Quantum cascade lasers (QCLs) are unipolar lasers based on intersubband transition (ISBT) in a multi-quantum-well heterostructure, which enable one to obtain lasing action at wide spectral range from mid-infrared to THz region. QCLs have been much developed during the past fifteen years after their first demonstration. This rapid progress is mostly due to the quickly advancing understanding of how 'band-structure engineering', which means designing quantum well and barrier thicknesses and band-offset, can be most successfully used to control the electron flow and thus increase population inversion.

At present, QCLs seem to have reached their mature state, at least in the mid-infrared region. Therefore, the band-structure engineering as a driving force for major performance improvements will have diminishing returns. However, there are still a room to make improvements in areas such as high power single mode lasers, wide-tunable single mode lasers, and QCL-based functional devices, such as optical modulators, amplifiers, and switching devices. In order to develop these devices, not only traditional approaches based on band-structure engineering, but also new approaches to efficiently manipulate photons are required.

One of the promising ways to develop high-functional intersubband devices is to utilize novel cavity structures, such as microcavities and coupled cavities. The microcavities enable single-mode operation, allow easy monolithic integration of the lasers with other components, and have very low energy consumption owing to their small sizes. Moreover, it is possible to achieve switching operation of lasing wavelength in the coupled cavity structures.

When these cavity structures are to be applied to QCLs, it is necessary to deeply understand the unique electrical and optical characteristics of QCLs caused by intersubband carrier relaxations and optical transitions. Objectives of this thesis are to investigate the effects of the exploitation of the microcavity structures and the couple cavity structures on the characteristics of the intersubband lasers, and to develop the performance of mid-infrared QCLs with these cavity structures.

This thesis presents original research work on design, fabrication, and characterization of mid-infrared QCLs with these cavity structures. It is organized into eight chapters. Chapters 1-3 give fundamentals of this thesis. Chapter 1 presents the research background and thesis objectives. In Chapter 2, basics of ISBT in quantum wells and its applications, especially to intersubband lasers are introduced. Chapter 3 describes calculation methods for electronic properties in quantum cascade structures, such as energy levels, carrier scattering times, and optical dipole matrix elements. Calculation methods for optical properties of the cavity structures, such as resonant frequencies, Q-factors, and field distributions are also introduced in this chapter.

In Chapter 4, crystal growth, fabrication, and characterization methods and experimental results on fundamental characteristics of the ridge waveduide lasers are described. Details on the crystal growth by molecular beam epitaxy (MBE) and analysis of the grown wafers by X-ray diffractometer (XRD) and scanning electrical microscope (SEM) are given. Methods to precisely control layer thickness, composition, interface roughness, and doping density are also presented. Then the fabrication processes of ridge-waveguide cavity structures by photolithography, wet-etching, sputtering deposition of dielectric material, metal deposition, thinning of wafer, and wire-bonding are described in details. Finally, electrical properties, output power characteristics, and emission spectrum of the pulsed operated ridge waveguide QCLs are given. The emission from the devices is detected by mercury cadmium telluride (MCT) detector and lock-in amplifier technique. Lasing spectra are measured by Fourier transformed infrared spectroscopy. Demonstration of room temperature lasing operation around λ~ 11.5 μm is shown.

Chapter 5 concerns two-segment ridge waveguide QCLs. The objective of this chapter is to investigate the possibility of realization of intra-cavity optical power modulation and bistable operation utilizing intersubband absorption property in two-segment QCLs. When the reverse bias voltages are applied to one of the two cavities with smaller size while keeping the longer cavity lased, output power is clearly attenuated. Dependence of absorption coefficient in the quantum cascade structure on applied reverse bias voltage is calculated and showed that the absorption coefficient strongly depends on the bias voltage. The reason is related to quantum-confined Stark effect. This calculation result reveals that the optical power attenuation is caused by the intersubband absorption. Feasibility of QCL-based bistable devices utilizing the saturable intersubband absorption properties is discussed. It is also numerically shown that it is possible to realize QCL-based bistable devices by using two-segment cavity structures, although the condition is strict due to intraband ultra-fast carrier relaxation processes.

