有機材料を含む微小共振器の強結合および超強結合状態における光学特性 Optical Properties of Microcavities Containing Organic Materials in the Strong and Ultra-strong Coupling Regime

論文要旨光の波長程度の共振器長を持つ光共振器は微小共振器と呼ばれる．微小共振器中において物質と光の相互作用は自由空間とは異なる性質を示す．例えば共振器の光子寿命よりも物質と光のエネルギー交換の速度が大きくなる領域においては強結合と呼ばれる相互作用が生じる．強結合状態においては物質と光がコヒーレントに相互作用することにより，共振器ポラリトンのような新たな量子状態が形成される．この時，微小共振器の共振モードが2つに分裂する真空Rabi分裂が観測される．さらに，この真空Rabi分裂が大きくなり物質の遷移エネルギーと同程度となった場合，超強結合と呼ばれる相互作用が観測される．超強結合状態においては強結合を扱う際には大きな意味を持たなかった反回転項やA2項による相互作用を無視できなくなる．これに伴って基底状態の変調や超放射相転移など，強結合状態では観測不可能な現象を観測できる可能性が理論研究によって指摘されている．従来，強結合に関する研究は主に半導体量子井戸構造などの無機物を用いて行われてきた．一方で有機物を用いた系においても100meV程度の大きな真空Rabi分裂の観測が1990年代末に報告された．さらに2010年台には1eVを超える巨大な真空Rabi分裂の観測が報告された．一方でポラリトン分枝間の遷移エネルギーは真空Rabi分裂の大きさに対応し，強結合領域においてはテラヘルツ領域に位置することが多い．また反転対称性を持つ系においては分枝間の遷移は禁制となる．従ってポラリトン分枝間の遷移は観測が難しく，その性質はこれまで調べられてこなかった．しかし上述のような超強結合系における巨大な真空Rabi分裂エネルギーは，分光実験が容易な近赤外領域に対応する．加えて有機色素分子には反転対称性がないものが多く，選択則が破れている可能性がある．そこで超強結合系における巨大な真空Rabi分裂を利用してポラリトン分枝間遷移を観測することを目指した．まずpseudoisocyanie色素分子J会合体を含む1次元フォトニック結晶微小共振器（1DPC）を作製した．J会合体は分子が1次元鎖状に配列した構造であり，分子間相互作用による振動子強度が増大を期待して強結合の観測によく用いられる．作製した試料に対して線形透過分光を行ったところ，真空Rabi分裂エネルギーは概ね色素濃度の1/2乗に比例して増加するものの，色素濃度が高くなってくると会合体間のエネルギー移動に由来すると考えられる真空Rabi分裂の飽和が起こり，超強結合状態には至らないことが明らかになった．また，非線形透過特性についても調査を行い，Spectral Triplet状態の色素濃度依存性を調べた．Spectral Tripletとは共振器ポラリトンを高強度の光で共鳴励起した際に，ポラリトンに由来する2つのモードの間に3つ目のモードが形成される現象である．この研究により，色素濃度にかかわらず，Spectral Tripletが形成されることが明らかになった．次に，prerylene系液晶性有機半導体を用いた系に関して実験を行った．実験に用いたperylene系液晶性有機半導体分子は室温でrectangular columnar相を形成し，分子は自発的に配向・配列する．このような状態は会合体と類似しており，振動子強度の増大が期待できるのではないかと考えた．そこで，perylene系液晶性有機半導体分子を1DPCの共振層に挿入し，線形透過分光を行ったところ，187meVという比較的大きな真空Rabi分裂エネルギーを観測し，強結合状態が実現された．さらに，friction transfer法を用いて分子配向を制御した薄膜を1DPC微小共振器の共振層に挿入し，真空Rabi分裂エネルギーの偏光依存性を調べた．この実験では光の偏光方向を変えることで真空Rabi分裂エネルギーの大きさを大きく変化させることに成功した．最後にホスト薄膜中に単分散させたLemke色素を金属微小共振器（MMC）に挿入した．線形透過分光の結果，1eVを超える真空Rabi分裂を観測し，真空Rabi分裂エネルギーと励起子遷移エネルギーの比は40%を超えた．透過分光により得られた分散関係は反回転項やA2項を含む理論計算と一致し，超強結合状態の観測に成功した．次に作製したMMCに対してポラリトン下枝に共鳴した光で励起し，真空Rabi分裂エネルギーに対応する光子エネルギーを持つ光でprobeする2色pump probe分光を行った．この結果，ポラリトン分枝間の遷移に由来する可能性がある光誘導吸収を観測することができた．本論文では，第1章で研究の背景と目的について述べる．第2章では微小共振器中の光と物質の相互作用を取り扱う．第3章で微小共振器の性質について議論する．第4章でpseudoisocyanie J会合体を含む1DPCの線形および非線形分光の結果について述べる．第5章ではperylene系液晶性有機半導体を含む1DPCの線形透過分光について記述する．第6章ではLemke色素を含むMMC微小共振器の線形・非線形分光について述べる．最後に第7章にて総括を行う．

