High-speed metasurface modulator using critically coupled bimodal plasmonic resonance

Free-space electro-optic (EO) modulators operating at gigahertz and beyond are attractive for a wide range of emerging applications, including high-speed imaging, free-space optical communication, microwave photonics, and diffractive computing. Here we experimentally demonstrate a high-speed plasmonic metasurface EO modulator operating at a near-infrared wavelength range with a gigahertz modulation bandwidth. To achieve efficient intensity modulation of reflected light from an ultrathin metasurface layer, we utilize the bimodal plasmonic resonance inside a subwavelength metal-insulator-metal grating, which is precisely tuned to satisfy the critical coupling condition. As a result, perfect absorption of -27 dB (99.8%) and a high quality (Q) factor of 113 are obtained at a resonant wavelength of 1650 nm. By incorporating an EO polymer inside the grating, we achieve a modulation depth of up to 9.5 dB under an applied voltage of $\pm$30 V. The 3-dB modulation bandwidth is confirmed to be 1.25 GHz, which is primarily limited by the undesired contact resistance. Owing to the high electrical conductivity of metallic gratings and a compact device structure with a minimal parasitic capacitance, the demonstrated device can potentially operate at several tens of gigahertz, which opens up exciting opportunities for ultrahigh-speed active metasurface devices in various applications.Comment: Main text: 18 pages, 3 figures, 39 references Supplementary material: 3 pages, 2 figures

A Platform for Practical Nanophotonic Systems Nitrides and Oxides for Integrated Plasmonic Devices

The fields of nanophotonics and metamaterials have revolutionized the way we think of optical space (ε,µ), enabling us to engineer the refractive index almost at will, to confine light to the smallest of volumes, as well as to manipulate optical signals with extremely small foot prints and energy requirements. Throughout the past, this field of research has largely been limited to the use of noble metals as plasmonic materials, largely due to the high conductivity (low loss) and wide availability in research institutions. However, the research which follows focuses on the development of two alternative material platforms for nanophotonics: namely the transition metal nitrides and the transparent conducting oxides. Through this research, we have explored the nonlinear optical properties of thin films, demonstrating unique and ultrafast dynamic response, and have designed and realized high performance integrated plasmonic devices. Ultimately, this work aims to demonstrate the impact and potential of alternative plasmonic materials for numerous nanophotonic applications

On-Chip Nanoscale Plasmonic Optical Modulators

In this thesis work, techniques for downsizing Optical modulators to nanoscale for the purpose of utilization in on chip communication and sensing applications are explored. Nanoscale optical interconnects can solve the electronics speed limiting transmission lines, in addition to decrease the electronic chips heat dissipation. A major obstacle in the path of achieving this goal is to build optical modulators, which transforms data from the electrical form to the optical form, in a size comparable to the size of the electronics components, while also having low insertion loss, high extinction ratio and bandwidth. Also, lap-on-chip applications used for fast diagnostics, and which is based on photonic sensors and photonic circuitry, is in need for similar modulator specifications, while it loosens the spec on the modulator’s size. Silicon photonics is the most convenient photonics technology available for optical interconnects application, owing to its compatibility with the mature and cheap CMOS manufacturing process. Hence, building modulators which is exclusively compatible with this technology is a must, although, Plasmonics could be the right technology for downsizing the optical components, owing to its capability in squeezing light in subwavelength dimensions. Hence, our major goal is to build plasmonic modulators, that can be coupled directly to silicon waveguides. A Plasmonic Mach-Zehnder modulator was built, based on the orthogonal junction coupling technique. The footprint of the modulator is decreased to 0.6 4.7, extinction ratio of 15.8 dB and insertion loss of 3.38 dB at 10 volts was achieved in the 3D simulations. The voltage length product for the modulator is 47 V. The orthogonal junction coupler technique minimized the modulator’s footprint. On the other hand, photonic sensors favorably work in the mid-infrared region, owing to the presence of a lot of molecules absorption peaks in this region. Hence, III-V semiconductor media is used for this type of applications, owing to the availability of laser sources built of III-V media, and to the lower losses that these materials have in mid-infrared region. Hybrid plasmonic waveguide, formed of doped InAs, AlAs and GaAs is studied extensively. Based on this waveguide an electro-absorption modulator is built. The device showed an extinction ratio of 27 dB at 40 length, and 1.2 dB of insertion loss. The small device footprint predicts a much lower energy consumption

Enhanced Light Absorption and Electro-absorption Modulation Based on Graphene and Conductive Oxide

