Nonlinear and wavelength-tunable plasmonic metasurfaces and devices

textWavelength-tunable optical response from solid-state optoelectronic devices is a desired feature for a variety of applications such as spectroscopy, laser emission tuning, and telecommunications. Nonlinear optical response, on the other hand, has an important role in modern photonic functionalities, including efficient frequency conversions, all-optical signal processing, and ultrafast switching. This study presents the development of optical devices with wavelength tunable or nonlinear optical functionality based on plasmonic effects. For the first part of this study, widely wavelength tunable optical bandpass filters based on the unique properties of long-range surface plasmon polaritons (LR SPP) are presented. Planar metal stripe waveguides surrounded by two different cladding layers that have dissimilar refractive index dispersions were used to develop a wide wavelength tuning. The concept was demonstrated using a set of index-matching fluids and over 200nm of wavelength tuning was achieved with only 0.004 of index variation. For practical application of the proposed concept, a thermo-optic polymer was used to develop a widely tunable thermo-optic bandpass filter and over 220 nm of wavelength tuning was achieved with only 8 ºC of temperature variation. Another novel approach to produce a widely wavelength tunable optical response for free-space optical applications involves integrating plasmonic metasurfaces with quantum-electronic engineered semiconductor layers for giant electro-optic effect, which is proposed and experimentally demonstrated in the second part of this study. Coupling of surface plasmon modes formed by plasmonic nanoresonators with Stark tunable intersubband transitions in multi-quantum well structures induced by applying bias voltages through the semiconductor layer was used to develop tunable spectral responses in the mid-infrared range. Experimentally, over 310 nm of spectral peak tuning around 7 μm of wavelength with 10 ns response time was achieved. As the final part of this study, highly nonlinear metasurfaces based on coupling of electromagnetically engineered plasmonic nanoresonators with quantum-engineered intersubband nonlinearities are proposed and experimentally demonstrated. In the proof-of-concept demonstration, an effective nonlinear susceptibility over 50 nm/V was measured and, after further optimization, over 480 nm/V was measured for second harmonic generation under normal incidence. The proposed concept shows that it is possible to engineer virtually any element of the nonlinear susceptibility tensor of the nonlinear metasurface.Electrical and Computer Engineerin

Effective-Medium-Clad Dielectric Components Towards Terahertz Integrated Platform

Over the past few decades, optics-based time-domain spectroscopic systems have significantly promoted the developments of terahertz science and technology. Despite their success in physics, the bulky and costly optical systems are not readily amendable to various applications such as communications, imaging, sensing, and radar. These applications require devices with structural compactness, integrability, and portability. Leveraging both electronic and photonic technologies, terahertz integrated circuits have emerged and gradually bridged the gap between ’concept’ and ’application’. To realise multifunctional terahertz integrated circuits, efficient and broadband platforms able to accommodate various passive and active components are in great demand, while interconnects with low loss, low dispersion, and broad bandwidth are vital. To this end, this thesis focuses on an efficient and broadband terahertz integrated platform based on silicon. Firstly, a class of self-supported substrateless dielectric waveguides are proposed based on the effective medium theory. The effective-mediumclad dielectric waveguides are purely built into a high-resistivity intrinsic float-zone silicon wafer to achieve extremely low loss and low dispersion. The effective medium is realised by periodically perforating the silicon slab with a deep subwavelength spacing, leading to a tailorable effective relative permittivity tensor. Consequently, an additional degree of freedom is granted in this design to manipulate the waveguides’ modal indices and adapt to different guiding scenarios. Through in-depth investigations of various propagation characteristics, the proposed waveguides show a potential to establish a terahertz integrated platform with a high level of design flexibility. Benefiting from the concept of effective medium to create this new waveguide platform, various fundamental building blocks and functional components are proposed including bends, crossings, directional couplers, filters, and polarisation splitters. All these components inherit high efficiency and broad bandwidth, which are much needed for terahertz applications that typically leverage a vast available bandwidth with limited source power. The proposed concepts can benefit terahertz integrated circuits at large, in analogy to the silicon-on-insulator platform for integrated photonics.Thesis (Ph.D.) -- University of Adelaide,School of Electrical and Electronic Engineering, 202

`THz Torch' technology: secure thermal infrared wireless communications using engineered blackbody radiation

