Investigation of nanophotonic structures for imaging and sensing

The ability to image micro/nano scale objectives with miniaturized optical components has always been of great interest due to its great potential in applications such as microscopy, nanofabrication, and biomedical monitoring. However, in traditional practice using dielectric lenses, the focal size is inevitably limited by the Abbe’s diffraction limit (0.51fλ/ρ). Here, λ is the wavelength in vacuum, and f and ρ are the focal length and the radius of the lens, respectively. Moreover, the performance of conventional spherical lenses deteriorates as their sizes approach the wavelength. On the other hand, owing to the recent advances in micro/nano fabrication techniques, miniature sensors have received much attention, which are highly desirable in many sensing applications for physical, chemical, and biomedical parameter measurements. However, the performance of miniature sensors usually suffers from the similar difficulty as miniaturized imaging systems. Recently nanophotonic structures have been explored for the development of miniaturizing imaging and sensing systems due to their capability of confining and manipulating light at a subwavelength scale. In this dissertation work, several different mechanisms that nanophotonic structures can be used to help enhance the performance of imaging and sensing in miniaturized systems are investigated. First, plasmonic lens utilizing the nanophotonic structure to achieve the subwavelength focusing ability is studied. Three different regions in the plasmonic lens design are defined. Furthermore, a plasmonic lens in the Fresnel’s region is designed and k.ed to achieve a sub-diffraction limit focus. Second, radially polarized light generated by the TEM mode in the annular aperture in metal is investigated, which can further enhance the focusing ability. Third, in terms of sensing, an ultra-thin plasmonic interferometer constructed with a nano-hole array is fabricated on a fiber facet. By using this structure, the multi-parameter sensing capability of this interferometer is demonstrated; high sensitivity refractive index and temperature sensing are achieved. Finally, a novel sensor design based on the cladding modes and buffer modes generated by the planar grating on the fiber facet is proposed. Experimental studies of this sensor demonstrate its superior temperature sensitivity and the potential of multi-parameter sensing

Nanoantennas for visible and infrared radiation

Nanoantennas for visible and infrared radiation can strongly enhance the interaction of light with nanoscale matter by their ability to efficiently link propagating and spatially localized optical fields. This ability unlocks an enormous potential for applications ranging from nanoscale optical microscopy and spectroscopy over solar energy conversion, integrated optical nanocircuitry, opto-electronics and density-ofstates engineering to ultra-sensing as well as enhancement of optical nonlinearities. Here we review the current understanding of optical antennas based on the background of both well-developed radiowave antenna engineering and the emerging field of plasmonics. In particular, we address the plasmonic behavior that emerges due to the very high optical frequencies involved and the limitations in the choice of antenna materials and geometrical parameters imposed by nanofabrication. Finally, we give a brief account of the current status of the field and the major established and emerging lines of investigation in this vivid area of research.Comment: Review article with 76 pages, 21 figure

Plasmonic properties and applications of metallic nanostructures

Plasmonic properties and the related novel applications are studied on various types of metallic nano-structures in one, two, or three dimensions. For 1D nanostructure, the motion of free electrons in a metal-film with nanoscale thickness is confined in its normal dimension and free in the other two. Describing the free-electron motion at metal-dielectric surfaces, surface plasmon polariton (SPP) is an elementary excitation of such motions and is well known. When further perforated with periodic array of holes, periodicity will introduce degeneracy, incur energy-level splitting, and facilitate the coupling between free-space photon and SPP. We applied this concept to achieve a plasmonic perfect absorber. The experimentally observed reflection dip splitting is qualitatively explained by a perturbation theory based on the above concept. If confined in 2D, the nanostructures become nanowires that intrigue a broad range of research interests. We performed various studies on the resonance and propagation of metal nanowires with different materials, cross-sectional shapes and form factors, in passive or active medium, in support of corresponding experimental works. Finite- Difference Time-Domain (FDTD) simulations show that simulated results agrees well with experiments and makes fundamental mode analysis possible. Confined in 3D, the electron motions in a single metal nanoparticle (NP) leads to localized surface plasmon resonance (LSPR) that enables another novel and important application: plasmon-heating. By exciting the LSPR of a gold particle embedded in liquid, the excited plasmon will decay into heat in the particle and will heat up the surrounding liquid eventually. With sufficient exciting optical intensity, the heat transfer from NP to liquid will undergo an explosive process and make a vapor envelop: nanobubble. We characterized the size, pressure and temperature of the nanobubble by a simple model relying on Mie calculations and continuous medium assumption. A novel effective medium method is also developed to replace the role of Mie calculations. The characterized temperature is in excellent agreement with that by Raman scattering. If fabricated in an ordered cluster, NPs exhibit double-resonance features and the double Fano-resonant structure is demonstrated to most enhance the four-wave mixing efficiency

