||Hikaru Saito, Hugo Lourenço-Martins, Noémie Bonnet, Xiaoyan Li, Tracy C. Lovejoy, Niklas Dellby, Odile Stéphan, Mathieu Kociak, Luiz Henrique Galvão Tizei, Emergence of point defect states in a plasmonic crystal, Physical Review B, 10.1103/PhysRevB.100.245402, 100, 24, 2019.12, Plasmonic crystals are well known to have band structure including a band gap, enabling the control of surface plasmon propagation and confinement. The band dispersion relation of bulk crystals has been generally measured by momentum-resolved spectroscopy using far field optical techniques while the defects introduced in the crystals have separately been investigated by near field imaging techniques so far. Particularly, defect related energy levels introduced in the plasmonic band gap have not been observed experimentally. In order to investigate such a localized mode, we performed electron energy-loss spectroscopy (EELS) on a point defect introduced in a plasmonic crystal made up of flat cylinders protruding out of a metal film and arranged on a triangular lattice. The energy level of the defect mode was observed to lie within the full band-gap energy range. This was confirmed by a momentum-resolved EELS measurement of the band gap performed on the same plasmonic crystal. Furthermore, we experimentally and theoretically investigated the emergence of the defect states by starting with a corral of flat cylinders protrusions and adding sequentially additional shells of those in order to eventually form a plasmonic band-gap crystal encompassing a single point defect. It is demonstrated that a defectlike state already forms with a crystal made up of only two shells..
||Hikaru Saito, Daichi Yoshimoto, Hugo Lourenco-Martins, Naoki Yamamoto, Takumi Sannomiya, Hybridization of Gap Modes and Lattice Modes in a Plasmonic Resonator Array with a Metal-Insulator-Metal Structure, ACS Photonics, 10.1021/acsphotonics.9b00977, 2019.01, Plasmonic resonator arrays have attracted a great interest as a platform to enhance light-matter interaction and have been examined for their applicability to various types of optical devices, such as sensors, light emitter, and photocatalyst, to name a few. In a plasmonic resonator array, localized and propagating plasmon modes can hybridize, which is known to result in an anticrossing of the plasmon bands in the dispersion curves. However, it was so far unclear how the modal symmetry affects such a hybridization, especially when it occurs at a specific reciprocal lattice point with a high degree of symmetry, for example, the Î" point. In this work, we used momentum-resolved cathodoluminescence-scanning transmission electron microscopy to comprehensively characterize the modal hybridization at the Î" point. Our study reveals theoretically and experimentally the existence of mode symmetry selection rules that specify hybrid pairs of the lattice mode and localized mode..
||Daichi Yoshimoto, Hikaru Saito, Satoshi Hata, Yoshifumi Fujiyoshi, Hiroki Kurata, Characterization of Nonradiative Bloch Modes in a Plasmonic Triangular Lattice by Electron Energy-Loss Spectroscopy, ACS Photonics, 10.1021/acsphotonics.8b00936, 5, 11, 4476-4483, 2018.11, Electron beam spectroscopy has recently attracted much attention in the modal analysis of nanophotonic and plasmonic systems. In principle, electron energy-loss spectroscopy (EELS) provides information about the electromagnetic local density of states of all sorts of electromagnetic modes as well as nonradiative modes. However, there have not been many examples related to the EELS analyses for electromagnetic Bloch modes. Herein, EELS measurements are performed for the characterization of nonradiative band-edge modes facing the first bandgap in a plasmonic crystal with a triangular lattice, which is well-known for its full bandgap formation. The obtained spectrum images clearly determine the characteristics of the lower and upper band-edge modes, compared with an analytical model based on group theory and a numerical simulation by the finite-difference time-domain method. The EELS spectra also reveal differences in the plasmonic density of states between the lower and upper band-edges, which provides information about the band deformation near the bandgap..
||Kana L. Hasezaki, Hikaru Saito, Takumi Sannomiya, Hiroya Miyazaki, Takashi Gondo, Shinsuke Miyazaki, Satoshi Hata, Three-dimensional visualization of dislocations in a ferromagnetic material by magnetic-field-free electron tomography, Ultramicroscopy, 10.1016/j.ultramic.2017.07.016, 182, 249-257, 2017.11, In conventional transmission electron microscopy, specimens to be observed are placed in between the objective lens pole piece and therefore exposed to a strong magnetic field about 2 T. For a ferromagnetic specimen, magnetization of the specimen causes isotropic and anisotropic defocusing, deflection of the electron beam as well as deformation of the specimen, which all become more severe when the specimen tilted. Therefore electron tomography on a ferromagnetic crystalline specimen is highly challenging because tilt-series data sets must be acquired without changing the excitation condition of a specific diffraction spot. In this study, a scanning transmission electron microscopy (STEM) tomography method without magnetizing a ferromagnetic specimen has been developed for three-dimensional (3D) visualization of dislocations in α-Fe, which is a typical ferromagnetic material. Magnetic-field-free environment down to 0.38 ± 0.07 mT at the specimen position is realized by demagnetizing the objective lens pole piece of a commercial STEM instrument. By using a spherical aberration corrector with the magnetic-field-free environment, an “aberration corrected Low-Mag STEM mode” with no objective lens field reaches a convergence semi angle ∼1 mrad and a spatial resolution ∼5 nm, and shows an adequate performance of imaging dislocations under a two-beam excitation condition for a low-index diffracted beam. The illumination condition for the aberration corrected Low-Mag STEM mode gives no overlap between the direct beam disk (spot) and neighboring diffraction disks. An electron channeling contrast imaging technique, in which an annular detector was located at a doughnut area between the direct beam and the neighboring diffracted beams, was effectively employed with the aberration corrected Low-Mag STEM mode to keep image intensity high enough even at large specimen-tilt angles. The resultant tomographic observation visualized 3D dislocation arrangements and active slip planes in a deformed α-Fe specimen..
