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Schematic of the Optical Gating technique used to measure the time-resolved intensity with sub-picosecond resolution. Two femtosecond ultrashort pulses are produced synchronously. The probe pulse interacts with the material resulting in a longer signal, while the gate pulse is delayed. A Non-linear Crystal (NLC) operates a sum-frequency upconversion. For a set delay Δτi the intensity of the sum-frequency signal ω3 will be proportional to the convolution between the scattered intensity through the sample and the gate pulse.
Transient imaging measurements of the back-scattered intensity distribution recorded at different times for a scattering sample. The relative time delay between consecutive frames is 1 ps. Each frame is normalized to its maximum and has lateral dimension of 5 mm.
Time of flight spectroscopy
Time of flight spectroscopy
Time domain techniques allow the observation of light as it travels through and interacts with optically disordered media. These techniques provide critical information about the medium's scattering properties and internal structure. In particular, in time-resolved spectroscopy ultrashort laser pulses are directed into a scattering medium, and the emerging light is measured as a function of time. By using pulses with a duration of only a few tens of femtoseconds, it it is possible to capture how light scatters and diffuses over short transients. This ultrafast interaction of light with matter through scattering, absorption, and re-emission reveals details about the optical and structural properties of the medium.
Our research group aims to expand the capabilities of time-resolved techniques by resolving also light transport in the space domain. This provides a comprehensive understanding of the light diffusion process. Specifically, we employ a technique called Transient Imaging based on an ultrafast Optical Gating mechanism, which allows us to capture the spatial profile of scattered light at different moments in time. By precisely controlling the time window for detection, optical gating enables us to visualize the evolution of a single focused pulse as it diffuses through the medium, taking snapshots of the intensity patterns. This multidomain capability reveals information that would otherwise remain hidden. For instance, insights on single-scattering event properties is provided by resolving the early transients of propagation within the few first picoseconds, before the diffusive regime fully sets in.
The research topics we investigated through the years include diagnostics of photonic circuits, anomalous light transport in Lévy glasses, light transport structurally anisotropic media, and time-resolved depolarization.
Contacts:
Contacts:
Lorenzo Pattelli
Ernesto Pini
Main collaborators:
Main collaborators:
Dr. Alexander Gatto (SONY Europe B.V., Stuttgart, Germany)
Main publications:
Main publications:
Pini, E., Naglič, P., Bürmen, M., Gatto, A., Schäfer, H., Wiersma, D.S. and Pattelli, L., 2024. Experimental determination of effective light transport properties in fully anisotropic media. Advanced Photonics Nexus, 3(5), 056017.
Pini, E., Martelli, F., Gatto, A., Schäfer, H., Wiersma, D.S. and Pattelli, L., 2024. Diffusion of light in structurally anisotropic media with uniaxial symmetry. Physical Review Research, 6(2), 023051.
Pattelli, L., Savo, R., Burresi, M. and Wiersma, D.S., 2016. Spatio-temporal visualization of light transport in complex photonic structures. Light: Science & Applications, 5(5), e16090
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