Liquid crystals (LCs) are self-assembled soft materials composed of certain anisotropic molecules with orientational orders. Their widespread applications include information displays and photonic devices, such as spatial light modulators for laser beam steering and tunable-focus lens, where achieving desired LC alignment is pivotal. In general, LC alignment is influenced by several factors, including chemical bonding, dipolar interactions, van der Waals interactions, surface topographies, and steric factors. Here, we focus on three alignment techniques for aligning rod-like LC molecules and highlights the photonic devices enabled by these techniques: 1) Two-photon polymerization direct-laser writing-induced alignment, 2) Weigert effect-based reversible photoalignment, and 3) electric field-assisted alignment in polymer-dispersed liquid crystal (PDLC) systems. With the help of advanced two-photon polymerization systems, nano-grooves with arbitrary orientations can be easily created on a variety of surfaces. The geometric topography helps align the LC molecules parallel to the groove direction. Alignment on a planar surface, on a curvilinear surface, and even in the bulk can be realized. Based on the patterning ability, three photonic devices are highlighted: a switchable geometric phase microlens array, a tunable compound microlens array, and a polarization-independent phase modulator. For Weigert effect-based reversible photoalignment, how to achieve space-variant linear polarization field is crucial. Here, two approaches are investigated: the direct projection method and the counter-propagating wave interference exposure method. Using the direct projection method, an LC Dammann grating with pixelized binary phase profile is achieved. Such a method relies on a spatial light modulator and is convenient for creating pixelized alignment that has abrupt changes from pixel to pixel. On the other hand, the interference exposure method can generate continuously and smoothly changing LC alignment. By such a method, two miniature high-quality microlens arrays are fabricated and further assembled into a planar telescope. Further characterizations reveal the high optical quality of the fabricated devices, which not only ensures their adoption in practical applications, but proves the powerful planar alignment patterning capability of the photoalignment materials. For a traditional PDLC system, the LC alignment is random from droplet to droplet, and the operation voltage of the active PDLC is too high to be employed in practical applications. Here, we establish a method to perfectly align LC droplets in a PDLC system and use it as a passive film. The well-aligned passive PDLCs exhibit polarization- and angle-dependent light scattering that can be engineered through composition tuning. Two kinds of selective scattering films are demonstrated: The first kind scatters obliquely incident light but is highly transparent for normally incident light, and the second kind scatters normally incident light but is more transparent for obliquely incident light.

Propagation of electromagnetic radiation through atmospheric turbulence has been a subject of study for over eight decades. With ever expanding applications of lasers, more attention has been paid recently to the interaction between atmospheric turbulence and laser beams propagating over greater and greater distances. For applications in communication, directed energy weapons and wireless power transmission the focused laser beam geometry is of particular interest. To increase understanding of the interaction between atmospheric turbulence and propagating laser beams a series of field campaigns were designed and conducted. These field campaigns provided a focused beam configuration propagated over different ranges and at different intensities of atmospheric turbulence. Collimated laser data was also collected to corroborate the findings. These field campaigns generated temperature spectral data that did not agree with existing temperature spectral models near the ground. Given the relationship between temperature spectral models and refractive index, a previously unexplored refractive index spectral model is examined. The unexplored refractive index spectral model provides a better fit to experimental temperature spectral data. Existing second order weak and strong fluctuation theory is modified to accommodate a novel refractive index spectral model. The results from the modified second order weak and strong fluctuation theory are compared to field campaign laser data and to split step wave optics simulations.

