Shutaro Ishida

Nano-particle manipulation using a plasmonic multimer nano-structure

We have demonstrated a nano-particle rotation above a plasmon-resonant gold multimer nano-structures with a nano-gap and a circularly polarized laser. We have confirmed that the rotational direction depends on the number of triangle nano-structure. And the multiple position trapping has been realized with a control of triangle nano-structure.

Trapping and Deposition of Dye–Molecule Nanoparticles in the Nanogap of a Plasmonic Antenna

Plasmonic nanostructures, which allow light focusing at the deep subwavelength scale, and colloidal nanoparticles with unique optoelectronic properties are nowadays fabricated with nanometer precision. However, to fully control and exploit nanoscale light–matter interactions in hybrid plasmonic–nanophotonic devices, both materials must be assembled in heterostructures with similar precision. Near-field optical forces have recently attracted much attention, as they can precisely trap and position nanoparticles at plasmonic hotspots. However, long-range attraction and the surface bonding of nanoparticles usually require other specific techniques, such as electrothermal heating and surface chemical treatments. This Letter reports on the optical trapping and deposition of dye–molecule nanoparticles in the nanogap of a gold antenna. The nanoparticles are captured by focusing a near-infrared laser beam on a targeted plasmonic antenna. This single-step deposition process requires only a few seconds under 1.4–1.8 MW·cm–2 continuous-wave illumination and shows a polarization dependence smaller than expected. Fluorescence and electronic microscopy observations suggest that nanoparticle deposition arises from a trade-off between optical and thermal effects.

Nano-particle rotation using a gap-mode plasmonic field of nano-structure

Nano-particle rotation above a plasmonic gold trimer nano-structure with a nano-gap has been demonstrated. We designed the plasmonic trimer nano-structure which has a resonant frequency matched to excitation and made it with electron beam lithography with a metal lift-off process. At first, with an actively rotating linearly polarized beam excitation, we have realized a rotational motion of a trapped nano-particle synchronized to a polarization of beam. Next, we have observed a nano-particle rotation using a circularly polarized beam. From the auto-correlation of position time trace with sinusoidal fitting, we have confirmed a faster rotation of nano-particle than that of the actively rotating linearly polarized beam.

Localized field control at the nano-scale

We investigate the wavelength dependence of localized plasmonic field distributions in a gold nanodimer structure under total internal reflection condition. Although a gold dimer structure is well known to induce strong localized mode at a nanogap, we find that the higher-order plasmonic modes are excited by the oblique light incidence and their interference effect enables us to observe the modification of localized filed distributions at the nano-scale even in a simple gold nanodimer structure depending on the detection wavelength. This change in the plasmonic field distribution would provide important knowledge for their potential applications such as plasmonic trapping, spectroscopy, and sensing.

Near-field optical forces-assisted molecular nanoparticle deposition in the nanogap of plasmonic nanoantennas

In this work, we demonstrate an original single-nanoparticle deposition process based on near-field optical forces arising from much localized plasmonic resonant gap-mode. At first, nanoparticles exclusively made of fluorescent dye molecules are fabricated in aqueous colloidal suspension. Near-field optical forces are then used to attract and deposit single nanoparticles in the nanogap of plasmonic nanoantennas. This one-step deposition process allows targeted deposition of nanoscale materials directly from a colloidal dispersion to a few-nanometer large area of interest.

Analysis of a nano-particle rotation using a plasmonic trimer nano-structure

In this paper, we have demonstrated a nano-particle rotation above a plasmonic gold trimer nano-structure with a nanogap. We designed the plasmonic trimer nano-structure which has a resonant frequency matched to excitation and made it with electron beam lithography with metal lift-off process. At first, with an actively rotating linearly polarized beam excitation, we have realized a rotational motion of a trapped nano-particle synchronized to a polarization of beam. Next, we observed a nano-particle rotation using a circularly polarized beam. From the auto-correlation of position time trace with sinusoidal fitting, we confirmed a faster rotation of nano-particle than that of an actively rotating linearly polarized beam.

Optical Nanomanipulation Using Nanoshaped Plasmonic Fields

Plasmonic trapping has attracted significant attention because of its applicability to nanoparticle manipulations such as single-molecule trapping and quantum-dot sorting. In this presentation, we report super-resolution trapping where nanoparticles are optically manipulated in nanoscale space smaller than the diffraction limit [1]. We performed two-dimensional mapping of optical trapping potentials experienced by a 100-nm dielectric particle above a plasmon resonant gold nanoblock pair with a gap of several nanometers. The experimental results demonstrated that the potentials have nanometer-sized spatial structures that reflect the near-field landscape of the nanoblock pair. When an incident polarization parallel to the pair axis is rotated by 90°, a single potential well turns into multiple potential wells separated by a distance of approximately 230 nm (<λ/2). We show that the trap stiffness can be enhanced by approximately 3 orders of magnitude compared to that with conventional far-field trapping. In addition, we propose new concept for controlling spatial profiles of gap-mode localized plasmonic fields toward the flexible nanomanipulation. We theoretically and experimentally show that the field distributions within hot spots are formed by constructive and destructive interferences of dipolar, quadrupolar, and higher-order multipolar plasmonic modes, which can be drastically altered by adjusting parameters of the excitation optical system [2-4]. Optical switching of hot spots separated by an 80-nm distance is also demonstrated using a double-nanogap plasmonic structure.