Femtosecond pump-probe spectroscopy is a key tool to reveal incredibly accurate details about the ultrafast dynamics in various physical systems, with applications ranging from biology to material science. However, spatial information is completely missing in standard ultrafast spectroscopy, preventing a complete characterization of the system under study. Ultrafast microscopy provides the sub-micron spatial resolution needed to study material’s heterogeneity and spatially dependent transient phenomena. However, megapixel cameras are incapable of acquiring every single shot of the laser, resulting in an acquisition that is highly affected by the laser intensity fluctuations. For this reason, current pump-probe microscopes use single-pixel detectors and raster scanning of the sample or cameras with very few pixels, thus limiting the observations to very small 2D areas.

In our lab we have recently developed a solution to this problem based on multiplexed off-axis holography, which lets us capture shot-noise limited widefield transient images regardless of the frame rate and number of pixels, thus acting as an all-optical lock-in camera. We have applied this to measure the femtosecond response of single gold nanoparticles and also to study the ultrafast carrier diffusion in new semiconductor materials for photovoltaics. Importantly, since it is a holographic technique, both amplitude and phase images are obtained, which gives us access to all tools of modern digital holography. This means that when imaging single nanoobjects in a three-dimensional context, objects that are out of focus can be analyzed post-image acquisition and their precise depth position can be characterized. We have demonstrated this by shooting movies of the 3D Brownian motion of dozens of individual gold nanoparticles in solution over a volume of view of 100х100х100 μm³.

In this Master’s thesis, the student will be involved in further developing the all-optical lock-in camera by turning it hyperspectral. Currently, the holographic microscope operates at a single color with imaging pulse bandwidths of around 10 nm, which corresponds to 100 femtoseconds. In hyperspectral imaging, instead of acquiring the amplitude and phase for each pixel, a broadband light source will be placed and a full spectrum for each pixel will be measured via Fourier transform spectroscopy. This will prepare the ground for the holographic microscope to operate using sub-10 femtosecond pulses. The student will spend plenty of time in the lab, gaining familiarity with ultrafast optics and hyperspectral imaging techniques.

If you want to have a chat and visit the lab, or more information on how the technique works, please email franco.camargo@cnr.it.

See also:

• M. Liebel, F. V. A. Camargo, G. Cerullo, and N. F. van Hulst, Nano Letters 21, 4, 1666–1671 (2021)
• M. Liebel, F. V. A. Camargo, G. Cerullo, and N. F. van Hulst, Nanoscale 14, 3062-3068 (2022)
• M. Hörmann, F. Visentin, …, G. Cerullo, and F. V. A. Camargo, Ultrafast Science 3, 0032 (2023)