Wavefront control by stacked metal-dielectric hole array with variable hole shapes.

A stacked metal-dielectric hole array (SHA) containing rectangular holes whose shape gradually varies in-plane is proposed as a means of achieving wavefront control. The dependence of the transmitted phase on the frequency can be tuned by the hole shape, in particular the length of the sides that are normal to the incident polarization. The combination of periodic holes along the polarization direction and the gradual change in hole shape normal to the polarization direction produce an inclined wavefront for 1-dimensional beam steering. An in-plane phase difference of 0.6π using an SHA with a thickness of one-sixth of the wavelength has been experimentally demonstrated.

Transmission phase control by stacked metal-dielectric hole array with two-dimensional geometric design.

Transmission phase control is experimentally demonstrated using stacked metal-dielectric hole arrays with a two-dimensional geometric design. The transmission phase varies drastically with small frequency shifts due to structural resonances. Laterally propagating surface plasmon polaritons excited by the periodic hole array roughly determine the resonance frequency, whereas localized resonances in each hole determine the dispersion. The transmission phase at various frequencies is directly evaluated using interferometric microscopy, and the formation of an inclined wavefront is demonstrated using a beam steering element in which the hole shapes gradually change in-plane from square to circular.

Kobe earthquake and post-traumatic stress in school-aged children.

BACKGROUND: The psychological reactions to catastrophic events are not known well in children. PURPOSE: The present study was performed to quantify the core features of post-traumatic stress reactions in schoolchildren after the Kobe earthquake. METHODS: Children's psychological reactions to the Kobe earthquake were examined in a total of 8,800 schoolchildren attending the third, fifth, or eighth grade in the disaster areas. The control subjects were 1,886 schoolchildren in the same grades in distant areas minimally affected by the earthquake. A self-report questionnaire was developed with reference to the Diagnostic and Statistical Manual of Mental Disorders-IV and the post-traumatic stress disorder reaction index and was used to score psychological reactions rating them from 1 to 4 depending on the frequency of the symptom. The survey was conducted four times, from 4 months to 2 years after the earthquake. RESULTS: Three factors were consistently extracted by factor analysis on the results of each study. Factor 1 was interpreted as relating to direct fear of the disaster and general anxiety, factor 2 as relating to depression and physical symptoms, and factor 3 as social responsibility such as feelings of sympathy for those who are suffering more severely and guilt for surviving. Young schoolchildren displayed particularly high scores on these factors. Furthermore, these factors were significantly associated with injuries of the children themselves, fatalities/injuries of family members, and the experience of being rescued or staying in shelters. CONCLUSIONS: Psychological and comprehensive interventions should be directed at the most vulnerable populations of young children after future earthquakes.

Enhanced third-order nonlinear effects in slow-light photonic-crystal slab waveguides of line-defect.

We experimentally studied enhancement of the third-order nonlinear optical phenomena, i.e., self-phase modulation due to optical Kerr effect and two-photon absorption (TPA) in a small group-velocity (V(g)) line-defect guided-mode of AlGaAs-based photonic-crystal slab waveguide. We found that the phase shift Deltaphi or nominal Kerr constant n'(2) and TPA coefficient beta were strikingly enhanced due to small V(g) as the band edge was approached, such that they were proportional to (V(g))(-2); the nonlinear refractive index n(2) is enhanced proportional to V(g)(-1). We also observed that owing to this enhancement as well as an extremely small cross-section area, the energy required for inducing pi-phase shift was very small, being of an order of a few pJ for 5 ps optical pulse and for a 0.5-mm long sample. Based on those results, we discuss the possibility of developing ultra-fast and ultra-small all-optical switches that operate due to cross-phase modulation.

High transmission recovery of slow light in a photonic crystal waveguide using a hetero groupvelocity waveguide.

High transmission of slow-light in a photonic crystal (PC) waveguide (WG) using a hetero group-velocity (Ht-V(g)) PC-WG was proposed and experimentally investigated. The Ht-V(g) WG, which comprises a low-group-velocity (L-V(g)) PC-WG section between two identical high-group- velocity (H-V(g)) PC-WGs, is designed to decrease the impedance mismatch of the L-V(g) PC-WG. The increase in transmittance of a propagating pulse was confirmed in the Ht-V(g) PC-WG even in the vicinity of the band-gap, whereas the homogeneous PC-WG showed a gradual decrease in transmittance with the pulse wavelength approaching the band-gap. The group index (n(g)) of the L-V(g) region in the Ht-V(g) PC-WG was measured by the cross-correlation method and attained a value above 20. On the other hand, the transmittance of the Ht-V(g) structure recovered approximately 16dB compared to the homogeneous L-V(g) WG having same n(g), 17. This recovery is mainly dominated by the coupling improvement due to the Ht-V(g) structure, around 12dB. These results indicate the effectiveness of the Ht-V(g) structure to use slow light in a PC-WG, which leads to various applications in PC-based optical devices.

In situ mask designed for selective growth of InAs quantum dots in narrow regions developed for molecular beam epitaxy system.

We have developed an in situ mask that enables the selective formation of molecular beam epitaxially grown layers in narrow regions. This mask can be fitted to a sample holder and removed in an ultrahigh-vacuum environment; thus, device structures can be fabricated without exposing the sample surfaces to air. Moreover, this mask enables the observation of reflection high-energy electron diffraction during growth with the mask positioned on the sample holder and provides for the formation of marker layers for ensuring alignment in the processes following the selective growth. To explore the effectiveness of the proposed in situ mask, we used it to grow quantum dot (QD) structures in narrow regions and verified the perfect selectivity of the QD growth. The grown QDs exhibited high optical quality with a photoluminescence peak at approximately 1.30 mum and a linewidth of 30 meV at room temperature. The proposed technique can be applied for the integration of microstructures into optoelectronic functional devices.

