Koo-Chul Je

Observation of red-shifted strong surface plasmon scattering in single Cu nanowires.

Surface plasmon scattering spectra of chemically produced single Cu nanowires were obtained using a total internal reflection microscope. In particular, we have observed a strong surface plasmon peak in the far red and a red-shift of the surface plasmon resonance with increasing nanowire diameter. We believe that the most reasonable origin for the red-shift of comparably large diameter nanowires is the phase retardation effect.

Nondestructive Photoacoustic Measurement of Doping Densities in Bulk GaAs

The frequency-dependent photoacoustic (PA) signals and phase spectra of GaAs samples with different doping densities are measured and analyzed. In particular, we find that the magnitude differences of the PA signal and phase between intrinsic GaAs and doped GaAs materials decrease linearly with the doping density in the frequency region of nonradiative bulk recombination. The results show that the PA spectroscopic technique can be used to measure doping density in bulk semiconductors with reasonable accuracy.

Thermal Properties for Hot-Pressed n-type (Bi1-xSbx)2Te3 Alloys

We have measured the thermal properties of an n-type hot-pressed Bi1.8Sb0.2Te3 alloy with a carrier concentration of 3.1×1019 cm-3. With the adjustable parameters determined from these experimental data, the dependence of thermal properties on the carrier concentrations is investigated, using the two-band-conduction model with a mixed scattering mechanism and a small fluctuation theory. The maximum dimensionless figures of merit of the alloy in a carrier concentration range of 0.8–3×1019 cm-3 have values over 0.7 at temperatures between 250 K and 300 K and a minimum thermal conductivity of 1 W/Km. These alloys are comparable to single crystals, showing the possibility that they provide higher thermoelectric performance than the single crystals, since their microstructure can be controlled by hot-press processing.

Theoretical Comparison of Electronic and Lattice Thermal Conductivities in n-type (Bi1-xSbx)2Te3 Solid Solutions

We have theoretically investigated the electronic and lattice thermal conductivities for various carrier concentrations in the n-type (Bi1-xSbx)2Te3 solid solutions using the two-band conduction model with a mixed scattering mechanism. For operation of thermoelectric cooling at low temperatures, the carrier concentration must be controlled so that the electronic thermal conductivity makes the total thermal conductivity have the minimum at an applicable temperature. Under this condition, the electronic thermal conductivity is over 25% of the lattice thermal conductivity and results in the gradual increase of the thermal conductivity. These effects are almost independent of the composition x under the same carrier concentration.

Field-induced Stark effects in Ag-coated CdS quantum dots

Field-induced Stark effects in Ag-coated CdS quantum dot structures are presented. We observe clear exciton peaks due to the quantum confinement effect and the surface plasmon effect in Ag-coated CdS quantum dot fabricated by gamma ray irradiation method. In addition, we observed also a dominant red-shift of the CdS exciton absorption peak as the Wannier Stark effect, implying the strong local field effect in the metal-coated semiconductor composite quantum dot system. The Stark shift of the exciton peak is investigated as a function of the local field for different silver thickness and various sizes of quantum dots based on the effective-mass Hamiltonian using the numerical-matrix-diagonalization method.

Total Internal Reflection Microscopy for Surface Plasmon Scattering of a Single Cu Nanowire

A total internal reflection microscope was constructed to study surface plasmon scattering spectra of a single Cu nanowire. In particular, we have observed a strong surface plasmon peak in deep red region and the red-shift of the surface plasmon resonance as the diameter increases.

Deformation of Dynamical Stark Ladders and CW Optical Absorption in DC–AC Biased Semiconductor Superlattices

We have investigated the importance of the relative phase (φ) between the THz field and the optical field, which is used to monitor the interband absorption, and cw optical absorption spectrum in biased semiconductor superlattices. All of the absorption spectra have Stark ladder structures and the peaks of the Stark ladders are changed for the different phases. Especially, it is demonstrated that the changes of the Stark ladders from those φ=0 occur periodically in four ladders for φ=π and in eight ladders for φ=π/2 in the absence of dynamical localization. The cw optical absorption spectrum, which is obtained as an average over the phases by using short pulse technique, have no gain peaks. Because this procedure leads to a cancellation of regions of negative absorption which may occur in the coherent THz-driven system.