Many of the features of photonic devices, including some of the most ubiquitous components such as resonators and waveguides, are usually thought to be intrinsically dependent on their geometry and constitutive materials. As such, the behaviour of an optical field interacting with such devices is dictated by the boundary conditions imposed upon the field. For instance, the resonant wavelengths and linewidths of a planar cavity are expected to be set by the mirrors' reflectivity, cavity length, and refractive index. Henceforth, satisfying a longitudinal phase-matching condition allows for incident light to resonate with the cavity. As another example, consider the planar waveguide; the field is confined along one transverse dimension, but diffracts along the other unbounded dimension. We have recently introduced several strategies for challenging these long-held intuitions that may be collected under the moniker 'space-time (ST) photonics', whereby the response of a photonic device is tailored post-fabrication in useful ways by sculpting the spatio-temporal structure of the incident optical field. In fact, introducing a prescribed relationship between the spatial frequencies and the temporal frequencies can help overcome the constraints imposed by the boundary conditions. We refer to such pulsed beam configurations as ST wave packets. In one scenario, introducing carefully designed angular dispersion into a pulsed field allows the realization of omni-resonance: the pulse traverses the cavity without spectral filtering even if the pulse bandwidth is larger than the cavity resonant linewidth after the entire bandwidth resonates with it. A similar strategy enables a new class of planar waveguide modes we refer to as 'hybrid guided ST modes' where the field is confined along the unbounded dimension through ST coupling. Crucially, the spatio-temporal structure introduced into the field along the unbounded dimension enables overturning the impact of the boundary conditions along the other dimension. For example, the modal size, index, and dispersion can all be engineered independently of the thickness and refractive index of the planar waveguide; i.e., the impact of the boundary conditions is overturned.

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In this work, we employ machine learning, signal identification, and signal classification to infer network processes governing packet transmission in dense, non-cooperative wireless networks. We exploit signal features in radio frequency (RF) transmissions to generate fingerprints that can enable the characterization of transmission events in a non-cooperative cognitive radio network or in a cognitive adaptive electronic attack scenario. In these situations, we have anticipated a need to depend heavily on identifying RF features that correspond to the way in which devices access spectrum channels and to the interactions of transmitted signals with the devices' surroundings. We develop improved signal processing for detection, estimation, and RF fingerprinting of wireless communications, and employ machine learning techniques for interpretation and classification of complex signals. We then use high-performance computing to create models and simulations of RF interactions with the environment to augment our study of the effects of scatterers in urban environments on the operations of communications networks due to mobility, multipath, absorption, and diffraction.

Ultrafast laser ablation has enabled high-precision processing of a wide range of materials including metals, semiconductors, dielectrics, and polymers. Several laser nanostructuring methods exist, including those based on optical near-fields, special material properties, surface plasmons, and multiphoton absorption (MPA). Among these methods, the MPA method has the potential for nanoscale direct laser writing by using a simple experimental setup. However, the understanding of the fundamental mechanism involved in the laser ablation process is still incomplete, and it remains challenging to obtain a feature size much smaller than the diffraction-limited spot size. The goal of this research is to understand how ultrafast laser pulses interact with different types of materials (including metal, dielectrics, and semiconductors), and to look for a repeatable method to achieve feature size deep below the diffraction limit. To this end, this dissertation describes novel approaches that enable the localization of laser energy at micron to nanoscale utilizing laser-matter interaction under unusual exposure conditions. Spatiotemporally separated double pulses are used to control the free-electron dynamics during laser ablation and self-trapped-excitons are found to be significant for localizing the spatial distribution of electron density when the delay between pulses is much longer than the free-electron lifetime. Moreover, the same relationship between laser-induced feature size and laser energy is found in metal, semiconductors, and dielectrics. The minimum controllable feature size is the smallest for the material with a larger bandgap, and this feature size highly depends on how well the laser energy is controlled. To produce nano/sub-micron feature size, an "ultrashort pulse burst" is used to achieve energy localization by tuning the delay between pulses. An experimental setup consisting of a fourfold Michelson interferometric system is built and characterized. Laser ablation experiments on fused silica are conducted and nano/sub-micron pits on the surface of fused silica samples are produced. The formation of these structures is attributed to the surface defects that provide energy levels within the bandgap through which selective excitation is achieved by choosing an excitation pathway that favors a long carrier lifetime. This result provides another means for tailoring surface structures at the sub-micron and nanoscale.