Chapter 6 presents experimental results on ridge-waveguide QCLs coupled with microcylindrical cavity. Compared with the coupled cavity structure presented in the previous chapter, one cavity is replaced by the microcylindrical-shaped two-dimensional cavity and is located close to the one-dimensional ridge-waveguide cavity. First half of this chapter gives the evidence of mode-coupling between two cavities and the latter half presents a demonstration of mode-switching over wide-spectral range. Several samples with different spacing between the two cavities are investigated to optimize the cavity spacing. When current is injected only into the microcylindrical cavity, threshold currents of those devices clearly depend on the spacing. The increment of threshold current is observed in the devices with the spacing shorter than 4μ-m. It is considered that increase of threshold current in the short spacing devices is an indirect evidence of the mode-coupling between the two cavities. Dependence of Q-factor of the integrated microcylindrical cavity on the spacing is calculated by three-dimensional finite-difference time-domain (3D-FDTD) method. As the spacing is decreased, the abrupt decrease of Q-factor after 4 μm is obtained because of the optical coupling with the ridge-waveguide modes. This calculated result explains the experimental result well. Regarding the 1-μm-spacing device, switching of the lasing wavelength is observed when the currents injected into both cavities are controlled. This switching behavior is a unique property of coupled cavity lasers. The switchable range of wavelength is over 120 nm, which is very wide compared with the value reported so far. These wide-range mode-switching is resulted from the large free spectral range of the integrated cavity with small size.

Chapter 7 serves for work on further reduction of the cavity size of the QCLs by exploiting two-dimensional photonic crystals (PCs). By carefully designing photonic crystal-based microcavities, the cavity size can be significantly decreased, and consequently, the consumption energy of QCLs can be reduced. Futhermore, emission direction can be tailored by controlling Q-factors for each direction. By intentionally introducing a defect into the perfect crystal, defect modes with high-Q factors can be formed in the photonic crystal. However, because the emission caused by ISBT in quantum wells is polarized in transverse magnetic- (TM-) modes, in which conventional two-dimensional photonic crystal structures do not possess a photonic bandgap, it is difficult to develop intersubband lasers with photonic crystal defect-mode microcavities. Therefore, several photonic crystal structures are investigated and optimized for the TM-like polarized light. One of the structures is a triangular-lattice PC cavity with triangular-shaped air holes. It is shown that it is difficult to apply these structures to QCLs because higher guided modes close the photonic band gaps. Then a triangular lattice PC cavity with circular-shaped dielectric pillars is optimized. In the pillar based structures, since there are wide photonic band gaps for the TM- like polarized light, main effort is dedicated to suppressing radiation losses in the vertical direction. Delocalization of cavity mode distributions in the real space by modifying line-defect cavities enhances the vertical Q-factor, which is due to the suppression of the momentum components of the cavity mode within the light cone. The last structure is a graded square-lattice PC cavity with circular-shaped air holes. The highest Q-factor (~2200) among three structures mentioned above is achieved even there is no photonic band gap for lateral direction in this structure. This value of Q is shown to be sufficiently high for lasing action. The origin of high-Q is attributed to the decoupling between the cavity modes and the guided modes, and also radiation-modes in the momentum space. Finally, design and characterization of coupled cavity lasers with these photonic crystal defect-mode microcavities are also discussed.

In Chapter 8, conclusions to this thesis are presented. Implications of the results presented in this thesis are discussed. The outlook for future research and development is also given.