AbstractA microcavity confines light in a narrow space comparable to the wavelength of the light. In microcavities, interactions between matter and light are unlike those in free space. For example, a strong coupling regime is realized when the rate of the energy exchange between the matter and light is greater than the photon lifetime of the microcavity. In the strong coupling regime, these coherent interactions give rise to new quantum states, such as cavity polaritons. In such a case, the resonant mode of the microcavity splits into two. This phenomenon is called vacuum Rabi splitting. Furthermore, if the vacuum Rabi splitting energy becomes comparable to the transition energy of the matter, the interaction is termed an ultrastrong coupling. Unlike the strong coupling regime, the rotating-wave approximation breaks down and the square of the vector potential A of the light field (A2 term) cannot be neglected in the ultrastrong coupling regime. Some researchers have theoretically predicted that such a difference might enable the observation of unique phenomena. For instance, the generation of an entangled photon pair from the polariton vacuum and superradiant phase transition.In the early days, the strong coupling regime was mainly studied with the use of inorganic materials such as semiconductor quantum well structures. A large vacuum Rabi splitting of approximately 100 meV was reported at the end of the 1990s based on the use of organic materials. Moreover, a huge vacuum Rabi splitting exceeding 1 eV was observed after 2010. However, the peculiar optical properties of the ultrastrong coupling regime have not been investigated to date. For example, even the conditions required to realize ultrastrong coupling remain unclear. Therefore, there is a need to achieve ultrastrong coupling in more systems and investigate their properties.On the other hand, the transition energies between polariton branches, which correspond to the vacuum Rabi splitting energy, are often located in the terahertz spectral region in the strong coupling region. Additionally, transitions between polariton branches are forbidden in inversion symmetric systems. Therefore, transitions between polariton branches are difficult to observe, and their properties have yet to be investigated. However, the huge vacuum Rabi splitting energies in ultrastrongly coupled systems, as described above, correspond to the near-infrared region in which spectroscopic experiments are easier to perform. In addition, many organic dye molecules do not have inversion symmetry and the selection rule might be broken. Therefore, this doctoral dissertation aims to achieve an ultrastrong coupling regime with a huge vacuum Rabi splitting and observe a transition between polariton branches.First, we fabricated one-dimensional photonic crystal (1DPC) microcavities containing a pseudoisocyanie J aggregate. The J ggregate is a one-dimensional chain superstructure of aligned molecules. These structures are often used to observe strong coupling because it is expected that their oscillator strength becomes large owing to intermolecular interactions. We performed linear transmission spectroscopy on the prepared samples. The vacuum Rabi splitting energy increased in proportion to the square root of the dye concentration in the low concentration region; however, when the dye concentration increased, saturation of the increase of the vacuum Rabi splitting energy was observed. Unfortunately, the ultrastrong coupling regime was not realized in this system. In addition, we also investigated the dye concentration dependence of the spectral triplet state. The spectral triplet is the state in which the new mode appears between the polariton doublet owing to absorption saturation in the strong excitation condition. Nonlinear transmission spectroscopy revealed that the dye concentration did not greatly affect the triplet formation.Next, we investigated the optical properties of the liquid-crystalline (LC) perylene tetracarboxylic bisimide (PTCBI) derivative. The LC PTCBI derivatives have a rectangular columnar LC phase at room temperature and the molecules spontaneously align. We considered that such a structure is similar to the aggregate, thus, their oscillator strength may be increased. We inserted LC PTCBI derivatives into the 1DPC microcavity and performed linear transmission spectroscopy. This sample showed a relatively large vacuum Rabi splitting energy of 187 meV, and strong coupling was realized. We also report on the polarization dependence of the vacuum Rabi splitting energy of a 1DPC microcavity containing a film made from aligned LC PTCBI derivatives by a friction transfer method. We successfully controlled the magnitude of vacuum Rabi splitting by changing the polarization-directions in this experiment.Finally, Lemke dyes were dispersed homogeneously in a host polymer thin films, which were inserted into metal microcavities (MMC). The vacuum Rabi splitting above 1 eV was observed by the linear transmission spectroscopy, and the ratio of the vacuum Rabi splitting energy to excitonic transition energy exceeded 40 %. The dispersion relation determined by transmission spectroscopy agreed with the theoretical calculations, including the anti-rotating and A2 terms, and we succeeded in observing the ultrastrong coupling regime. Next, we performed dual-color pump-probe spectroscopy on the prepared MMC. In this experiment, we excited the sample with resonant light over the lower polariton branch and probed with light having a photon energy corresponding to the vacuum Rabi splitting energy. In this experiment, we observed photo-induced absorption that might result from transitions between polariton branches.We describe the construction of this doctoral dissertation. Chapter 1 outlines the background and purpose of the research. Chapter 2 discusses the interaction between the matter and light in microcavities. Chapter 3 deals with the nature of microcavities. Chapter 4 shows the results of linear and nonlinear spectroscopy of 1DPC microcavities containing pseudoisocyanie J aggregate. Chapter 5 describes the linear transmission spectroscopy of 1DPC microcavities containing PTCBI derivatives. Chapter 6 demonstrates linear and nonlinear spectroscopy of MMC containing Lemke dyes. Finally, Chapter 7 summarizes this dissertation.

