326 research outputs found

    Beam scanning by liquid-crystal biasing in a modified SIW structure

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    A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

    Quantum imaging and polarimetry with two-color photon pairs

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    Verschränkte Photonenpaare, gemeinhin als Signal- und Idler-Photonen bezeichnet, wurden als Grundlage für Quantum imaging with undetected photons (QIUP) und Quantum ghost imaging (QGI) verwendet. Mit QIUP können wir ein Objekt abbilden, indem wir nur die Signal-Photonen messen, die nie mit dem Objekt wechselwirken, während die Idler-Photonen, die das Objekt beleuchten, undetektiert bleiben. Bei QGI werden die beleuchtenden Idler-Photonen von einem räumlich nicht auflösenden Detektor gemessen, während die nicht wechselwirkenden Signal-Photonen von einer Kamera gemessen werden und das Bild dann nur aus den Koinzidenzen von Signal und Idler rekonstruiert wird. Nennenswert ist hier die Verwendung von zweifarbigen Photonenpaaren, welche es uns ermöglichen, Komplikationen bei der Bildgebung in Wellenlängenbereichen zu überwinden, in denen Kameras nur eine geringe Effizienz aufweisen. Daraus ergibt sich ein enormes Potenzial für die Biosensorik, bei der empfindliche Proben, die für Strahlungsschäden anfällig sind, mit herkömmlichen Einzelphotonen-Kameras abgebildet werden können, wie zum Beispiel im sichtbaren Spektralbereich, während die Probe von Photonen mit viel geringerer Energie beleuchtet wird. In dieser Arbeit wurden drei Lücken in der Literatur zur Quantenbildgebung und Polarimetrie geschlossen: (1) Die fundamentale transversale Auflösungsgrenze von QIUP und QGI, die zweifarbige Photonenpaare verwenden, wurde diskutiert. (2) Ein linsenloses QGI-Verfahren wurde vorgestellt, das sich speziell für die Abbildung in Wellenlängenbereichen eignet, für die weniger Linsen zur Verfügung stehen, wie zum Beispiel im Terahertz-Bereich. Wir haben es Pinhole QGI genannt, da wir gezeigt haben, dass es analog zur klassischen Lochkamera ist. (3) Ein Quantum ghost polarimetry (QGP) Schema wurde vorgeschlagen, bei dem dielektrische Metaoberflächen verwendet werden können, um den Einsatz rekonfigurierbarer optischer Elemente zu vermeiden

    Tailoring Light-Matter Interaction via Advanced Nanophotonic Structures : From Passive to Dynamically Tunable Systems

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    Light-matter interaction is the fundamental principle of photonics that governs numerous disruptive applications. Dynamically tuning the light-matter interaction is key to designing advanced photonic devices with improved and enhanced functionalities. Specifically, having active control of the amplitude, wavelength, phase, and polarization of light is vital. It essentially addresses the key pillars of photonics, ranging from generating, guiding, manipulating, amplifying, and detecting light. This thesis presents a framework and platform to model, tailor, and enhance the light-matter interactions in nanophotonic structures. Epsilon-near-zero (ENZ) materials, plasmonic nanostructures, and metal-insulator-metal (MIM) cavities were utilized as a light-matter interaction platform. First, the underlying mechanism of emission enhancement was unravelled by integrating fluorescent dye with the MIM cavity. This study suggests a pathway for engineering the emission properties of an emitter through both Purcell and excitation rate enhancement. Following this, dynamic emission tuning was achieved, whereby a fluorescent dye containing hydrogel integrated MIM cavity was utilized. The thickness of the insulator layer was tuned by changing the ambient humidity, which resulted in spectral tuning of cavity resonance, hence the active tuning of emission. The coupling strength quantifies the light-matter interaction, so tuning the coupling strength is another way to tailor the light-matter interaction. By developing a novel electrical gating scheme, an active tuning of the coupling strength was demonstrated in a strongly coupled system comprised of ENZ materials that support ENZ mode and gold nanorods supporting the localized surface plasmon mode. Lastly, by harnessing the vanishing index of the ENZ material, less sensitivity of the spectral position of photonic resonance towards the geometrical perturbations was obtained through a polarization-independent plasmonic structure on an ENZ substrate. Overall, this thesis shows broad opportunities for using nanophotonic systems to tailor light-matter interactions dynamically

