Kota Yamada

Passive millimeter-wave imaging for security and safety applications

77 GHz passive millimeter wave (PMMW) imaging camera for the purpose of security is developed. In order to detect concealed objects in clothes without hindrance to flow of people at airport security checks, video rate imaging is realized using one-dimensional imaging sensor array of 25 elements and a flapping reflector. As receiving antennas, novel antipodal Fermi antenna (APFA) having required characteristics for passive imaging such as broad bandwidth to obtain enough power, axially symmetric directivity with 10dB beam width of 35 degrees to obtain optimum coupling with dielectric lens, narrow width geometry for high spatial resolution of imaging is used. Real-time calibration (RTC) technique is introduced to eliminate the drift of receiving circuits. Interpolation technique to improve the quality of image and marking software for screening of suspicious objects are also developed. High spatial resolution of 20 mm is obtained by using developed imaging camera.

Development of 77 GHz millimeter wave passive imaging camera

77 GHz millimeter wave passive imaging camera for the purpose of security is developed. In order to detect concealed objects in clothes without hindrance to flow of people at airport security checks, video rate imaging is realized using one-dimensional imaging sensor array of 25 elements and a flapping reflector. As receiving antennas, novel antipodal Fermi antenna (APFA) having required characteristics for passive imaging such as broadband to obtain enough power, axially symmetric directivity with 10dB beam width of 35 degrees to obtain optimum coupling with dielectric lens, narrow width geometry for high spatial resolution of imaging is used. High spatial resolution of 20 mm (width of finger) is obtained by using developed passive imaging camera.

Si-based photonic crystals and photonic bandgap waveguides

We experimentally demonstrate the structural tuning of the waveguiding modes of line defects in photonic crystal slabs. By tuning the defect widths, we realized efficient single-mode waveguides that operate within photonic band gap frequencies in SOI photonic crystal slabs. The observed waveguiding characteristics agree very well with 3D finite- difference time-domain calculations. The propagation loss of the line defect waveguides is experimentally determined to be 6 dB/mm. In addition, we measure group velocity dispersion of line defects by using Fabry-Perot resonance of the sample. Extremely large group dispersion is observed, and the traveling speed of light is reduced down to 1/100 of the light velocity in air.

Combined neutron scattering and muon-spin rotation investigations of the Abrikosov state of high-temperature superconductors

The magnetic phase diagram of high-temperature superconductors can contain many exotic vortex phases not observed in conventional superconducting materials. For example, the familiar vortex lattice may melt at high temperatures into a vortex liquid. The influence of defects, which pin the vortices, is of particular interest from both a theoretical and an experimental point of view. We have used a combination of small angle neutron scattering (SANS) and muon-spin rotation to probe the order of the vortex system on a microscopic scale and have succeeded, for the first time, to measure a well-ordered vortex lattice (VL) structure at all doping regimes of LSCO. In the optimally to overdoped regime a field-induced transition from hexagonal to square coordination is reported. The possible connections of our neutron results to photoemission data, as well as the implications for various competing theoretical models will be discussed. In the underdoped regime we observe, as a function of applied magnetic field, a transition from an ordered vortex state to a vortex glass phase that results from the presence of random pinning. Finally, recent measurements of the VL on electron doped high-temperature superconductors are presented.