Observation of ultrastrong-coupling regime in the Fabry–Pérot microcavities made of metal mirrors containing Lemke dye

Dual-colour pump-probe spectroscopy to observe the transition between polariton branches in an ultrastrongly coupled microcavity containing organic dye molecules

This article includes the following articles; 1) "Observation of ultrastrong-coupling regime in the Fabry-Perot microcavities made of metal mirrors containing Lemke dye"; Applied Physics Letters; Volume 114, Issue 19 (2019); DOI:10.1063/1.5080623 , which has been published in final form at https://doi.org/10.1063/1.5080623 . 2) "Dual-colour pump-probe spectroscopy to observe the transition between polariton branches in an ultrastrongly coupled microcavity containing organic dye molecules"; Japanese Journal of Applied Physics; (2019), which has been published in Accepted Manuscript at https://iopscience.iop.org/article/10.7567/1347-4065/ab53c8/meta.

https://doi.org/10.1063/1.5080623

https://iopscience.iop.org/article/10.7567/1347-4065/ab53c8/meta

論文要旨 光の波長程度の共振器長を持つ光共振器は微小共振器と呼ばれる．微小共振器中において物質と光の相互作用は自由空間とは異なる性質を示す．例えば共振器の光子寿命よりも物質と光のエネルギー交換の速度が大きくなる領域においては強結合と呼ばれる相互作用が生じる．強結合状態においては物質と光がコヒーレントに相互作用することにより，共振器ポラリトンのような新たな量子状態が形成される．この時，微小共振器の共振モードが2つに分裂する真空Rabi分裂が観測される．さらに，この真空Rabi分裂が大きくなり物質の遷移エネルギーと同程度となった場合，超強結合と呼ばれる相互作用が観測される．超強結合状態においては強結合を扱う際には大きな意味を持たなかった反回転項やA2項による相互作用を無視できなくなる．これに伴って基底状態の変調や超放射相転移など，強結合状態では観測不可能な現象を観測できる可能性が理論研究によって指摘されている． 従来，強結合に関する研究は主に半導体量子井戸構造などの無機物を用いて行われてきた．一方で有機物を用いた系においても100meV程度の大きな真空Rabi分裂の観測が1990年代末に報告された．さらに2010年台には1eVを超える巨大な真空Rabi分裂の観測が報告された． 