    Roadmap on Label-Free Super-resolution Imaging

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    Label-free super-resolution (LFSR) imaging relies on light-scattering processes in nanoscale objects without a need for fluorescent (FL) staining required in super-resolved FL microscopy. The objectives of this Roadmap are to present a comprehensive vision of the developments, the state-of-the-art in this field, and to discuss the resolution boundaries and hurdles that need to be overcome to break the classical diffraction limit of the label-free imaging. The scope of this Roadmap spans from the advanced interference detection techniques, where the diffraction-limited lateral resolution is combined with unsurpassed axial and temporal resolution, to techniques with true lateral super-resolution capability that are based on understanding resolution as an information science problem, on using novel structured illumination, near-field scanning, and nonlinear optics approaches, and on designing superlenses based on nanoplasmonics, metamaterials, transformation optics, and microsphere-assisted approaches. To this end, this Roadmap brings under the same umbrella researchers from the physics and biomedical optics communities in which such studies have often been developing separately. The ultimate intent of this paper is to create a vision for the current and future developments of LFSR imaging based on its physical mechanisms and to create a great opening for the series of articles in this field.Peer reviewe

    Electro-Optic Excitations in van der Waals Materials for Active Nanophotonics

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    van der Waals materials are emerging due to their unique properties such as atomic thickness, diverse quasiparticle optical resonances, and no requirement for lattice matching. While there is a vast variety of materials, semiconductors hold a special place for opto-electronic and linear/non-linear optical studies. Black phosphorus (BP), a 2D quantum-well with direct bandgap and puckered crystal structure, is a compelling platform for this research direction. In this thesis, we investigate fundamental optical excitations in novel low-dimensional quantum materials to achieve strong light-matter interaction and integrate with nanophotonic motifs for low-footprint, reconfigurable optical technology, focusing primarily on black phosphorus and transition metal dichalcogenides. The thesis begins with the 'thin film limit' of van der Waals materials, between 5 and 20 nm thickness range. Chapters 2 and 3 explore how few-layer black phosphorus hosts interband and intraband optical excitations that can be strongly modified with gate-controlled doping and electric field, displaying epsilon near zero and hyperbolic behavior in the mid and far-infrared. In atomic thickness, strongly bound excitonic quasiparticles dominate the optical response. In Chapter 4, we investigate electrically tunable excitons in tri-layer black phosphorus, demonstrating a reconfigurable birefringent material that, when coupled with a Fabry-Perot cavity, enables the realization of a versatile and broadband polarization modulator. In Chapter 5, we examine the ultimate limit of a monolayer, studying MoTe2 via photoluminescence measurements and first-principles GW+BSE calculations, highlighting the Rydberg series associated with the exciton and its gate-tunability to understand strong electron-exciton interactions. In Chapter 6, we show how such excitons in monolayer black phosphorus can be strongly quantum confined at natural edges of exfoliated flakes, leading to highly temporally coherent emission. This emission is gate-tunable and understood via transmission electron microscopy and first-principles GW+BSE calculations of phosphorene nanoribbons to be originating from atomic reconstructions of the edge coupled with strain and screening effects. Overall, our work highlights the potential of van der Waals materials for various electro-optical excitations and their applications in active nanophotonics. </p

    Holography Measurement for Crossed-Dragone Type Telescope & its Application to the Fred Young Submm Telescope

