Masahiro Ishigami

IR permittivities for silicides and doped silicon

The complex permittivity for Pt, Pd, Ni, and Ti-silicide films as well as heavily doped p- and n-type silicon were determined by ellipsometry over the energy range 0.031 eV to 4.0 eV. Fits to the Drude model gave bulk plasma and relaxation frequencies. Rutherford backscattering spectroscopy, X-ray diffraction, scanning electron microscopy, secondary ion mass spectrometry, and four-point probe measurements complemented the optical characterization. Calculations from measured permittivities of waveguide loss and mode confinement suggest that the considered materials are better suited for long-wavelength surface-plasmon-polariton waveguide applications than metal films.

Graphene as an atomically thin interface for growth of vertically aligned carbon nanotubes.

Growth of vertically aligned carbon nanotube (CNT) forests is highly sensitive to the nature of the substrate. This constraint narrows the range of available materials to just a few oxide-based dielectrics and presents a major obstacle for applications. Using a suspended monolayer, we show here that graphene is an excellent conductive substrate for CNT forest growth. Furthermore, graphene is shown to intermediate growth on key substrates, such as Cu, Pt, and diamond, which had not previously been compatible with nanotube forest growth. We find that growth depends on the degree of crystallinity of graphene and is best on mono- or few-layer graphene. The synergistic effects of graphene are revealed by its endurance after CNT growth and low contact resistances between the nanotubes and Cu. Our results establish graphene as a unique interface that extends the class of substrate materials for CNT growth and opens up important new prospects for applications.

Infrared surface waves on semiconductor and conducting polymer

Conductors with infrared plasma frequencies are potentially useful hosts of surface electromagnetic waves with sub-wavelength mode confinement for sensing applications. Such materials include semimetals, semiconductors, and conducting polymers. In this paper we present experimental and theoretical investigations of surface waves on doped silicon and the conducting polymer polyaniline (PANI). Resonant absorption features were measured in reflection from lamellar gratings made from doped silicon for various p-polarized CO2 laser wavelengths. The angular reflectance spectra for doped silicon was calculated and compared with the experiments using experimental complex permittivities determined from infrared (IR) ellipsometry data. Polyaniline films were prepared, optical constants determined, and resonance spectra calculated also. A specific goal is to identify a conductor having tight mode confinement, sharp reflectivity resonances, and capability to be functionalized for biosensor applications.

Effects of Polymer Infusion and Characteristic Length Scale on Gold-Black Long-Wave and Far-Infrared Absorbance

Infrared absorbance is investigated in gold-black, a porous nano-structured conducting film. A two level full factorial optimization study with evaporation-chamber pressure, boat current, substrate temperature, and degree of polymer infusion (for hardening) was performed. Polymer infusion was found generally to reduce absorbance in the long wave IR but has little effect at THz wavelengths, although for samples with the highest absorbance there is a slight improvement in the absorbance figure of merit (FOM) in both wavelength regimes. The characteristic length scales of the structured films vary considerably as a function of deposition parameters, but the IR FOM is found to be weakly correlated with these distributions, which are determined by wavelet analysis of scanning electron micrographs.

Plasmon Absorption in grating-coupled InP HEMT and Graphene sheet for tunable THz Detection

Tunable resonant absorption by plasmons in the two-dimensional electron gas (2DEG) of grating-gated InP- and Graphene-based HEMTs are investigated. Fourier-spectrometer-obtained transmission resonances are observed over a wide spectral band from mm wavelengths to THz frequencies. These results are found to be consistent with grating period and 2DEG sheet charge density dependent theoretical calculations. The temperature dependence of these transmission resonances as a function of temperature is also reported for both devices. Such devices have potential as a chip-scale frequency-agile THz imaging spectrometers for man-portable or space-based spectral-sensing applications.