This thesis encompasses a broad area of physics including linear and nonlinear optics, photonics and quantum physics. It combines the phenomena of nonlinearoptical frequency conversion with waveguiding and coupling, taking advantage of new opportunities presented by advances in fabrication technologies of micro- and nano-waveguides. In this dissertation an in-depth analysis of quantum and classical properties of light traveling in nonlinear optical waveguides, directional couplers and waveguide arrays is performed. The concepts of spatial and temporal dispersion, waveguiding in structures with subwavelength dimensions and nonlinear interactions between different frequencies of light are studied both theoretically and experimentally. Some sections of this thesis include development and implementation of novel physical ideas, while other sections are focused on comprehensive experimental and numerical analysis of advanced theoretical concepts. The results presented in this dissertation demonstrate new physical phenomena with potential applications in the areas of telecommunications and quantum information. The research performed in this thesis opens opportunities for frequency conversion with world-leading power efficiency, including operation with ultrashort pulses for a variety of wavelengths to suit a wide range of perspective application requirements. It also shows an approach for simple and energy efficient spatio-temporal optical signal control, which can find applications in next generation telecommunications networks. Furthermore, the results obtained in this dissertation demonstrate the possibility for flexible shaping of quantum statistics of photons generated in photonic waveguiding structures through spontaneous frequency conversion, contributing to the development of integrated quantum circuits. The new methods of frequency conversion in micro- and nano-scale waveguides and optical circuits have potential to advance the performance, energy efficiency, and security of future optical communication networks and computing systems.
This thesis encompasses a broad area of physics including linear and nonlinear optics, photonics and quantum physics. It combines the phenomena of nonlinearoptical frequency conversion with waveguiding and coupling, taking advantage of new opportunities presented by advances in fabrication technologies of micro- and nano-waveguides. In this dissertation an in-depth analysis of quantum and classical properties of light traveling in nonlinear optical waveguides, directional couplers and waveguide arrays is performed. The concepts of spatial and temporal dispersion, waveguiding in structures with subwavelength dimensions and nonlinear interactions between different frequencies of light are studied both theoretically and experimentally. Some sections of this thesis include development and implementation of novel physical ideas, while other sections are focused on comprehensive experimental and numerical analysis of advanced theoretical concepts. The results presented in this dissertation demonstrate new physical phenomena with potential applications in the areas of telecommunications and quantum information. The research performed in this thesis opens opportunities for frequency conversion with world-leading power efficiency, including operation with ultrashort pulses for a variety of wavelengths to suit a wide range of perspective application requirements. It also shows an approach for simple and energy efficient spatio-temporal optical signal control, which can find applications in next generation telecommunications networks. Furthermore, the results obtained in this dissertation demonstrate the possibility for flexible shaping of quantum statistics of photons generated in photonic waveguiding structures through spontaneous frequency conversion, contributing to the development of integrated quantum circuits. The new methods of frequency conversion in micro- and nano-scale waveguides and optical circuits have potential to advance the performance, energy efficiency, and security of future optical communication networks and computing systems.
Integrated optical quantum circuits based on photonic waveguiding structures are increasingly gaining attention as a possible solution for scalable quantum technologies with important applications to quantum simulations. Quantum communication provides secure information transmission, but the distance over which quantum states of light can be distributed without significant disturbance is limited due to inescapable losses and noise in optical elements. Loss is the greatest challenge facing the implementation of integrated photonic technologies, and it is inescapable in experimental reality. In recent years there is a rise of interest in structures with spatially inhomogeneous losses. Light propagation in waveguiding structures with spatially distributed sections of loss can be used for implementation of quantum plasmonic circuits, which are able to strongly confine light to sub-wavelength dimensions, as well as for parity-time (PT) symmetric structures, with phase transition associated with PT-symmetry breaking, which opens new possibilities for light manipulation. The PhD thesis contains research on the controllable classical and quantum dynamics of optical frequency conversion processes in quadratically nonlinear photonic integrated circuites in the presence of losses. Namely, I discuss spontaneous parametric down-conversion (SPDC), sum-frequency generation (SFG) and optical parametric amplification (OPA) in nonlinear structures governed by non-Hermition Hamiltonians. I explore the fundamental features of multi-photon generation in integrated nonlinear waveguides. I have been shown that arrays of coupled nonlinear waveguides can serve as a robust integrated platform for the generation of entangled photon states with nonclassical spatial correlations through spontaneous parametric down-conversion (SPDC), and that the operation of such quantum circuit is tolerant even to relatively high losses. Furthermore, I have studied the bi-photon multimode quantum emission tomography in waveguide structures with spatially inhomegeneous losses. The PhD thesis also covers the research on the effect of these losses in waveguide couplers possessing parity-time (PT) symmetry. I have identified an anti-PT spectral symmetry of a parametric amplifier based on those couplers. Finally, I describe the single-photon conversion to a photon pair, and identify opportunities for the efficient enhancement of this process.
