Micro-fabricated devices for manipulating terahertz radiation

This thesis reports on the design, fabrication and testing of microstructured devices for the manipulation of terahertz radiation. In particular, there is an emphasis on the fabrication and test of diffractive optics type components; including a surface micromachined, multilevel SU-8 based Fresnel lens and a micromilled aluminium Fresnel Zone Plate Reflector (FZPR). For both of these devices, the focal spot is characterized by measuring the electric field intensity and phase as a function of distance along the optical axis. This is carried out using a THz Vector Network Analyzer with associated free space optics. The results are compared directly with Finite Difference Time Domain simulations. A commercial FDTD solver, Lumerical, is used throughout the thesis. FDTD is first introduced for the design of antireflective subwavelength surfaces. These surface structures are bulk micromachined in silicon and their performance experimentally validated using THz Time-Domain Spectroscopy and Durham's THz VNA. A compact THz VNA based S11 measurement configuration is presented which uses the FZPR and a single parabolic mirror. This reflection configuration is used for the characterization of liquid samples (e.g. water and Isopropyl Alcohol mixtures) in microfluidic channels. Two types of channels are presented; one is formed using bulk micromachined silicon whereas the other type uses acetate films to create low cost, disposable devices. The results from the compact measurement configuration are compared with those obtained using a more conventional four parabolic mirror transmission arrangement (as found in THz Time-Domain Spectroscopy systems). Even in the compact configuration, the alignment of the components is found to be a significant factor in determining the measurement performance. Consequently, a six-axis micropositioner (Hexapod), is used to automatically sweep the reflector with the aim of producing a self-aligning system

Doctor of Philosophy

dissertationMetamaterials have gained significant attention over the last decade because they can exhibit electromagnetic properties that are not readily available in naturally occurring materials. This dissertation describes our work on design, fabrication and characterization of liquid metal-based metamaterials with focus on their applications in the terahertz (THz) frequency range. In contrast to the more conventional approaches to fabricating these structures, which rely on vacuum deposited solid metal films, we used metals that are liquid at room temperature. This family of materials is especially attractive for such applications, since it enables large-scale reconfigurability in the overall geometry of the device. We demonstrate a number of unique plasmonic and metamaterial devices. Within the topic of plasmonics, we demonstrate a device that allows for mechanical stretching that is reversibly deformable. In an analogous structure, we can change the geometry dramatically by injecting or withdrawing liquid metals from specific area of the pattern. We also developed a liquid metal-based reconfigurable THz metamaterial device that is not only pressure driven, but also exhibits pressure memory. As an alternate approach to demonstrate reconfigurability, we developed a technique for creating dramatic configuration changes in a device via selective erasure and refilling of metamaterial unit cells that utilizes hydrochloric acid. While the approach is successful in changing the geometry, it does not allow for fine spatial control of the pattern. Thus, we have refined the approach by developing an electrolytic process to change the geometry of a liquid metal-based structured device in a more localized and controlled manner. Since liquid metals can be solidified under certain conditions, we have demonstrated a novel technique for fabrication of free-standing two-dimensional and three-dimensional terahertz metamaterial devices using injection molding of gallium. Finally, we developed a technique of printing three-dimensional solid metal structures by pulling liquid gallium out of a reservoir via solid/liquid interface. Based on these results, we are currently extending our work towards development of metamaterials that can be used in real-world applications. Based on the significant progress made the THz field over the last two decades, the likelihood of THz systems level applications is much brighter

Dispersion Based Real-Time Analog Signal Processing (R-ASP) and Application to Wireless Communications