Observation of Raman scattering in GaAs photonic-crystal slab waveguides.

We have observed Raman spectra of optical phonons at 1.34 mum in an air-bridge type of GaAs photonic-crystal (PC) slab waveguides (WGs) of 0.6 mm in length. Unlike the bulk GaAs case, both longitudinal (LO)- and transverse (TO) phonons were observed not only in the forward direction, but also in the backward direction. This anomalous feature can be well interpreted as arising from strong confinement of light in PC WGs. The scattering efficiency of LO phonon at 292 cm(-1) is estimated as 1.9 x 10(-7) cm(-1)-sr(-1) in the forward scattering. Based on the present information, the possibility of developing an optical amplifier with use of stimulated Raman scattering in a future ultrafast and ultrasmall PC WGs-based optical circuit is discussed.

Broadband waveguide intersection with low crosstalk in two-dimensional photonic crystal circuits by using topology optimization.

Topology optimization has been used to design intersections in two-dimensional photonic crystal slab waveguides. We have experimentally confirmed that the optimized intersection displays high-transmittance with low-crosstalk for the straightforward beam-propagation line.

High-precision optical interference in Mach-Zehnder-type photonic crystal waveguide.

Excellent optical interference was experimentally demonstrated in the near infrared region using asymmetric Mach-Zehnder (MZ) type GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides with directional couplers (DCs). As one of two MZ arm lengths changed in units of the lattice constant, the output intensities exhibited sinusoidal curves in excellent agreement with coupled-mode theory. In another experiment where the DCs were operated by two incident optical beams with externally controlled phase's difference, a sinusoidal change was observed also in output intensities according to the theory of the DC. These results were obtained by virtue of excellent nano-fabrication of the 2DPC structures and pave the way to successful operation of a PC-based ultra-small symmetrical MZ (SMZ) all-optical switch.

Low propagation loss of 0.76 dB/mm in GaAs-based single-line-defect two-dimensional photonic crystal slab waveguides up to 1 cm in length.

Straight single-line-defect photonic crystal (PC) waveguides on GaAs slabs with lengths of 1, 4, and 10 mm have been fabricated. By controlling the Al content of a sacrificial AlGaAs clad layer and the wet etching duration, a PC core layer with a very smooth surface was obtained. Atomic force microscope images indicate that the roughness on the top surface is less than 1 nm. An extremely low propagation loss of 0.76 dB/mm for the GaAs-based PC waveguide was achieved.

InAs quantum-dot laser utilizing GaAs photonic-crystal line-defect waveguide.

We have observed laser action from optically-pumped InAs-quantum-dots embedded in a line-defect waveguide in an air-bridge type GaAs-photonic-crystal slab (an array of air-holes). The lasing is found to occur without any optical cavity such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the W3 defect-mode enables us to specify the lasing wavelength as that at the band edge. From this fact it follows that distributed feedback mechanism at the band edge with a vanishingly small group-velocity should be responsible for the present lasing. Usefulness of this kind of compact laser in a future ultrafast planar photonic integrated circuit is discussed.

Ultra-fast photonic crystal/quantum dot alloptical switch for future photonic networks.

We demonstrated a novel two-dimensional photonic crystal (PC) based Symmetric Mach Zehnder type all-optical switch (PC-SMZ) with InAs quantum dots (QDs) acting as a nonlinear phase-shift source. The 600- ?m-long PC-SMZ having integrated wavelength-selective PC-based directional couplers and other PC components exhibited a 15-ps-wide switching-window with 2-ps rise/fall time at a wavelength of 1.3 ?m. Nonlinear optical phase shift in the 500-?m-long straight PC waveguide was also achieved at sufficiently low optical-energy (e.g., ??phase shift at ~100-fJ control-pulse energy) due to the small saturation energy density of the QDs, which is enhanced in the PC waveguide, without using conventional measures such as SOAs with current-injected gain. The results pave the way to novel PC- and QD-based photonic integrated circuits including multiple PC-SMZs and other novel functional devices.

Cultural variations on optimistic and pessimistic bias for self versus a sibling: is there evidence for self-enhancement in the west and for self-criticism in the east when the referent group is specified?

A culturally relevant framework was used to examine variations on optimistic and pessimistic bias in Westerners and Easterners. Study 1 showed that 136 European Americans compared with 159 Japanese were more likely to predict typical positive events to occur to self than to a sibling. The opposite pattern emerged in the prediction of typical negative events. Study 2 replicated these findings on the basis of predictions for atypical events in 175 European Americans and 130 Japanese. Across both studies, within-groups analyses indicated that European Americans held an optimistic bias in the prediction of positive and negative events, whereas Japanese held a pessimistic bias for negative events. These findings are taken to offer support for presumed cultural differences in self-enhancement and self-criticism between Westerners and Easterners, respectively.

Optical characterization of photonic crystal delay lines based on one-dimensional coupled defects.

We performed optical characterizations of optical delay lines based on photonic crystal waveguides. The delay lines were composed of cylindrical air holes in silicon-on-silicon-dioxide ridge waveguides, defects were periodically created by means of increasing the separation of two neighboring air holes, and the structure was designed to have flat-transmittance impurity band. We clearly observed an impurity band with a bandwidth of ~30 nm and a maximum transmittance of ~0.5 at an operating wavelength of ~1.55 microm . A 600-fs delay was confirmed with a 20-microm -long delay line. A good agreement was confirmed between the experimental results and the calculations.