Electromagnetic waves carry information in multiple degrees of freedom, such as amplitude, phase, polarization, coherence, etc. When light encounters physical matter, its properties generally fluctuate in the spatial or the temporal domain. If the structure of matter is complex, these fluctuations may appear random at first glance. However, information about the light-matter interaction can still be recovered from such noise-like signals under certain conditions. Optical sensing or imaging tasks of different approaches can be taken depending on the specific physical problem. In this dissertation, we provide original solutions to several sensing problems based on measurements of intensity fluctuations. First, we will discuss how temporal intensity fluctuations can be used to infer the structural evolution of dynamic scattering media. Then, we will introduce a new and efficient experimental approach for retrieving this dynamic information from complex media in a geometry-independent manner and across a broad range of scattering regimes. In addition, using the process of protein polymerization/depolymerization as an example, we will demonstrate how temporal fluctuations of scattered light can be used to quantify the dynamics of a thermal hysteresis process. The second part of the thesis will discuss the characteristics of intensity fluctuations in both spatial and temporal domains. We will theoretically propose and experimentally demonstrate the statistical nonstationarity of intensity fluctuations in strong scattering media where the mechanisms of recurrent scattering and the near field coupling compete. Furthermore, we will present an experimental procedure for simultaneously assessing the mechanical and optical properties of complex media experiencing structural phase transitions.

Advances in integrated photonic devices require low loss, easy-to-integrate solutions for chip-to-chip and chip-to-fiber interfacing. Among the most common solutions are traditional lenses. However, circular lenses require additional mounting mechanisms to ensure proper alignment. Additionally, the beam routing functionality cannot be added to the traditional lenses unless they are combined with mirrors and operate in the reflection mode. In this dissertation, we investigate lens-embedded photonic crystals (LEPCs) as a solution to flat and multifunctional lenses. The concept is demonstrated by creating self-collimating lattices containing a gradient refractive index lens (GRIN-LEPC), a binary-shaped lens (B-LEPC), and a Fresnel-type binary-shaped lens (F-B-LEPC). The devices are fabricated in a photopolymer by multi-photon lithography with the lattice spacing chosen for operation around the telecom wavelength of 1550 nm. Both the experimentally observed optical behaviors and simulations show that the device behaves like a thin lens, even though the device is considerably thick. The thickness of a B-LEPC was reduced threefold by wrapping phase in the style of a Fresnel lens. Embedding a faster-varying phase profile enables tighter focusing, and NA = 0.59 was demonstrated experimentally. Furthermore, we demonstrate experimentally that a Fresnel lens can also be combined into a bender, so one PC performs both bending and focusing functions, further reducing the footprint of the PC devices. We also explored a hexagonal lattice and demonstrated wide-angle and broad-band self-collimation. The PCs are fabricated using the same material and method as that of the LEPCs. Optical characterization shows that the device strongly self-collimates light at near-infrared wavelengths that span from 1360 nm to 1610 nm. Self-collimation forces light to flow along the extrusion-direction of the lattice without diffractive spreading, even when light couples into the device at high oblique angles. Numerical simulations corroborate the experimental findings.

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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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No description available.

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.

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.