Research Review ’00-’02 (2003年3月26日刊行)

Preface

This is the second research review of Baba Laboratory, which summarizes activities from 2000 to 2002. It also introduces facilities equipped in between 1994 to 2002.

The first big event in 2000 – 2002 is an international workshop on photonic crystals (PC) named PECS (Photonic and Electromagnetic Crystal Structures), which was a follow-up meeting of WECS in 1999. It was held at Sendai on March, 2000 with participants over 200, more than twice that of WECS. With many authorities coming from overseas, English presentations and discussions were surprisingly active and natural. I participated the workshop as a committee member, as a chairperson of the final session and a presenter of three papers including an invited one. Main topics of this workshop were waveguides and lasers in PC slabs, both of which were first experimentally demonstrated after WECS by our group and Caltech, respectively. Many contributed papers from Japan well demonstrated the activity in Japan. The second workshop called PECS III, which was numbered from WECS, was held at St. Andrews in June, 2001. Here, participants were limited to ~100 (80 was refused to participate) to keep the workshop style. The topic was strongly concentrated on the PC waveguide studied toward a large scale photonic integration. This trend was maintained in IEEE JQE special issue on PCs in July, 2002, which was edited by Thomas Krauss, Univ. St. Andrews and I myself. In PECS IV held in UCLA on October, 2002, the trend slightly calmed down, while some previous topics were refocused, i.e. the negative refractive index and the superprism. Besides the series of the workshop, NATO conference “Photonic Crystals and Light Localization” in June, 2000 (just after PECS) was also impressive. I was happy to visit beautiful Crete island of Greece again, since 1995 for the previous conference. Korean Photonics Conference on November, 2001 was also a unique experience. Although presentations were made in Korean language, I could feel lively sessions and activities in universities and companies of Korea.

In these three years, our research focused on the PC waveguide as a main theme. The introduction of FE-type EB writer and ICP etch well improved the stability and the accuracy of the fabrication process. The experimental results were well explained by photonic bands obtained by the FDTD method. Still there remain many issues to be investigated, e.g., reduction in propagation loss, compatibility to a singlemode fiber, and low reflection and radiation losses at sharp bends, branches, etc. in a wide spectral range. Also, we addressed the analyses and the optimum design of light conductive PCs, i.e. superprisms and light deflection devices, high efficiency interfaces, and third order nonlinear functions, all of which were the first to be investigated. The PC LED was a simple application to demonstrate its high efficiency. To suppress the surface recombination in a highly processed PC made of GaInAsP-InP, we studied some passivation techniques, and discovered the effectiveness of the methane plasma irradiation. However, for the complete suppression of such nonradiative effects, we rather employed the surface-grating-type 2D PC. I was happy to find that it could improve not only the LED efficiency but also those of any spontaneous-emission-based light emitters. Another important result was the successful lasing in point defect lasers at the end of 2002, although it was the fifth achievement in the world and the result is not included in this research review. All of these results were based on 2D PCs. We also studied 1D and 3-D PCs. The laser diode with semiconductor and air distributed Bragg reflectors, which is considered to be a 1D PC, was improved by using the ICP etch. The smooth etch profile allowed the demonstration of a clear advantage of this mirror compared with a cleaved mirror. I was also happy to contribute to the fabrication of a woodpile 3D crystal lead by RIKEN team. They successfully constructed the crystal by a micromanipulation technique and observed the photonic bandgap.

During 2000 – 2002, the Priority Area Research and the Research for the Future of the Ministry of Education, Culture, Sports, Science and Technology ended, while new big projects CREST (Prof. Noda, Kyoto Univ., as a leader) of Japan Science and Technology Corporation, Nanoelectronics Research Center (Prof. Arakawa, Univ. Tokyo, as a leader), and 21st Century COE program (Prof. Kohno, Yokohama Nat’l Univ., as a leader) started from 2000, 2002 and 2002, respectively. The continuous grow of the Japanese activity can also be seen in the number of papers in annual meetings of the Japan Society of Applied Physics. As program committee members, I and Dr. Notomi of NTT founded a new session named Photonic Nanostructures and Phenomena. It accumulated many papers, so the number increased to nearly 70 in 2002, almost twice that in 2000. The first release of the Technology Roadmap, which was edited by Prof. Noda, I myself and Dr. Kosaka of NEC, published from Optoelectronic Industry and Technology Development Association, and distributed to more than 800 industrial groups, became a powerful driving force for the development of PCs. Due to the rapid development in these three years, we had to renew the contents and release the second version in 2002 with additional editor, Dr. Notomi. I hope this trend is still maintained when the next research review is planned.

