Tetsuyuki Ochiai

Optical transitions in quantum ring complexes

High Magnetic Field Center, National Institute for Materials Science, Sakura 3-13, Tsukuba 305-0003, Japan(Dated: February 2, 2008)Making use of a droplet-epitaxial technique, we realize nanometer-sized quantum ring complexes, consistingof a well-defined inner ring and an outer ring. Electronic structure inherent in the unique quantum system isanalyzed using a micro-photoluminescence technique. One advantage of our growth method is that it presentsthe possibility of varying the ring geometry. Two samples are prepared and studied: a single-wall ring and aconcentric double-ring. For both samples, highly efficient photoluminescence emitted from a single quantumstructure is detected. The spectra show discrete resonance lines, which reflect the quantized nature of the ring-type electronic states. In the concentric double–ring, the carrier confinement in the inner ring and that in theouter ring are identified distinctly as split lines. The obse rved spectra are interpreted on the basis of singleelectron effective mass calculations.

Photonic analog of graphene model and its extension: Dirac cone, symmetry, and edge states

This paper investigates the topological phase transition in honeycomb lattice photonic crystals with and without time-reversal and space-inversion symmetries through extensive analysis on bulk and edge states. In the system with both the symmetries, there appear multiple Dirac cones in the photonic band structure, and the mass gaps are controllable via symmetry breaking. The zigzag and armchair edges of the photonic crystals can support novel edge states that reflect the symmetries of the photonic crystals. The dispersion relation and the field configuration of the edge states are analyzed in detail in comparison to electronic edge states. Leakage of the edge states to free space, which is inherent in photonic systems, is fully taken into account in the analysis. A topological relation between bulk and edge states, which has been discussed in the context of electronic quantum Hall effect, is also examined in the photonic system with leaky edge states.

Evaluation of effective electric permittivity and magnetic permeability in metamaterial slabs by terahertz time-domain spectroscopy.

We established a novel method to evaluate effective optical constants by terahertz (THz) time domain spectroscopy and suggested a strict definition of optical constants and an expression for electromagnetic energy loss following the second law of thermodynamics. We deduced the effective optical constants of phosphor bronze wire grids in the THz region experimentally and theoretically. The results depend strongly on the polarization of the THz waves. When the electric field is parallel to the wires, we observed Drude-like electric permittivities with a plasma frequency reduced by a factor of 10(-3), whereas when the field is perpendicular, the sample behaved as a simple dielectric film. We also observed unexpected magnetic permeabilities, which originate from the non-resonant real magnetic response of finite size-conductors.

Acceleration and suppression of photoemission of GaAs quantum dots embedded in photonic crystal microcavities

We fabricated optical microcavities in a photonic crystal slab embedded with GaAs quantum dots by electron beam lithography and droplet epitaxy. The Purcell effect of exciton emission of GaAs quantum dots was confirmed for the first time by microphotoluminescence and lifetime measurements.

Subnanomolar fluorescent-molecule sensing by guided resonances on nanoimprinted silicon-on-insulator substrates

We have produced nanoimprinted silicon-on-insulator (SOI) substrates with high precision and applied them for fluorescence (FL) enhancement of typical dye molecules of Rhodamine series. We experimentally found that the FL signals on the nanoimprinted SOI substrates are enhanced by more than 200 fold as compared with the signals on Si wafers. The FL enhancement mechanism was investigated using spectroscopic and theoretical approaches. The results indicated that guided resonances with large density of states significantly contribute to the FL enhancement. We also substantiated that the FL-enhancing substrates well serve as rapid-sensing platforms for a very dilute FL-molecule solution with subnanomolar concentration. The nanoimprinted SOI substrates were repeatedly reused without degradation.

Theory of Light Scattering in Axion Electrodynamics

Taking account of the axion term in the Maxwell Lagrangian, we present a rigorous theory of light scattering in piecewise-constant axion fields. In particular, we focus on axionic substances with confined and/or curved geometries, and the scattering matrices of an axionic slab, cylinder, and sphere are derived analytically. The axion term generates a surface current with off-diagonal optical conductivity, giving rise to a new type of photospin–orbit interaction. As a result, various novel light-scattering phenomena can take place. We demonstrate enhanced Faraday rotation, parity-violating light scattering, and strong perturbation of dipole radiation.

Scaling law of enhanced second harmonic generation in finite Bragg stacks.

The second-harmonic generation (SHG) in finite Bragg stacks of alternating linear and nonlinear optical films is studied with the exact Green function under the assumption of perturbation theory. Various mechanisms of enhanced SHG are investigated analytically, and the scaling law with respect to the number N of stacking layers is derived for each mechanism. In particular, it is shown that there is a simple mechanism of the enhanced SHG having N;4 scaling, in which both the enhancement of the Green function and the phase matching condition peculiar to finite Bragg stacks are fulfilled simultaneously.

Topological properties of bulk and edge states in honeycomb lattice photonic crystals: the case of TE polarization

Topological properties of bulk and edge states in honeycomb lattice photonic crystals are investigated theoretically for transverse-electric (TE) polarization. Breaking of space-inversion and time-reversal symmetries is considered at optical frequencies. The bulk band structure exhibits a topological phase transition by changing the degree of the broken symmetries. The resulting phase diagram correlates with zigzag and armchair edge states, and the so-called bulk-edge correspondence is verified. The effects of flat interfaces near the edges are also discussed.

Electron energy loss and Smith-Purcell radiation in two- and three-dimensional photonic crystals

A theoretical description of the electron energy loss and the Smith-Purcell radiation is presented for an electron moving near a two-dimensional photonic crystal slab and a three-dimensional woodpile photonic crystal. The electron energy loss and the Smith-Purcell radiation spectra are well correlated with the photonic band structures of these crystals and thus can be used as a probe of them. In particular, there is a selection rule concerning the symmetries of the photonic band modes to be excited when the electron moves in a mirror plane of the crystals. In the woodpile, a highly directional Smith-Purcell radiation is realized by using the planar defect mode inside the complete band gap.

Advanced quantum dot and photonic crystal technologies for integrated nanophotonic circuits

Two types of nanophotonic technologies-two-dimensional photonic crystal (2D PC) slab waveguides (WGs) and quantum dots (QDs)-were developed for key photonic device structures in the future. For an ultrafast digital photonic network, an ultrasmall and ultrafast symmetrical Mach-Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip-flop device (PC-FF) have been developed. To realize these devices, one method is to develop a selective-area molecular beam epitaxial growth QD technique by employing a metal mask method. Another method is to establish a new design method, i.e., topology optimization of the 2DPC WG with a wide and flat bandwidth, high transmittance, and low reflectivity. We also fabricated an optical microcavity in a photonic crystal slab embedded with GaAs QDs by droplet epitaxy. The Purcell effect on the exciton emission of GaAs QDs was confirmed by microphotoluminescence and lifetime measurements.