RÉSUMÉ Nous sommes confrontés à une demande explosive de systèmes radio plus rapides, plus fiables et plus écoénergétiques, pour la communication sans fil 5G par exemple. On s’attend à ce que la capacité des données mobiles dépasse 1000 fois ce qu’elle est actuellement dans la prochaine décennie. Un tel volume de données nécessite un grand spectre de bande passante. Aux fréquences radio-fréquences (RF) faibles, le spectre est congestionné par des milliards d’appareils radio. Dans les hautes fréquences, le spectre de bande passante ultra large (UWB) est moins congestionné. Cependant, le traitement d’un tel signal UWB RF pose de grands défis au niveau du traitement du signal (DSP) numérique, qui est habituellement utilisé pour les basses fréquences et les bandes passantes étroites. Les problèmes dont souffre le DSP pour les signaux hautes fréquences sont la limitation de la vitesse, le coût élevé et la forte consommation d’énergie pour la conversion analogique / numérique (ADC). Par conséquent, une technique de traitement en temps réel et purement analogique est souhaitable. En optique, les gens ont traité des signaux RF UWB avec des approches photoniques hyperfréquences en temps réel, mais cela impliquait une conversion électrique / optique coûteuse. Le traitement de signal analogique d’une onde radio en temps réel (R-ASP) est une alternative attrayante et moins exploitée. Le premier chapitre présente l’état de l’art de la technologie R-ASP ainsi que la contribution de la thèse. Le composant au coeur du traitement R-ASP s’appelle "phaseur", un composant qui fournit un retard de groupe spécifié � (!) à une onde radio. Un phaseur, en réponse à un signal d’excitation large bande, réorganise les composants spectraux dans le temps. La façon dont un phaseur réorganise le spectre dépend de la fonction de retard de groupe, � (!). Différentes applications R-ASP peuvent nécessiter des profils de retard de groupe différents. Le chapitre 2 introduit le concept de retard de groupe, présente différentes technologies phaseur, et présente une méthode pour augmenter la quantité de délai de groupe en utilisant des phaseurs réfléchissants passifs. Un phaseur passif et passe-tout (qui ne filtre aucune fréquence) affiche une perte qui est proportionnelle au retard de groupe, ce qui entraîne une distorsion du signal. Le chapitre trois présente une solution à ce problème, qui consiste en une mise en cascade d’un phaseur ayant du gain et un phaseur ayant des pertes.---------- ABSTRACT We are facing exploding demands for faster, more reliable, more energy-efficient radio systems, such as for instance 5G wireless communication. It is expected that for the next decade the mobile data capacity would exceed 1000 times higher than it is right now. Such high data volume requires large bandwidth spectrum resources. In low RF frequencies, the precious spectrum have been congested by zillions of radio devices. In high frequencies, such as millimeter wave, ultra wideband (UWB) spectrum is much easier available. However, processing UWB RF signal poses great difficulties on conventional digital signal processing (DSP) technique that has prevailed for low frequency and small bandwidth processing. For instance, DSP suffers limited speed, high cost and high power consumption for analog/digital conversion (ADC). Therefore, real-time and purely analog processing technique is desirable. In optics, people have been processing UWB RF signal with microwave photonics approaches, which is real-time, but involves expensive and lossy electrical/optical conversion. The direct radio Real-time Analog Signal Processing (R-ASP) is thus tractive but less exploited. Chapter 1 presents the advancements of R-ASP along with the contributions of the thesis. The core of R-ASP is “phaser”, which is a group delay engineered component that provides specified group delay function � (!). A phaser, in response to a wideband signal excitation, rearranges spectral components in time. The way a phaser arranges spectral components is controlled by the group delay function, � (!). Different R-ASP applications may require different group delay profiles. Chapter 2 introduces the concept of group delay engineering, different phaser technologies, and presents an R-ASP resolution (group delay swing) enhancement example using passive reflective phaser units. Passive phaser exhibits loss that is proportional to the group delay, i.e. imbalance amplitude, which typically results in undesired processing distortion. It is found that a phaser unit loaded with gain (G) and another loaded with equalized loss (L = 1/G) provide symmetric amplitudes (about 0 dB) and identical group delays. Cascading such gain and loss pair yields real all-pass amplitude. Moreover, the group delay can be tuned by the gain and loss. Chapter 3 introduces the gain-loss equalization concept, mathematically presents the device modeling, and experimentally demonstrated the prototype

