Site: University of Tokyo
Institute of Industrial Science
Department of Electrical Engineering
Minato-ku, Tokyo, 106 Japan
Date Visited: November 14, 1994
Report Author: R. Hickernell
Prof. Y. Arakawa
Prof. H. Sakaki
Prof. T. Kamiya
Prof. K. Tada
The University of Tokyo, established in 1877, consists of 10 facilities, 12 institutes, and 11 graduate schools. The Institute of Industrial Science (IIS) is the largest among the institutes, with 48 full professors, 37 associate professors, and 313 graduate students (as of year-end 1992). Researchers from five departments, including the Department of Electrical Engineering and Electronics, participate in IIS. There are a total of 70 faculty members in the graduate school of Electrical Engineering and Electronics. The IIS graduates approximately 90 Master's degree students and 35 PhD students each year, including 15 noncitizen PhDs. Eleven professors in Electrical Engineering are involved in optoelectronics research at the University of Tokyo. They are located on the campuses of Roppongi (IIS), Hongo (Faculty of Engineering), and Komaba (the Research Center for Advanced Science and Technology, RCAST).
The research of Professor Kunio Tada and Assistant Professor Yoshiaki Nakano falls into three categories: (1) compound semiconductor optical modulators and switches, (2) distributed-feedback semiconductor lasers, and (3) materials, processing, and novel devices for photonics applications. In the first category, they have developed traveling-wave GaAs/AlGaAs directional coupler modulators with bandwidths of 9 GHz at 1.06 microns, and carrier-injected optical switches using a double-heterostructure bipolar transistor waveguide structure.
Most of Dr. Tada's presentation centered on his group's work on distributed feedback (DFB) semiconductor lasers. The DFB lasers use gain or absorption coupling (periodic perturbation of the gain/loss coefficient) to remove the two-mode degeneracy of conventional index-coupled DFB lasers. The project, started in 1988, has concluded that loss gratings produce lasers with longer lifetimes than gain gratings. The lasers are insensitive to facet and external reflections, operate with a single longitudinal mode, and exhibit self-suppression of spatial hole burning. During high-power, short-pulse operation, loss saturation yields a higher intrinsic modulation bandwidth and lower chirping. Future applications foreseen for the gain-coupled DFB lasers are as a source for the subscriber loop and as a source in photonic integrated circuits. Professor Tada also discussed his work on semiconductor polarization-independent waveguide switches.
Professor Takeshi Kamiya explained some of the government funding avenues for university researchers. The Ministry of Science and Technology funds "big science" efforts and the ERATO program. MITI is currently funding the Real World Computing project and a femtosecond technology project that is in the planning stage. The Ministry of Education funds the basic universities' budgets, specific research grants, and currently the Ultrafast and Ultraparallel Optics (UUO) project, which involves 40 university groups across Japan. The UUO project's funding level is $6 to 7 million for three years, to be applied only toward purchases of equipment, materials, and supplies. Professor Kamiya's group has been involved in the UUO, RWC, and femtosecond technology projects. His research topics include ultrafast optoelectronics, OEICs, the physics and electronics of low-dimensional systems, and liquid crystal characterization and application. In collaboration with NTT, Oki Electric, and the University of Melbourne, Professor Kamiya's group is developing semiconductor laser pulse sources, concentrating on laser gain switching and fiber solution compression techniques. They have generated pulse widths less than 0.6 ps with pulse energies larger than 30 pJ. In the area of OEICs, they are improving the triggering sensitivity of optically addressed flip-flop circuits based on GaAs IC technology. Design, modeling, and mask fabrication is performed at the university; the devices are fabricated by an outside supplier.
The research of Professor Hiroyuki Sakaki and collaborators centers on the investigation of the fabrication and the electron transport and photonic properties of semiconductor quantum wires and dots. Professor Sakaki has a joint appointment at IIS, RCAST, and the Quantum Wave Project of ERATO (the program for Exploratory Research for Advanced Technology of the Research and Development Corporation of Japan). The quantum wire and dot structures are GaAs/AlGaAs grown by MBE over photolithographically patterned substrates. The quality of the structures and the practical techniques for fabrication have improved greatly over the last few years, increasing the confidence of the technical community in the reliability of quantum devices.
The work of Professor Yasuhiko Arakawa and his group falls into four categories: (1) fundamentals to control electrons and photons, (2) semiconductor nanotechnologies using MOCVD, e-beam, and STM, (3) quantum effect semiconductor devices (lasers, in particular), and (4) ultrafast optoelectronics technologies. Professors Arakawa and Sakaki first proposed the concept of quantum wire and quantum dot lasers and their improved threshold current characteristics in 1982. Recently, Arakawa's group has fabricated GaAs quantum wires with a 10 nm lateral dimension and highly uniform InGaAs/GaAs quantum dots with 15 nm dimensions by an MOCVD selective growth technique. He compared his quantum dot results with those of UCSB researchers, who have fabricated 20 nm dots by MBE. He has made what is believed to be the first demonstration of the interaction of low-dimensional electrons and low-dimensional photons. The quantum wires are embedded in the spacer region of an AlGaAs vertical-cavity, surface-emitting microlaser, which operates at 77 deg. K.
Professor Arakawa reviewed the recent funding history of nanoelectronics, in which IIS is involved, supported by the Japanese Ministry of Education:
Professor Arakawa is the head of both the quantum nanoelectronics project and the international collaboration with Imperial College of London. Professor Arakawa is also on the quantum electronics steering committee. When asked about future equipment expenditures from project funds, he said that he would purchase an ultra-high-vacuum STM.
Professor Yoichi Fujii was not available for our visit, but his work was represented in the handout given to the JTEC team. It includes studies of soliton applications in fiber communications and computing, and measurements of the electro-optic properties and photorefractive sensitivity of lithium niobate, lithium tantalate, and KTP waveguides. Professor Fujii's group is also investigating the growth of MgO-doped lithium niobate films by liquid-phase epitaxy.
The panel was given a tour of some of the laboratories and facilities of IIS at the University of Tokyo. It included the MOCVD growth facilities and laser laboratories. Panelists were told that the regulations for toxic gas safety are especially strict in the inner city areas, and that the government funds the use of safety equipment and methods. The last stop on the tour was a demonstration of micromechatronics research (equivalent to micro-electro-mechanical systems (MEMS) research in the United States), and a brief look at a modular cleanroom recently installed adjacent to a demolished building tower. The premium on space was evident in the crowded conditions of many of the labs we visited.
The research in compound semiconductor optoelectronics at the university continues to be very impressive. The funding outlook for optoelectronics appeared to be quite good, especially compared to recent years. The JTEC panel was informed that, coincident with the Japanese recession, the funding for universities is increasing. This is in part due to an increased priority on graduate education by the Japanese government. One of the team members observed that the laboratories appeared to be much better equipped than at the time of his visit a few years ago.