An optical near field should promote phonon-assisted multiple excitation in nanoscale structures. With the phonon-assisted process, greater catalytic activity is expected without heating. To confirm this effect, photo-induced current generation using platinum black electrodes in ferricyanide solution (an absorption band-edge wavelength of 470 nm) under visible light irradiation continuous wave [(CW), λ=532 nm ] was observed. Higher order dependence of the generated current density on the incident light power was observed, indicating two-step activation of electron transfer, which originated from the phonon-assisted near-field effect on the nanostructured surface of the electrode.
A nanophotonic hierarchical hologram works in both optical far-fields and near-fields, the former being associated
with conventional holographic images, and the latter being associated with the optical intensity distribution based on a
nanometric structure that is accessible only via optical near-fields. In principle, a structural change occurring at the
subwavelength scale does not affect the optical response functions, which are dominated by propagating light. Therefore,
the visual aspect of the hologram is not affected by such a small structural change on the surface, and retrieval in both
fields can be processed independently. We propose embedding a nanophotonic code, which is retrievable via optical
near-field interactions involving nanometric structures, within an embossed hologram. Due to the one-dimensional grid
structure of the hologram, evident polarization dependence appears in retrieving the code. Here we describe the basic
concepts, numerical simulations, and experimental demonstrations of a prototype nanophotonic hierarchical hologram
with a nanophotonic code and describe its optical characterization.
We demonstrated a high-quality single-photon emitter based on excitation energy transfer between two different-sized CuCl quantum dots (QDs). It is operated under triple blockade mechanisms. The mechanisms are tuning the incident light to the smaller QD and using an electric dipole forbidden excitation energy level in the larger QD, using the optical near-field interaction to transfer energy from the smaller QD to the electric dipole forbidden level of the larger QD, and using a single exciton emission level in the larger QD. These mechanisms are supported by the large binding energy of the exciton molecule. A 99.3% plausibility of single-photon emission was confirmed with 99.98% accuracy based on a photon correlation experiment with 80-MHz repetition frequency using an optical fiber probe.
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
We have developed new type of photolithography based on a nonadiabatic photochemical process that exposes an ultraviolet-photoresist using a visible optical near field. Investigating the exposure dependence of the developed depth using nonadiabatic photolithography, we found that the depth increased with the exposure threshold. To explain this result, the optical field intensity was simulated by using the finite-difference time-domain method. The evolution of the developed depth was proportional to the optical field intensity and its spatial gradient, agreeing closely with the simulated result that took into account the nonadiabatic processes. Another experimental result is to support our explanation, that in nonadiabatic photolithography, a component of the exposure progresses inside the photoresist.
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.
Using illumination-collection mode optical near-field spectroscopy, a a spectral shift of photoluminescence of silicon (Si) nanocrystals compared with the far-field measurement was observed, due to the near-field coupling of the Si nanocrystals and the probe.
Here we show our architectural approaches to nanophotonics to benefit from unique physical properties obtained by local interactions between nanometric elements, such as quantum dots, via optical near fields, that provide ultra high-density integration capability beyond the diffraction limit of light. We discuss a memory-based architecture and a simple hierarchical architecture. By using resonant energy levels between quantum dots and inter-dot interactions, nanometric data summation and broadcast architectures are demonstrated including their proof-of-principle experimental verifications using CuCl quantum dots. Through such architectural and physical insights, we are seeking nanophotonic information systems for solving the integration density limited by diffraction limit of light and providing ultra low-power operations as well as unique functionalities which are only achievable using optical near-field interactions.
Near-field optical chemical vapor deposition (NFO-CVD), proposed by us, is a kind of optical CVD using the optical near field (ONF). Its application to nanostructure fabrication has the potential to realize high-density nanometric structures with extremely high accuracies in size and position. So far, we have deposited 20-nm-wide Zn wire, 40-nm Zn and 25-nm Al dots, and ZnO dot with 85-nm spot size of UV emission. The localized property of ONF also causes a unique photochemical reaction. Conventional optical CVD is based on the adiabatic photochemical process and requires the UV light in order to excite molecules from the ground electronic state to the excited state for dissociation (the Frank-Condon principle). For NFO-CVD, however, nonadiabatic photodissociation can take place, i.e., even by a visible light, which arises from the steep spatial gradient of optical power of ONF. We succeeded to deposit 20-nm Zn dot by using this nonadiabatic process, which can be explained by the exciton-phonon polariton model. According to this model, ONF generated at the apex of the fiber probe can directly excite the molecular vibrational state with its photon energy. Such nonadiabatic process rejects the requirement of resonant light for photochemical reaction. This unique process makes it possible to use visible lights and optically inactive gas sources to deposit a variety of nanometric materials, and is also applicable to other photochemical processes, e.g., photolithography.