In this thesis, nonlinear periodic structures and their applications in both classical and quantum regime are investigated. New theoretical models are developed, and novel applications of nonlinear periodic structures are proposed and demonstrated. The theoretical studies, both design and simulation, are based on but not limited to InGaAsP material. A new method, namely the time-domain transfer-matrix (TDTM), is presented to simulate optical pulse propagation in layered media with resonant nonlinearity. As there were no satisfactory methods in the literature to model this problem, in order to validate and compare with the TDTM method, the standard FDTD method is generalized to include the rate equation in the analysis of semi-conductors. Also in this work, optical manipulation of absorption in periodic structures is studied for the first time. Thanks to the large accessible nonlinearity that results from the absorption saturation and frequency selectivity of periodic structures, a sensitive and compact optical limiter is designed. The novel design and modeling work developed in this thesis has provided new insights and tools to the utilization of resonant nonlinearities in compact all-optical devices.The experimental studies are based on quasi-phase matched AlGaAs superlattice waveguides. These devices have been previously designed and used for classical optical wavelength conversion such as second harmonic generation and difference frequency generation. In this work, these devices are exploited for spontaneous down conversion, which is a quantum effect, for the first time, through this process, entangled photon pairs are generated. Unprecedented performance, in terms of brightness and purity, of III-V semiconductor-based entangled photon sources has been demonstrated here. Moreover, the quantum properties of these entangled photons are characterized. The experimental studies presented in this thesis open up new application areas for III-V nonlinear optical devices as quantum sources and convincingly demonstrate the promising role of such devices will play in future quantum technologies.
Recent advances in nonlinear nanophotonics have enabled optical frequency conversion at energy scales remarkably close to the few-photon regime of quantum optics. A key technique in these efforts is dispersion engineering, allowing dynamical confinement of light into ultrashort pulses with high peak intensities. In this talk, we explore quantum effects expected to emerge in next-generation devices, with a focus on understanding how these phenomena interplay with the multimode physics of femtosecond pulses. Drawing inspiration from recent experiments in thin-film lithium-niobate nanophotonics, we present several numerical studies of broadband frequency conversion in chi(2)-nonlinear waveguides. Our analysis provides a detailed understanding of the dynamics and modal structure of quantum noise and entanglement, e.g., in parametric and supercontinuum generation. This talk also surveys some recent results we obtained for harnessing exotic quantum non-Gaussian states of light in ultrafast nonlinear nanophotonics. Proper engineering of these quantum effects may generate entirely new functionalities for nonlinear optics at and beyond the classical-quantum transition.
