Browsing by Author "Yu, Han"

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Control of plasmon dephasing time using stacked nanogap gold structures for strong near-field enhancement
(2019-03-01)
Ueno, Kosei
;
Yang, Jinghuan
;
Sun, Quan
;
Aoyo, Daisuke
;
Yu, Han
The construction of metallic nanostructures with strong near-field enhancement is becoming increasingly significant for the practical use of plasmonic devices, such as plasmonic sensors and light-energy conversion systems. Importantly, the near-field enhancement effect depends on the plasmon dephasing time. Here, we propose a method for controlling plasmon dephasing time by utilizing plasmonic coupling for stronger near-field enhancement. Ordered arrays of stacked nanogap gold (Au) structures composed of a metal/insulator/metal nanostructure were fabricated by electron beam lithography and dry etching processes on a niobium-doped titanium dioxide substrate. The dark plasmon mode was excited by the near-field coupling between the upper and lower Au nanostructures separated by an alumina layer with a thickness of 15 nm. A strong near-field enhancement effect was induced by the localization of the electromagnetic field between the upper and lower Au nanostructures and the longer plasmon dephasing time based on the excitation of the dark plasmon mode. It is noteworthy that the dephasing time of the dark plasmon mode measured by time-resolved photoemission electron microscopy was extended 3-fold compared with that of the plasmon mode of the Au nanoblock, which can be controlled by the structural design of the stacked nanogap Au structures. (C) 2018 Elsevier Ltd. All rights reserved.
Dissecting the Few-Femtosecond Dephasing Time of Dipole and Quadrupole Modes in Gold Nanoparticles Using Polarized Photoemission Electron Microscopy
(2016-03)
Sun, Quan
;
Yu, Han
;
Ueno, Kosei
;
Kubo, Atsushi
;
Matsuo, Yasutaka
Dipole and quadrupole modes are the two lowest orders of localized surface plasmon resonance (LSPR) eigenmodes in metallic nanoparticles. Of these two modes, the quadrupole mode is forbidden for symmetric metallic nanoparticles excited by linearly polarized light at normal incidence. Here, we demonstrate excitation of the quadrupole mode in symmetrical gold (Au) nanoblocks shined with s -polarized light at oblique incidence. In particular, we probe the near -field LSPR in Au nanoblocks using photoemission electron microscopy (PEEM) and find that at oblique incidence, the dipole and quadrupole modes can be selectively excited, in terms of near -field enhancement, by manipulating the light polarization state. More importantly, by time -resolved PEEM measurements, we experimentally demonstrate that the quadrupole mode in symmetrical Au nanoblocks has longer dephasing time than that of the dipole mode.
Experimental research on purifying ultrafine nanoparticle by SBA-15 and its filtration mechanism
(2018-05-01)
Xing, Yi
;
Yu, Han
;
Lu, Pei
;
Li, Ziyi
;
Liu, Yingshu
Airborne ultrafine nanoparticle (UFNP) is a key factor causing heavy haze pollution in Beijing-Tianjin-Hebei, which is very harmful to human health and the environment. Removal of environmental UFNPs by porous material is an important method of air purification treatment. However, the specific mechanism by which porous material filters UFNPs is not clear. In this study, UFNPs with 2-10 nm diameter were removed by SBA-15 mesoporous silica molecular sieves with different pore sizes, and the filtration mechanism was analyzed using transmission electron microscopy (TEM), specific surface area tests (BET), and X-ray photoelectron spectroscopy (XPS). The analyses showed a distinctive filtration of UFNPs over the interior mesopore channels. The removal efficiencies of SBA-15 with different pore sizes were also compared, demonstrating the influence of the pore size and structure on most penetrating particle size (MPPS). The UFNP filtration efficiency of SBA-15 increased with the increase of pore size. Moreover, SBA-15-14.6 could effectively purify UFNPs, and it had the highest fractional diffusive filtration efficiency of 98.79% and a total efficiency of 92.58% for UFNPs. This study provides a technical basis for the filtration of UFNPs using mesoporous materials. (C) 2018 Elsevier B.V. All rights reserved.
Exploring Coupled Plasmonic Nanostructures in the Near Field by Photoemission Electron Microscopy
(2016-11)
Yu, Han
;
Sun, Quan
;
Ueno, Kosei
;
Oshikiri, Tomoya
;
Kubo, Atsushi
The extraordinary optical properties of coupled plasmonic nanostructures make these materials potentially useful in many applications; thus, they have received enormous attention in basic and applied research. Coupled plasmon modes have been characterized predominantly using far-field spectroscopy. In near-field spectroscopy, the spectral response of local field enhancement in coupled plasmonic nanostructures remains largely unexplored, especially experimentally. Here, we investigate the coupled gold dolmen nanostructures in the near field using photoemission electron microscopy, with wavelength tunable femtosecond laser pulses as an excitation source. The spatial evolution of near-field mapping of an individual dolmen structure with the excitation wavelength was successfully obtained. In the near field, we spatially resolved an anti-bonding mode and a bonding mode as the result of plasmon hybridization. Additionally, the quadrupole plasmon mode that could be involved in the formation of a Fano resonance was also revealed by spatially resolved near-field spectra, but it only contributed little to the total near-field enhancement. On the basis of these findings, we obtained a better understanding of the near-field nanostructures, where the plasmon hybridization and the plasmonic Fano resonance were mixed.
Near-field spectral properties of coupled plasmonic nanoparticle arrays
(2017-03-20)
Yu, Han
;
Sun, Quan
;
Yang, Jinghuan
;
Ueno, Kosei
;
Oshikiri, Tomoya
We investigated the grating effect in complex gold dolmen structures, in which multiple plasmon modes are present due to plasmon hybridization, experimentally from both the far field and the near field. In particular, the near-field properties were investigated using photoemission electron microscopy, and it was demonstrated that two hybridized plasmon modes on the dolmen structures could be influenced by the grating effect. For comparison, we also investigated the grating effect in arrays of simple nanoblocks and heptamer structures, which were supposed to support a strong bright plasmon mode and a strong dark plasmon mode, respectively, in the near field. We found that the spectral responses of the two hybridized modes on the dolmen structures as the pitch size changed evolved in a manner similar to that of the bright dipole mode on the nanoblocks, whereas the dark mode on the heptamer structures is less sensitive to the pitch size. (C) 2017 Optical Society of America
Spatial evolution of the near-field distribution on planar gold nanoparticles with the excitation wavelength across dipole and quadrupole modes
(2017-06-01)
Yang, Jinghuan
;
Sun, Quan
;
Yu, Han
;
Ueno, Kosei
;
Misawa, Hiroaki
We investigate the superposition properties of the dipole and quadrupole plasmon modes in the near field both experimentally, by using photoemission electron microscopy (PEEM), and theoretically. In particular, the asymmetric near-field distributions on gold (Au) nanodisks and nanoblocks under oblique incidence with different polarizations are investigated in detail. The results of PEEM measurements show that the evolutions of the asymmetric near-field distributions are different between the excitation with s-polarized and p-polarized light. The experimental results can be reproduced very well by numerical simulations and interpreted as the superposition of the dipole and quadrupole modes with the help of analytic calculations. Moreover, we hypothesize that the electrons collected by PEEM are mainly from the plasmonic hot spots located at the plane in the interface between the Au particles and the substrate in the PEEM experiments. (C) 2017 Chinese Laser Press