Browsing by Subject "Metasurfaces"
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Item Open Access Actively Tunable Plasmonic Nanostructures(2020) Wilson, Wade MitchellActive plasmonic nanostructures with tunable resonances promise to enable smart materials with multiple functionalities, on-chip spectral-based imaging and low-power optoelectronic devices. A variety of tunable materials have been integrated with plasmonic structures, however, the tuning range in the visible regime has been limited and small on/off ratios are typical for dynamically switchable devices. An all optical tuning mechanism is desirable for on-chip optical computing applications. Furthermore, plasmonic structures are traditionally fabricated on rigid substrates, restricting their application in novel environments such as in wearable technology.
In this dissertation, I explore the mechanisms behind dynamic tuning of plasmon resonances, as well as demonstrate all-optical tuning through multiple cycles by incorporating photochromic molecules into plasmonic nanopatch antennas. Exposure to ultraviolet (UV) light switches the molecules into a photoactive state enabling dynamic control with on/off ratios up to 9.2 dB and a tuning figure of merit up to 1.43, defined as the ratio between the spectral shift and the initial line width of the plasmonic resonance. Moreover, the physical mechanisms underlying the large spectral shifts are elucidated by studying over 40 individual nanoantennas with fundamental resonances from 550 to 720 nm revealing good agreement with finite-element simulations.
To fully explore the tuning capabilities, the molecules are incorporated into plasmonic metasurface absorbers based on the same geometry as the single nanoantennas. The increased interaction between film-coupled nanocubes and resonant dipoles in the photochromic molecules gives rise to strong coupling. The coupling strength can be quantified by the Rabi-splitting of the plasmon resonance at ~300 meV, well into the ultrastrong coupling regime.
Additionally, fluorescent emitters are incorporated into the tunable absorber platform to give dynamic control over their emission intensity. I use optical spectroscopy to investigate the capabilities of tunable plasmonic nanocavities coupled to dipolar photochromic molecules. By incorporating emission sources, active control over the peak photoluminescence (PL) wavelength and emission intensity is demonstrated with PL spectroscopy.
Beyond wavelength tuning of the plasmon resonance, design and characterization is performed towards the development of a pyroelectric photodetector that can be implemented on a flexible substrate, giving it the ability to be conformed to new shapes on demand. Photodetection in the NIR with responsivities up to 500 mV/W is demonstrated. A detailed plan is given for the next steps required to fully realize visible to short-wave infrared (SWIR) pyroelectric photodetection with a cost-effective, scalable fabrication process. This, in addition to real-time control over the plasmon resonance, opens new application spaces for photonic devices that integrate plasmonic nanoparticles and actively tunable materials.
Item Open Access Analytical Modeling of Waveguide-fed Metasurfaces for Microwave Imaging and Beamforming(2018) Pulido Mancera, Laura MariaA waveguide-fed metasurface consists of an array of metamaterial elements excited by a guided mode. When the metamaterial elements are excited, they in turn leak out a portion of the energy traveling through the waveguide to free space. As such, a waveguide-fed metasurface acts as an antenna. These antennas possess a planar form factor that offers tremendous dexterity in forming prescribed radiation patterns; a capability that has led to revolutionary advances in antenna engineering, microwave imaging, flat optics, among others.
Yet, the common approach to model and design such metasurfaces relies on effective surface properties, a methodology that is inspired by initial metamaterial designs. This methodology is only applicable to periodic arrangements of elements, and the assumption that the neighboring elements are identical. In the scenarios where the metasurface consists of an aperiodic array, or the neighboring elements are significantly different, or the coupling to the waveguide structure changes; the aforementioned approaches cannot predict the electromagnetic response of the waveguide-fed metasurface. In this thesis, I have implemented a robust technique to model waveguide-fed metasurfaces without any assumption on the metamaterial elements' geometry or arrangement. The only assumption is that the metamaterial elements can be modeled as effective dipoles, which is usually the case given the subwavelength size of metamaterial elements.
Throughout this document, the simulation tool will be referred to Dipole Model. In this framework, the total response of each dipole, representing a metamaterial element, depends on the mutual interaction between elements, as well as the perturbation of the guided mode. Both effects are taken into account and, by using full-wave simulations, I have confirmed the validity of the model and the ability to predict radiation patterns that can be used for beamforming as well as for microwave imaging.
Once the capabilities of the dipole model are compared with full wave simulations of both traditional antenna designs as well as more elaborated waveguide-fed metasurfaces, I develop an analysis on the use of these metasurfaces for microwave imaging systems. These systems are used to form images of buried objects, which is crucial in security screening and synthetic aperture radar (SAR). Traditionally, the hardware needed for many imaging techniques is cumbersome, including large arrays of antennas or bulky, moving parts. However, one attractive alternative to overcome these problems is to use dynamic metasurface antennas. By quickly varying the radiation patterns generated by these antennas, enough diverse measurements can be made in order to produce high quality images in a fraction of the time.
The compact size and speed come with a trade-off: a computationally intensive optical inverse problem has to be solved, which has so far prohibited these antennas from enjoying widespread use. I address this problem by reformulating the problem to make it similar to a SAR scenario, for which fast image reconstruction algorithms already exist. By adapting an algorithm known as the Range Migration to be compatible with these metasurfaces, I can cut down on real-time computation significantly. The computer simulations performed are highly promising for the field of microwave imaging, since it is demonstrated that diffraction-limited images can be acquired in a fraction of the time, in comparison with other imaging techniques.
Item Embargo Electromagnetic Metamaterials for Wave Manipulation(2024) Rozman, Natalie AnnThe overarching problem addressed in this dissertation is the restricted number of devices that operate in the millimeter wave, terahertz, and infrared regimes using conventional materials. Devices designed for operation at these wavelengths are incredibly valuable across various applications such as material characterization, imaging technologies, and communication systems. However, the scarcity of devices is attributed, in part, to the limited availability of naturally occurring materials that can operate in these ranges. Therefore, there is an exciting opportunity to tailor electromagnetic metamaterials for millimeter wave, terahertz, and infrared manipulation.
Electromagnetic metamaterials have been shown to enable unique scattering effects leading to advancements in next-generation devices. One important feature of metamaterials is the ability to tune the geometry and engineer the scattered response for nearly any range of the electromagnetic spectrum. Therefore, the exploration and development of advanced electromagnetic metamaterials for use in millimeter wave, terahertz, and infrared regimes is of great importance.
Chapter 1 provides a discussion on the importance of millimeter wave, terahertz, and infrared radiation. In addition, this chapter provides an introduction to electromagnetic metamaterials. Chapters 2 and 3 discuss two metamaterials designed for operation at millimeter wavelengths. In Chapter 2, a metamaterial coherent detector is presented and in Chapter 3, a metamaterial gradient index lens is introduced.
Several metamaterials for operation in the terahertz range are studied and discussed in Chapter 4, 5, and 6. In Chapter 4, exotic physics is studied and results in the excitation of high-quality factor modes. Chapter 5 introduces an electromagnetic absorber for radiometric calibration applications. Lastly, Chapter 6 presents a metamaterial strain sensor. A reflective and transmissive metamaterial diffuser is studied in Chapter 7 for use in the infrared regime. An in-depth discussion on the fabrication of all presented metamaterials is included in Chapter 8. Finally, a summary of all presented works and concluding thoughts is included in Chapter 9.