Waseda University · Materials Science
Professor Kohei Imura's research lab specializes in the nanoscale optical characterization of plasmonic nanostructures, focusing on the near-field optical properties of gold nanoparticles such as nanorods, nanospheres, and nanoplates. The lab employs advanced scanning near-field optical microscopy (SNOM) techniques to visualize and analyze localized surface plasmon resonances, electromagnetic field distributions, and ultrafast electron dynamics at the nanoscale. Key research directions include two-photon-induced photoluminescence, plasmon mode engineering, and real-space imaging of optical local density of states, with applications in nanophotonics and optoelectronic devices. The lab combines experimental measurements with theoretical simulations to uncover the fundamental optical behaviors of metallic nanostructures.
Figures are computed from collected data and may differ slightly.
We investigated the two-photon-induced photoluminescence properties of single gold nanorods by scanning near-field spectroscopy. The process was found to be initiated by a sequential one-photon absorption for creating a pair of an electron and a hole in the sp and d bands. Photoluminescence is then radiated when the electron near the Fermi surface recombines with the hole near the X and L symmetry points. The polarization characteristics of emitted photons from the X and L regions were found to
We demonstrate visualization of localized intense electromagnetic fields in real space in well-tailored dimeric and trimeric gold nanospheres by using near-field optical techniques. With two-photon induced luminescence and Raman measurements, we show that the electric field is confined at an interstitial site in the aggregate. We also demonstrate optical switching operations for the electric-field localized sites in the trimer structure.
We have investigated two-photon-induced photoluminescence images and spectra of single gold nanorods by using an apertured scanning near-field optical microscope. The observed PL spectrum of single gold nanorod can be explained by the radiative recombination of the electron-hole pair near the X and L symmetry points. PL images reveal characteristic features reflecting an eigenfunction of a specific plasmon mode as well as electric field distributions around the nanorod.
We have investigated optical properties of single gold nanorods by using an apertured-type scanning near-field optical microscope. Near-field transmission spectrum of single gold nanorod shows several longitudinal surface plasmon resonances. Transmission images observed at these resonance wavelengths show oscillating pattern along the long axis of the nanorod. The number of oscillation increases with decrement of observing wavelength. These spatial characteristics were well reproduced by calcula
We investigated the near-field optical properties of single gold nanorods. Transmission images observed near the surface plasmon resonances agree qualitatively with the calculated maps of the optical local density of states and are reflected by spatial characteristics of plasmon wave functions. Ultrafast temporal responses have been observed by combining near-field microscopy with a time-resolved technique. It has been found that the behavior of the collective motion of electrons in the center o
We have investigated two-photon-induced photoluminescence (PL) properties of single gold nanoplates by using an apertured scanning near-field optical microscope. The nanoplates show PL much stronger than nanorods. The near-field PL images show characteristic spatial features, which depend on the incident polarization. These PL images are in good agreement with the calculated spatial distribution of the electric fields adjacent to the particles at the excitation wavelength. We attribute the obser
We investigated properties of photoluminescence from single gold nanorods induced by near-field two-photon excitation, using scanning near-field optical microscopy. Photoluminescence spectrum showed two peaks around 550 and 650 nm in wavelengths regardless of the rod dimensions, whereas the intensity ratio of the two spectral components varies from rod to rod. We discussed the observed spectral features of photoluminescence based on electromagnetic calculations and found that the observed spectr
Abstract We observed near-field images of aggregates of gold nanospheres, by detecting TPI-PL from gold and Raman scattering from Rhodamine 6G adsorbed on the nanoparticles. The results were analyzed in relation to the localized Raman active site (hot spot) of surface-enhanced Raman scattering.
Abstract We have developed novel methods of near-field microspectroscopy by combining near-field optical microscopy with linear and non-linear optical techniques. The developed near-field microscope achieves high spatial resolution and high time resolution, and is applied to studies of local excitations and wave functions of single noble metal nanoparticles. We demonstrate that the plasmon wave functions and the optical fields in the vicinity of nanoparticles are visualized by the near-field met
Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundam
We experimentally investigated the reciprocity of scanning near-field optical microscopy between illumination and collection modes. Near-field transmission images of single gold spheres and nanorods observed by the two modes are found to be equivalent to each other in the region from visible to near infrared. This result shows that reciprocity holds for the near-field scattering problems. We found that the conventional optical selection rule for far-field excitations does not apply not only unde
We report an anomalous light transmission phenomenon for a nanoaperture on an opaque screen when the aperture is covered with an opaque cap. In conventional optics, light transmission must decrease when the aperture is capped. However, we found that light transmission is enhanced when the nanodisk is in close proximity to the aperture at a wavelength close to the plasmon resonance. This effect even occurs when the disk is larger than the aperture.
We investigated the spectral and spatial characteristics of plasmons induced in chemically synthesized triangular gold nano- and microplates by aperture-type scanning near-field optical microscopy. Near-field transmission images taken at plasmon resonance wavelengths showed two-dimensional oscillating patterns inside the plates. These spatial features were well reproduced by the square moduli of calculated eigen functions confined in the two-dimensional triangular potential well. From the irredu
Precise understanding of the spatiotemporal characteristics of plasmons is essential for the development of applications of plasmonic nanoparticles. In this study, we investigated the spatiotemporal properties of high-order plasmon modes induced in a gold triangular nanoplate by static and dynamic near-field measurements. The near-field transmission measurements revealed that in-plane and out-of-plane polarized plasmon modes were simultaneously excited and these modes spectroscopically and spati
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