"Nanophotonics" uses the local interaction between nanometric particles via optical near-fields to bring "qualitative
innovation" to the field of optical technology. Optical near-field interactions respond hierarchically at the nanometer
scale, allowing unique nanophotonic functions. We defined two kinds of hierarchical optical near-field interactions:
those between optical far- and near-fields, and those in the optical near-field only. We demonstrated these hierarchical
effects numerically and experimentally using several prototype "nanophotonic architectures." The first, a "hierarchical
hologram," operated in both the far- and near-fields with few adverse effects. We also demonstrated hierarchical effects
in the optical near-field by core-shell metal nanostructures. Hierarchical nanoscale architectures could allow single
optical devices to perform multiple functions. The practical realization of such devices could have a major impact, for
example, in the field of optical security.
To decrease the sizes of photonic devices beyond the diffraction limit of light, we propose nanophotonic devices based
on optical near-field interactions between semiconductor quantum dots (QDs). To drive such devices, an optical signal
guide whose width is less than several tens of nanometers is required. Furthermore, unidirectional signal transfer is
essential to prevent nanophotonic devices operating incorrectly due to signals reflected from the destination. For
unidirectional signal transfer at the nanometer scale, we propose a nanophotonic signal transmitter based on optical nearfield
interactions between small QDs of the same size and energy dissipation in larger QDs that have a resonant exciton
energy level with the small QDs. To confirm such unidirectional energy transfer, we used time-resolved
photoluminescence spectroscopy to observe exciton energy transfer between the small QDs via the optical near-field, and
subsequent energy dissipation in the larger QDs. We estimated that the energy transfer time between resonant CdSe/ZnS
QDs was 135 ps, which is shorter than the exciton lifetime of 2.10 ns. Furthermore, we confirmed that exciton energy did
not transfer between nonresonant QD pairs. These results indicated that the proposed nanophotonic signal transmitters
based on optical near-field interactions and energy dissipation could be used to make multiple transmitters and selfdirectional
Exciton energy transfer between quantum dots via an optical near-field and subsequent dissipation was observed. Two sizes of CdSe/ZnS quantum dots with resonant energy levels were mixed to confirm the energy transfer and dissipation using time-resolved photoluminescence spectroscopy. It was estimated that the energy transfer time was 135 ps, which is shorter than the exciton lifetime of 2.10 ns. This indicates that CdSe quantum dots are promising material for nanophotonic devices.
We have developed a near-field optical probe by introducing the metallized pyramidal structure of a Si probe with a slit-shaped tip for high-density optical storage. Numerical analysis using the finite-difference time-domain method showed that the optical spot generated at the aperture measured 13×30 nm. We fabricated a slit-type Si probe and evaluated the spot size using fluorescence imaging of a single dye molecule. The full-width at half maximum of the signal profiles was 16 nm×26 nm, which corresponds to a data density of 1.5 Tbit/in2. Furthermore, a large extinction coefficient depending on the polarization was confirmed.
Particles several tens of nanometers in size were aligned in the desired positions in a controlled manner by using capillary force interaction and suspension flow. Latex beads 40-nm in diameter were aligned linearly around a 10-μm-hole template fabricated by lithography. Further control of their position and separation was realized using colloidal gold nanoparticles by controlling the particle-substrate and particle-particle interactions using an optical near field generated on the edge of a Si wedge, in which the separation of the colloidal gold nanoparticles was controlled by the direction of polarization.
To realize a nanometer-scale optical waveguide for far-/near-field conversion, we proposed a nanodot coupler which is the linear array of closely spaced metallic nanoprticles in order to transmit the optical signal to a nanophotonic device. In comparison with metallic core waveguide, the use of nano-dot coupler is expected to realize lower energy loss due to the resonant in the metallic nanoparticles. First, to optimize the efficiency in the nanodot coupler, we checked whether the single Au nanoparticles led to efficient scattering. The Au nanoparticles on the glass substrate were fabricated by the focused ion beam milling technique. The optical near-field intensity for the Au nanoparticles in diameter range from 100 to 300nm with constant height of 50nm were observed by the collection mode near-field optical microscope (NOM) at λ = 785nm. Near-field intensity took the maximum for the Au nanoparticle with 200nm in diameter, and this result is in good agreement with the calculated value of plasmon resonance by Mie's theory for an Au prolate spheroid. Next, we examined the plasmon-polariton transfer of nanodot couplers whose diameter range from 150 to 300nm by the collection mode NOM. The efficient energy transfer was observed only in the nanodot coupler with 200nm in diameter. This result agreed well with that of single Au nanoparticles. From these results, efficient energy transfer along nanodot coupler was confirmed by the near-field coupling between plasmon-polariton in the nanoparticles.