Photonic generation of ultra-low phase noise and frequency-tunable microwave or millimeter-wave (mm-wave) signals has been a topic of interest in the last few years. Advanced photonic techniques, especially the recent advancement in photonic components, have enabled the generation of microwave and mm-wave signals at high frequencies with a large tunable range and ultra-low phase noise. In this thesis, techniques to generate microwave and mm-wave signals in the optical domain are investigated, with an emphasis on system architectures to achieve large frequency tunability and low phase noise. The thesis consists of two parts. In the first part, techniques to generate microwave and mm-wave signals based on microwave frequency multiplication are investigated. Microwave frequency multiplication can be realized in the optical domain based on external modulation using a Mach-Zehnder modulator (MZM), but with limited multiplication factor. Microwave frequency multiplication based on external modulation using two cascaded MZMs to provide a larger multiplication factor has been proposed, but no generalized approach has been developed. In this thesis, a generalized approach to achieving microwave frequency multiplication using two cascaded MZMs is presented. A theoretical analysis leading to the operating conditions to achieve frequency quadrupling, sextupling or octupling is developed. The system performance in terms of phase noise, tunability and stability is investigated. To achieve microwave generation with a frequency multiplication factor (FMF) of 12, a technique based on a joint operation of polarization modulation, four-wave mixing and stimulated-Brillouin-scattering-assisted filtering is also proposed. The generation of a frequency-tunable mm-wave signal from 48 to 132 GHz is demonstrated. The proposed architecture can even potentially boost the FMF up to 24. In the second part, techniques to generate ultra-low phase noise and frequency-tunable microwave and mm-wave signals based on an optoelectronic oscillator (OEO) are studied. The key component in an OEO to achieve low phase noise and large frequency-tunable operation is the microwave bandpass filter. In the thesis, we first develop a microwave photonic filter with an ultra-narrow passband and large tunability based on a phase-shifted fiber Bragg grating (PS-FBG). Then, an OEO incorporating such a microwave photonic filter is developed. The performance including the tunable range and phase noise is evaluated. To further increase the frequency tunable range, a technique to achieve microwave frequency multiplication in an OEO is proposed. An mm-wave signal with a tunable range more than 40 GHz is demonstrated.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21569号 / 理博第4476号 / 新制||理||1642(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 橋田 昌樹, 教授 阪部 周二, 教授 鶴 剛 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Terahertz (THz) spectroscopy is an emerging technology with promising applications in imaging, homeland security, and material detection and quantification. Frequencies in the THz region can be generated by optical rectification of ultrafast near-infrared laser pulses in the presence of a nonlinear optical (NLO) materials such as organic crystals. Non-centrosymmetric organic THz generating crystals such as DAST, HMQ-TMS, and OH1 have received special attention due to the strong generated fields on the order of MV/cm. The cation of these organic salts is designed by connecting electron-donating with electron-accepting groups via a highly planar aromatic system. To improve the performance of organic crystals for THz generation, the molecular hyperpolarizability (β) can be optimized by introducing modifications in the architecture of these push-pull chromophores. However, the large dipole moments associated with molecules that have a large β promote the formation of NLO inactive centrosymmetric molecular alignments in the crystal state. This dissertation provides important insights into the design of new push-pull chromophores that feature a) higher β values compared with state-of-the-art organic crystals, and b) non-centrosymmetric molecular packing in the crystalline state. The first strategy presented on this dissertation relates to the introduction of a triple bond instead of a double bond in the cation of DAST to improve the β parameter. The newly designed 4DEP core was combined with different anions to promote non-centrosymmetric molecular packing with almost ideal arrangements for THz generation. However, large single crystals were difficult to obtain and high THz generation was not achieved. The second strategy presented in this dissertation raises the value of β by extending the π-conjugation length in different cations with dimethylamino and methoxy electron-donating groups. A new molecular cation, 6MNEP, was found to have large β value combined with ideal non-centrosymmetric molecular packing. Combining these two factors, a ~ 75% higher performance for THz generation is expected for 6MNEP compared with DAST. Currently, we are testing different crystallization techniques to grow large single crystals of 6MNEP. In addition to the strategies developed to increase the β parameter value, we also introduce a new molecular modification to induce non-centrosymmetric packing in organic salt THz generating crystals. This is achieved by substituting a methyl by an ethyl group in the quaternary nitrogen of hydrogen-bonded crystals. We showed the applicability of this method for changing molecular packing in the crystal state from centrosymmetric to non-centrosymmetric in two different molecular cations. We also demonstrated the generation of strong THz fields in the novel NLO crystal EHPSI-4NBS.