Plasmonic Superconducting Single Photon Detector

A theoretical model with experimental verification is presented to enhance the quantum efficiency of a superconducting single-photon detector without increasing the length or thickness of the active element. The basic enhancement framework is based on: (1) Utilizing the plasmonic nature of a superconducting layer to increase the surface absorption of the input optical signal. (2) Enhancing the critical current of the nanowires by reducing the current crowding at the bend areas through optimally rounded-bend implementation. The experimental system quantum efficiency and fluctuation rates per second are assessed and compared to the proposed theoretical model. The model originated from an accurate description of the different liberation mechanisms of the nano-patterned superconducting films (vortex hopping and vortex-antivortex pairing). It is built complimentary to the existing, well-established models by considering the effects of quantum confinement on the singularities' energy states. The proposed model explains the dynamics of singularities for a wide range of temperatures and widths and describe an accurate count rate behavior for the structure. Furthermore, it explains the abnormal behaviors of the measured fluctuation rates occurring in wide nano-patterned superconducting structures below the critical temperature. In accordance to this model, it has been shown that for a typical strip width, not only is the vortex-antivortex liberation higher than the predicted rate, but also quantum tunneling is significant in certain conditions, and cannot be neglected as it has been in previous models. Also it is concluded that to satisfy both optical guiding and photon detection considerations of the design, the width and the thickness of the superconducting wires should be carefully determined in order to maintain the device sensitivity while crossing over from the current crowding to vortex-based detection mechanisms.1 yea

Mechanisms for enhancing the optical transmission through a single

Este trabajo está dedicado al estudio teórico de las propiedades ópticas mediadas por ondas electromagnéticas superficiales en nanoestructuras metálicas perforadas con agujeros anulares. Con el método analítico de la Expansión Modal, se describen redes infinitas periódicas con agujeros y sistemas finitos (agujeros aislados o Geometrías Ojo de Buey: un agujero circular rodeado de surcos anulares concéntricos). Asimismo, las propiedades de transmisión en tiras de metal combinadas con dieléctricos no lineales de tipo Kerr se estudian usando el método numérico FDTD

Latest Advances in Nanoplasmonics and Use of New Tools for Plasmonic Characterization

Nanoplasmonics is an area that uses light to couple electrons in metals, and can break the diffraction limit for light confinement into subwavelength zones, allowing for strong field enhancements. In the last two decades, there has been a resurgence of this research topic and its applications. Thus, this Special Issue presents a collection of articles and reviews by international researchers and is devoted to the recent advances in and insights into this research topic, including plasmonic devices, plasmonic biosensing, plasmonic photocatalysis, plasmonic photovoltaics, surface-enhanced Raman scattering, and surface plasmon resonance spectroscopy

High index difference polymer waveguide Mach-Zehnder interferometer biosensor, compatible with injection molding and spin-coating