This thesis delves into innovative active photonic integrated devices and circuits on the thin-film lithium niobate (TFLN) platform, focusing on their applications and potential future advancements. We introduce a new family of electrooptic modulators (EOMs), the Four-Phase Electrooptic Modulators (FEOMs), which are fabricated using the TFLN platform. These devices effectively mitigate bandwidth and dynamic-range constraints in optical communication systems by reducing dispersion penalties and common-mode noises. Their functionality is demonstrated in a photonic time-stretch system. A dual-polarization variant further exemplifies the mitigation of both dispersion penalties and common-mode noises in long-haul communication links, marking significant strides towards the practical implementation of coherent optical communication. We also engineer dual-channel, tunable ultra-narrow linewidth filters using phase-shifted Bragg grating structures on the TFLN platform. These filters act as key components for optical communication, sensing systems, and emerging quantum photonic applications. The device boasts a high extinction ratio, closely spaced channels with narrow linewidths, and efficient central wavelength tuning via the electrooptic effect. This makes it beneficial for finely adjusting high-precision photonic integrated circuits (PICs). The experimental results align well with the design and simulations, indicating promising potential for integration into advanced PICs for future quantum photonic applications and the development of multiple-channel ultra-narrowband filters with active tuning capabilities. Additionally, the thesis includes the design and simulation of a fully packaged TFLN EOM, catering to the rising demand for high-performance optical modulators in telecom and RF photonics applications. Lastly, we delve into a pioneering micro-electromechanical systems (MEMS) photonic switch that uses TFLN, harnessing LN's piezoelectric properties and outstanding operational bandwidth. These features have the potential to propel significant advancements in optical communication systems and other fields that necessitate precise light signal control.

The potential of high-energy sources within the mid-infrared region (3-8 μm) has garnered significant attention for diverse research and industrial applications. Millijoule pulses extending beyond 3 μm can facilitate the production of x-rays with photon energies in the keV range through high harmonic generation (HHG). These high-energy x-ray pulses enable the characterization of electron dynamics within molecules and condensed matter materials. Additionally, the atmospheric transmission window between 3-5 μm allows lasers within this spectral range to deliver energy efficiently to distant targets via optical filaments without divergence, highlighting promising prospects for defense applications. In contrast to laser amplifiers, which are restricted to several wavelengths within the mid-infrared spectral region, nonlinear optical effects allows the generation of pulses of similar energy but with a more adjustable spectrum. This flexibility makes amplification schemes, such as the Optical Parametric Chirped Pulse Amplifier (OPCPA), especially fitting for mid-Infrared applications. However, the efficiency of most existing systems operating above 3 μm is comparatively low due to the reliance on a near-infrared pump (0.75-1.4 μm). This thesis describes a novel tabletop OPCPA system, using ZnGeP2 pumped by a Ho:YLF chirped pulse amplifier (CPA) operating at 2 µm and seeded by intra-pulse difference frequency generation. The output energy and beam quality from the Ho:YLF laser are optimized by advancing the cooling system to reach lower operational temperatures, ensuring the quality of the OPCPA. Through the optimization of the ZGP's phase-matching bandwidth via a non-collinear configuration, and the enhancement of conversion efficiency with the aid of the top-hat pump, the OPCPA system can deliver 4-mJ, 50-fs pulses at a 1 kHz frequency. This system attains an unprecedented overall efficiency of 15% at this wavelength. The detection of harmonics up to the seventh order upon focusing the output in air substantiates the system's competence in conducting strong field experiments.

<p>New types of laser using the basic structure of FP cavity are designed and modeled, to achieve high SMSR single-mode lasing that can be immune to high level of optical feedbacks for optical network communication applications.</p> <p>The work includes design of asymmetric Bragg reflection waveguide laser that employs wavelength selective Bragg reflectors as the claddings to confine and filter desired FP longitudinal modes for amplification and lasing. Si-rich SiO₂ single-mode laser based on this structure is also proposed and analyzed.</p> <p>To optimize a recent design of discrete mode laser that is re-growth free and feedback-perturbation insensitive, a comprehensive implementation of the time domain transfer matrix method, including temperature and feedback effects, is carried out. The model helps to obtain a optimized DM structure that is balanced between high SMSR and low feedback sensitivity.</p> / Doctor of Science (PhD)