Microwave Photonic Signal Processing Using On-Chip Nonlinear Optics

The field of microwave photonics (MWP) emerged as a solution to the challenges faced by electronic systems when dealing with high-bandwidth RF and microwave signals. Photonic devices are capable of handling immense bandwidths thanks to the properties of light. MWP therefore employs such devices to process and distribute the information carried by RF and microwave signals, enabling significantly higher capacity compared to conventional electronics. The photonic devices traditionally used in MWP circuits have mainly comprised bulky components, such as spools of fibre and benchtop optical amplifiers. While achieving impressive performance, these systems were not capable of competing with electronics in terms of size and portability. More recently, research has focused on the application of photonic chip technology to the field of MWP in order to reap the benefits of integration, such as reductions in size, weight, cost, and power consumption. Integrated MWP however is still in its infancy, and ongoing research efforts are exploring new ways to match integrated photonic devices to the unique requirements of MWP circuits. This work investigates the application of on-chip nonlinear optical interactions to MWP. Nonlinear optics enables light-on-light interactions (not normally possible in a linear regime) which open a vast array of powerful functionalities. In particular, this thesis focuses on stimulated Brillouin scattering, resulting from the interaction of light with hypersonic sound waves, and four-wave mixing, where photons exchange energies. These two nonlinear effects are applied to implement MWP ultra-high suppression notch filters, wideband phase shifters, and ultra-fast instantaneous frequency measurement systems. Experimental demonstrations using integrated optical waveguides confirm record results

Investigation of Single-Section InAs/InP Quantum Dot Mode-Locked Lasers

The study of mode-locking in generating short pulses began in the 1960s. Since then, the advances have been remarkable over almost 50 years and some of the mode-locked lasers (MLLs) have been commercialized. Short pulses from sub-picoseconds to femtoseconds have been successfully demonstrated from crystal and fiber based lasers. The diverse applications of MLLs have been pushing the development of MLLs in high bit rate transmission, optical time division multiplexed transmission, optical clock recovery, ultrafast signal processing and frequency comb, etc. Semiconductor lasers have advantages of simplicity, compactness and high efficiency. They have attracted interests in the application of optical communications. Until now, semiconductor MLLs are mainly based on bulk and quantum well (QW) structures. More recently, quantum dot (QD) based MLLs have attracted more and more attentions. The main characteristic of QD is the delta-function-like density of states with electrons confined in all three dimensions. It is promising in ultrashort and ultrafast pulse generations as a result of inhomogeneous gain broadening, broad gain bandwidth and fast carrier dynamics. In passive mode-locking, a two-section structure is usually used. A saturable absorber section is essential in the lasing cavity to initiate and shape pulses, which is also the case in almost all QD MLLs. However, without the absorber, passive mode-locking can also be achieved in single-section QD cavity, which has not been well studied yet. This thesis focuses on investigating single-section InAs/InP QD MLLs. It aims at improving the laser performance by both experimental and theoretical analyses. The following works have been done in this thesis. Firstly, as an important parameter of all semiconductor lasers, the linewidth enhancement factor is measured using two methods: the Hakki-Pauli method that is used for a laser below threshold and injection-locking technique for a laser above threshold. The results from the two methods agree with each other, and it is found that the linewidth enhancement factor of our QD lasers is much smaller than that of QW based lasers. Secondly, the time-domain travelling-wave model is used to investigate the single-section QD MLLs. By introducing an equivalent saturable absorber, the pulse generation and evolution are successfully simulated. Furthermore, this model is improved by including the effects of group-velocity dispersion (GVD) and self-phase modulation. It is found that the GVD effect plays an important role in the pulse width evolution of our mode-locked lasers. The improved model can be widely extended to other types of semiconductor lasers and amplifiers. Thirdly, high-repetition-rate pulse trains of up to 1 THz are generated from a QD laser combined with fiber-Bragg-grating (FBG) external cavities. The QD laser is used for multi-mode gain and several specific modes are selected by the FBGs. The pulse train is measured by using the time-domain autocorrelator, and the repetition rate is in agreement with the frequency spacing of the FBGs. Finally, tunable terahertz beat waves of up to 2.1 THz are generated also using FBG external cavities. This method may find applications for generating microwave, millimeter wave and terahertz wave

Laser-induced forward transfer (LIFT) of water soluble polyvinyl alcohol (PVA) polymers for use as support material for 3D-printed structures

