Ultrafast scanning electron microscopy

Ultrafast scanning electron microscopy (UFSEM) combines two microscopic modalities, Pump-probe microscopy and Scanning electron microscope, to gather temporal and spatial resolution phenomena. The technique uses ultrashort laser pulses for pump excitation of the material and the sample response will be detected by an Everhart-Thornley detector. Acquiring data depends mainly on formation of images by raster scan mode after pumping with short laser pulse at different delay times. The characterization of the output image will be done through the temporal resolution aspect.^{[clarification needed]} Thus, the idea is to exploit the shorter DeBroglie wavelength in respect to the photons which has great impact to increase the resolution about 1 nm.^[1] That technique is an up-to-date approach to study the dynamic of charge on material surfaces.

Ernst Ruska was a pioneer German scholar who won the Nobel prize in 1986 for his work on the development of an electron microscope in 1933 in collaboration with Max Knoll.^[2] Nowadays, electron microscopy is miscellaneous used tool due to enhancement not only the spatial resolution respect to the optical microscope but also high imaging contrast and remarkable sensitivity due to the fact that the robustness of electrons impact on the matter in comparison with photons.^{[clarification needed]}} Proceeding from that concept, the technology of ultrafast scanning electron microscopy has been modified by assistance of Ultrashort pulse laser which allows the scientists to investigate material dynamic in short and ultra-short scale of time. Nowadays, pump-probe microscopy has been improved after Ahmed Zewail's discovery of femtosecond time scale for chemical reaction and has awarded the Nobel Prize for his historical discovery.^{[3][4][5][6][7]}

Pump-probe microscopy phenomenon, widely known as transient absorption microscopy, is a sort of nonlinear process starting by excitation of the material by very short pulse laser beam (pump), which induces internal transition.^[8] A probe beam follows the pump beam to trace the progress that has been done inside the material also in very short time. In reality, that response could be changed by manipulating the time delay between pump and probe and by this way the concept of Time-resolved spectroscopy will be used to trace dynamic process evolution as a function of time. ^[9] The fascinating in Ultrafast scanning electron microscopy is how powerful it obtains by combining high spatial resolution of the electrons and temporal resolution of ultra-fast pump-probe microscopy.^[10]

The fundamental idea that measurement has been built to exploit the Spatial resolution of electron microscopy and temporal resolution for ultrafast optical pump probe.^[11] The setup simply consists of scanning electron microscopy machine always works in ultra-high vacuum that regarding on electron beam as a probe and ultrashort laser beam as pump.^[12] Firstly, Schottky emission gun is almost common to use as source of primary beam due to high beam brightness after passing through electromagnetic lens. Secondly, femtosecond Powerful fibre laser with repetition rates from KHZ to few of MHz splits by nonlinear process into third and fourth harmonic generation 343 nm and 257 nm, respectively. During the measurement, the tip emission is less than thermal emission limit to acquire photoemission mode. That photoemission mode improves by allow forth harmonic generation beam to interact the tip which generates more electrons. On the other hand, another third harmonic generation will be used to excite the sample itself. The time-resolved measurement will be acquired by detecting the secondary electron emission in image shape at different delay time between third and fourth harmonic beam. The final acquired intensity must be normalized by subtraction from the background. It is important to acquire the measurement at different delay time forward and reverse that a good tool for checking the stability and reproducibility. ^[13]

• Time-resolved mass spectrometry
• Terahertz time-domain spectroscopy
• Ultrafast laser spectroscopy
• Surface states
• Attophysics
• Femtotechnology

• Ultrafast Scanning Electron Microscopy (USEM)-Dipartimento di Fisica - Politecnico di Milano
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