Takeo Kitamura

Broadly tunable subterahertz emission from internal branches of the current-voltage characteristics of superconducting Bi2Sr2CaCu2O(8+δ) single crystals.

Continuous, coherent subterahertz radiation arises when a dc voltage is applied across a stack of the many intrinsic Josephson junctions in a Bi2Sr2CaCu2O(8+δ) single crystal. The active junctions produce an equal number of I-V characteristic branches. Each branch radiates at a slightly tunable frequency obeying the Josephson relation. The overall output is broadly tunable and nearly independent of heating effects and internal cavity frequencies. Amplification by a surrounding external cavity to allow for the development of a useful high-power source is proposed.

High Temperature Superconductor Terahertz Emitters: Fundamental Physics and Its Applications

Coherent and continuous radiation sources of the electromagnetic (EM) waves at terahertz (1 THz = 1012 c/s) frequencies using a mesa structure fabricated from high temperature superconducting Bi2Sr2CaCu2O8+? single crystals are described with a special emphasis on the physics of the radiation mechanism and the applications. After the intensive studies of many mesas fabricated with different conditions, it is revealed that the ac-Josephson effect works as a primary driving mechanism of the radiation and the cavity resonance needed for stronger radiation plays an additional role to the mechanism, although both are working together while radiating. A prototype of the imaging machine for multipurpose uses has successfully been developed.

Terahertz imaging system using high-Tc superconducting oscillation devices

Microwatt power oscillation devices at sub-terahertz frequency region between 0.3 and 1.0 terahertz (THz) were fabricated from high-Tc superconducting single crystalline Bi2Sr2CaCu2O8+δ and used as a source of the transmission terahertz imaging system. As test examples, terahertz images of coins and a razor blade placed inside the brownish paper envelopes with the spatial resolution of 1 mm are presented. The signal-to-noise ratio exceeds 130 in these images. Using a simple wedge-shaped interferometer and analysing the interference fringe pattern, the wavelength of the terahertz wave is calibrated within 0.1% accuracy. This interferometer also provides a simple method to measure the absorption coefficient of the liquid sample. Two test measurements for distilled water and ethanol are demonstrated and their absorption coefficients are obtained with 99.2% accuracy. This suggests that our terahertz imaging system can be applied to many practical applications, such as biological and biomedical imaging, enviro...

Generation of electromagnetic waves from 0.3 to 1.6 terahertz with a high- Tc superconducting Bi2Sr2CaCu2O8+δ intrinsic Josephson junction emitter

To obtain higher power P and frequency f emissions from the intrinsic Josephson junctions in a high-Tc superconducting Bi2Sr2CaCu2O8+δ single crystal, we embedded a rectangular stand-alone mesa of that material in a sandwich structure to allow for efficient heat exhaust. By varying the current-voltage (I-V) bias conditions and the bath temperature Tb, f is tunable from 0.3 to 1.6 THz. The maximum P of a few tens of μW, an order of magnitude greater than from previous devices, was found at Tb∼55 K on an inner I-V branch at the TM(1,0) cavity resonance mode frequency. The highest f of 1.6 THz was found at Tb=10 K on an inner I–V branch, but away from cavity resonance frequencies. A possible explanation is presented.

Reflection type of terahertz imaging system using a high-Tc superconducting oscillator

A reflection type of imaging system is shown at sub-terahertz frequencies generated from high-Tc superconducting intrinsic Josephson junction mesa structures fabricated by single crystalline Bi2Sr2CaCu2O8+δ to demonstrate how the sub-terahertz imaging technique using monochromatic radiation is powerful and unique for the variety of practical applications. Several examples are discussed in detail and are compared to other terahertz imaging systems.

Computed tomography image using sub-terahertz waves generated from a high-Tc superconducting intrinsic Josephson junction oscillator

A computed tomography (CT) imaging system using monochromatic sub-terahertz coherent electromagnetic waves generated from a device constructed from the intrinsic Josephson junctions in a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ was developed and tested on three samples: Standing metallic rods supported by styrofoam, a dried plant (heart pea) containing seeds, and a plastic doll inside an egg shell. The images obtained strongly suggest that this CT imaging system may be useful for a variety of practical applications.

Broadly tunable, high-power terahertz radiation up to 73 K from a stand-alone Bi2Sr2CaCu2O8+δ mesa

High-power, continuous, broadly tunable THz radiation from 0.29 to 1.06 THz, was obtained from the outer current-voltage characteristic (IVC) branch of a single stand-alone mesa of the high-transition temperature Tc superconductor Bi2Sr2CaCu2O8+δ. The particular metallic film structures placed both beneath and atop the mesas resulted in more efficient heat dissipation, higher allowed applied dc voltages, larger IVC loops, wider emission temperature ranges, and much broader emission frequency tunability than obtained previously.

Tunable terahertz emission from the intrinsic Josephson junctions in acute isosceles triangular Bi₂Sr₂CaCu₂O₈+δ mesas.

In order to determine if the mesa geometry might affect the properties of the coherent terahertz (THz) radiation emitted from the intrinsic Josephson junctions in mesas constructed from single crystals of the high-temperature superconductor, Bi₂Sr₂CaCu₂O₈+δ, we studied triangular mesas. For equilateral triangular mesas, the observed emission was found to be limited to the single mesa TM(1,0) mode. However, tunable radiation over the range from 0.495 to 0.934 THz was found to arise from an acute isosceles triangular mesa. This 47% tunability is the widest yet observed from the outer current-voltage characteristic branch of such mesas of any geometry. Although the radiation at a few of the frequencies in the tunable range appear to have been enhanced by cavity resonances, most frequencies are far from such resonance frequencies, and can only be attributed to the ac-Josephson effect.

A high-Tc intrinsic Josephson junction emitter tunable from 0.5 to 2.4 terahertz

Strong, monochromatic, coherent and continuous terahertz (THz) radiation was generated from the intrinsic Josephson junctions in a cylindrical stand-alone mesa sandwich structure fabricated from a single crystal of the high-temperature superconductor Bi2Sr2CaCu2 O8+δ. By varying the base temperature and the dc bias current-voltage characteristic (IVC) points, the emission frequency is tunable from 0.5 to a record high 2.4 THz observed on two inner IVC branch points. Strong emission power peaks were observed at 1.0 THz and 1.6 THz. This device is hence an excellent candidate to fill the “THz gap” between ∼1.4 and 2.0 THz.

Influence of the local heating position on the terahertz emission power from high-T c superconducting Bi2Sr2CaCu2O8+ δ mesas

Simultaneous measurements of spectroscopic terahertz emissions from and SiC photoluminescent local temperature T(r) distributions of high transition temperature Tc superconducting Bi2Sr2CaCu2O8+δ rectangular mesa devices were made. A local region with T(r) > Tc known as a hot spot can emerge with current bias changes. When the hot spot position was moved to a mesa end by locally heating the mesa surface with a laser beam, the intensity of the emission increased, but no changes to its frequency or line width were observed. These results suggest that higher power radiation is attainable by adjusting the hot spot position.