The development of integrated photonics is limited by bulky and inefficient photonic component compared to their electronic counterparts due to weak light-matter interactions. As the key devices that determine the performance of integrated photonic circuits, electro-optical (EO) modulators are inherently built on the basis of enhancing light-matter interactions. Current EO modulators often deploy conventional materials with poor EO properties, or ring resonator structure with narrow bandwidth and thermal instability, so their dimensions and performance have nearly reached their physical limits. Future integrated photonic interconnects require EO modulators to be ultra-compact, ultra-fast, cost-effective and able to work over a broad bandwidth. The key to achieving this goal is to identify an efficient and low-cost active material. Meanwhile, novel waveguides and platforms need to be explored to significantly enhance light-active medium interaction. As widely investigated novel materials, graphene and conductive oxide (COx) have shown remarkable EO properties. The objective of this dissertation is to realize enhanced light-matter interaction based on these two novel materials and waveguiding platforms, and further develop ultra-compact, ultra-fast EO modulators for future photonic integrated circuits. The first part of this dissertation covers the theory of EO modulation mechanisms, several types of EO materials including graphene and COx, as well as fabrication techniques. The second part demonstrates greatly enhanced light absorption based on mono-/multi-layer graphene. The third part proposes the theoretical study of nanoscale EA modulators based on ENZ-slot waveguide. The fourth part explores the field effect within a MOS-like structure, and verifies the ENZ behavior of COx. The fifth part experimentally demonstrates both plasmonic and dielectric configurations for ultra-compact and ultra-fast EA modulators. The final part summarizes the work presented in this dissertation and also discusses some future work for photonic applications

