Owing to its centrosymmetric structure, the critical optical component of a silica fiber cannot to possess a second-order nonlinear optical susceptibility, Χ(²), preventing a silica fiber from many potential applications. Here, we theoretically and experimentally demonstrate a new technique to generate large and thermodynamically stable second-order nonlinearity into silica optical tapered fibers without breaking the centrosymmetry of the silica glass. The nonlinearity is introduced by surface layers with high polar-ordering fabricated by a novel hybrid covalent/ionic self-assembly multilayer technique. Despite the overall rotational symmetry of the nonlinear fiber, we observe significant second harmonic generation with ~ 400–500 fold enhancement of the SHG power compared to the traditional tapers. Phase matching for a SHG process in second-order nonlinear tapered fibers is also realized by the compensation of waveguide modal dispersion with material chromatic dispersion, which occurs only for submicron tapers where the modal dispersion is large. In addition, quasi-phase-matching for a nonlinear taper can be accomplished by introducing a periodic pattern into the nonlinear film coating. We use UV laser ablation for the controlled removal of particular nonlinear film segments on a taper surface in order to produce a Χ(²) grating structure. A resulting SHG enhancement from quasi-phase-matching is observed over a broadband spectrum of the pump light mainly due to the non-uniform shape of a taper waveguide. The laser ablation is a clean and fast technique able to produce well-define patterns of polymer films on either flat or curved substrate geometry. With surface layers containing reactive functional groups e.g. primary amines, we demonstrate that the resulting patterned film obtained from the laser ablation can be used as a template for further self-assembly of nanoparticles with high selectivity. A pattern feature size down to ~ 2μm or smaller can be fabricated using this approach. We also discuss preliminary results on a novel technique to further improve spatial accuracy for selective self-assembly of nanoparticles at an unprecedented level. Different types of nanoparticles are joined in order to form well-defined, molecular-like superstructures with nanoscale accuracy and precision. The technique is based on a selective surface functionalization of photosensitive molecules coated on metallic nanoparticles utilizing enhanced two-photon photocleavage at the plasmonically-active sites (hot spots) of the nanoparticles in resonance with an applied electromagnetic wave. As a result, the surface functional groups at the nanoparticle hot spots are different from the the other areas, allowing other kinds of nanoparticles to self-assemble at the hot spots with high degree of selectivity. / Ph. D.

<p dir="ltr">This work presents the development and application of novel laser absorption spectroscopy sensors that were deployed to make high-rate (1-15 kHz) measurements of temperature, CO, NO, CO₂, and air-fuel ratio in internal combustion engine (ICE) systems. These sensors provided measurements with unprecedented time resolution in ICE exhaust that allowed for individual cylinder firing events to be detected which will greatly improve understanding of ICE systems and allow for emissions reduction strategies to be tested. </p>

The nonlinear beam deflection (BD) technique is used to directly measure and time-resolve the nonlinearly-induced phase shift in a variety of materials. In this technique, a weak probe beam is spatially overlapped, while slightly displaced, with a strong excitation beam while the temporal delay is scanned. The excitation-induced index gradient, which for 3rd-order nonlinearities is proportional to the nonlinear refractive index 16 n2"> of the medium, deflects the weak probe beam. This deflection is determined using a position sensitive segmented detector after propagation to the far field. In this dissertation, we expand our previous work on BD theory to include the effects of the Gaussian spatial beam profile of the excitation, as opposed to a constant index gradient. We also explore the BD signal as we allow the spatial size and relative position of the probe with respect to the excitation beam, r, at the sample to vary to maximize the calculated signal. While the analysis requires numerical solutions, we find a simple empirical fitting function for the BD signal that allows determination of the nonlinear phase shift and thus the nonlinear refraction. We performed BD experiments at near-degenerate photon energies for various spot size ratios which resulted in very good agreement with our simulation results. In order to examine our empirical function the BD signal for various r (0.2-0.6) is measured while keeping the phase shift relatively constant. This helped us isolate the effect of spot size ratio on the BD signal. Our results showed the correct trend for the growth of BD signal as r increases, which is what is expected from our model. We also studied nondegenerate two-photon absorption (ND-2PA) in bulk silicon. We present the results of spectroscopic pump-probe measurements of ND-2PA in silicon across the indirect-gap (1.12 eV). We observed enhancement of the 2PA coefficient as the degree of degeneracy of pump and probe photon energies increased, and the dispersion compares favorably with our recently-developed semi-empirical theoretical model for the dispersion of indirect ND-2PA in silicon. Additionally, we experimentally investigated WTe2 which is a Weyl semimetal. Here, we prepared very thin flakes (10s of microns thick) of WTe2 and investigated the possibility of observing circular dichroism (CD) in pump-probe measurements, pumping at near IR and probing at mid-IR. Although we did not observe any CD, we believe this is because our pump photon energy is far from Weyl nodes and that we need to pump at mid-IR range.

