Optical Near-field Energy Transfer Research Articles

Analysis of optical near-field energy transfer by stochastic model unifying architectural dependencies

We theoretically and experimentally demonstrate energy transfer mediated by optical near-field interactions in a multi-layer InAs quantum dot (QD) structure composed of a single layer of larger dots and N layers of smaller ones. We construct a stochastic model in which optical near-field interactions that follow a Yukawa potential, QD size fluctuations, and temperature-dependent energy level broadening are unified, enabling us to examine device-architecture-dependent energy transfer efficiencies. The model results are consistent with the experiments. This study provides an insight into optical energy transfer involving inherent disorders in materials and paves the way to systematic design principles of nanophotonic devices that will allow optimized performance and the realization of designated functions.

Temperature dependence of optical near-field energy transfer rate between two quantum dots in nanophotonic devices

We have obtained a simple numerical model that explains the temperature behavior of multi-quantum-dot (QD) nanophotonic devices whose operations are based on optical near-field (ONF) interactions between any two resonant QDs that are in thermal equilibrium. This model involves a set of coupled rate equations that govern the temporal behavior of the QDs' energy level occupancies. Under a certain operating condition, this simple model can substitute for the more complex density matrix (DM) approach in modeling the temperature dependence of the ONF energy transfer rate (R(ONF)) between any two resonant QDs in thermal equilibrium. The same applies for modeling the system state-filling time (tau(S)). Applying our simple model to a two-QD system, we have derived analytical formulas for the interdot and the intradot transfer rates at finite temperatures (T > or = 0). Furthermore, by assuming a unidirectional energy transport operating condition, we have also derived an analytic formula for calculating tau(S) for a two-QD system. To the best of our knowledge, this work is the first instance of reporting such analytic equations. Approximated values of tau(S) obtained from our simple analytic equation are in reasonable agreements with those calculated by the DM approach.

Efficient optical near-field energy transfer along an Au nanodot coupler with size-dependent resonance

We confirmed that the light intensity (λ=785 nm) scattered from an isolated hemispherical Au nanoparticle was resonantly enhanced at a diameter of 200 nm and a height of 50 nm, as observed experimentally using a collection-mode near-field optical microscope. The experimental results agreed with the calculated results using Mie’s theory. Furthermore, we observed resonant energy transfer of the optical near-field energy along a chain of Au nanoparticles. The magnitude of the transferred energy increased resonantly at the size of resonant light scattering for an isolated Au nanoparticle (200 nm diameter with 240 nm center-to-center separation).

Demonstration of a nanophotonic switching operation by optical near-field energy transfer

We have demonstrated the operation of a nanophotonic switch that uses three CuCl quantum cubes with a size ratio of 1:√:2. The switching mechanism is based on resonant optical near-field energy transfer between the resonant quantized excitonic energy levels of the quantum cubes. Using near-field optical spectroscopy, we observed a switching rise time of less than 100 ps and a repetition rate of 80 MHz. These results suggest the possibility of making a nanophotonic switching device smaller than 20 nm with a figure of merit 10–100 times higher than that of a conventional photonic switch.

Nanophotonics: design, fabrication, and operation of nanometric devices using optical near fields

This paper reviews progress in nanophotonics, a novel optical nanotechnology, utilizing local electromagnetic interactions between a few nanometric elements and an optical near field. A prototype of a nanophotonic integrated circuit (IC) is presented, in which the optical near field is used as a carrier to transmit a signal from one nanometric dot to another. Each section of this paper reviews theoretical and experimental studies carried out to assess the possibility of designing, fabricating, and operating each nanophotonic IC device. A key device, the nanophotonic switch, is proposed based on optical near-field energy transfer between quantum dots (QDs). The optical near-field interaction is expressed as the sum of the Yukawa function, and the oscillation period of the nutation of cubic CuCl QDs is estimated to be less than 100 ps. To guarantee one-directional (i.e., irreversible) energy transfer between two resonant levels of QDs, intrasublevel transitions due to phonon coupling are examined by considering a simple two-QD plus phonon heat bath system. As a result, the state-filling time is estimated as 22 ps for CuCl QDs. This time is almost independent of the temperature in the Born-Markov approximation. Using cubic CuCl QDs in a NaCl matrix as a test sample, the optical near-field energy transfer was experimentally verified by near-field optical spectroscopy with a spatial resolution smaller than 50 nm in the near-UV region at 15 K. This transfer occurs from the lowest state of excitons in 4.6-nm QDs to the first dipole-forbidden excited state of excitons in 6.3-nm QDs. To fabricate nanophotonic devices and ICs, chemical vapor deposition using an optical near field is proposed; this is sufficiently precise in controlling the size and position of the deposited material. A novel deposition scheme under nonresonant conditions is also demonstrated and its origin is discussed. In order to confirm the possibility of using a nanometric ZnO dot as a light emitter in a nanophotonic IC, spatially and spectrally resolved photoluminescence imaging of individual ZnO nanocrystallites was carried out with a spatial resolution as high as 55 nm, using a UV fiber probe, and the spectral shift due to the quantum size effect was found. To connect the nanophotonic IC to external photonic devices, a nanometer-scale waveguide was developed using a metal-coated silicon wedge structure. Illumination (wavelength: 830 nm) of the metal-coated silicon wedge (width: 150 nm) excites a TM plasmon mode with a beam width of 150 nm and propagation length of 2.5 /spl mu/m. A key device for nanophotonics, an optical near-field probe with an extremely high throughput, was developed by introducing a pyramidal silicon structure with localized surface plasmon resonance at the metallized probe tip. A throughput as high as 2.3% was achieved. Finally, as an application of nanophotonics to, a high-density, high-speed optical memory system, a novel contact slider with a pyramidal silicon probe array was developed. This slider was used for phase-change recording and reading, and a mark length as short as 110 nm was demonstrated.