量子カスケードレーザとは半導体多重量子井戸構造中に形成されるサブバンド間での光学遷移を利用した半導体レーザである。現在までの量子カスケードレーザの著しい発展は、バンド内での効率的な電子の流れを追求する「バンド構造エンジニアリング」の発展によって支えられたといえる。それに対し、単一モードレーザや波長可変レーザ、変調器などの光機能デバイスへの展開を図るには、サブバンド間遷移に起因する発光を制御する光共振器構造も合わせて研究することが望まれる。本論文は「Fabrication and Characterization of GaAs-based Mid-Infrared Quantum Cascade Lasers with Coupled Microcavity Structures (結合微小共振器構造を有するGaAs系中赤外量子カスケードレーザの作製と評価)」と題し、光結合共振器構造を利用した中赤外量子カスケードレーザの機能化について論じており、全8章から構成され英文で書かれている。

第1章は「Introduction」と題し、量子井戸におけるサブバンド間遷移に関する基礎研究、及びサブバンド間遷移レーザへの応用に関する研究の歴史が論じられた後、本研究の目的が述べられている。

第2章は「Basics of Intersubband Transition, Quantum Cascade Lasers, and Coupled Cavities」と題し、サブバンド間光学遷移に関する数式的取り扱い、量子カスケードレーザに関する基礎、及び結合共振器構造を有する半導体レーザの特徴が述べられている。

第3章は「Numerical Analysis on Gain/Absorption and Current-Output Power Characteristics of Quantum Cascade Lasers」と題し、量子カスケード構造中でのサブバンド間吸収を利用した新規光機能デバイスの実現を目的として、量子カスケード構造中でのサブバンド間遷移に起因する吸収係数の印加電圧依存性の計算を行い、吸収係数はバイアス電圧に強く依存し、また逆バイアス状態では大きな吸収が存在することを示している。

第4章は「Fabrication and Characterization of Quantum Cascade Lasers with Single Ridge-Waveguide Cavity」と題し、GaAs系中赤外量子カスケードレーザの作製手法、及び試作と評価結果を示している。試作したデバイスは、パルスモードによる電流注入により、波長11.5μm、最高動作温度310Kでレーザ発振を確認することができた。

第5章は「Intracavity Amplitude Modulation in Coupled Ridge-Waveguide Cavity Quantum Cascade Lasers」と題し、長さが異なる2つのリッジ導波路共振器からなる結合共振器量子カスケードレーザにおける、共振器内光強度変調の実証について述べている。長い導波路部に電流を注入し、レーザ発振状態を保ちながら、他方の短い導波路部分に逆バイアス電圧を印加したときに、光強度変調動作の実現を確認した。共振器内変調にもかかわらず、Qスイッチングによる短パルス発生を伴わずに光強度変調動作を実現した意義は大きく、これは量子カスケードレーザにおける非発光遷移過程が支配的であるを示している。

第6章は「Wide-range Mode Switching in Ridge-Waveguide Cavity Quantum Cascade Lasers Coupled with Micro-Cylindrical Cavity」と題し、リッジ型導波路とマイクロシリンダー型微小共振器構造からなる結合共振器量子カスケードレーザにおける、発振波長の制御について述べている。集積化した微小共振器の大きな自由スペクトル領域(FSR)を活用し、広い範囲での波長のスイッチングを目的として、レーザ発振状態を保ちながら、微小共振器に正バイアスを印加したとき、120nm離れた2つのモード間で、発振波長をスイッチングすることに成功した。

第7章は「Design of Photonic Crystal Defect-Mode Microcavity for Application to Quantum Cascade Lasers」と題し、量子カスケードレーザに適した極微小共振器構造の設計として、2次元フォトニック結晶構造の活用を検討し、半径変調型円孔正方格子配列フォトニック結晶構造を用い伝搬モードとの結合強度を弱めることにより、レーザ発振に必要な2200という高いQ値を得ることが可能であることを示した。

第8章は「Conclusions and Future Outlook」と題し、各章の主要な成果をまとめて総括し、本論文の結論、及び将来展望について述べている。

以上、これを要するに、本論文は、量子カスケードレーザの機能化について論じ、結合共振器構造レーザにおける光強度変調動作、ならびに広帯域波長スイッチング動作を実現するとともに、共振器の小型・高性能化に向けてフォトニック結晶による高Q値微小共振器構造の設計について論じたものであり、電子工学に貢献するところが少なくない。

よって、本論文は、博士(工学)の学位請求論文として合格と認められる。