With a growing demand in industry for cost effective, increased data handling capabilities great attention has been paid to the study of various polymer systems for use in optical telecommunications. Inorganic crystals, currently used in such systems, have high performance, but are more expensive and less obtainable than organic materials. Recent advances in techniques for developing highly efficient and inexpensive organic polymeric electro-optic (EO) devices compatible with current state-of-the-art electronics have created an interest in the commercialization of such electro-optic devices. In light of the many advantages of utilizing organic materials for electro-optic applications, numerous methods have been developed to produce nonlinear optically (NLO)-active polymeric films for such purposes. Ionic self-assembled multilayer (ISAM) films are a recently developed class of materials that allows detailed structural and thickness control at the molecular level, combined with ease of manufacturing and low cost. However, the layer-by-layer deposition technique utilized for this method currently requires lengthy processing times that challenge the feasibility of fabricating a thick film suitable for EO modulator device fabrication. This study focuses on addressing the influence of several pertinent processing variables affecting these challenges for application to electro-optic device fabrication. This study investigated (1) the effect of forced convection, varying deposition time and varying dye concentration on the properties of PAH/Procion Brown films fabricated via the hybrid reactive deposition scheme, (2) the automation and optimization of the fabrication of thick NLO active films and (3) the use of the hybrid covalent-electrostatic deposition scheme to fabricate a polymeric waveguide device with an electro-optic coefficient comparable to that of lithium niobate (LiNbO₃). At fixed deposition time and concentration conditions, the presence of convection had little demonstrated effect on films with deposition times shorter than 2 minutes. For the 5 minute case, the presence of convection correlated with a ~45% increase in Ï (2)zzz values values and a 25% increase in absorbance per bilayer. At a constant dye concentration of 5 mg/ml, the deposition time had little effect on SHG for deposition times less than two minutes. In the presence of convection, the increase in deposition time from 2 minutes to 5 minutes showed a 57% increase in Ï (2)zzz values and a 30% increase in absorbance per bilayer. For a deposition time of 2 minutes in the presence of convection, the dye solution concentration was successfully reduced 5-fold (from 5 mg/ml to 1 mg/ml) with less than a 5% difference in Ï (2)zzz values, less than a 15% decrease in absorbance per bilayer and no detriment to film quality. These results strongly indicate that the deposition conditions remain well outside of the transport-limited regime at a dye concentration of 1 mg/ml. Rather, the surface reaction rate apparently is controlling. Depositing slides at an elevated temperature (~35°C), had an undetermined effect on Ï (2)zzz values, but showed a 15% increase in absorbance per bilayer. An automatic dipper was programmed to replicate the current manual deposition method to fabricate a film suitable for EO modulator devices. Utilizing the optimal conditions for the processing variables, an optically-homogeneous, 100 nm-thick film was fabricated utilizing the automated process, yielding a Ï (2)zzz values~ 23 x 10⁻⁹ esu. A three-layer coplanar electro-optic device was fabricated utilizing the hybrid reactive deposition method. For this device, the presence of added salt was found to increase the electro-optic coefficient r33 by a factor of 3 compared to its value when made with no added salt. The electro-optic coefficient of the added salt case was found to be about 1/2 that of lithium niobate (LiNbO3). / Master of Science

