Results 2001



This is an annual report on the research activities in the field of optical communications and high-speed electron devices for 2001 at the Graduate School of Science and Engineering, the Interdisciplinary Graduate School of Science and Engineering, and Research Center for Quantum Effect Electronics, Tokyo Institute of Technology.

These activities are initiated by Professor Y. Suematsu (emeritus, the former president), and Professor S. Arai [Group A], mainly in the field of Low- Dimensional Quantum-Structure Lasers, Advanced Lasers for Photonic Integrations, and also in the fabrication of ultra-fine structures.

This report consists of a brief introduction of the research activities and a collection of the research papers published in 2001.

Publication List

Group members

  • Professor Emeritus
    • Yasuharu SUEMATSU D.E.
  • Professor
    • Shigehisa ARAI D.E.
  • Secretaries
    • Kyoko KASUKAWA B.A.
  • Visiting Researcher
    • Jong-In SHIM1 D.E. Jul.-Aug.
  • Post-Doctoral Research Fellow
    • Bo CHEN1 Ph.D. -Feb.
    • Nobuhiro NUNOYA D.E Oct.-
  • Graduate Students
    Doctor Course)
    • Jorg WIEDMAN2 M.S. -Sep.
    • Nobuhiro NUNOYA M.E. -Sep.
    • Hyo-Chang KIM M.E. Apr.-
    • Hideki YAGI M.E. Apr.-
  • Graduate Students
    Master Course)
    • Koji EBIHARA3 B.E. -Mar.
    • Kensuke MATSUI4 B.E. -Mar.
    • Monir MORSHED5 B.E. -Mar.
    • Hideki YAGI B.E. -Mar.
    • Kengo MURANUSHI B.E.
    • Takeshi OKAMOTO B.E.
    • Kazuya OHIRA B.E. Apr.-
    • Yuich ONODERA B.E. Apr.-
  • Undergraduate Students
    • Kazuya OHIRA -Mar.
    • Masataka OHTA6 -Mar.
    • Yuich ONODERA -Mar.
    • Hiroshi KANJO Apr.-
    • Akihiro ONOMURA Apr.-
    • Takuya SANO Apr.-

Present Address

  • 1) Demeter Technologies, Inc. (USA)
  • 2) QDI Germany GmbH (Germany)
  • 3) Fujitsu Laboratories Ltd.
  • 4) Hitachi Cable, Ltd.
  • 5) The Furukawa Electric. Co., Ltd.
  • 6) Assoc. Prof. Tomoyuki Miyamoto’s Lab., Tokyo Inst. Tech.


Low Dimensional Quantum Structure Lasers

Staffs: Y. Suematsu, S. Arai, S. Tamura
Post-Doctoral Research Fellow: B. Chen, N. Nunoya (from Oct.)
Students: N. Nunoya, M. Morshed, H. Yagi, K. Muranushi, K. Ohira, A. Onomura, T. Sano

GaInAsP/InP strained-quantum-film, -wire and -box lasers have been studied. Distributed feedback (DFB) lasers consisting of wirelike active regions fabricated by the same fabrication process as Quantum-Wire lasers have been also studied.

Results obtained in this research are as follows:

(1) 1.5-µm-wavelength partially strain-compensated GaInAsP/InP 5-layered quantum-wire lasers with the wire width of 23 nm in the period of 80 nm were realized for the first time by electron beam lithography, CH4/H2-reactive ion etching and organometallic vapor-phase-epitaxial regrowth. The threshold current density of 774 A/cm2 and differential quantum efficiency of 40 % were obtained under a pulsed condition at room temperature. From measurement of spontaneous emission spectra, the blue shift at the peak wavelength was 38 meV, which was much larger than a calculated value, and the spontaneous emission spectral width was almost constant at temperatures between 103 K and 253 K, indicating a lateral quantum confinement effect. Finally, the spontaneous emission efficiency below the threshold was almost comparable to that of the Q-Film lasers up to 85°C, that revealed low-damage property of the etched/regrown interfaces.

(2) GaInAsP/InP partially strain-compensated multiple-quantum-wire lasers with the wire widths of 18 nm and 27 nm in the period of 80 nm were also realized. Size fluctuations of these quantum-wire structures were measured by scanning electron microscope views, from which the standard deviation was obtained to be less than 2 nm. The differential quantum efficiencies of these quantum-wire lasers were almost the same as that of the 5-quantum-well lasers at room temperature. From EL spectra of various wire widths lasers, a larger energy blue shift than that from a simple analysis model was observed, which can be attributed to residual compressive strain between the active region and surrounding InP layer.

(3) High-performance operation of 1.55 µm wavelength GaInAsP/InP strongly index-coupled and gain-matched distributed feedback lasers with periodic wirelike active regions was realized, whose index-coupling coefficient was more than 300 cm-1. In order to design lasers for low threshold current operation, threshold current dependences on the number of quantum wells and the wire width were investigated both theoretically and experimentally. A record low threshold current of 0.7 mA was realized at room temperature CW condition for a 2.3-µm-wide buried heterostructure with a 200-µm-long cavity. We also confirmed stable single-mode operation due to a gain matching effect between the standing-wave profile and the wirelike active region.

(4) A CW life test of 1550 nm gain-matched DFB laser, which consists of wire-like active regions and exhibits sub-mA threshold, was carried out. No degradation was observed in the output and the spectral characteristics after 8500 hrs operation at a bias current around 10 times the threshold.

(5) A distributed reflector (DR) laser consisting of wirelike active regions with asymmetric output characteristic was realized for the first time. To realize an asymmetric output property while maintaining low threshold current operation, a l/4 shifted grating and modulated active region widths were introduced into the grating structure. Threshold current as low as 1.8 mA, asymmetric output ratio of 8, and a sub-mode suppression-ratio (SMSR) of 33 dB at I = 1.2Ith were obtained for the cavity length of 200 µm and the stripe width of 2.3 µm under a RT-CW condition.

