We report a method for measuring spot size and focusing conditions of the femtosecond (fs) excitation laser in situ at the specimen location in 4D ultrafast electron microscopy (UEM). The method makes use of threshold laser ablation behaviors of thin amorphous carbon membranes. For Gaussian beam profiles and for ablation threshold fluence values, we analytically derive expressions describing the relationship between ablated hole size and the actual laser spot size. Using these expressions, we developed experimental procedures for characterizing the shape and spot size of the pump beam at the specimen. We demonstrate the viability of the approach for incident excitation wavelengths of 343 nm and 515 nm, thus illustrating the applicability of the method to a range of optical wavelengths without modification. Further, we show that by measuring ablated hole size as a function of focusing condition, a full metrological characterization of the Gaussian beam propagation properties can be performed. Finally, we find good agreement for spot sizes determined with this method and with those determined by extrapolation from measurements taken outside the microscope column. Overall, this method is a simple, cost-effective means for accurate and precise determination of key pump-beam parameters in situ at the specimen location in UEM experiments.
Bibliographical noteFunding Information:
This material is based upon work supported by the U.S. Department of Energy , Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0018204 . This work was supported partially by the U.S. Department of Energy through the UMN Center for Quantum Materials under Grant No. DE-SC-0016371 . Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of the research under Award No. 60584-ND10 .
- In-situ TEM
- Ultrafast electron diffraction
PubMed: MeSH publication types
- Journal Article