超伝導エレクトロニクスは、これまでに、高感度磁気センサー、テラヘルツ検出器、Ｘ線検出器などを創出し、また、単一磁束量子論理回路や超伝導量子ビットなど、次世代革新技術としても研究開発が進められている。一方、近年のオプトエレクトロニクスの発展は、光通信を中心として科学技術の進展に大きく貢献してきた。それら“超伝導エレクトロニクス”と“オプトエレクトロニクス”を融合させる“超伝導フォトニクス工学”の創成は、わが研究室が世界に先駆けて、提案・推進している研究領域であり、新しい技術革新をもたらし、超伝導エレクトロニクスの発展にブレークスルーをもたらすものことが期待されます。
　具体的には、超高速光検出器、超高速光−SFQインターフェイス、光超伝導量子ビット、超伝導量子ビット光インターフェイス、テラヘルツ検出デバイス・イメージングデバイス、テラヘルツ通信デバイス・基盤技術などかいはつにいより、情報通信、安全・安心社会への貢献、バイオ・医療ならびに基礎科学に対する高度分析器機開発などの分野において、大きな波及効果をもたらすことが期待されます。

I. Introduction

Tonouchi laboratory studies material science, novel devices and systems for ultrafast photonics and terahertz applications. One of the important missions is to open new research field of “Superconductive Photonics”, in which, for instance, high-speed optical interface to single flux quantum circuits, terahertz emission devices, optical-terahertz signal generation and detection systems, and so on are included. For the material science, we prepare high quality thin films made of, mainly, strongly correlated electron systems by means of laser ablation technique. We characterize them in terms of both optical and terahertz functionality, carrier dynamics, and new phenomenon. As for the devices and systems, we develop the optical interface, photomixing local oscillator, basic systems for wireless terahertz wave communications, and quantum computing elements based on the magnetic flux quantum control. The experimental tools for those researches have been proposed and developed, such laser-terahertz emission microscope. Shown in "Project/Fund" section is our main research

II. Material Science

We grow most of the perovskite thin films by laser ablation, and characterize them by means of X-ray diffraction, magnetization, AFM, X-ray fluorescence spectrometer, femtosecond optical pump and probe transient reflection measurement,terahertz emission spectroscopy, pump and probe terahertz emission spectroscopy, dielectric parameter measurements using inter-digital electrodes, and so on.

The prepared films are characterized by means of Terahertz time-domain spectroscopy (THz-TDS) at various temperatures. The followings are notable findings we obtained so far; Pseudo-gap opening above 200 K in BSCCO thin films was observed in the temperature dependence of the imaginary part of conductivity; Pseudogap was observed in La2-2xSr1+2xMn2O7 (x=0.3); Charge density waves were excited in PCMO; Contrary to theprevious reports, SrTiO3 shows no hardening effect of thephonon soft mode below structural phase transitionat around 110K(pdf).

Terahertz emission properties are studied to understand carrier dynamics in various electronic materials. We found that moderate decrease in carrier density of YBCO enhances THz emission efficiency(pdf). The THz emission from high Tc superconductors obeys the rules that the maximum frequency of the emitted waves never exceed Josephson plasma frequencies of the materials. THz emission properties reflect spin behavior of PCMO. Photo-induced phase transition from metal state to insulating could provide unique function of the THz switches made of PCMO thin films(pdf). Recently we dicovered THz emission effect multiferroic materials with memorry effect(pdf).

We develop the system of pump and probe terahertz emission spectroscopy (PPTES) at low temperatures. Semiconductors, GaAs, InP, and LT-GaAs, are well characterized by PPTES, which has been proven as a powerful tools to study ultrafast carrier dynamics while exciting terahertz waves. PPTES has been employed to explore the physics in YBCO and PCMO as well as semiconductors.

D. Complex Dielectrics Measurements for Epitaxial Films at Various Temperatures

We study complex dielectric constants of the epitaxial thin films employing interdigital electrodes. The system measurable between 20K and 873K was developed. Mainly, dielectric and ferroelectric films are studied.

E. Exploration in ultrafast material science to discover new phenomena

General physics is organized on the basis of the minimum free energy of the ensemble system. Recent progress in ultrafast laser is now breaking the rules of the ensemble system. As one of the interesting approach, we try to manipulate magnetic flux quantum in superconductors by illumination with femtosecond laser. As a new effect, we realize vortex generation and its control inside high-Tc superconductos, breaking the rule of superconductivity(pdf).

III. Novel Devices and Systems

A. Semiconductor Terahertz Switches

We develop a-Ge and InGaAs-photoconductive-switches(pdf) for femtosecond fiber laser excitation at a wavelength of 1.5μm. Fe implanted InGaAs works efficiently as a THz emitter but inefficiently as a detector. Optimization of both annealing and implantation is now underway.

B. Photomixer/High-Tc Josephson Junction Hybrid System

A photomixer coupled with a high-Tc Josephson junction has been developed for photonic local oscillator and optical input interface to SFQ circuits. Conventional photodiode and uni-travelling-carrier photodiode (UTC-PD) are utilized. Up to now, over 50GHz signal generation and detection through a coplanar strip transmission line was realized.We also develop flux-flow-transitor for optical interface(pdf).

C. Wireless Terahertz Communication

We generate over 100GHz electromagnetic waves by photomixing and the waves which travel in free space are detected by high-Tc Josephson junction(pdf). This provides important techniques and information for the research of Terahertz wireless communication.

IV. Application Systems

We develop compact THz-TDS systems; mobile THz- imaging head coupled with optical fiber (pdf) and all 1.5-μm-wavelenght fiber-laser-based THz-TDS (pdf). Both system enable us to apply the system for many applications. For instance, the system fits in cryostat for the study of physics in strongly correlated electron systems at low temperatures.

One can observe terahertz emission from various kinds of materials excited with a femtosecond laser. Images can be obtained by scanning laser spot on them, which we refer as to “Laser Terahertz Emission Microscope (LTEM)”. Since terahertz emission properties reflect carrier dynamics and intrinsic nature in the materials, we will be able to visualize the new type of physical information. Recently we developed scanning-probe LTEM(pdf).

C. Magneto-Optical Microscope

We are developing a new type of magneto-optical (MO) microscope to study vortex dynamics (pdf) in the optically excited high-Tc superconductors. The system will be applied to develop the optical output interface of SFQ.

V. Summary

Tonouchi laboratory explores ultrafast science and looks for novel terahertz/photonic functions in new materials. The final targets of the research are for ultrafast phtonics and terahertz applications, especially in the field of solid state photonics/electronics such as Terahertz Wireless Communications.

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