Hidehiro Asai

Intense terahertz emission from intrinsic Josephson junctions by external heat control

A practical method for realizing intense terahertz (THz) emission from intrinsic Josephson junctions (IJJs) by utilizing external local-heating is proposed and demonstrated theoretically. An artificial temperature distribution induced by local heating strongly excites Josephson plasma waves inside IJJs. Accordingly, the emission power of the THz wave is enhanced drastically, and it can reach the order of mW. Our result indicates that the use of local heat control is a powerful method to realize practical solid-state THz-emitters based on IJJs.

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.

Proposal of terahertz patch antenna fed by intrinsic Josephson junctions

We propose a THz patch antenna, in which THz ac current is fed by intrinsic Josephson junctions (IJJs). The radiation power of the antenna for three different feed positions is calculated using the finite-difference time-domain method. We predict that the antenna will radiate sub-milliwatt terahertz waves with high radiation efficiency of over 20%. The maximum radiation power will depend on the position of the feed. We also show that the radiation characteristics of the antenna are described well by the equivalent inductance-capacitance-resistance circuit model.

Effects of lasing in a one-dimensional quantum metamaterial

Electromagnetic pulse propagation in a quantum metamaterial, an artificial, globally quantum coherent optical medium, is numerically simulated. We show that a one-dimensional quantum metamaterial based on superconducting quantum bits, initialized in an easily reachable factorized excited state, demonstrates lasing in the microwave range, accompanied by the chaotization of qubit states and generation of higher harmonics. These effects may provide a tool for characterization and optimization of quantum metamaterial prototypes.

Theory of macroscopic quantum tunneling with Josephson-Leggett collective excitations in multiband superconducting Josephson junctions

Collective excitations reveal fundamental properties and potential applications of superconducting states. We theoretically study macroscopic quantum tunneling (MQT) in a Josephson junction composed of multiband superconductors, focusing on a phase mode induced by interband fluctuations: the Josephson-Leggett (JL) collective excitation mode. Using the imaginary-time path-integral method, we derive a formula for the MQT escape rate for low-temperature switching events. We clarify that the JL mode has two major effects on the MQT: (i)thezero-pointfluctuationsenhancetheescaperate,and(ii)thequantumdissipationinducedbythecouplingsto the gauge-invariant phase difference suppresses the quantum tunneling. We show that the enhancement exceeds the suppression for a wide range of junction parameters. This enhancement originates from the single-mode interaction between the tunneling variable and the interband fluctuations.

Emission of Circularly Polarized Terahertz Wave From Inhomogeneous Intrinsic Josephson Junctions

We have theoretically demonstrated the emission of circularly polarized terahertz (THz) waves from intrinsic Josephson junctions (IJJs), which are locally heated by an external heat source such as laser irradiation. We focus on a mesa-structured IJJ, whose geometry slightly deviates from a square, and find that the local heating makes it possible to emit circularly polarized THz waves. In this mesa, the inhomogeneity of critical current density induced by the local heating excites the electromagnetic cavity modes TM (1,0) and TM (0,1), whose polarizations are orthogonal to each other. The mixture of these modes results in the generation of circularly polarized THz waves. We also show that the circular polarization dramatically changes with the applied voltage. The emitter based on IJJs can emit circularly polarized and continuum THz waves by the local heating and will be useful for various technological applications.

Control of circularly polarized THz wave from intrinsic Josephson junctions by local heating

This paper reports a practical method of generating circularly polarized terahertz (THz) waves from intrinsic Josephson junctions (IJJs) and controlling their polarization states by external local heating. We theoretically find that a mesa-structure IJJ whose geometry is almost square can emit circularly polarized THz waves by local heating of the mesa. Moreover, we demonstrate that the polarization states of the THz waves change dramatically with the local heating position. Our results indicate that the use of local heating can provide a high level of controllability of the THz emissions and significantly extend the range of applications of IJJ-based THz emitters.

Demonstrating performance improvement of complementary TFET circuits by I on enhancement based on isoelectronic trap technology

We improved the performance of a complementary circuit comprising Si-based tunnel field-effect transistors (TFETs) by using isoelectronic trap (IET) technology. IET technology was found to increase the ON current (ION) 5 times in P-TFETs and 2 times in N-TFETs. The ION enhancement improved the inverter performance. In addition, ring oscillator (RO) circuit operation with the complementary TFET inverters was experimentally demonstrated for the first time. The RO circuit with IET-TFETs exhibited a higher operation frequency than that with conventional TFETs. IET technology provides a breakthrough towards realizing complementary circuits with Si-TFETs.

Compact model of ferroelectric-gate field-effect transistor for circuit simulation based on multidomain Landau–Kalathnikov theory

We report a new compact model for a ferroelectric-gate field-effect transistor (FeFET) considering multiple ferroelectric domain structures that can be thermally activated. The dynamics of the electric polarization and the thermal activation rate are calculated on the basis of the Landau–Khalatnikov (LK) theory. We implement this compact model in a circuit simulator, SmartSPICE, using Verilog-A language for analog circuit simulations. The device characteristics of FeFETs reported in experiments are well fitted by our compact model. We also perform the circuit simulation for the inverter utilizing FeFETs by using this compact model. Unlike normal inverters composed of MOSFETs, the switching speed of the inverter changes with the voltage pulse before the operation.

Suppression of tunneling rate fluctuations in tunnel field-effect transistors by enhancing tunneling probability

This paper discusses the impact of the tunneling probability on the variability of tunnel field-effect transistors (TFETs). Isoelectronic trap (IET) technology, which enhances the tunneling current in TFETs, is used to suppress the variability of the ON current and threshold voltage. The simulation results show that suppressing the tunneling rate fluctuations results in suppression of the variability. In addition, a formula describing the relationship between the tunneling rate fluctuations and the electric field strength is derived based on Kanes band-to-band tunneling model. This formula indicates that the magnitude of the tunneling rate fluctuations is proportional to the magnitude of the fluctuations in the electric field strength and a higher tunneling probability results in a lower variability. The derived relationship is universally valid for any technologies that exploit enhancement of the tunneling probability, including IET technology, channel material engineering, heterojunctions, strain engineering, etc.