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. 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. That technique is an up-to-date approach to study the dynamic of charge on material surfaces.

Time resolved in scanning electron spectroscopy
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. 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.} 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. There was an early attempt to initiate this technique by Larry D.Flesner in a US patent in 1990, he incorporated the scanning electron microscopy and modulated light to study semiconductor surface photovoltaic in both time and space scale. 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.

Scanning electron microscopy
Scanning electron microscopy is a powerful technique for mapping of sample surface topography and material content in very wide range metal, semiconductor even organic samples in a vacuum or low-pressure environment. The principle depends on scanning the specimen with a beam of electrons a few nanometers in size. If the thickness of the sample is within few micrometers, the electron beam will be completely attenuated by interaction with electrons or atoms of the specimen. The interaction of an electron with the specimen may be elastic or inelastic. In the first case, there is low loss of energy, and the electrons may be backscattered out of the specimen. In the inelastic interaction process, low-energy electrons are ejected with energies up to about 30 KeV. The shown picture summarizes all kinds of possible interactions and their related depth to the sample. For example, x-rays may be generated from some depth or an Auger electron may be generated close to the surface. Specific detectors are used to detect the type of energy emitted and convert its intensity to an electronic signal. The final image, acquired and reconstructed by raster scan mode contains no colour information and is usually presented in presented in grayscale. Since secondary electrons have an energy less than 50 eV it only provides information from within a few nanometres of the surface.

Pump probe microscopy


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. 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. Nowadays, the appreciated impact to reach high progress in that phenomenon is directly coming from the nonlinear optics. There are many ways for nonlinear process interaction, for example second-harmonic generation, Coherent anti-Stokes Raman or two-photon-excited fluorescence. 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.

Measurement methodology
The fundamental idea that measurement has been built to exploit the Spatial resolution of electron microscopy and temporal resolution for ultrafast optical pump probe. 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. 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.

Applications
The powerfulness of that technique meets the requirement for investigation of innovative materials for electronics, sustainable energy harvesting and photonics that enables us to study the charge dynamic in deep for semiconductors materials which have been stimulated by ultrashort laser beam. It has powerful accessibility to carrier recombination and trapping in condensed matter physics that allows more progress in photovoltaics fabrication.