Kazuhiro Yamaki

Evidence for a Dual-Source Mechanism of Terahertz Radiation from Rectangular Mesas of Single Crystalline Bi2Sr2CaCu2O8+δ Intrinsic Josephson Junctions

The THz radiation emitted from rectangular mesas of single-crystal Bi 2 Sr 2 CaCu 2 O 8+δ was studied using angular distribution measurements and Fourier transform infrared spectroscopy. Unlike the recent theoretical predictions, the results provide strong evidence for a dual-source mechanism in which the uniform and non-uniform parts of the ac-Josephson current act as electric and magnetic current sources, respectively. The latter synchronizes with cavity modes of the mesa with integral harmonics of the fundamental radiation.

High-power terahertz electromagnetic wave emission from high-Tc superconducting Bi2Sr2CaCu2O8+δ mesa structures

In this paper, we report intense electromagnetic wave emissions generated by the rectangular mesa structure of intrinsic Josephson junctions with high-T(c) superconducting Bi2Sr2CaCu2O(8+δ). The radiated power is an order of magnitude stronger than that of the previously observed power of a few μW. Two emission regions, reversible (R-type) and irreversible (IR-type), with comparable intensities can be observed at different I-V curve locations in the same mesa. We find peculiar temperature dependences of the emission power in both the R- and IR-type radiations, suggesting that a non-equilibrium thermodynamic state may be realized through the dc input current. The R-type emission is quite stable, whereas the IR-type emission is rather unstable. Although the emitted frequency for both cases obey the cavity resonance conditions, this sharp contrast in emission stability is indicative of two different states in a multi-stacked intrinsic Josephson junction system.

Geometrical Full-Wavelength Resonance Mode Generating Terahertz Waves from a Single-Crystalline Bi2Sr2CaCu2O8+\delta Rectangular Mesa

The terahertz radiation from a rectangular mesa of single-crystalline Bi 2 Sr 2 CaCu 2 O 8+δ with dimensions of 80 ×320 ×1.6 µm 3 fabricated by Ar and focused ion bean milling techniques was studied by Fourier transform infrared (FTIR) spectroscopy and angular distribution measurement of the radiation intensity. The emission measured using the FTIR spectrometer is centered at 0.9219 THz, which corresponds to exactly twice of that previously observed from mesas of the same width. The spectral width was as narrow as 7.5 GHz, which is limited by the instrumental resolution. The radiation obeys the ac Josephson relation and has a tunability of about 7% in frequency. The angular distribution of the emission power was analyzed using the conventional patch antenna theory. Both results strongly suggest that the excitation mode with a standing full-wavelength may be observed across the mesa width as the fundamental mode, instead of the half-wavelength mode commonly observed previously.

Thickness Dependences of Resistivity and Temperature Coefficient of Resistance for Ge Thin Films Sandwiched between Si Layers for Uncooled Infrared Imaging Sensor

An RF magnetron sputtering technique was used to deposit Ge and stacked Si/Ge films for infrared imaging sensors; the electrical characteristics of these films were estimated. The cross-sectional scanning electron microscope (SEM) image obtained confirmed that a layered Si/Ge structure was deposited on the SiO2 substrate. The layered film, annealed in an Ar atmosphere, exhibited a large temperature coefficient of resistance (TCR) (-3.63%/K) and a low resistivity (64.5 Ωcm). The conductivity and TCR of Si/Ge films depend on the thickness of the Ge layer. A significant improvement in TCR was achieved by decreasing the thickness of the Ge layer. Ge thin films sandwiched between amorphous silicon layers facilitate the realization of a noncooled bolometer.