A critical electromagnetic response of a self-complementary structure was investigated. The nearly perfect selfcomplementary checkerboard patterns were fabricated by the electron-beam lithography and their electromagnetic responses are measured in the terahertz region. The electromagnetic responses are affected drastically by the small structural difference even though the differences are less than 0.1% of the wavelength of the incident electromagnetic waves. The sample most close to the self-complementary checkerboard pattern shows a less frequency dependent response, which is expected for the self-complementary structures. In this sample, the metallic squares seem to be connected randomly from the SEM observation. The effect of the structural randomness in metal mesh structures is also investigated to obtain the common electromagnetic properties in randomly connected systems.
Alternating stacks of metal and dielectric films with nano-hole arrays, called fishnet structures, control the propagation
of electromagnetic waves. In such a structure, changing a dimension or a shape, especially the change in shape of nanoholes,
affect propagation constants. In this study, we report the dispersivity of fishnet structures is controllable with
different hole shapes, by measuring the interferometric fringe in various wavelengths. Two structures were fabricated,
which consist of five alternating stacks of aluminum and silicon dioxide with nano-hole arrays. The holes in one of the
structures are circular with diameters of 500nm, and the other are square with 500nm sides. The lattice constant in each
case is 1,000nm. Since fishnet structures are wavelength-dependent structures, the variable-wavelength interferometric
microscope was set up. The phase shift of the circular hole and the square hole fishnet were about 110 degrees and 85
degrees, respectively, within a tunable wavelength from 1,470nm to 1,545nm. These values were equivalent to a
refractive-index-change of 0.8 and 0.6, respectively. From these results, fishnet structures indicate high dispersivity
within target wavelengths. The dispersion of fishnet structure can be controlled by the shape of the hole.
Electromagnetic wave propagation in structured metals has attracted strong attention in wide wavelength regions from microwave to visible. We have investigated transmission properties of metal hole arrays and related structures in the terahertz region by using the terahertz time-domain spectroscopy (THz-TDS). We have found a variety of transmission properties depending on the periodic structure of metals, i. e. extraordinary transmission, large polarization conversion, large optical activity, etc. Some of the properties are explained by the surface plasmon-polariton and/or the local structure of holes.
We have measured the terahertz (THz) absorption spectra of MBBA (4-Methoxybenzylidene-4'-n-butylaniline) and its
homologues by using THz time-domain and polarized fourier transform far-infrared (FT-IR) spectroscopy, and observed
the absorption peak due to the permanent dipole moments perpendicular to the molecular long axis. In addition, we also
measured the THz absorption spectra of CCN47 (4'-trans-butyl-4-cyano-4-trans-heptyl-1,1'-bicyclohexane) which has a
large permanent moment perpendicular to the long axis, by using polarized far-infrared FT-IR spectroscopy. The relation
between the vibration modes and the absorption in the THz region is discussed.
Ultrashort electromagnetic pulses with approximately 0.5 ps width have been radiated into free space from current biased superconducting YBa2Cu3O7 (YBCO) films by exciting femtosecond laser pulses. The Fourier spectrum of them extends up to approximately 3 THz. The characteristics of the radiation are studied in detail and the mechanism of the radiation is discussed in relation to the relaxation of photoexcited quasi particles By the improvement of the device structure and the collecting optics, the radiation power can be increased to approximately 0.5 (mu) W. A new type THz radiation from YBCO films under an external magnetic field or in the flux-trapped state is found and its preliminary result is described.
A nulling bridge-type dispersive interferometric spectrometer has been constructed to measure complex refractiveindices of solids in the millimeter wave region. The complex refractive indices of doped Si and GaAs wafers are obtained in the110 - 170 GHz region and their dispersion is analyzed based on the Drude model. The electrical resistivity estimated from theDrude-model fitting agrees well with that measured by the dc four-point probe method.
We have constracted a Mach-Zehnder type interferometric spectrometer in the millimeter wave region (75 - 170 GHz) to measure optical constants of solids. Using this spectrometer, we have measured complex refractive indices of silicon crystals with various resistivities (1 - 300 Ω -cm). The density and relaxation time of carriers are determined by the experimental dispersion of the refractive indices using the Drude model. The refractive indices in this region are compared with those measured by the far infrared Fourier spectrometer.
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