The additive microfabrication method of laser-induced forward transfer (LIFT) permits the creation of functional microstructures with feature sizes down to below a micrometre [1]. Compared to other additive manufacturing techniques, LIFT can be used to deposit a broad range of materials in a contactless fashion. LIFT features the possibility of building out of plane features, but is currently limited to 2D or 2½D structures [2–4]. That is because printing of 3D structures requires sophisticated printing strategies, such as mechanical support structures and post-processing, as the material to be printed is in the liquid phase. Therefore, we propose the use of water-soluble materials as a support (and sacrificial) material, which can be easily removed after printing, by submerging the printed structure in water, without exposing the sample to more aggressive solvents or sintering treatments. Here, we present studies on LIFT printing of polyvinyl alcohol (PVA) polymer thin films via a picosecond pulsed laser source. Glass carriers are coated with a solution of PVA (donor) and brought into proximity to a receiver substrate (glass, silicon) once dried. Focussing of a laser pulse with a beam radius of 2 µm at the interface of carrier and donor leads to the ejection of a small volume of PVA that is being deposited on a receiver substrate. The effect of laser pulse fluence , donor film thickness and receiver material on the morphology (shape and size) of the deposits are studied. Adhesion of the deposits on the receiver is verified via deposition on various receiver materials and via a tape test. The solubility of PVA after laser irradiation is confirmed via dissolution in de-ionised water. In our study, the feasibility of the concept of printing PVA with the help of LIFT is demonstrated. The transfer process maintains the ability of water solubility of the deposits allowing the use as support material in LIFT printing of complex 3D structures. Future studies will investigate the compatibility (i.e. adhesion) of PVA with relevant donor materials, such as metals and functional polymers. References: [1] A. Piqué and P. Serra (2018) Laser Printing of Functional Materials. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA. [2] R. C. Y. Auyeung, H. Kim, A. J. Birnbaum, M. Zalalutdinov, S. A. Mathews, and A. Piqué (2009) Laser decal transfer of freestanding microcantilevers and microbridges, Appl. Phys. A, vol. 97, no. 3, pp. 513–519. [3] C. W. Visser, R. Pohl, C. Sun, G.-W. Römer, B. Huis in ‘t Veld, and D. Lohse (2015) Toward 3D Printing of Pure Metals by Laser-Induced Forward Transfer, Adv. Mater., vol. 27, no. 27, pp. 4087–4092. [4] J. Luo et al. (2017) Printing Functional 3D Microdevices by Laser-Induced Forward Transfer, Small, vol. 13, no. 9, p. 1602553

Microwave Photonic Signal Processing Using On-Chip Nonlinear Optics

The field of microwave photonics (MWP) emerged as a solution to the challenges faced by electronic systems when dealing with high-bandwidth RF and microwave signals. Photonic devices are capable of handling immense bandwidths thanks to the properties of light. MWP therefore employs such devices to process and distribute the information carried by RF and microwave signals, enabling significantly higher capacity compared to conventional electronics. The photonic devices traditionally used in MWP circuits have mainly comprised bulky components, such as spools of fibre and benchtop optical amplifiers. While achieving impressive performance, these systems were not capable of competing with electronics in terms of size and portability. More recently, research has focused on the application of photonic chip technology to the field of MWP in order to reap the benefits of integration, such as reductions in size, weight, cost, and power consumption. Integrated MWP however is still in its infancy, and ongoing research efforts are exploring new ways to match integrated photonic devices to the unique requirements of MWP circuits. This work investigates the application of on-chip nonlinear optical interactions to MWP. Nonlinear optics enables light-on-light interactions (not normally possible in a linear regime) which open a vast array of powerful functionalities. In particular, this thesis focuses on stimulated Brillouin scattering, resulting from the interaction of light with hypersonic sound waves, and four-wave mixing, where photons exchange energies. These two nonlinear effects are applied to implement MWP ultra-high suppression notch filters, wideband phase shifters, and ultra-fast instantaneous frequency measurement systems. Experimental demonstrations using integrated optical waveguides confirm record results

Enabling Real-Time Terahertz Imaging With Advanced Optics and Computational Imaging

