Researc Review ’94-’99 (2000年3月23日刊行)

Preface

This is the first research review of Baba Laboratory in Yokohama National University, Division of Electrical and Computer Engineering, which summarizes activities from 1994 to 1999. This laboratory started from 1993 physical year in Japan, as a research group for optoelectronics. Main research topics now are (1) microcavity semiconductor lasers and the spontaneous emission control, (2) photonic crystals and photonic bandgaps, (3) microdisk injection lasers and near field optics, (4) micro-fabricated semiconductor light sources and their applications, (5) vertical cavity surface emitting lasers (VCSELs) for fiber communications, and (6) passive components with micromachine technologies.

One of the most remarkable events in these six years is the great advance in the research field of photonic crystal, especially in Japan. This field originated in Japan at the end of 70’s, but became a new topic mainly in the American Physics Society from the end of 80’s to the early 90’s. During this period, the main interest of people was the analogy of photonic bands to electronic bands, and limited discussions were devoted to technologies and applications. Some research groups in USA and Europe moved toward such new trends after the publication of the memorial special issue of JOSA in 1993. In 1994, I discussed photonic crystals with fabrication methods and possible applications, for the first time, at the spring annual meeting of JSAP, for the purpose of the engineering of spontaneous emission in light emitters. Far beyond my first expectation, the field spread rapidly, involving optic theories, quantum physics, solid state materials, numerical simulations, synthesis algorisms, semiconductor processing techniques, optoelectronic components including fibers, electromagnetic waves, micro-mechanics, organic chemistries, and so on. In 1998, I received happy news that the research on photonic crystals proposed by the group of Hokkaido University, Chiba University, Kyoto University, Yokohama National University, Shinshu University, and RIKEN was selected as the first subject for the Grant-In-Aid Priority Area (B) just founded. This accelerated the expansion of the research community in Japan almost exponentially. Now, the number of papers presented at JSAP exceeds 40. During these six years, we have developed micro-fabrication technologies and numerical computation programs, and have succeeded in demonstrating the efficient light propagation in photonic crystal waveguides with steep bends and the strong enhancement of light extraction efficiency in photonic crystal light emitters. I was happy that I could join relatively closed workshops by NATO in 1995 as the only Japanese, and also join the memorial workshop WECS (Workshop on Electromagnetic Crystal Structures) at Laguna Beach, CA, as one of the five committee members. I believe that activities of our group contributed something to the development of this research field.

Another successful research was the microdisk injection laser. It was invented by a group of AT&T Bell Labs. in 1992. After that, however, the number of reports on this laser gradually decreased due to the difficulty in the continuous wave (cw) lasing and due to the inefficient light output. In 1996, we succeeded in the room temperature pulsed operation in a 3-micron-diameter device, the smallest injection laser at that time. We also achieved the first cw operation of this laser in 1998 with a record low threshold current of 150 microamperes. Since the geometry is very simple and the high reflectivity mirror is easily obtained, this device is promising to the efficient control of spontaneous emission. Now, we are studying toward a very large spontaneous emission factor > 0.1 expected in this type of device. We also proposed and numerically simulated the efficient light output by using the evanescent coupling of laser light to other microdisk elements. This concept was used for a new type of optical near field probe sensor. A high probe resolution beyond the diffraction limit was calculated and experimentally demonstrated.

Beyond our expectation, a laser diode with semiconductor and air distributed Bragg reflectors, which we proposed in 1994, was accepted by many groups as a research target of an access network laser or a photonic crystal laser. The stable fabrication of this reflector and the development of the moduling technology are next key issues. Besides these, we studied the thermal problem in VCSELs, the enhancement of LED efficiency by the photon recycling effect, wafer bonding techniques for the monolithic/hybrid photonic integration, micro-mirrors for the optical deflection, optical interconnects for neural computers, and so on.

I wish to acknowledge Prof. Y. Kokubun, Yokohama National University, and Prof. K. Iga and Associate Prof. F. Koyama, Tokyo Institute of Technology, for their great support of our research and many valuable suggestions. Also, I would like to thank Prof. Y. Hirose, Prof. M. Toki, Associate Prof. H. Arai and Mr. T. Abe of Yokohama National University, Prof. S. Arai, Assistant Prof. T. Miyamoto, Mr. T. Sakaguchi, Dr. A. Matsutani of Tokyo Institute of Technology, and Dr. A. Kasukawa, The Furukawa Electric Company Ltd., for their supports. Our work was supported by the Ministry of Education, Science, Sports and Culture, and many grants of foundations, companies, etc.

March 23, 2000

Toshihiko Baba, Associate Professor

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