Thesis (Ph.D.) - University of St Andrews, September 2008.

Thesis (Ph. D.)--Ohio State University, 2008.

Thesis (Ph.D.)--City University of Hong Kong, 2009. / "Submitted to Department of Mathematics in partial fulfillment of the requirements for the degree of Doctor of Philosophy." Includes bibliographical references (leaves [85]-91)

Thesis (M.S.)--West Virginia University, 2010. / Title from document title page. Document formatted into pages; contains viii, 95 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 89-95).

Defect control in colloidal crystals is essential for these nanostructures to be effective as photonic bandgap (PBG) materials. We have used in-situ monitoring of the PBG of a colloidal crystal to study the structural changes during colloidal self assembly, with a focus on the formation of macroscopic defects such as cracks. These findings allow us to model the final stages of colloidal self assembly and explain the formation of growth defects in colloidal crystal. Our model suggests that cracks are intrinsic to self assembly growth methods. . However, by tuning the interaction potential between the colloids, it is possible to minimize the cracks in colloidal crystals. / Singapore-MIT Alliance (SMA)

This thesis describes work to create a variety of thin-film photonic devices based upon liquid crystalline materials. Firstly, a variety of liquid crystal phases are polymer- templated by combining a liquid crystalline material with photo-polymerisable reactive mesogens. Upon photo-curing, a polymer scaffold, which is a template of the original phase, is formed with liquid crystal molecules in interstitial sites. This liquid crystal is removed to yield a polymer scaffold which can be used to template the original phase. Here, polymer-templating is used to template the smectic A liquid crystal alignment onto nematic liquid crystals for the first time; this results in materials with improved contrast ratios and faster response times than conventional nematic devices. Next, a study is performed to compare the electro-optic properties of polymer-templated and polymer-stabilised chiral nematic liquid crystals. The enhanced tuning range of polymer-templated liquid crystals is applied to create a polymer-templated liquid crystal laser and to electrically tune its emission wavelength. Subsequently, thin-film elastomeric liquid crystal lasers are created. The lasing wavelength of these films can be reversibly and selectively tuned without hysteresis by subjecting them to a mechanical stress. Finally, work is performed to study the potential of inorganic materials for use in liquid crystal lasers. Transition metal clustomesogens (liquid crystalline materials that contain highly emissive molybdenum clusters) and inorganic-organic perovskites are considered here. The dispersal and emissive properties of clustomesogens in liquid crystals are studied, and they are used to create circularly polarised light sources with a polarisation that can be controlled using electric fields. Layered structures of inorganic- organic perovskite and liquid crystal are created; these exhibit enhanced amplified spontaneous emission. Then, perovskites are used as the gain materials in distributed feedback lasers for the first time. These lasers may be wavelength-tuned by varying the grating spacing of the structure.

The fields of telecommunications and optoelectronics are under constant pressure to shrink devices and reduce power consumption. Micro-scale photonic and plasmonic structures can trap light and enhance the brightness of active emitters; thus, these types of structures are promising avenues to accomplishing the goals of miniaturization and efficiency. A deeper understanding of specific structures is important in order to gauge their suitability for specific applications. In this dissertation, two types of microstructures are explored: one-dimensional silicon photonic crystals and self-assembled indium islands. This dissertation will provide novel characterization of these structures and a description of how to utilize or compensate for the observed features. A photonic crystal can act as a tiny resonator for certain wavelengths, making it a promising structure for applications that require extremely small lasers. However, because of silicon’s indirect bandgap, a silicon photonic crystal cavity would require the addition of an active emitter to function as a light source. Attempts to incorporate erbium into these cavities, and the observation of an unusual coupling phenomenon, will be discussed. Self-assembled indium islands are plasmonic structures that can be grown via molecular beam epitaxy. In theory, these islands should be pure indium nanoantennas on top of a smooth gallium arsenide substrate. In practice, the component materials are less segregated than predicted, giving rise to unexpected hollow dome shapes and a sub-surface indium layer. Although these features were not an intended result of indium island growth, they provide information regarding the island formation process and potentially contribute additional applications.

The aim of the research presented in this thesis is to develop photonic integrated circuit (PIC) for Radio-over-Fiber (RoF) application. As such, at the beginning of the thesis, a dual-function photonic integrated circuit for microwave photonic applications is proposed. The photonic circuit is arranged to have two separate output ports, and depending upon the RF input signal strength, it provides either tunable millimeter wave carriers by frequency octo-tupling of the RF signal or frequency up-conversion of a microwave signal from the electrical to the optical domain. The circuit exploits the intrinsic relative phases between the ports of multi-mode interference couplers (MMI) to provide all the static optical phases needed, hence drift free. In the middle of thesis, a generalized architecture having N parallel phase modulators driven electrically with a progressive 2π⁄N phase shift is analyzed. The proposed design is justified by computer simulation for N=8 architecture with properly determined optical phase shifts to generate frequency multiplication of an electrical signal. The front- and back-end of the circuit comprises 4×4 MMI couplers enclosing an array of four pairs of phase modulators and 2×2 MMI couplers. The proposed design for frequency multiplication requires no optical or electrical filters; the operation is not limited to carefully adjusted modulation indexes. Later on, a generalized approach for achieving frequency multiplication using two cascades MZM is presented. The proposed design consists of a Mach-Zehnder interferometer with each arm containing a pair of Mach-Zehnder modulators (MZM) in series as a means of optoelectronic frequency multiplication (octo-tupling and quattourviginti-tupling). The circuit requires no electrical or optical filters. There is no requirement to carefully adjust the modulation index to achieve correct operation of the octo-tupler. A comparison is made with an alternative functionally equivalent single-stage parallel MZM circuit discussed herein the thesis. Finally, the thesis describes the generation of the same magnitude but opposite sign high order single optical side band from its output ports by using a RF source. A single stage parallel Mach-Zehnder Modulator (MZM) and a two-stage series parallel MZM architecture is described and their relative merits and demerits discussed. As an illustration of a prospective application it is shown how the circuit may be used to transport radio signals over fibre for wireless access; generating remotely a mm-wave carrier modulated by digital IQ data. A detail calculation of symbol error rate is presented to characterise the system performance. A mathematical analysis is provided to describe the principle of operation for all the proposed design and validated by commercially available industrial standard simulation tool.