La bande des térahertz est une région particulière du spectre électromagnétique comprenant les fréquences entre 0.1 THz à 10 THz, pour des longueurs d’onde respectives de 3 mm à 30 um. Malgré tout l’intérêt que cette région a suscité au cours de la dernière décennie, de grands obstacles demeurent pour une application plus généralisée de la radiation THz dans les applications d’imagerie. Cette thèse aborde le problème du temps d’acquisition d’une image THz. Notre objectif principal sera de développer des technologies et techniques pour permettre l’imagerie THz en temps réel. Nous débutons cette thèse avec une revue de littérature approfondie sur le sujet de l’imagerie THz en temps réel. Cette revue commence par énumérer plusieurs sources et détecteurs THz qui peuvent immédiatement être utilisés en imagerie THz. Nous détaillons par la suite plusieurs modalités d’imagerie développés au cours des dernières années : 1) Imagerie THz en transmission, en réflexion et de conductivité, 2) imagerie THz pulsée, 3) imagerie THz par tomographie computationnelle et 4) imagerie THz en champ proche. Nous discutons par la suite plus en détail à propos de technologies habilitantes pour l’imagerie THz en temps réel. Pour cela, nous couvrons trois différents axes de recherche développés en littérature : 1)Imagerie en temps réel de spectroscopie THz dans le domaine du temps, 2) caméras THz et 3) imagerie en temps réel avec détecteur à pixel unique. Nous présentons ensuite le système d’imagerie que nous avons développé pour les démonstrations expérimentales de cette thèse. Ce système est basé sur la spectroscopie THz en temps réel et permet donc d’obtenir des images hyperspectrales en amplitude et en phase. Il utilise des antennes photoconductrices pour l’émission et la détection de la radiation THz. En outre, le détecteur est fibré, ce qui permet de le déplacer spatialement pour construire des images. Nous couvrons aussi brièvement plusieurs techniques de fabrication avancées que nous avons utilisées : impression 3D par filament, stéréolithographie, machinage CNC, gravure/découpe laser et transfert de métal par toner. Nous portons ensuite notre attention à l’objectif principal de cette thèse à travers trois démonstrations distinctes. Premièrement, nous concevons des composants THz à faibles pertes en utilisant des matériaux poreux. L’absence de détecteurs THz ultra-sensibles implique que les pertes encourues dans un système d’imagerie sont hautement indésirables. En effet, un moyennage temporel est généralement fait pour extraire de faibles signaux THz sévèrement enfouis sous le bruit technique. Ceci a pour impact de diminuer le nombre d’images à la seconde. ----------Abstract The terahertz band is a region of the electromagnetic spectrum comprising frequencies between 0.1 THz to 10 THz for respective wavelengths of 3 mm to 30 um. Despite all the interest and potential generated in the past decade for applications of this spectral band, there are still major hurdles impeding a wider use of THz radiation for imaging. This thesis addresses the problem of image acquisition time. Our main objective is to develop technologies and techniques to achieve real-time THz imaging. We start this thesis with a comprehensive review of the scientific literature on the topic of realtime THz imaging. This review begins by listing some off-the-shelf THz sources and detectors that could be readily used in THz imaging. We then detail some key imaging modalities developed in the past years: 1) THz transmission, reflection and conductivity imaging, 2) THz pulsed imaging, 3) THz computed tomography, and 4) THz near-field imaging. We then discuss practical enabling technologies for real-time THz imaging: 1) Real-time THz timedomain spectroscopy imaging, 2) THz cameras, and 3) real-time THz single-pixel imaging. We then present our fiber-coupled THz time-domain spectroscopy imaging setup. This system is used throughout the thesis for experimental demonstrations. We also briefly overview many advanced fabrication techniques that we have used, namely fused deposition modeling,stereolithography, CNC machining, laser cutting/engraving and metal transfer using toner. We then turn to the main objective of this thesis with three distinct demonstrations. First, we design low-loss THz components using porous media. The losses incurred in the imaging system are highly undesirable due to the lack of sensitive THz detectors. Indeed, time averaging is generally performed in order to retrieve THz signals severely buried under noise,which in return reduce the framerate. We propose to use low-refractive index subwavelength inclusions (air holes) in a solid dielectric material to build optical components. We show that these components have smaller losses than their all-solid counterparts with otherwise identical properties. We fabricate a planar porous lens and an orbital angular momentum phase plate, and we use our imaging system to characterize their effects on the THz beam. Second, we demonstrate a spectral encoding technique to significantly reduce the required number of measurements to reconstruct a THz image in a single-pixel detection scheme