This thesis explored fundamental concepts of linear optics focusing on the modal interaction within waveguide/microresonator systems. In addition, it investigated a nonlinear process of stimulated degenerate four-wave mixing in a channel waveguide exploiting the analogy between photons and cold boson atoms. The backscattering phenomenon due to the surface wall roughness of a microresonator is addressed by adding to the usual conservative (Hermitian) coupling coefficient, a dissipative (non-Hermitian) term. This allows explaining the experimental measurements of a multimodal microresonator, which exhibits an asymmetrical resonance splitting characterized by a difference in the peak depths of the transmission spectra. It is shown theoretically, numerically and experimentally that the stochastic nature of the roughness along with the inter-modal dissipative coupling could give rise to a different exchange of energy between the co-propagating and the counter-propagating mode. The unbalanced exchange of energy between the two modes with opposite angular momenta can generate a different reflection by swapping the injection of the light between the input and the output ports. This effect lies at the heart of the realization of an unidirectional reflection device and it finds an explanation in the physics of the exceptional points. The realization of an optical setup based on a Mach-Zehnder interferometer, which exploits some particular techniques of data acquisition, allows obtaining a full knowledge of the complex electric field of a propagating mode. In this way, the spectrum of a wedge microresonator vertically coupled to a bus waveguide is explained using analysis methods based on parametric phasors and inverse complex representations. In addition, the energy exchange between the co-propagating and counter-propagating modes is studied from a temporal point of view by extrapolating a simple model based on the Green function. In particular, it is discussed the analytical temporal response of a microring resonator excited through a bus waveguide by an optical rectangular pulse. Here, it is shown theoretically and experimentally, how the temporal response leads to the characterization of the coupling regime simply from the knowledge of the electric field intensity. In this thesis, the isomorphism between the Schroedinger’s equation and the Helmholtz wave equation is analyzed in the nonlinear case. Considering a bulk nonlinear medium of the Kerr type, the complex amplitude of the optical field is a slowly varying function of space and time, which satisfies a nonlinear Schroedinger equation. The well-known nonlinear optical phenomenon of stimulated degenerate four wave mixing is reformulated in the language of the Bogoliubov theory. This parallelism between photons and cold atoms allows showing that the phase of the signal assumes a peculiar sound-like dispersion under proper assumptions.

2025 / The development of fiber amplifiers plays a critical role in a wide range of applications, including high-energy systems, weak signal sensing and imaging, and optical communications, where Erbium-doped fiber amplifiers (EDFAs) are commonly utilized. In particular, the increasing demand for amplifiers capable of supporting a high number of data channels is essential to avoid the looming “capacity crunch” in information networks. However, any significant expansion in capacity will inevitably drive a substantial rise in energy consumption. Consequently, the integration of additional data channels in telecommunications must be approached with energy efficiency in mind. Spatial division multiplexing (SDM) has emerged as a promising solution, leveraging spatial dimensions such as modes or fiber cores to enable data parallelism, and is becoming the preferred technology for reducing energy usage in optical networks. This thesis examines the amplification dynamics and properties of multimode (MMF) ring-core fibers (RCFs) that can stably support spatial modes carrying orbital angular momentum (OAM), which can serve as independent, uncoupled signal channels. Notably, RCFs featuring topologically confined modes (TCMs) have demonstrated the highest uncoupled mode capacity among MMFs to date. We explore how these fibers can be turned into amplifiers by utilizing χ^(3) material nonlinearities and by developing doped MMF-EDFAs.In this work, we investigate the nonlinear effects of OAM modes in multimode fibers, with a particular focus on acousto-optic interactions between these modes and phonons, leading to the generation of Stimulated Brillouin Scattering (SBS). Traditionally, SBS in single-mode fiber amplifies a narrowband Stokes signal using a powerful pump, operating through a self-phase matching process. However, by utilizing OAM modes, we exploit their distinctive phase characteristics to exert greater control over this interaction. This leads to the introduction of a novel OAM conservation law, which governs the modulation of inter-modal interactions between the pump, phonons, and Stokes, allowing for adjustable nonlinear gain. Furthermore, the chiral properties of OAM modes enable the launch of superposition-state pumps in RCFs, resulting in polarization rotation, a phenomenon known as optical activity. This optical activity, characterized as a stable birefringent interference effect due to its geometrodynamic nature, creates a special phase-matching and polarization-selective condition. This condition allows for complete spatial phase conjugation of the pump state, as the Stokes signal must retrace the pump’s polarization rotation to achieve significant gain. This mechanism also provides control over Stokes growth and the gain threshold condition. Overall, our analysis demonstrates that OAM modes offer up a versatile degree of freedom for controlling amplification through fiber nonlinearities. RCFs and OAM modes present significant potential for developing high-data-capacity SDM-EDFAs, offering key advantages for stimulated emission-based amplification. The strong confinement of these modes within a doped fiber core enhances their interaction with erbium ions, facilitating the creation of highly absorbing and emitting amplifiers that outperform their single-mode and multi-core EDFA counterparts. Another benefit of using OAM modes lies in their similar intensity profiles, with their orthogonality primarily derived from distinct phase characteristics. Since EDFA amplification depends on intensity rather than phase, this architecture enables high, equalized gain and low-noise amplification across numerous spatial channels. We experimentally characterize an RCF-EDFA that leverages these advantages and topological confinement to achieve high-gain amplification across a record number of uncoupled OAM modal channels. Furthermore, simulations of an optimized, deployment-ready version of the EDFA further demonstrate its ability to amplify numerous spectral and spatial data channels simultaneously while maintaining high energy efficiency. This performance is made possible through a proposed pumping scheme in which the pump consists of a superposition of OAM fiber modes, like the signal, benefiting from the large and stable mode ensemble. By sculpting the modal distribution of the pump, the amplifier architecture is optimized to increase pump-signal overlap, achieving both high, equalized gain and low noise figures while reducing pump power requirements. This thesis explores this parameter space, through both simulations and experimental investigations, with the aim of developing optimal SDM fiber amplifiers that address the capacity, energy efficiency, and cost demands of future optical fiber networks.