Active control of surface plasmons in hybrid nanostructures

Plasmonics nanostructures are becoming remarkably important as tools towards manipulating photons at the nanoscale. They are poised to revolutionize a wide range of applications ranging from integrated optical circuits, photovoltaics, and biosensing. They enable miniaturization of optical components beyond the "diffraction limit'' as they convert optical radiation into highly confined electromagnetic near-fields in the vicinity of subwavelength metallic structures due to excitation of surface plasmons (SPs). These strong electromagnetic fields generated at the plasmonic "hot spots'' raise exciting prospects in terms of driving nonlinear effects in active media. The area of active plasmonics aims at the modulation of SPs supported at the interface of a metal and a nonlinear material by an external control signal. The nonlinear material changes its refractive index under an applied control signal, thereby resulting in an overall altered plasmonic response. Such hybrid nanostructures also allow for the creation of new kinds of hybrid states. This not only provides tools for designing active plasmonic devices, but is also a means of re-examining existing conventional rules of light-matter interactions. Therefore, the need for studying such hybrid plasmonic nanostructures both theoretically and experimentally cannot be understated. The present work seeks to advance and study the control of SPs excited in hybrid systems combining active materials and nanometallics, by an external optical signal or an applied voltage. Different types of plasmonic geometries have been explored via modeling tools such as frequency domain methods, and further investigated experimentally using both near-field and far field techniques such as scanning near field optical microscopy and leakage radiation microscopy respectively. First, passive SP elements were studied, such as the dielectric plasmonic mirrors that demonstrate the ability of gratings made of dielectric ridges placed on top of flat metal layers to open gaps in the dispersion relation of surface plasmon polaritons (SPPs). The results show very good reflecting properties of these mirrors for a propagating SPP whose wavelength is inside the gap. Another passive configuration employed was a plasmonic resonator consisting of dielectric-loaded surface plasmon polariton waveguide ring resonator (WRR). Also, a more robust variant has been proposed by replacing the ring in the WRR with a disk (WDR). The performance in terms of wavelength selectivity and efficiency of the WDRs was evaluated and was shown to be in good agreement with numerical results. Control of SPP signal was demonstrated in the WRR configuration both electro-optically and all-optically. In the case of electro-optical control, the dielectric host matrix was doped with an electro-optical material and combined with an appropriate set of planar electrodes. A 16% relative change of transmission upon application of a controlled electric field was measured. For all-optical control, nonlinearity based on trans-cis isomerization in a polymer material is utilized. More than a 3-fold change between high and low transmission states of the device at milliwatt control powers ( ~100 W/cm^2 intensity) was observed. Beyond the active control of propagating surface plasmons, further advancement can be achieved by means of nanoscale plasmonic structures supporting localized surface plasmons (LSP). Interactions of molecular excitations in a pi-conjugated polymer with plasmonic polarizations are investigated in hybrid plasmonic cavities. Insights into the fundamentals of enhanced light-matter interactions in hybrid subwavelength structures with extreme light concentration are drawn, using ultrafast pump-probe spectroscopy. This thesis also gives an overview of the challenges and opportunities that hybrid plasmonic functionalities provide in the field of plasmon nano optics.Las nanoestructuras plasmónicas han adquirido una importante relevancia como herramientas capaces de manipular los fotones en la nanoescala, y pueden llegar a revolucionar un amplio abanico de aplicaciones tales como los circuitos ópticos integrados, la fotovoltaica o los dispositivos biosensores. Dichas estructuras hacen posible la miniaturización de los componentes ópticos más allá del “límite de difracción” de la luz, ya que convierten la radiación óptica en campos electromagnéticos fuertemente confinados en la proximidad de estructuras metálicas de tamaño inferior a la longitud de onda mediante la excitación de plasmones de superficie (SPs). Estos campos electromagnéticos tan intensos generados en los llamados “puntos calientes” plasmónicos brindan perspectivas muy interesantes para la generación de efectos no lineales en medios activos. El área de investigación denominado plasmónica activa busca la modulación de los SPs sostenidos por la intercara entre un metal y un material no lineal mediante una señal de control externa. El índice de refracción del material no lineal cambia bajo la aplicación de la señal de control, lo cual da lugar a la modificación de la respuesta plasmónica. Estas nanoestructuras híbridas también hacen posible la aparición de nuevos tipos de estados híbridos, lo cual proporciona tanto herramientas para diseñar dispositivos plasmónicos activos como mecanismos que permiten re-examinar las reglas convencionales de la interacción luz materia. Por lo tanto, es necesario el estudio de dichas nanoestructuras plasmónicas híbridas de manera teórica y experimental. En este trabajo de tesis se analiza el control de los SPs excitados en sistemas híbridos que combinan materiales activos y nanoestructuras metálicas mediante una señal óptica externa o un voltaje aplicado. Se han investigado distintos tipos de geometrías plasmónicas utilizando herramientas de simulación basadas en métodos en el dominio de la frecuencia, y posteriormente se han caracterizado experimentalmente dichas geometrías mediante técnicas de campo cercano y de campo lejano tales como la microscopía óptica de campo cercano y la microscopía basada en pérdidas radiativas, respectivamente. En primer lugar se estudiaron elementos plasmónicos pasivos, en particular espejos plasmónicos dieléctricos que demuestran la capacidad que tienen las redes periódicas de caballones de material dieléctrico colocados sobre una superficie metálica plana para abrir intervalos prohibidos en la relación de dispersión de los plasmones de superficie propagantes o plasmones-polaritones de superficie (SPPs). Los resultados muestran que dichos espejos poseen muy buenas propiedades reflectantes para SPPs cuya energía está en el intervalo prohibido. Otra configuración pasiva analizada fueron los resonadores plasmónicos basados en anillos de guía de onda plasmónica fabricada a partir de estructuras dieléctricas sobre metal (WRR, del inglés waveguide ring resonator ). Asimismo, se propone una versión más robusta de resonador plasmónico, basada en la sustitución del anillo del WRR por un disco (WDR, del inglés waveguide disk resonator). Se ha evaluado el funcionamiento de los WDRs en términos de selectividad en longitud de onda y de eficiencia, y los resultados obtenidos presentan un buen acuerdo con las predicciones numéricas. Pasando a las configuraciones activas, se demuestra el control de la señal plasmónica en configuración WRR por medios tanto electro-ópticos como completamente ópticos. En el caso del control electro-óptico, el material dieléctrico que compone el WRR estaba dopado con un componente electro-óptico y a la estructura pasiva se le añadió un conjunto de electrodos planos. Bajo la aplicación de un campo eléctrico externo, se midió un cambio relativo en la transmisión a través de la guía plasmónica del 16%. En cuanto al control puramente óptico, se utilizó la no linealidad de un material polimérico con origen en una isomerización trans-cis. En este caso se detectó un factor 3 entre los estados de alta y baja transmisión del dispositivo con potencias de control del orden de milivatios (intensidad del haz óptico de control de unos 100W/cm2 aproximadamente). Además del control activo de los plasmones de superficie propagantes, la utilización de nanoestructuras plasmónicas que poseen resonancias plasmónicas localizadas puede dar lugar a nuevos fenómenos. En esta tesis también se han estudiado las interacciones entre las excitaciones moleculares en un polímero pi-congujado con las polarizaciones plasmónicas en nanocavidades plasmónicas híbridas. Utilizando espectroscopia de tipo bombeo-sonda con pulsos ultrarrápidos, se han analizado diversos aspectos del aumento en la interacción luz-materia para estructuras híbridas de dimensiones inferiores a la longitud de onda sometidas a concentraciones de luz muy altas. Por último, esta tesis también proporciona una visión general de los desafíos y posibilidades que las funcionalidades plasmónicas híbridas ofrecen en el campo de la nano-óptica basada en plasmones de superfície

