Researchers from the College of Advanced Interdisciplinary Studies at the National University of Defense Technology developed a vacuum ultraviolet (VUV) laser system for scanning photoemission microscopy. The 177-nm VUV system was developed with a focal spot of less than 1 μm at a long focal length of approximately 45 mm using a spherical aberration-free zone plate.
Based on the setup, the team also developed an off-axis fluorescence detection platform that exhibits substantial capability in revealing subtle features of quantum materials when compared to conventional laser systems.
The system is well suited for the evaluation of electronic structures of 2D quantum materials such as twisted bilayer graphene, monolayer copper superconductors, and quantum spin Hall materials. Traditionally, a technique called angle-resolved photoemission spectroscopy (ARPES) is used to measure the energy and momentum of electrons photoemitted from samples illuminated by x-ray or VUV light sources. Although the x-ray-based spatially resolved ARPES offers spatial resolution at approximately 100 nm, its energy resolution is greater than 10 meV, making it difficult to use to visualize finer details of the electronic structure in quantum materials.
Complementary to x-ray light sources, VUV laser-based light sources deliver improved energy resolution at approximately 0.2 meV, deeper depth of detection, and lower cost compared to synchrotron sources. The longer wavelengths of the VUV light source tends to deteriorate its spatial resolution, though, making it insufficient for characterizing small flake samples or spatially nonhomogenous materials such as those that might be magnetic, electronic, or composite.
Compared to the current source, the researchers’ 177-nm VUV laser could help ARPES span a larger momentum space with better energy resolution, though spatial resolution remains a problem area.
According to the researchers, the first obstacle lies in the severe spherical aberrations existing in a high numerical aperture (NA) lens. The second is that there is a very limited number of materials that can be used in optics for correcting the spherical aberration due to the strong adsorption at VUV frequencies.
Checking the quality (collimation, uniformity, and efficient diameter) of the incident, as well as the alignment of the optical elements, is also difficult; the VUV beam is invisible and the optical elements must be placed in a vacuum or a sealed chamber filled with inert gas.
The VUV laser focusing system consists of five essential parts: a 355-nm laser, a second harmonic generation stage, a beam-shaping stage, a polarization adjustment part, and a focusing element of the flat lens. The researchers avoided spherical aberration by introducing planar diffractive lenses that can realize tight focusing of light by fine tuning the interference from multiple beams. The system is able to achieve long focal length at around 45 mm, submicron spatial resolution at approximately 760 nm, ultrahigh energy resolution at about 0.3 meV, and ultrahigh brightness around 355 MWm−2. It can be directly applied to scientific research instruments such as photoemission electron microscopes, ARPES, and deep ultraviolet laser Raman spectrometers.
Currently, the system has been integrated with the ARPES at ShanghaiTech University where it is able to reveal the fine energy band features of novel quantum materials, such as quasi-1D topological superconductors and magnetic topological insulators, the researchers said.
The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00463-3).