No description available.

Quantum memories are key components in photonics-based quantum information processing networks. Their ability to store and retrieve information on demand makes repeat-until-success strategies scalable. Warm alkali-metal vapours are interesting candidates for the implementation of such memories, thanks to their very long storage times as well as their experimental simplicity and versatility. Operation with the Raman memory protocol enables high time-bandwidth products, which denote the number of possible storage trials within the memory lifetime. Since large time-bandwidth products enable multiple synchronisation trials of probabilistically operating quantum gates via memory-based temporal multiplexing, the Raman memory is a promising tool for such tasks. Particularly, the broad spectral bandwidth allows for direct and technologically simple interfacing with other photonic primitives, such as heralded single photon sources. Here, this kind of light-matter interface is implemented using a warm caesium vapour Raman memory. Firstly, we study the storage of polarisation-encoded quantum information, a common standard in quantum information processing. High quality polarisation preservation for bright coherent state input signals can be achieved, when operating the Raman memory in a dual-rail configuration inside a polarisation interferometer. Secondly, heralded single photons are stored in the memory. To this end, the memory is operated on-demand by feed-forward of source heralding events, which constitutes a key technological capability for applications in temporal multiplexing. Prior to storage, single photons are produced in a waveguide-based spontaneous parametric down conversion source, whose bespoke design spectrally tailors the heralded photons to the memory acceptance bandwidth. The faithful retrieval of stored single photons is found to be currently limited by noise in the memory, with a signal-to-noise ratio of approximately 0.3 in the memory output. Nevertheless, a clear influence of the quantum nature of an input photon is observed in the retrieved light by measuring the read-out signal's photon statistics via the g⁽²⁾-autocorrelation function. Here, we find a drop in g⁽²⁾ by more than three standard deviations, from g⁽²⁾ ~ 1.69 to g⁽²⁾ ~ 1.59 upon changing the input signal from coherent states to heralded single photons. Finally, the memory noise processes and their scalings with the experimental parameters are examined in detail. Four-wave-mixing noise is determined as the sole important noise source for the Raman memory. These experimental results and their theoretical description point towards practical solutions for noise-free operation.

<p dir="ltr">Optical frequency combs have revolutionized metrology and advanced other fields such as RF photonics and astronomy. While powerful, they can be bulky, expensive, and difficult to manufacture. This tends to limit uses in real-world scenarios. Within the last decade or so, coherent frequency combs have begun to be generated in millimeter-scale, CMOS fabrication-compatible nonlinear crystals. These so-called “microcombs” have led to hopes of overcoming deployability constraints of more traditional bulk combs.</p><p dir="ltr">One of the first applications for \textit{bulk} frequency combs after their explosion in 2000 was the optical atomic clock. It promised extreme long-term time stability better than that of the Cesium clock that currently defines the SI second. More recently, interest in a fully portable optical atomic clock has grown. Such a device could reliably keep time even without the aid of GPS references, and potentially with greater accuracy than current GPS synchronization can provide.</p><p dir="ltr">Frequency combs have also been used to sample electrical signals more rapidly than traditional electronics can accomplish. This has been used to achieve dramatically increased effective frequency bandwidths for signal detection architectures. One can imagine how this capability would be beneficial in a portable (microcomb-driven) form: a lightweight, comb-enhanced receiver able to capture a broadband snapshot of its surrounding electromagnetic environment could be a powerful tool.</p><p dir="ltr">Timekeeping and RF photonics are the primary applications of microcombs focused upon here. I will attempt to roughly summarize important concepts and highlight relevant work in both subjects in the Introduction. Then I will move a step closer to the hands-on lab work that has largely kept me preoccupied over the last several years and describe important or commonly-employed Methods for experiments. A collection of three journal manuscripts (two published, and the third recently submitted) will follow in the Publications chapter, highlighting some experimental results. Finally, I will conclude with a brief Outlook.</p>