一方でポラリトン分枝間の遷移エネルギーは真空Rabi分裂の大きさに対応し，強結合領域においてはテラヘルツ領域に位置することが多い．また反転対称性を持つ系においては分枝間の遷移は禁制となる．従ってポラリトン分枝間の遷移は観測が難しく，その性質はこれまで調べられてこなかった．しかし上述のような超強結合系における巨大な真空Rabi分裂エネルギーは，分光実験が容易な近赤外領域に対応する．加えて有機色素分子には反転対称性がないものが多く，選択則が破れている可能性がある．そこで超強結合系における巨大な真空Rabi分裂を利用してポラリトン分枝間遷移を観測することを目指した． まずpseudoisocyanie色素分子J会合体を含む1次元フォトニック結晶微小共振器（1DPC）を作製した．J会合体は分子が1次元鎖状に配列した構造であり，分子間相互作用による振動子強度が増大を期待して強結合の観測によく用いられる．作製した試料に対して線形透過分光を行ったところ，真空Rabi分裂エネルギーは概ね色素濃度の1/2乗に比例して増加するものの，色素濃度が高くなってくると会合体間のエネルギー移動に由来すると考えられる真空Rabi分裂の飽和が起こり，超強結合状態には至らないことが明らかになった．また，非線形透過特性についても調査を行い，Spectral Triplet状態の色素濃度依存性を調べた．Spectral Tripletとは共振器ポラリトンを高強度の光で共鳴励起した際に，ポラリトンに由来する2つのモードの間に3つ目のモードが形成される現象である．この研究により，色素濃度にかかわらず，Spectral Tripletが形成されることが明らかになった． 次に，prerylene系液晶性有機半導体を用いた系に関して実験を行った．実験に用いたperylene系液晶性有機半導体分子は室温でrectangular columnar相を形成し，分子は自発的に配向・配列する．このような状態は会合体と類似しており，振動子強度の増大が期待できるのではないかと考えた．そこで，perylene系液晶性有機半導体分子を1DPCの共振層に挿入し，線形透過分光を行ったところ，187meVという比較的大きな真空Rabi分裂エネルギーを観測し，強結合状態が実現された．さらに，friction transfer法を用いて分子配向を制御した薄膜を1DPC微小共振器の共振層に挿入し，真空Rabi分裂エネルギーの偏光依存性を調べた．この実験では光の偏光方向を変えることで真空Rabi分裂エネルギーの大きさを大きく変化させることに成功した． 最後にホスト薄膜中に単分散させたLemke色素を金属微小共振器（MMC）に挿入した．線形透過分光の結果，1eVを超える真空Rabi分裂を観測し，真空Rabi分裂エネルギーと励起子遷移エネルギーの比は40%を超えた．透過分光により得られた分散関係は反回転項やA2項を含む理論計算と一致し，超強結合状態の観測に成功した．次に作製したMMCに対してポラリトン下枝に共鳴した光で励起し，真空Rabi分裂エネルギーに対応する光子エネルギーを持つ光でprobeする2色pump probe分光を行った．この結果，ポラリトン分枝間の遷移に由来する可能性がある光誘導吸収を観測することができた． 本論文では，第1章で研究の背景と目的について述べる．第2章では微小共振器中の光と物質の相互作用を取り扱う．第3章で微小共振器の性質について議論する．第4章でpseudoisocyanie J会合体を含む1DPCの線形および非線形分光の結果について述べる．第5章ではperylene系液晶性有機半導体を含む1DPCの線形透過分光について記述する．第6章ではLemke色素を含むMMC微小共振器の線形・非線形分光について述べる．最後に第7章にて総括を行う．