This doctoral dissertation focuses on the nonlinear optical response of water vapour as well as some solids at terahertz (THz) frequencies. In this study, the propagation of broadband single-cycle THz pulses through a medium with the third-order nonlinear optical response is theoretically investigated. Also, a technique to measure the nonlinear response of transparent materials based on the time-domain THz spectroscopy is developed, which provides frequency dispersion curves of the nonlinear Kerr coefficient (n₂). A numerical model is used to simulate the THz pulse propagation. This model takes into account non-paraxial effects, self-focusing, and diffraction, as well as dispersion, in both the linear and nonlinear optical regimes. The contribution of non-instantaneous Kerr-type nonlinearity to the overall instantaneous and delayed Kerr effect at the THz frequencies is investigated. It is shown how increasing the nonlinear relaxation time and its dispersion modifies the THz pulse after the propagation through a transparent medium. The effect of linear dispersion on self-action during pulse propagation is also discussed. Moreover, the nonlinear spectroscopy of water vapour at THz frequencies is reported. Atmospheric water vapour has a rich spectrum with several strong resonances at frequencies below 3 THz, falling within the range of operation of most existing THz sources. An extremely large nonlinear response to THz radiation is observed at the positions of these resonances. Using the optical Kerr model for the nonlinear response, a minimum nonlinear refractive index of the order of 10² m²/W is estimated. The results provide insight into the energy levels of the water molecule and give a more accurate picture of its response to electromagnetic radiation, paving the way to more accurate THz spectroscopy, imaging, and sensing systems, and thereby facilitating future emerging THz technologies. Finally, the nonlinear response of solids at THz frequencies is studied. It has been shown that a phonon-induced THz Kerr effect can result in a larger nonlinear refractive index than the nonlinear refractive index at the visible or near-infrared range (optical Kerr effect). This pronounced nonlinear optical behavior is verified using a time-domain characterization approach. The results indicate a large delay occurred to the THz fields as they transmit through some of the material samples. In the frequency domain, the induced nonlinear phase shift of the intense THz field is shown to be relatively large of the order of 0.1 rad. From the phase information, the nonlinear phase is extracted by which the dispersion profile of n₂ is obtained.

Les communications aux fréquences terahertz sont étudiées dans un contexte de transmissions en espace libre. L’objectif de la thèse est de proposer une étude comparative de différentes topologies d’électrodes supérieures appliquées aux photodiodes UTC ainsi qu’un module d’intégration de ces composants. Ces composants permettent de réaliser des liens de communications sans fils aux fréquences terahertz. Une étude bibliographique évalue les domaines d’applications de ces fréquences, dont le nombre est croissant cette dernière décennie. Les sources terahertz existantes sont ensuite étudiées en fonction des performances et de leur domaine spectrale d’émission. Les photoréponses et puissances émises pour différentes structures de photodiodes PIN, UTC et TTR utilisées en photomélangeur sont comparées au-delà de 60 GHz. Les modules d’intégrations développées et les différentes méthodes d’intégrations sont détaillés dans la dernière partie du premier chapitre. Le second chapitre présente une étude théorique, numérique et expérimentale des photodiodes UTC réalisées. L’utilisation d’une électrode supérieure dédiée et l’étude de différents motifs est développée afin de limiter la dépendance à la polarisation optique de la structure, afin d’en augmenter la fiabilité dans un contexte de transmission de données vidéos aux fréquences terahertz. Un module d’intégration dédiée, en guide WR-3, pour ces photodiodes, est étudié numériquement sous le logiciel CST ainsi que les lignes de transmissions au récepteur. Enfin, les liens terahertz réalisés et les performances correspondantes sont résumées dans le dernier chapitre. / The terahertz frequency communications are studied in a context of free space transmissions. The objective of this thesis is to propose a comparative study of different topologies of top electrode applied to UTC photodiode and a module for packaging this type of components. Those components permit to produce wireless communications links at terahertz frequencies. A bibliographic study evaluates the areas of applications for this frequency band. Then, current terahertz sources are studied according to the performances and their spectral emission domain. The photoresponses and the power emitted for different structures of PIN, UTC, and TTR photodiodes used like photomixer are compared beyond 60 GHz. The packaging and the different modules of integration are detailed in the last part of this first chapter. The second chapter presents a theoretical, numerical and experimental study of UTC photodiodes. The use of a dedicated top contact and the study of different patterns are developed in order to limit the optical polarization dependence of this structure, in order to increase the reliability in a context of data video transmission at terahertz frequencies. A dedicated WR-3 packaging module for these photodiodes is studied numerically under CST software and lines of transmission are studied in receiver. Finally, the terahertz links and the corresponding performances are summarized in the last chapter.

