surface are shown in the movie theater below.