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Japanese Journal of Applied Physics

Optical Responses of Josephson Vortex Flow Transistors under Irradiation of Femtosecond Laser Pulses

Yasushi Doda, Iwao Kawayama, Hironaru Murakami, Masayoshi Tonouchi

Published Apr 1, 2007• 3 citations
Publisher

Abstract

We have fabricated Josephson vortex flow transistors (JVFTs) that consist of a parallel array of Josephson junctions and a control current line to apply the external magnetic field at the junctions. We observed modulations of the flow voltage with a DC current input. From these results, we estimated the mutual inductance and the self-inductance, and calculated the operation frequency of the JVFT. Next, femtosecond laser pulses modulated by an optical chopper were irradiated to the Josephson junction array. As a result, we observed an increase in the output voltage in the current-voltage characteristics, and the clear modulation of the output voltage with the same period as the optical chopping rate. These results suggest that a JFVT can operate as an optical input interface for superconducting circuits.

DOI
10.1143/jjap.46.2381

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Journal of Applied Physics•Dec 1, 1994•19 citations
We have fabricated Josephson vortex flow transistors (JVFTs) that consist of a parallel array of Josephson junctions and a control current line to apply the external magnetic field at the junctions. We observed modulations of the flow voltage with a DC current input. From these results, we estimated the mutual inductance and the self-inductance, and calculated the operation frequency of the JVFT. Next, femtosecond laser pulses modulated by an optical chopper were irradiated to the Josephson junction array. As a result, we observed an increase in the output voltage in the current-voltage characteristics, and the clear modulation of the output voltage with the same period as the optical chopping rate. These results suggest that a JFVT can operate as an optical input interface for superconducting circuits.
We fabricated Josephson vortex flow transistors (JVFTs) with a parallel array of Josephson junctions that were prepared using c-axis-oriented 100-nm-thick YBa 2 Cu 3 O 7-δ (YBCO) thin films grown on 24° bicrystal MgO (100) substrates. We observed clear modulations of the critical current and the flow voltage with DC current input to the control line that was inductively coupled to the array of junctions. From the results, we estimated the parameters of the device, e.g., the mutual inductance and the self-inductance, and calculated the operation frequency at which the device potentially exhibited these parameters. Moreover, the current gain and the transresistance were evaluated and found to be 0.5 and 0.15 Ω, respectively. In addition, we observed the high-frequency responses of the JVFT to the input AC current of the sine wave or the square pulse wave. A clear oscillation of the output voltage could be observed with a 1 MHz sine wave and 250 kHz square pulse wave. We also discussed the feasibility of higher frequency operation by using it as an input interface for a single flux quantum (SFQ) logic circuit.
The dynamics of fluxons in the vortex flow transistor (VFT) has been much studied. The fluxon transit time determines the fundamental speed limit of operation. Since fluxons can travel at the velocity of electromagnetic waves in the junction, the VFT has the potential for operating in high frequency systems ( ≈ 100 GHz). However, owing to the low input and output impedance of the VFT, use of the device in a conventional circuit would be quite limited. A distributed amplifier configuration consisting of many VFTs has been proposed to remedy the problems of low impedance levels. However, the realization of such an amplifier circuit at microwave or millimetre wave frequencies depends on obtaining a circuit model. In this paper, microwave superconducting VFT distributed amplifiers using the balanced control technique is reviewed. This kind of amplifier has the advantage that the capacitive feedthrough effect is decreased to a negligible extent. This is the major limiting factor for high frequency applications. The self-field effect which makes the current step inclined is reduced by injecting bias current only around the region of one end of the junction thus obtaining steeper I- V characteristics. With the asymmetric geometry, the slope of the current step is about one hundred times steeper than those obtained with the conventional overlap geometry. Owing to the diamagnetic behaviour of the Josephson junction, a little of the magnetic field induced by the current in the control line is allowed to penetrate the junction. Thus, the transresistance rm of a VFT is very small. Some methods for maximizing rm and minimizing the output impedance r0 also appear in this review. The feasibility of fabricating VFT distributed amplifiers using low-Tc superconducting material has been demonstrated. The power gain of the amplifier can be as high as 15 dB with a flat frequency response.
An optical input/output interface system for a Josephson junction integrated circuit is fabricated and tested. The system consists of a superconducting optical detector, a dc powered Josephson circuit, a dc powered Josephson high voltage circuit, a liquid-He-cooled semiconductor amplifier, and a liquid-He-cooled semiconductor laser. Features of the system are use of an ultrathin NbN film for the optical detector and adoption of the dc powered Josephson circuits for logic operation circuits. Correct optical output signal is detected by a liquid-He-cooled semiconductor photodiode. The optical input/output interface has the advantage of low heat penetration and low crosstalk compared to the interface using conventional coaxial lines. Moreover, dc powered Josephson circuits have an advantage of low crosstalk from power supply lines compared to conventional Josephson circuits, which are driven by ac supply current.>
A niobium-based superconducting amplifier consisting of a series array of 60 vortex flow transistors (VFT) was fabricated and tested. Each VFT consisted of a long Josephson junction biased in the flux flow state, magnetically coupled to a current based on a distributed amplifier configuration. The transresistance of the amplifier in a 50- Omega environment for frequencies up to 1 MHz was measured. The transresistance of the array was less than the expected value calculated from measurements obtained on a single VFT. The difference was probably due to nonuniformities in the fabrication process, which caused some of the junctions not to be biased in the flux flow state or receive the same amount of magnetic field from the control line. However, measurements of the array showed that a large number of VFTs could be connected in series and simultaneously be biased into the flux flow state.>
We fabricated a Josephson junction array with an additional inductive coupling line to detect the high frequency and power of Josephson self-radiation. The YBa2Cu3O y Josephson junctions are fabricated by a pulsed laser deposition method on bicrystal MgO substrates. We measured the submillimeter wave and the Josephson self-radiation power in an external waveguide system. The Josephson junction arrays showed the resistively shunted junction (RSJ)-like current–voltage characteristics and good frequency-locking of Josephson self-radiation. As the number of Josephson junctions increased, the maximum detected power was increased. We observed the maximum frequency of above 6 THz for Josephson self-radiation in the array with 11 Josephson junctions from the Shapiro steps under 22 GHz irradiation.
We have measured vortex motion in a long one-dimensional array of small underdamped niobium tunnel junctions which are coupled in parallel by superconducting wires. In the current-voltage characteristic, a resonant step occurs similar to the Eck peak in long continuous junctions. Due to the discrete nature of our samples, the voltage of this resonant step follows a sinusoidal dependence on the magnetic field rather than the linear dependence found in long continuous junctions. With a control current applied parallel to the array of junctions, the device can be operated as a vortex-flow transistor or as a tunable vortex-flow oscillator. Compared to long continuous junctions, the performance of discrete transistors is better with regard to the threshold current and transresistance, but worse with regard to the output resistance. Discrete oscillators have the advantage over their continuous counterparts in that the output impedance is of the order of 10 Ω. Numerical simulations indicate power levels around 0.5 μW and frequencies above 500 GHz.