Die selektive Detektion von Biomolekülen, wie z.B. Proteinen oder Teilen von DNA, ist von essenzieller Bedeutung für die medizinische Diagnostik. Optische Biosensoren werden aktuell als vielversprechende Kandidaten gehandelt, um die Beschränkungen teurer und zeitaufwändiger state-of-the-art diagnostischer Tests zu überwinden. In dieser Arbeit wird die Entwicklung eines vollständig integrierten Mach-Zehnder Interferometer Biosensors, basierend auf Hochindexkontrast Polymerwellenleitern, dargelegt. Das Design des optischen Sensors, basierend auf optischen Beugungsgittern für die Kopplung von Licht in und aus dem Sensor sowie monomodigen Wellenleitern, ist vollständig kompatibel mit weit verfügbaren und kosteneffizienten Technologien zur Massenproduktion von Polymer-basierten Bauteilen, wie Spritzgießen und Aufschleudern, was diesen Sensor zu einer interessanten Alternative zu anorganischen Sensorkonzepten macht. Die Elemente des Sensors wurden vollständig simuliert und optimiert. Um die Koppeleffizienz kleiner Gitterkoppler in Materialsystemen mit niedrigem Indexkontrast zu erhöhen, wurde der neuartige Zugang einer Hochindex Beschichtung von Gitterkopplern angewandt. In den Simulationen zeigte sich eine Steigerung der maximalen Koppeleffizienz mittels Gitterkopplern in schmale, monomodige Wellenleiter durch die Aufbringung dieser Hochindex Beschichtung um nahezu 8 dB. Der positive Effekt der Hochindex Beschichtung wurde anschließend experimentell verifiziert. Zuletzt wurde der auf Polymerwellenleitern basierte Sensor für die markerfreie Echtzeitmessung der Bindungsprozesse von Biotin-Streptavidin bei einer Wellenlänge von 1310 nm eingesetzt. Für die Binding von Streptavidin auf der Sensoroberfläche während dieser Experimente wurde die Sensoroberfläche mit 3-mercaptopropyl trimethoxy Silan und Malemid markiertem Biotin funktionalisiert. Trotz der großen Wellenlänge und des geringen verfügbaren Brechungsindexkontrastes in Polymer-Materialsystemen, verglichen mit anorganischen Materialsystemen, konnten Streptavidin Konzentrationen ab 0.1 µg/ml erfolgreich detektiert werden.The selective detection of biomolecules, such as proteins or DNA strands, is of vitally importance for medical diagnosis. Optical biosensors are believed to be a promising way to overcome the limitations of expensive and time-consuming state-of-the-art diagnostic tests. In this thesis, the development of a fully integrated Mach-Zehnder interferometer biosensor based on high index contrast polymer waveguides is reported. The optical sensor design, based on grating waveguide couplers for light in- and out-coupling as well as single mode polymer waveguides, is fully compatible with cost-efficient mass production technologies for polymers such as injection molding and spin coating, which makes the sensor an attractive alternative to inorganic optical sensors. The sensor elements are rigorously simulated and optimized. For an improved efficiency of small grating couplers in material systems with comparatively low index contrasts, the novel approach of a high index coating on top of grating couplers was applied. Simulations revealed a nearly 8 dB increase in the maximum coupling efficiency into narrow single mode waveguides by means of grating couplers due to the high index coating. The positive effect of the high index coating was then experimentally verified. Finally, the polymer waveguide based biosensor was applied for label-free online measurements of biotin-streptavidin binding processes on the sensor surface at a wavelength of 1310 nm. For the binding experiments, the surface of the polyimide waveguide core layer was functionalized with 3-mercaptopropyl trimethoxy silane and malemide tagged biotin. Despite the large wavelength and the comparatively low surface sensitivity of the sensor system due to the low index contrast in polymer material systems compared to inorganic material systems, streptavidin concentrations down to 0.1 µg/ml were resolved

Active Infrared Nanophotonics in van der Waals Materials

Two-dimensional van der Waals materials have recently been introduced into the field of nanophotonics, creating opportunities to explore novel physics and realize first-of-their kind devices. By reducing the thickness of these materials, novel optical properties emerge due to the introduction of vertical quantum confinement. Unlike most materials, which suffer from a reduction in quality as they are thinned, layered van der Waals materials have naturally passivated surfaces that preserve their performance in monolayer form. Moreover, because the thickness of these materials is below typical charge carrier screening lengths, it is possible to actively control their optical properties with an external gate voltage. By combining these unique properties with the subwavelength control of light-matter interactions provided by nanophotonics, new device architectures can be realized. In this thesis, we explore van der Waals materials for active infrared nanophotonics, focusing on monolayer graphene and few-layer black phosphorus. Chapter 2 introduces gate-tunable graphene plasmons that interact strongly with their environment and can be combined with an external cavity to reach large absorption strengths in a single atomic layer. Chapter 3 builds on this, using graphene plasmons to control the spectral character and polarization state of thermal radiation. In Chapter 4, we complete the story of actively controlling infrared light using graphene-based structures, introducing graphene into a resonant gold structure to enable active control of phase. By combining these resonant structures together into a multi-pixel array, we realize an actively tunable meta-device for active beam steering in the infrared. In Chapters 5 and 6, we present few layer black phosphorus (BP) as a novel material for active infrared nanophotonics. We study the different electro-optic effects of the material from the visible to mid-infrared. We additionally examine the polarization-dependent response of few-layer BP, observing that we can tune its optical response from being highly anisotropic to nearly isotropic in plane. Finally, Chapter 7 comments on the challenges and opportunities for graphene- and BP-integrated nanophotonic structures and devices.</p