Momentum-resolved EELS and CL study on 1D-plasmonic crystal prepared by FIB method

Microscopy, 2024, 73(6), 473–480 DOI: https://doi.org/10.1093/jmicro/dfae022 Advance Access Publication Date: 4 May 2024 Article Momentum-resolved EELS and CL study on 1D-plasmonic crystal prepared by FIB method Akira Yasuhara1,* , Masateru Shibata1 , Wakaba Yamamoto1 , Izzah Machfuudzoh2 , Sotatsu Yanagimoto2 and Takumi Sannomiya2,* 1 JEOL Ltd, 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama, Tokyo 226-8503, Japan 2 To whom correspondence should be addressed. E-mail: , Abstract We investigate a one-dimensional plasmonic crystal using momentum-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) techniques, which are complementary in terms of available optical information. The plasmonic crystal sample is fabricated from large aluminum grains through the focused ion beam method. This approach allows curving nanostructures with high crystallinity, providing platforms for detailed analysis of plasmonic nanostructures using both EELS and CL. The momentum-resolved EELS visualizes dispersion curves outside the light cone, confirming the existence of the surface plasmon polaritons and local modes, while the momentum-resolved CL mapping analysis identified these surface plasmon polaritons and local modes. Such synergetic approach of two electron-beam techniques offers full insights into both radiative and non-radiative optical properties in plasmonic or photonic structures. Key words: momentum-resolved EELS, momentum-resolved CL, surface plasmon, dispersion relation, FIB Introduction Electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) represent two primary electron microscopy techniques capable of probing the optical properties of materials and devices at spatial resolutions below the diffraction limit of light [1–3]. EELS provides insights into excitation, while CL furnishes information about emission processes [4,5]. Consequently, EELS and CL serve as complementary tools for investigating optical properties, each offering access to the excitation and emission processes, respectively [6–8]. In addition to the conventional spectral measurements, both methods support momentum-resolved measurements, enabling the analysis of the in-plane momentum of the generated particles (e.g. photons) or quasi-particles [9–11]. This in-plane momentum analysis becomes crucial for systems with surface propagating waves, such as surface plasmon polaritons (SPPs), which find widespread applications [12–15]. Notably, periodic structures supporting SPPs, known as plasmonic crystals (PlCs), including metamaterials, exhibit intriguing optical properties like cavities, waveguides and topological features analogous to atomic crystals by introducing defects or by ‘doping’ [16–19]. When investigating PlCs using an electron beam, the CL approach delivers high spatial resolution for excitation with excellent momentum resolution of emission within the light line [20]. On the other hand, EELS allows access to the dispersion outside the light line, albeit with a trade-off between spatial resolution and momentumresolved measurement [21]. While there have been a number of momentum-resolved CL studies on PlCs, corresponding EELS approaches are scarce probably due to the requirement of the electron beam transmission through the sample material [22]. As the plasmonic materials, Al, Au, Ag and Cu nanostructures have garnered extensive attention due to their superior optical characteristics, offering enticing applications based on unique catalytic, electric and optical properties [15,23–25]. Despite their intrinsically high mobility of free electrons, the optical properties of these plasmonic materials are hindered by poor crystallinity resulting in poor propagation of SPPs [26]. This deterioration arises from electron scattering at grain orientation, boundaries or other atomic defects [27,28]. Additionally, challenges are also in the fabrication precision due to variations in grain size and crystalline orientation, impacting etching rates and mechanical stiffness. The quest for larger crystals, preferably single crystals, is driven by the desire to fully exploit plasmonic properties and achieve optimal structures. However, attaining such structures is nontrivial, given that metallic nanostructures for plasmonic devices are typically produced through thin film deposition processes, which usually produce polycrystalline films consisting of nanometer scale grains. This constraint emphasizes the necessity to navigate the intricacies of producing larger, well-defined structures to fully unleash the capabilities of plasmonic materials. In this work, we utilize momentum-resolved EELS and CL techniques to investigate one-dimensional (1D) aluminum PlCs, allowing analyzing both inside and outside the light line. Received 23 February 2024; Revised 16 April 2024; Editorial Decision 18 April 2024; Accepted 2 May 2024 © The Author(s) 2024. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/ licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site–for further information please contact . * 474 A. Yasuhara et al. Momentum-resolved EELS and CL study on 1D-PlC prepared by FIB method Through this comprehensive study of combined EELS and CL analysis, we not only clarify the optical property of 1D PlC but also review the advantages of both techniques. For the plasmonic material, we chose Al since it has high penetration for the electron for EELS measurement still having moderate SPP propagation [29]. We have achieved a high-crystallinity PlC by employing a focused ion beam (FIB) method and fabricating a 1D PlC structure out of bulk Al. Experimental methods Sample fabrication The fabrication of 1D PlC on a free-standing Al membrane for the transmission electron microscopy (TEM) observation was performed by FIB (JIB-PS500i, JEOL Ltd, Japan) supporting a drift compensation function, which enables precise processing. First, a 2-μm-thick Al block was meticulously extracted from bulk Al. Large crystals were chosen via observation by secondary ion microscopy in the FIB instrument. The lamella was subsequently positioned on a Mo grid using the in situ probe pickup method. Then, a 200-nm-thin Al lamella was manufactured by a focused Ga-ion beam. To achieve a precisely defined 1D crystal structure with a depth of 50 nm, fine control over the dose r (...truncated)