||Hikaru Saito, Naoki Yamamoto, Takumi Sannomiya, Waveguide Bandgap in Crystalline Bandgap Slows Down Surface Plasmon Polariton, ACS Photonics, 10.1021/acsphotonics.6b00943, 4, 6, 1361-1370, 2017.06, Next generation on-chip optical devices require light manipulation in time and space, that is, control of group velocity of light in subwavelength dimensions. A waveguide in plasmonic crystal fulfills such requirements offering nanoscale light confinement in the dispersion-tunable plasmonic crystal matrix. However, there has been no direct access to the local dispersion of the waveguide mode itself, and the group velocity of light could not be evaluated. Herein, for the first time, we experimentally clarify the dispersion of the waveguide modes by use of angle-resolved cathodoluminescence scanning transmission electron microscopy. Their group velocity can be extremely slowed down by the existence of a bandgap formed in the waveguide in the energy range of the plasmonic crystal bandgap..
||Hikaru Saito, Shohei Mizuma, Naoki Yamamoto, Confinement of Surface Plasmon Polaritons by Heterostructures of Plasmonic Crystals, Nano Letters, 10.1021/acs.nanolett.5b02623, 15, 10, 6789-6793, 2015.10, Square lattice plasmonic crystals (SQ-PlCs) composed of silver pillars generate large bandgaps for surface plasmon polaritons (SPPs). SPP confinement is demonstrated using one- and two-dimensional heterostructures of SQ-PlCs comprised of cylindrical pillars with different diameters in a common square lattice. Two kinds of localized modes are observed to appear in the heterostructures by photon map imaging using cathodoluminescence (CL) technique combined with a scanning transmission electron microscopy (STEM). Angle-resolved CL spectroscopy reveals contrasting characteristics of the two localized modes in their emission distributions, indicating that they originate from the band-edge A and E modes of the matrix SQ-PlC..
||Hikaru Saito, Naoki Yamamoto, Control of Light Emission by a Plasmonic Crystal Cavity, Nano Letters, 10.1021/acs.nanolett.5b01719, 15, 9, 5764-5769, 2015.09, Surface plasmon-polaritons (SPPs) localized inside a plasmonic crystal (PlC) cavity are probed by STEM-CL technique to characterize the influence of the surface shape parameters on the cavity modes. The results elucidate the formation mechanism of the cavity mode in terms of the symmetry and quality factor, which provide a clear design guide for the PlC cavity to control the coupling between SPPs and photons in plasmonic devices and future integrated circuits..
||Hikaru Saito, Naoki Yamamoto, Size dependence of bandgaps in a two-dimensional plasmonic crystal with a hexagonal lattice, Optics Express, 10.1364/OE.23.002524, 23, 3, 2524-2540, 2015.02, The optical properties of surface plasmon polaritons (SPPs) are investigated at the Γ point in a two-dimensional plasmonic crystal with a hexagonal lattice (Hex-PlC). The cathodoluminescence (CL) technique combined with a scanning transmission electron microscope (STEM) are used to produce spectral images of the SPP standing waves at the Γ point and identify the four types of band-edge modes predicted by group theory. The systematic measurement of the band-edge energies employed here is used to determine the characteristic dependence of each band-edge mode on the structure parameters, which provides some criteria for the design of plasmonic devices with Hex-PlCs..
||Hikaru Saito, Kyoko Namura, Motofumi Suzuki, Hiroki Kurata, Dispersion relations for coupled surface plasmon-polariton modes excited in multilayer structures, Microscopy, 10.1093/jmicro/dft047, 63, 1, 85-93, 2014.01, The coupled surface plasmon-polariton (SPP) modes excited in an Al/SiO2/ Al multilayer structure were analyzed using angle-resolved electron energy-loss spectroscopy (AREELS) with a relativistic electron probe. The dispersion relations for the coupled SPP modes were then directly observed and compared with predicted relations obtained via calculations. Good agreement was noted between the experimental and calculated results. In the multilayer structures, the dispersion relation for the coupled SPP modes was found to be sensitive to the thickness of each film, which could be interpreted qualitatively by the electron energy-loss probability calculated for thin aluminum (Al) films and narrow Al gaps using Kröger’s formula. It was demonstrated that significant differences in the excitation probability for SPPs could be observed depending on the coupling modes..
||H. Saito, C. H. Chen, H. Kurata, Optical guided modes coupled with Čerenkov radiation excited in Si slab using angular-resolved electron energy-loss spectrum, Journal of Applied Physics, 10.1063/1.4796140, 113, 11, 2013.03, Retardation effects in the valence electron energy-loss spectrum (EELS) of a Si slab are analyzed by angular-resolved EELS. The dispersion curves of the valence spectra excited in a slab are directly observed from a specimen area with several different thicknesses and are interpreted by performing a calculation of the dispersion relation using Krögers formula. The dispersion curves observed below about 3 eV are attributed to guided modes coupled with Čerenkov radiation (ČR). The coupling between guided modes and ČR is found to be dependent on the sample thickness (t). For the sample with t > 150 nm, the intensity of the guided modes increased linearly with thickness, revealing the coupling with ČR. For t 150 nm, however, the intensity of the guided modes rapidly decreased due to a diminished coupling with ČR, resulting from the thickness-dependent dispersion curves of the guided modes..