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    Microwave Holography is an accurate and efficient method for measuring the surface shape of large reflector antennas. The method is based on the Fourier transform relationship between the antenna's far-field diffraction beam pattern and its aperture field. Measuring the antenna's far-field beam both in amplitude and phase can deduce the aperture field distribution. The phase deviations of the aperture field are directly related to the antenna's surface shape. This technique has become a well-established method for surface metrology of large radio telescopes because of its high efficiency and measurement accuracy. However, employing the traditional holography cannot identify the surface deformity in a 'two-reflector' antenna system. This thesis investigates a new multi-map holography metrology to overcome this limitation. The new method is developed to align the Fred Young Sub-millimeter telescope (FYST), a coma-corrected Crossed-Dragone antenna with two 6-m off-axis reflectors. The surfaces of the two reflectors must be aligned to be better than 10.7um. The multi-map holography identifies the surface errors between the two reflectors by taking five holographic beam measurements by placing the receiver at well-separated points in the focal plane. The parallactic shift of the surface errors allows assigning them to either one of the two mirrors. A new data processing technique is developed using an inference technique to simultaneously analyze the five beams and convert them to two surface error maps. Extensive numerical simulations have been carried out to check the feasibility, measurement accuracy, and optimum set-up of the new holographic system by modeling the systematic errors in the system, such as random instrument noise and fluctuation of performance of the instruments. These indicate that a measurement accuracy of ~2um is achievable. The critical part of the data processing technique of the 'Multi-map' holography is to develop a fast and accurate beam simulation algorithm. The conventional physical optics method is very time-consuming for analyzing the FYST antenna. A new 'two-step' Kirchhoff-Fresnel diffraction method is developed, which, compared to the conventional physical optics analysis, can reduce the computational time by four orders of magnitude without noticeable accuracy degradation. The new multi-map holography and its data processing technique are implemented to measure the reflector errors for a 0.4-m diameter Crossed-Dragone antenna in the laboratory. The experiments prove that the errors on the two reflectors can be discriminated and accurately measured with a statistic error lower than 1um. The holographic measurements and reflector corrections also indicate that the large spatial errors existing on the two reflectors also can be measured

    Development of laser sources and interferometric approaches for polarization-based label-free microscopy

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    The project developed in this thesis describes the design and the experimental realization of optical methods which can probe the anisotropy of semitransparent media. The ability to manipulate polarized light enables a label-free imaging approach that can retrieve fundamental information about the sample structure without introducing any alteration within it. Such a potential is of great importance and methods like the ones based on polarization analysis are gaining more and more popularity in the biomedical and biophysical fields. Moreover, when they are coupled with fluorescence microscopy and nanoscopy, they may provide an invaluable tool for researchers. The optical method I developed mainly exploits the laser radiation emitted from tailored optical oscillators to dynamically generate polarization states. The realization of such states does not comprise any external active device. The resulting time-evolving polarization state once properly coupled to an optical system enables probing a sample to retrieve its anisotropies at a fast rate. The development of two different laser sources is presented together with the characterizations of their optical properties. One of them consists of a Helium-Neon laser modified by applying an external magnetic field to trigger the Zeeman effect in its active medium. The other one is a Dual-Comb source, that is a mode-locked (ML) laser generating a pair of mutually coherent twin beams. Moreover, the thesis delivers the theoretical model and the experimental realization of the optical method to probe the optical anisotropies of specimens. Finally, the technical realization of a custom laser scanning optical microscope and its imaging results obtained with such methods are reported

    1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

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    A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

    X-ray Phase Contrast Tomography : Setup and Scintillator Development

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    X-ray microscopy and micro-tomography (μCT) are valuable non-destructive examination methods in many disciplines such as bio-medical research, archaeometry, material science and paleontology. Besides being implemented at synchrotrons radiation sources, laboratory setups using an X-ray tube and high-resolution scintillation detector routinely provide information on the micrometre scale. To improve the image contrast for small and low-density samples, it is possible to introduce a propagation distance between sample and detector to perform propagation-based phase contrast imaging (PB-PCI). This contrast mode relies on a sufficiently coherent illumination and is characterised by the appearance of an additional intensity modulations (‘edge enhancement fringes’) around interfaces in the image. The strength of this effect depends on hardware as well as geometry parameters. This thesis describes the development of a laboratory setup for X-ray μCT with a PB-PCI option. It contains the theoretical and technical background of the setup design as well the characterization of the achieved performance.Moreover, the optimization of the PB-PCI geometry was explored both theoretically as well as experimentally for three different setups. A simple rule for finding the optimal magnification to achieve high phase contrast for edge features was deduced. The effect of the polychromatic source spectrum und detector sensitivity was identified and included into the theoretical model.Besides application and methodological studies, the setup was used to test and characterise new X-ray scintillator materials. Recently, metal halide perovskite nanocrystals (MHP NCs) have gained attention due to their outstanding opto-electronic performance. The main challenge for their use and commercialization is their low long-term stability against humidity, temperature, and light exposure. Here, a CsPbBr3 scintillator comprised of an ordered array of nanowires (NW) in an anodized aluminium oxide (AAO) membrane is presented as a promising new scintillator for X-ray microscopy and μCT. It shows a high light yield under X-ray exposure which improves with smaller NW diameter and higher NW length. In contrast to many other MHP materials this scintillator shows good stability under continuous X-ray exposure and changing environmental conditions over extended time spans of several weeks. This makes it suitable for tomography, which is demonstrated by acquiring the first high-resolution tomogram using a MHP scintillator with the presented laboratory setup