Regarding microdisk lasers, which has been another main subject of this group, the lasing threshold was steadily reduced within these three years. In 2000, a lowest record threshold of 40 microamperes for GaInAsP compound system was achieved in a 2.7-micron-diameter device by cw current injection at room temperature. It was attributed to the precisely vertical etching by the ICP method, which improved the uniform carrier diffusion. The integration of metal pad electrode was realized by using a polymer cladding. The device not only exhibited the lasing but also the reduction in thermal resistance and the athermal effect. An ideal athermal laser, in which the lasing wavelength is independent of the temperature, is theoretically predicted using a very thin disk. The cw lasing by photopumping was also obtained for the first time with a threshold power of 30 microwatts. In 2002. However, this record was easily broken by a microgear cavity to 17 microwatts, which is a microdisk having a rotationally periodic grating. This threshold is a renewed lowest record for lasers made of GaInAsP material system. One of the surprising theoretical discoveries was that the minute control of electric and magnetic field profiles in the microgear can improve the Q factor of the microdisk. Another unique characteristic found in these microdisk-type lasers was the strain relaxation phenomenon in the disk active layer. The lasing wavelength of these lasers often red shifted from the PL peak wavelength. This suggests the phenomenon, resulting in the threshold reduction by the drift current flowed from the center region to the disk edge where the lasing mode is localized. The spontaneous emission factor of over 0.1 was precisely evaluated from the output - pump characteristic and the wavelength - pump characteristic. This is one of the highest value so far reported for semiconductor lasers. Next important issue is to demonstrate the Purcell factor. It has been partially demonstrated at room temperature at the end of 2002. The result will be included in the next research review. As a functional device that utilizes the microdisk is the active near field optical probe. By putting an object close to a lasing microdisk and scanning the relative position, the shape of the object or the mode profile of the microdisk laser was successfully visualized through the change of the lasing power.

A new subject we started studying within these three years is the Si photonics, which is based on the Si photonic wire waveguide in an SOI substrate. The pioneering work was done by MIT and some other groups, but published papers are still very limited. We focused on this waveguide because the fabrication technique is similar to that for the PC waveguides, but the design and light propagation characteristics are much simpler than those of the PC waveguide. We were surprised at experimental results, which well agreed with 3D FDTD simulations. The singlemode propagation was observed in a submicron rectangular core. The Fabry-Perot resonance indicated that the group index of this waveguide can be much higher than the material index. A sharp bend with a radius of a few mm showed a negligible low bend loss. The polarization crosstalk was investigated and some optimized bends for a low crosstalk was found through the FDTD simulation. The bend-waveguide-type branch exhibited a low excess loss less than 0.3 dB and the Robustness for a fabrication error. This branch enabled the successful demonstration of an H-tree optical signal distribution circuit. Recently, we can feel that this waveguide has attracting almost comparable attention to that for the PC waveguide. With so developed basic technologies, I can now expect to demonstrate more sophisticate circuits in the near future.

I wish to acknowledge Prof. Y. Kokubun, Yokohama National University, Prof. K. Iga, Prof. S. Arai and Prof. F. Koyama, Tokyo Institute of Technology, Prof. K. Inoue, Hokkaido University, Prof. K. Ohtaka, Chiba University, Prof. S. Kawakami, Tohoku University, Prof. Y. Aoyagi, RIKEN, Prof. Y. Arakawa, University of Tokyo, Prof. S. Noda, Kyoto University, and Dr. M. Notomi, NTT, for their great support of our research and many valuable suggestions. Also, I would like to thank professors and staffs of the communication group C of Dept. Electrical and Computer Eng., Yokohama National University, professors and staffs of Precision and Intelligence Laboratory, Tokyo Institute of Technology, Dr. A. Kasukawa and other members of Yokohama Laboratory, The Furukawa Electric, and photonic crystal research members of NEC, Hitachi, NTT, RIKEN, FESTA, NIMS, etc., for their discussions and supports. Our work was supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan Society for the Promotion of Science, Japan Science and Technology Corporation, and many companies.

March 7, 2003

Toshihiko Baba, Associate Professor

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