<p dir="ltr">In recent years, nanosecond repetitively pulsed (NRP) plasma discharges have garnered significant interest due to their rapid generation of reactive excited-state species, reactive radicals, and localized heat release within nanosecond (ns) timescale. To effectively harness these plasmas for altering system-level thermal and chemical behavior, a thorough understanding of their governing physics is crucial. This knowledge enables the development of predictive plasma kinetic models for tailoring NRP plasmas to specific applications. However, achieving this requires high-fidelity experimental data to validate models and deepen our understanding of fundamental plasma physics. Advancing experimental spectroscopy and laser diagnostics methods is essential for probing such temporally highly dynamic and optically complex nonequilibrium environments. This includes developing novel <i>test platforms</i>, conducting <i>fundamental research</i> to address existing knowledge gaps, and constructing custom <i>ultrafast laser architectures</i> for probing plasma properties. </p><p dir="ltr">The pioneering development of Streak-based <i>test platform</i> in the diagnostics field of nanosecond pulsed plasmas and its successful application towards inferring the underlying ultrafast spatio-temporal evolution of nanosecond pulsed plasma discharges with an unprecedented time-resolution as short as ~25 ps is presented for the first time. Spectrally filtered, 1D line-imaging of nanosecond pulsed plasma discharges in a single-shot, jitter-free, continuously sweeping manner is obtained, and differences in discharge dynamics of air and N2 plasma environments are studied. Successive <i>test platform</i> advancement includes spectrally resolved Streak-spectroscopy measurements of thermal regime-transition evolution from early-nonequilibrium to local-thermal-equilibrium (LTE) to attain time-resolved quantitative insights into N2(C) state rotational/vibrational nonequilibrium temperatures, electron temperature/density, and spectral lifetime dynamics. </p><p dir="ltr">Ultrafast laser-based progression includes detailed <i>fundamental</i> investigation of higher-order optical nonlinearity perturbations of fs-EFISH by considering of – self-phase modulation induced spectral characteristic of fs-EFISH signal, calibration mapping during-below-and-beyond optical breakdown regime, optical Kerr effect consequences, impact of femtosecond (fs) laser seeding on the noninvasiveness of fs-EFISH, and spectral emission characteristics of fs laser filaments. To infer N2(X) state nonequilibrium of NRP pulsed plasmas, two hybrid fs/ps ro-vibrational coherent anti-Stokes Raman scattering (CARS) <i>ultrafast laser architectures</i> are developed. First architecture, single-laser-solution, reduces system’s energy budget by ~3 mJ/pulse for generating narrowband (~21 ps), high-energy (~420 μJ/pulse), 532 nm probe pulses through incorporation of custom built visible fs optical parametric amplifier (OPA) coupled with an Nd:YAG power amplifier module. The second architecture, two-laser-solution, improves system’s robustness through the development of a 1 kHz, 532 nm, high-energy (~600 μJ/pulse), low-jitter (<1 ps), narrowband (~27 ps), master-oscillator-power-amplification (MOPA) based picosecond probe pulse laser time-synchronized with fs master-oscillator. Single-shot, hybrid fs/ps narrowband ro-vibrational CARS demonstration in a combusting flame up to temperatures of ~2400 K is demonstrated. Experimental ro-vibrational CARS investigation includes polarization based nonresonant background suppression and demonstration of preferential Raman coherence excitation shift, a temperature sensitivity enhancing strategy for vibrationally hot mediums like nanosecond pulsed plasmas. Lastly, an ultrafast pulse-friendly optically accessible vacuum cell is designed and fabricated for controlled experiments of NRP fs/ps CARS. Special care is taken to prevent self-focusing and spectral-temporal chirp of fs CARS beams while maintaining Gaussian focusing beam caustic.</p>