Plasmonic-Organic and Silicon-Organic Hybrid Modulators for High-Speed Signal Processing

High-speed electro-optic (EO) modulators are key devices for optical communications, microwave photonics, and for broadband signal processing. Among the different material platforms for high-density photonic integrated circuits (PIC), silicon photonics sticks out because of CMOS foundries specialized in PIC fabrication. However, the absence of the Pockels effect in silicon renders EO modulators with high-efficiency and large modulation bandwidth difficult. In this dissertation, plasmonic and photonic slot waveguide modulators are investigated. The devices are built on the silicon platform and are combined with highly-efficient organic EO materials. Using such a hybrid platform, we realize compact and fast plasmonic-organic hybrid (POH) and silicon-organic hybrid (SOH) modulators. As an application example, we demonstrate for the first time an advanced terahertz communication link by directly converting data on a 360 GHz carrier to a data stream on an optical carrier. For optical transmitter applications, we overcome the bandwidth limitation of conventional SOH modulators by introducing a high-k dielectric microwave slotline for guiding the modulating radio-frequency signal which is capacitively-coupled to the EO modulating region. We confirm the viability of such capacitively-coupled SOH modulators by generating four-state pulse amplitude modulated signals with data rates up to 200 Gbit/s

Novel Metamaterials and Their Applications in Subwavelength Waveguides, Imaging and Modulation

The development of metamaterials has opened the door for engineering electromagnetic properties by subwavelength artificial atoms , and hence accessing new properties and functionalities which cannot be found among naturally occurring materials. In particular, metamaterials enable the flexibility of independently controlling the permittivity and permeability to be almost any arbitrary value, which promises to achieve deep subwavelength confinement and focusing of electromagnetic waves in different spectrum regimes. The next stage of this technological revolution will be focused on the development of active and controllable metamaterials, where the properties of the metamaterials are expected to be tuned by external stimuli. In this sense, some natural materials are also promising to provide the tunable capability, particularly in the near infrared and terahertz domains either by applying a voltage or shining light on the materials. The objective of this dissertation is to investigate novel metamaterials and explore three important applications of them: subwavelength waveguiding, imaging and modulation. The first part of this dissertation covers the theory, design and fabrication of several different types of metamaterials, which includes artificially designed metamaterials and some naturally existing materials. The second part demonstrates metal gratings functioning as designer surface plasmonic waveguides support deep subwavelength surface propagation modes at microwave frequency. The third part proposes multilayered metal-insulator stack as indefinite metamaterial that converts evanescent waves to propagating waves, hence deep subwavelength image can be observed. The fourth part explores the tunability of several natural materials - gallium (Ga), indium tin oxide (ITO) and graphene, and demonstrates electro-optical (EO) modulators based on these materials can be achieved on nano-scale. The final part summarizes the work presented in this dissertation and also discusses some future work for photodetection, photovoltaics, and modulation

Low Loss Plasmon-Assisted Integrated Photonics

Photonic integrated circuits (PICs), semiconductor chips with both photonic and electronic elements, are seeing rapid development and have the potential to transform several industries, such as autonomous driving, computing, telecommunication and quantum networks. However, realization and wide adoption of PICs across the various fields faces a key challenge – soze disparity between electronic (~0.01 um) and photonic components (~100’s of um). Plasmonics, a technology which confines light to the interface of metals and dielectrics, has a potential to address challenges. In particular, it has been shown to led to smaller devices (~10 um or less), enabling higher density optical circuits and devices on-chip. However, the technology is limited by quite extraordinarily high off-state transmission, wherein ~10% of an input signal makes it out of the device. This is simply too high to be practical. This thesis addresses this size disparity, while maintaining high speeds (100’s of GHz), low losses (\u3c 1dB) and high energy efficiency (~ 100 fJ/bit), through the concept of plasmon-assisted devices. The plasmon-assisted design philosophy is based on engaging and disengaging the lossy plasmonic component based on when active modulation is needed. As will be shown, the use of the plasmon-assisted approach generates proposed devices that have the potential to exhibit record performance, significantly elevating the capabilities of integrated photonic devices while greatly reducing the size disparity. For example, the all-oxide modulator can exhibit resistive-capacitive (RC) limited speeds of up to 333 GHz with a sub 0.2 dB insertion loss (IL), while the hybrid polymer-based modulator can exhibit RC limited speeds of 700 GHz but with narrow linewidth. The NOEM based devices can operate with record low energy consumption, down to a few 100 aJ/bit. In addition, this record-breaking performance can be achieved with device that are less than 40 um2 in size