Abstract A microcavity confines light in a narrow space comparable to the wavelength of the light. In microcavities, interactions between matter and light are unlike those in free space. For example, a strong coupling regime is realized when the rate of the energy exchange between the matter and light is greater than the photon lifetime of the microcavity. In the strong coupling regime, these coherent interactions give rise to new quantum states, such as cavity polaritons. In such a case, the resonant mode of the microcavity splits into two. This phenomenon is called vacuum Rabi splitting. Furthermore, if the vacuum Rabi splitting energy becomes comparable to the transition energy of the matter, the interaction is termed an ultrastrong coupling. Unlike the strong coupling regime, the rotating-wave approximation breaks down and the square of the vector potential A of the light field (A2 term) cannot be neglected in the ultrastrong coupling regime. Some researchers have theoretically predicted that such a difference might enable the observation of unique phenomena. For instance, the generation of an entangled photon pair from the polariton vacuum and superradiant phase transition. In the early days, the strong coupling regime was mainly studied with the use of inorganic materials such as semiconductor quantum well structures. A large vacuum Rabi splitting of approximately 100 meV was reported at the end of the 1990s based on the use of organic materials. Moreover, a huge vacuum Rabi splitting exceeding 1 eV was observed after 2010. However, the peculiar optical properties of the ultrastrong coupling regime have not been investigated to date. For example, even the conditions required to realize ultrastrong coupling remain unclear. Therefore, there is a need to achieve ultrastrong coupling in more systems and investigate their properties. On the other hand, the transition energies between polariton branches, which correspond to the vacuum Rabi splitting energy, are often located in the terahertz spectral region in the strong coupling region. Additionally, transitions between polariton branches are forbidden in inversion symmetric systems. Therefore, transitions between polariton branches are difficult to observe, and their properties have yet to be investigated. However, the huge vacuum Rabi splitting energies in ultrastrongly coupled systems, as described above, correspond to the near-infrared region in which spectroscopic experiments are easier to perform. In addition, many organic dye molecules do not have inversion symmetry and the selection rule might be broken. Therefore, this doctoral dissertation aims to achieve an ultrastrong coupling regime with a huge vacuum Rabi splitting and observe a transition between polariton branches. First, we fabricated one-dimensional photonic crystal (1DPC) microcavities containing a pseudoisocyanie J aggregate. The J ggregate is a one-dimensional chain superstructure of aligned molecules. These structures are often used to observe strong coupling because it is expected that their oscillator strength becomes large owing to intermolecular interactions. We performed linear transmission spectroscopy on the prepared samples. The vacuum Rabi splitting energy increased in proportion to the square root of the dye concentration in the low concentration region; however, when the dye concentration increased, saturation of the increase of the vacuum Rabi splitting energy was observed. Unfortunately, the ultrastrong coupling regime was not realized in this system. In addition, we also investigated the dye concentration dependence of the spectral triplet state. The spectral triplet is the state in which the new mode appears between the polariton doublet owing to absorption saturation in the strong excitation condition. Nonlinear transmission spectroscopy revealed that the dye concentration did not greatly affect the triplet formation.Next, we investigated the optical properties of the liquid-crystalline (LC) perylene tetracarboxylic bisimide (PTCBI) derivative. The LC PTCBI derivatives have a rectangular columnar LC phase at room temperature and the molecules spontaneously align. We considered that such a structure is similar to the aggregate, thus, their oscillator strength may be increased. We inserted LC PTCBI derivatives into the 1DPC microcavity and performed linear transmission spectroscopy. This sample showed a relatively large vacuum Rabi splitting energy of 187 meV, and strong coupling was realized. We also report on the polarization dependence of the vacuum Rabi splitting energy of a 1DPC microcavity containing a film made from aligned LC PTCBI derivatives by a friction transfer method. We successfully controlled the magnitude of vacuum Rabi splitting by changing the polarization-directions in this experiment. Finally, Lemke dyes were dispersed homogeneously in a host polymer thin films, which were inserted into metal microcavities (MMC). The vacuum Rabi splitting above 1 eV was observed by the linear transmission spectroscopy, and the ratio of the vacuum Rabi splitting energy to excitonic transition energy exceeded 40 %. The dispersion relation determined by transmission spectroscopy agreed with the theoretical calculations, including the anti-rotating and A2 terms, and we succeeded in observing the ultrastrong coupling regime. Next, we performed dual-color pump-probe spectroscopy on the prepared MMC. In this experiment, we excited the sample with resonant light over the lower polariton branch and probed with light having a photon energy corresponding to the vacuum Rabi splitting energy. In this experiment, we observed photo-induced absorption that might result from transitions between polariton branches. We describe the construction of this doctoral dissertation. Chapter 1 outlines the background and purpose of the research. Chapter 2 discusses the interaction between the matter and light in microcavities. Chapter 3 deals with the nature of microcavities. Chapter 4 shows the results of linear and nonlinear spectroscopy of 1DPC microcavities containing pseudoisocyanie J aggregate. Chapter 5 describes the linear transmission spectroscopy of 1DPC microcavities containing PTCBI derivatives. Chapter 6 demonstrates linear and nonlinear spectroscopy of MMC containing Lemke dyes. Finally, Chapter 7 summarizes this dissertation.