Seminars Archive
Charge carrier dynamics by secondary electron detection in Ultrafast Scanning Electron Microscopy
Department of Physics, Politecnico di Milano
Abstract
Ultrafast Scanning Electron Microscopy (USEM) aims at combining the temporal resolution of femtosecond laser spectroscopy and the nanometer spatial resolution of electron microscopy to characterize the dynamics of photo‑induced processes at surfaces and in ultra-thin films.
To provide time-resolved operation, the USEM apparatus works in pump-probe configuration, where the optical pump beam from a pulsed fiber laser in the femtosecond regime enters the SEM chamber and excites charge-related phenomena, that are probed by a scanned pulsed electron beam. The SE signal is detected by an Everhart-Thorley detector, either in current mode for time resolved imaging or by lock-in demodulation for time spectroscopy on selected areas.
As a calibrated temporal delay between the pump and probe beams is set and scanned, the pump-probe approach works in an equivalent time frame and retrieves the optically driven dynamical evolution in the SE contrast on temporal scales ranging from picoseconds to nanoseconds.
Our USEM apparatus issues from a collaboration between Politecnico di Milano and IIT, with the collaboration of CNR-IFN and is based on a modified UHV (10-9÷10-10 Torr) SEM, equipped with a ZrO‑coated W Field-Effect electron tip. The Third Harmonic (TH) and Fourth Harmonic (FH) UV beams of a fiber laser source (300 fs , operated at 10 MHz repetition rate) are used to excite respectively charge-related phenomena in the specimen under test, and the pulsed emission of electron bunches from the SEM tip. The pump-probe relative delay is tuned with sub-ps resolution by a mechanical delay stage. A signal rise time of about 10 ps has been demonstrated.
The Seminar will present the USEM experimental setup as well as results on photo-induced charge carrier dynamics under strong UV optical pumping at the Si(100) surface.
The constraints and artifacts resulting from the choice of the experimental conditions will also be discussed.