In recent years much effort has been carried out to make integrated photonics a widespread technology to be exploited in current optical communication industry. It is hoped by substituting microelectronics by photonic chips and keeping the light carried by optical fibers in light domain for further processing, the cost and speed of communications will be vastly improved. Although this transition is challenging in various aspects, here in this thesis some of these issues are discussed and addressed. In this thesis firstly the limitations of current simulation tools for analysis of wide range of photonic devices is pointed out. Structures based on photonic crystals are taken into consideration at this point which because of finely detailed structures have shown to be challenging to be analyzed by conventional tools. In this regard three different common structures based on photonic crystals in both resonant and non-resonant regimes have been considered: lamellar gratings, metamaterials for Lüneburg lens and Bragg gratings in a LC-DFB laser. For each structure, an analytical method or homogenization approach is proposed which is claimed to be faster for analysis of such components than numerical methods. Comparisons of the results with conventional numerical methods prove accuracies of each approach. Furthermore, fiber-to-chip coupling and polarization management are discussed as other important issues in the field of integrated photonics. Concerning polarization management, stepped waveguide approach will be introduced as the most promising approach for SOI and III-V substrates and designs based on this structure reported in literature are reproduced and inaccuracies are pointed out and corrected accordingly. Also regarding fiber-to-chip coupling, a critical appraisal of the most recent proposed structures for edge coupling will be offered and the results will be reproduced by simulation tools. At the end, based on detailed comparisons, the most encouraging approach with low insertion loss and easy fabrication steps is introduced and novel platform for easy butt coupling single mode fibers to the coupler structure is proposed.

Creating compact, efficient, highly-controllable optical systems has been one of the central goals of optics and photonics research. Integrated photonics provides a powerful platform for manipulating light efficiently and flexibly by guiding light in waveguide circuits on chip. Among the rich family of integrated photonic devices, integrated optical modulators and wavefront generators are two types of components for a great many applications such as optical communications, VR/AR displays, and LiDAR. Current approaches to creating these two types of devices – integrated optical modulators based on waveguides, active wavefront transceivers based on phased arrays, and passive wavefront transceivers based on grating couplers or integrated metasurfaces – suffer from large footprint, high power consumption, low beam quality, and limited controllability. It is desirable to improve the performance of such devices by exploring new device physics and architectures.In this thesis, we propose and investigate several novel approaches for efficient optical modulation and wavefront manipulation using integrated photonics. First, we show that efficient optical phase modulation can be achieved using a micro-resonator operating in the strongly over-coupled regime. Theoretical analysis, simulations, and experimental demonstrations of thermally tuned silicon nitride adiabatic micro-ring resonators operating at the visible and near-infrared wavelengths are conducted. Compared with traditional waveguide-based devices, our resonator-based phase modulators operating at the visible wavelengths showed order-of-magnitude reductions in both device footprint and power consumption. Through a statistical study of the device performance, our adiabatic micro-ring device architecture also showed significantly improved robustness against fabrication variations when compared with the regular micro-ring architecture. Second, we invent a new category of integrated wavefront-shaping devices – leaky-wave metasurfaces – that possess the simple form factor of a grating coupler and the capability of complete wavefront manipulation over all the four optical degrees of freedom: amplitude, phase, polarization ellipticity, and polarization orientation. The working principle of the leaky-wave metasurfaces is based on symmetry-broken photonic crystal slabs supporting quasi-bound states in the continuum (q-BICs). We extended the mechanism of q-BICs excited by free-space planewaves into q-BICs excited by guided waves, and developed a semi-analytical model describing the mapping between the four structural parameters and the four optical parameters of a meta-unit. We experimentally demonstrated multiple leaky-wave metasurface devices that convert light confined in an optical waveguide to an arbitrary optical pattern in free space, realizing custom polarization control, phase-amplitude control, and complete wavefront control, and validating the theory and capability of this platform. Lastly, we explore strategies to optimize the beam quality and efficiency of integrated optical phased arrays. We show that a two-dimensional disordered hyperuniform array layout is promising for generating a radiation pattern with high directionality with performance surpassing uniform arrays, constrained random arrays, and non-redundant arrays. We experimentally demonstrated a passive 32-channel phased array operating at the blue wavelength that showed a high percentage of power in the main beam and suppressed side lobes. We further propose and discuss the use of efficient, resonator-based modulators in phased arrays to improve the system compactness, power efficiency, and scalability. The approaches we investigated in this thesis provide a concrete set of solutions for interfacing free-space optics and integrated photonics. These two platforms have traditionally been studied by investigators from different subfields of optics and have led to commercial products addressing different needs. Our work suggests new ways to create “hybrid” systems consisting of partly integrated photonics and partly free-space optics and utilize the best of both worlds to address many emerging applications such as quantum optics, optogenetics, sensor networks, inter-chip communications, and holographic displays.