This thesis will explore the ultrafast modulation and optical tunability of plasmonic filters in the terahertz (THz) spectral region. First, the principles and functional design of THz metasurfaces are explored through plasmonic surface lattice resonance interactions and lumped-element circuit models. We will then describe the methodology of generating and detecting THz radiation through the nonlinear processes of optical rectification and electrooptic sampling, respectively. Next, the implementation of a THz time-domain spectroscopy technique is discussed in the context of pump-probe measurements and time-domain resonance analysis. We then show how THz probed materials can be characterized in terms of a temporal and spectral analysis. We will demonstrate how this time-domain technique can allow us to characterize the interaction of plasmonic resonators with optically active substrates and 2D nanomaterials. A completely tunable THz plasmonic notch resonance is modulated utilizing a static and dynamic method of optical tunability in silicon. Active tunability is also demonstrated in a graphene-based plasmonic resonator through the hot carrier multiplication effect. The significance of this work lies in the application of designing controllable devices for future THz communication technologies.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20380号 / 工博第4317号 / 新制||工||1669(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 北野 正雄, 教授 山田 啓文, 教授 松尾 哲司 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

No description available.

It is reported that terahertz radiation from 0.69 to 2.54 THz has been sensitively detected in a device consisting of bundles of carbon nanotubes containing single wall metallic carbon nanotubes, quasi-optically coupled through a lithographically fabricated antenna, and a silicon lens. The measured data are consistent with a bolometric detection process in the metallic tubes and the devices show promise for operation well above 4.2 K. Microwave measurements have also been done up to 20GHz. Voltage responsivity got here is comparable to that of the Schottky diode detector. The detection at microwave frequencies are consistent with the diode detection mode. S11 parameters of different devices were measured using microwave probing, and de-embedding process has been done to get the impedances of the SWNTs. A circuit model was fitted based on the measurement data, and different values of the elements of the circuit are extracted. Frequency response from the circuit model is consistent with the experimental data.

The far-infrared and millimeter wave (FIR/mmW) (wavelength 75 micrometer to 10 mm) portion of the electromagnetic spectrum is fairly underdeveloped technologically, owing to the large amount of atmospheric attenuation in that range. At present, the FIR/mmW region is lacking in compact, high-brightness radiation sources and practical imaging systems. This dissertation focuses on development of two complementary technologies in this area - an active mmW imaging system and high-reflectivity Bragg mirrors for the FIR p-Ge laser. The imaging system uses a vector network analyzer in the frequency range of 90-140 GHz as the radiation source and receiver. Raster scanning is used to map a two-dimensional field of view, demonstrating the detection and imaging of buried plastic landmines. Principal components analysis is used for hyperspectral signal processing, where a series of images is taken at discrete frequencies. Results are obtained as a function of depth and disturbance of the soil surface. In support of this study, various types of soils were characterized for scattering loss across the mmW/FIR region, with measured results compared to theory. This mmW imaging system was also used to demonstrate imaging through walls and other obscuring materials, as well as for imaging of rocks beneath volcanic sand, simulating the conditions encountered by an imaging system on a Mars rover vehicle. Furthermore, a high-reflectivity Si-etalon FIR mirror design was developed and demonstrated as a cavity mirror for the p-Ge laser. These components stand to have a number of systems-level impacts on FIR imagers. In the context of an active illuminator, they may allow narrowband selection from the broad emission spectrum of the p-Ge laser source. These mirrors can also be used in a Fabry-Perot FIR scanning spectrometer, where the resulting high finesse would give discrimination advantages in chemical sensing and astrophysical spectroscopy applications.