The thermal (emitted) infrared frequency bands, from 20 to 40 THz and 60 to 100 THz, are best known for applications in thermography. This underused and unregulated part of the spectral range offers opportunities for the development of secure communications. The `THz Torch' concept, operating between the THz and mid-infrared ranges, was recently introduced. This technology fundamentally exploits engineered blackbody radiation, by partitioning thermally-generated spectral power into pre-defined frequency channels; the energy in each channel is then independently pulsed modulated to create a robust form of short-range secure communications in the far/mid-infrared. In the thesis, the development of `THz Torch' wireless communications systems will first be introduced. State-of-the-art THz technologies, infrared sources and detectors, as well as near-infrared and visible light communications technologies, will be reviewed in Chapter 2. Basic single-channel architecture of the `THz Torch' technology will be presented in Chapter 3. Fundamental limits for the first single-channel proof-of-concept demonstrator will be discussed, and possible engineering solutions will be proposed and verified experimentally. With such improvements, to date, octave bandwidth (25 to 50 THz) single-channel wireless links have been demonstrated with >2 kbit/s data rate and >10 cm transmission distance. To further increase the overall end-to-end data rate and/or the level of security, multiplexing schemes for `THz Torch' technologies are proposed in Chapter 4. Both frequency division multiplexing (FDM) and frequency-hopping spread-spectrum (FHSS) working demonstrators, operating between 10 and 100 THz spectral range, will be implemented. With such 4-channel multiplexing schemes, measured bit error rates (BERs) of <10−6 have been achieved over a transmission distance of 2.5 cm. Moreover, the integrity of such 4-channel multiplexing system is evaluated by introducing four jamming, interception and channel crosstalk experiments. Chapter 5 gives a detailed power link budget analysis for the 4-channel multiplexing system. The design, simulation and measurement of scalable THz metal mesh filters, which have potential applications for multi-channel `THz Torch' technology, will be presented in Chapter 6. The conclusions and further work are summarised in the last chapter. It is expected that this thermodynamics-based approach represents a new paradigm in the sense that 19th century physics can be exploited with 20th century multiplexing concepts for low cost 21st century ubiquitous security and defence applications in the thermal infrared range.Open Acces

High Bit Rate Wireless and Fiber-Based Terahertz Communication

RÉSUMÉ Dans le spectre électromagnétique, la bande des térahertz s’étend de 100 GHz à 10 THz (longueurs d’onde de 3 mm à 30 μm). Des décennies auparavant, le spectre des THz était connu sous le nom de « gap térahertz » en raison de l’indisponibilité de sources et détecteurs efficaces à ces fréquences. Depuis quelques années, la science a évolué pour faire migrer la technologie THz des laboratoires aux produits commerciaux. Il existe plusieurs applications des ondes THz en imagerie, spectroscopie et communications. Dans cette thèse, nous nous intéressons aux communications THz à travers deux objectifs. Le premier objectif est de développer une source THz de haute performance dédiée aux communications et basée sur les technologies optiques avec des produits commerciaux uniquement. Le second objectif est de démontrer l’utilisation de fibres optiques afin de renforcer la robustesse des communications THz sans fil. Nous débutons cette thèse avec une revue de la littérature scientifique sur le sujet de la communications THz sans fil et filaire. D’abord, nous discutons des deux méthodes communément utilisées (électronique et optique) pour démontrer des liens de communications THz avec leurs avantages et inconvénients. Nous présentons par la suite la possibilité d’utiliser un système de spectroscopie THz pour des applications en communications avec des modifications mineures au montage. Nous présentons ensuite plusieurs applications gourmandes en bande passante qui pourraient bénéficier du spectre THz, incluant la diffusion en continu (streaming) de flux vidéo aux résolutions HD et 4K non compressés. Ensuite, nous discutons de la motivation d’utiliser de longues fibres THz et notamment du fait qu’elles ne sont pas destinées à remplacer les fibres optiques conventionnelles de l’infrarouge, mais plutôt à augmenter la robustesse des liens THz sans fil. En particulier, les fibres THz peuvent être utilisées pour garantir le lien de communication dans des environnements géométriques complexes ou difficile à atteindre, ainsi que pour immuniser le lien THz aux attaques de sécurité. Plusieurs designs de fibres et guides d’onde précédemment démontrées dans la littérature sont discutés avec, entre autres, leurs méthodes de fabrication respectives. Nous discutons ensuite de la possibilité d’utiliser un simple guide d’onde diélectrique et sous-longueur d’onde pour transmettre l’information à un débit de l’ordre de plusieurs Gbps sur une distance de quelques mètres.----------ABSTRACT The Terahertz (THz) spectral range spans from 100 GHz to 10 THz (wavelength: 3 mm to 30 μm) in the electromagnetic spectrum. Decades ago, the THz spectral range is often named as ‘THz gap’ due to the non-availability of efficient THz sources and detectors. In the recent years, the science has evolved in bringing the THz technology from lab scale to commercial products. There are several potential applications of THz frequency band such as imaging, spectroscopy and communication. In this thesis, we focus on THz communications by addressing two objectives. The first objective is to develop a high-performance photonics-based THz communication system using all commercially available components. The second objective is to demonstrate the THz-fiber based communications, which can be used to increase the reliability of THz wireless links. We begin this thesis with a scientific literature review on the subject of THz wireless and fiber-based communications. First, the two different methodologies (all electronics based and photonics-based THz system) that is commonly used in the demonstration of THz communications is discussed along with their advantages and challenges. We then present the flexibility of photonics-based THz system where it is possible to switch it with minor modifications for THz spectroscopic studies and THz communication applications. Several bandwidth hungry applications that demands the use of THz spectrum for next generation communications is detailed. This includes the streaming of uncompressed HD/4K and beyond high-resolution videos, where the THz spectrum can be beneficial. Next, the motivation of using long THz fibers is discussed and we convince the readers that the THz fibers are not meant to replace the fibers in the optical-infrared region but to increase the reliability of THz wireless links. Particularly, the THz fibers can be used to provide connectivity in complex geometrical environments, secure communications and signal delivery to hard-to-reach areas. Several novel fiber/waveguide designs along with their fabrication technologies from the literature are presented. We then show that a simple solid core dielectric subwavelength fiber can be used to transmit the information in the order of several Gbps to a distance of a few meters