New Types of Semiconductor Lasers for Photonic Integration

Staffs: Y. Suematsu, S. Arai, Y. Miyamoto, S. Tamura
Post-Doctoral Research Fellow: B. Chen, N. Nunoya (from Oct.)
Students: J. Wiedmann, N. Nunoya, H.-C. Kim, K. Ebihara, K. Matsui, T. Okamoto, M. Ohta, Y. Onodera, H. Kanjo

Semiconductor lasers with low threshold current, high efficiency, and single wavelength operation are very attractive for optical interconnection and a number of optoelectronics applications. New types of semiconductor lasers, such as multiple-micro-cavity (MMC) lasers, deeply etched distributed-Bragg-reflector (DBR) and vertical-grating distributed feedback (VG-DFB) lasers as well as vertical-grating distributed-reflector (VG-DR) lasers have been studied both theoretically and experimentally.

Membrane lasers consisting of very thin semiconductor core layer sandwiched by polymer/SiO2 cladding layers have been also studied.

Results obtained in this research are as follows:

(1) High-reflectivity semiconductor/BCB reflectors were fabricated by multiple sequential steps of CH4/H2 RIE etching and O2 plasma ashing. The reflectivity was estimated to be as high as 95%. Using these reflectors, highly uniform 1.55-µm-wavelength lasers with low threshold and high differential quantum efficiency were demonstrated. In addition, the reliability of such polymer-buried DBR lasers was investigated for the first time. The technology employed in this work is highly promising for the monolithic integration and batch processing of edge emitting lasers with other photonic devices through low-loss polymer waveguides.

(2) The novel design for obtaining single-mode operation by combing a DBR facet with multiple cavities was analyzed in theory and experiment. It was shown that the loss per groove is an important parameter for the best choice of the cavity number. Single-mode operation was obtained for different number of cavities. Increasing the number of cavities will decrease the efficiency drastically. The threshold current is lowest for two or three cavities. Therefore, it can be concluded that a CC laser is best for laser operation according to high efficiency and low threshold. For the CC laser an SMSR of 36dB was achieved at 1.8 Ith.

(3) A novel DR laser including a vertically etched grating was successfully fabricated. In case of a mesa width of Ws= 6 µm and mesa width variation of Δ Ws = 0.5 µm, a low threshold current of Ith= 12.4 mA and a high differential quantum efficiency ofhd = 42% were achieved with an SMSR of 33 dB.

(4) Distributed feedback lasers with a deeply etched first order vertical grating were realized for the first time. It was shown that we could obtain an effective coupling by reducing the stripe width. The sample with the cavity length of 430 µm, 1.8 µm stripe width and 0.2 µm grating depth on each lateral side exhibited a 12.5 mA threshold current, 37 % total differential quantum efficiency and an SMSR of 35 dB at a bias current of two times the threshold.

(5) By use of VG-DFB structure, it was clarified that structural birefringence can be completely eliminated. The grating coupling coefficient can also be polarization independent by adjusting grating depth.

(6) Novel semiconductor laser structure, that is, membrane laser which has the Benzocyclobutene (BCB) cladding layers, enables to increase optical confinement into active layer due to a large refractive index difference between the active and cladding layers. A RT-CW operation of membrane DFB laser consisting of deeply etched single-quantum-well wirelike active regions was demonstrated for the first time under optical pumping. A threshold power of 38 mW was obtained for 10.7 µm-wide and 40 mm-long device. From spontaneous emission spectrum, a large stop-band width of 65 nm and a low equivalent refractive index of 2.30, which are peculiar to a thin membrane waveguide structure, were observed.

(7) In order to realize single mode and low threshold operation of 1.5 mm-wavelength GaInAsP/InP membrane DFB laser, buried heterostructure (BH) was innovated by slightly changing the fabrication process. A threshold pump power of 4.8 mW and an SMSR of 39 dB were obtained for a 142 nm-thick semiconductor membrane core layer with a cavity length of 120 µm and a stripe width of 2 µm under RT-CW optical pumping. The corresponding threshold for current injection was roughly estimated to be 88 µA.

Financial Support

1. Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Grant-in-Aid for Research Center for Ultra-high Speed Electronics
Grant-in-Aid for Research Center for Quantum Effect Electronics
Grant-in-Aid for Scientific Research (A, B, C)
Grant-in-Aid for Exploratory Research
Grant-in-Aid for Encouragement of Young Scientists

2. Other Grant

Fellowship of the JSPS for Japanese Junior Scientists
Grant for “Development of Frequency Resources” from Ministry of Public Management, Home Affairs, Posts and Telecommunications
Seki Memorial Foundation for the Promotion of Science and Technology

3. Companies & Others

Fujikura Co., Ltd.
Furukawa Electric Industries Co., Ltd.
Hitachi Cable Co., Ltd.
NEC Co., Ltd.
Nippon Sanso Co., Ltd.
NTT Photonics Research Laboratories
Sumitomo Electric Industries Co., Ltd.

Nishiyama Laboratory
Quantum Nanoelectronics Research Core, Tokyo Institute of Technology

7F, S9-1, 2-12-1 O-okayama, Meguro-ku Tokyo 152-8552, Japan +81-3-5734-2555 ee.e titechnishiyama

Nishiyama lab. Student's room : South Bldg. 9 #701, #706, #707 | Measurement room : South Bldg. 9 #604, #502, #201 |
Clean room : South Bldg. 9 #202, B1F Exposure house | Research Laboratory of Ultra-High Speed Electronics