    Fast Terahertz Metamaterial/Graphene-Based Optoelectronic Devices for Wireless Communication

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    Research in the terahertz (THz) band, which is broadly defined as 0.1-10 THz, is an active area of research driven by applications in sixth generation (6G) and beyond for communications, spectroscopy, imaging, and sensing. In order to exploit the full potential of all these applications, fast integrated circuitry is required. This work revolves around removing this bottleneck. Achievement of efficient dynamic modulation requires the implementation of active material. Amongst many different approaches to achieve active modulation, metamaterials/graphene-based technology is establishing itself as a benchmark for THz operation due to its versatility, power efficiency, small footprint, and integration capabilities. Our devices have been modulated all-electronically, as described in Chapters 4 and 6, and all-optically as reported in Chapter 5. The fabrication of the novel design based on metamaterial (MM) and graphene for amplitude, phase, and polarization modulations is reported in Chapter 3. The optoelectronic behaviour of this modulator is tested in a THz time-domain spectroscopy (THz-TDS) setup as demonstrated in Chapter 4. By choosing the appropriate THz-TDS setup configuration, a spectral amplitude extinction ratio of >10 dB (>93%) at the resonant frequency of 0.8 THz is demonstrated. The spectral phase of THz radiations is actively tuned by >27o at 0.62 THz frequency. Linear to circular polarization conversion with nearly 100% of conversion efficiency is reported demonstrating almost an independent control of circular dichroism (CD) and optical activity (OA) as mentioned explicitly in Chapter 6. Dynamic changes of ellipticity are reported to exceed 0.3 in ratio at resonance. The OA of transmitted THz radiations is continuously rotated by >21.5o at 0.71 THz by varying the gate. These values are in line with acquainted literature with graphene-based or 2-dimensional electron gas modulators but with higher reconfiguration speed. The helicity, either right or left circular polarization states, of elliptical waves can be controlled. These results are of great importance for fundamental research of polarization spectroscopy, polarization imaging, or THz applications in the pharmaceutical and biomedical fields. An all-electronic controlled metamaterial-based THz modulator is demonstrated to achieve a recorded operating speed >3 GHz which is limited by the available instrumentation as illustrated in Section 7.1. The achievements in the modulation speed (in GHz range), amplitude extinction ratio (>10 dB), phase shift tuning (27o), and nearly decoupled control of OA and CD of THz waves are the key values of this device, which is undoubtedly meaningful for communication applications and has a certain impact on the THz modulator technology. The achieved GHz modulation speed of this hybrid MMs/graphene device is within very good agreement with previous literature reported on pristine graphene. This result provides an upper intrinsic limit of the maximum reconfiguration speed of these devices to 100s of GHz and, at the same time, reinforces the use of metamaterial/graphene optoelectronic devices for ultrafast modulation of terahertz waves. This overall remarkable performance of an optoelectronic modulator based on metamaterial/graphene resonators is capable of efficiently modulating THz radiation all-electronically with GHz-reconfiguration speed. It is worth highlighting that this exceptionally high reconfiguration speed, the highest reported so far to the best of our knowledge for a graphene-based integrated device, was not achieved at the expense of the other performances, e.g. amplitude and polarization modulation depths. These results represent great progress for several terahertz research and ultrafast photonic applications, such as the realization of fast deep, and efficient THz circuitry for the investigation of exotic quantum phenomena, wireless communications, and laser diodes stabilization in quantum electronics
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