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic

Taming optical parametric amplification

Due to a limited number of known laser media technically relevant femtosecond laser systems are restricted to a few laser frequencies only. Tunability based on these sources is enabled by the process of optical parametric amplification (OPA). A small portion of the laser output is used to generate a seed pulse. To enable tunability over a large spectral range a bulk continuum is used. With a strong pump pulse a selected spectral part can be amplified in a suitable nonlinear medium. In order to obtain highly stable and ultrashort pulses on the 10 fs scale a deep understanding of each individual step in the process is necessary. In this work Ti:sapphire and Yb-based laser systems are used to pump OPAs. In the used topology it is possible to utilize repetition rates from 1 kHz to 1 MHz and input energies from 10 to 300 µJ without major changes of the design. We obtain ultrashort pulses from the UV to the MIR. New aspects on bulk continuum generation are presented. For generation with 1030 nm light various crystals are compared regarding the visible and near infrared continuum side. The hitherto unconsidered GSO is found to be superior. Self-compression due to continuum generation is shown without the need for external compression. It bases on a 1 mm sapphire or YAG plate and an astigmatism-free, achromatic telescope . a Schiefspiegler. The self-compressed, unchirped continuum is used to seed a NOPA (noncollinear OPA) for ultra-broadband pulse generation. With this system quantum efficiencies of up to 45% in the amplification process and 18 µJ output are demonstrated and compression down to 6.7 fs. The transmitted seed light after optical parametric amplification and the amplified output can propagate in significantly different directions. This is explained as a consequence of Kerr lensing at the needed pump intensities. A pump induced Kerr lens is acting on the signal/seed pulse. This induces a deflection in the amplifier medium at imperfect input coupling. Locating the amplifier crystal behind the focal plane of the pump minimizes the self-lensing effect due to nonlinear balancing of the beam divergence. This also allows reducing the pump intensity simply by moving the crystal further away from the focal plane. For MHz repetition rates serious thermal issues in a NOPA arise due to the high average power that is needed for the nonlinear processes. For third harmonic generation (THG) of an Yb-fiber laser, second harmonic generation (SHG) with subsequent sum-frequency mixing is demonstrated. Two-photon absorption of the UV leads to measureable heat in the THG crystal. The phase-matching conditions change and the UV power decreases over time. With a time delay compensation plate (aBBO) between the SHG and THG crystal this effect can be minimized. By temporally pre-compensating the fundamental with respect to the second harmonic, the generation locus of the main UV power is shifted to the end of the THG crystal and the volume for absorption is minimized. Stable UV pulses result. This enables a long term stable UV pumped NOPA output even at 1 MHz repetition rate. An OPA for the generation of an octave spanning middle infrared pulse centered around 8 µm is presented. A 515 nm pumped NOPA with a subsequent collinear, 1030 nm pumped amplifier based on LGS is utilized. The chirp management is entirely by bulk material and selected optical filters. The MIR field is temporally characterized by electro-optical sampling. As gate pulse the self-compressed fundamental is used in an extremely simple setup. Electro-optical sampling reveals a compression down to 1.4 cycles of the MIR field and an intrinsically phase-locked CEP stability of better than 94 mrad over one hour