Entanglement is the hallmark of quantum mechanics. Quantum entanglement -- putting two or more identical particles into a non-factorable state -- has been leveraged for applications ranging from quantum computation and encryption to high-precision metrology. Entanglement is a practical engineering resource and a tool for sidestepping certain limitations of classical measurement and communication. Engineered nonlinear optical waveguides are an enabling technology for generating entangled photon pairs and manipulating the state of single photons. This dissertation reports on: i) frequency conversion of single photons from the mid-infrared to 843nm as a tool for incorporating quantum memories in quantum networks, ii) the design, fabrication, and test of a prototype broadband source of polarization and frequency entangled photons; and iii) a roadmap for further investigations of this source, including applications in quantum interferometry and high-precision optical metrology. \n \nThe devices presented herein are quasi-phase-matched lithium niobate waveguides. Lithium niobate is a second-order nonlinear optical material and can mediate optical energy conversion to different wavelengths. This nonlinear effect is the basis of both quantum frequency conversion and entangled photon generation, and is enhanced by i) confining light in waveguides to increase conversion efficiency, and ii) quasi-phase matching, a technique for engineering the second-order nonlinear response by locally altering the direction of a material's polarization vector. Waveguides are formed by diffusing titanium into a lithium niobate wafer. Quasi-phase matching is achieved by electric field poling, with multiple stages of process development and optimization to fabricate the delicate structures necessary for broadband entangled photon generation. \n \nThe results presented herein update and optimize past fabrication techniques, demonstrate novel optical devices, and propose future avenues for device development. Quantum frequency conversion from 1848nm to 843nm is demonstrated for the first time, with >75% single-photon conversion efficiency. A new electric field poling methodology is presented, combining elements from multiple historical techniques with a new fast-feedback control system. This poling technique is used to fabricate the first chirped-and-apodized Type-II quasi-phase-matched structures in titanium-diffused lithium niobate waveguides, culminating in a measured phasematching spectrum that is predominantly Gaussian (R^2 = 0.80), nearly eight times broader than the unchirped spectrum, and agrees well with simulations.
The aim of this thesis is to explore nonlinear nanostructures – photonic systems with one or more dimension with nanoscale feature size – as new platforms for parametric photon-pair generation. This is motivated by the need for photon-pair sources in many technological applications of quantum optics, such as quantum communication, imaging and computing. The initial hypothesis is that nonlinear nanostructures are a promising platform to complement established photon-pair source schemes based on bulk crystals or photonic integrated circuits in terms of quantum state-engineering, footprint, functional density etc. However, nonlinear pair generation, e.g. via spontaneous parametric down-conversion (SPDC), is a new subfield in nanophotonics with a very limited number of studies existing. This constitutes the knowledge-gap addressed in this thesis. Therefore, the objective is to provide first experimental demonstrations of SPDC in subwavelength-scale and nonlinear nanostructures. We investigate the ability of such systems to control the properties of two-photon quantum states such as frequency and angular spectrum or polarization entanglement. To this end we analyze nanostructures with different degrees of complexity, starting with an unstructured thin crystal with one nanoscale dimension, leading up to a 3D nano-scale resonator with subwavelength-sized features in all dimensions. Furthermore, we test different types of nonlinear materials such as lithium niobate, group III-V semiconductors and emerging, highly nonlinear materials such as molybdenum disulfide. Additionally, we also advance the theoretical understanding by developing a comprehensive model of the quantum process of nonlinear parametric pair-generation in photonic nanostructures.
This thesis describes a series of experimental research primarily revolving around the applications of periodically poled lithium niobate (PPLN). Initially, a set of work is described regarding the fabrication and characterisation of PPLN ridge waveguides designed for frequency doubling into the blue/UV range. These waveguides are fabricated via diffusion of zinc into the crystal structure. A precision dicing process creates the sidewall definition for modal confinement and also optical facet preparation. Preliminary results with the most efficient waveguide fabricated produced a maximum conversion efficiency of 6.9%/W in a 14 mm-long device, generating 391 nm light. This was performed with a novel near-IR VECSEL; designed and built as part of a collaboration with the research undertaken in this thesis to investigate new routes for miniaturised UV sources. The development of a non-destructive route to characterise the physical dimensions of a ridge waveguide for nonlinear applications is also presented. This presented methodology uses white light interferometry and standard edge recognition algorithms to determine the waveguide width profile. In combination with numerical propagation constant data for these waveguides, a numerical phasematching spectra can be calculated. A comparison to the experimental phasematching spectra of the second-harmonic generation process enables the discrimination between issues in the fabrication processes. Using bulk PPLN, a final application is presented for communications using mid-IR radiation and all commercial components. This configuration involves the use of differencefrequency generation followed by sum-frequency generation to create the transmitter and receiver systems, respectively. This enables final detection of 809 nm light (firmly within the silicon absorption band). Bit error ratios as low as 10−7 from single-pass nonlinear conversion for both systems without the need for an additional optical amplifier in the receiver. Finally, the propagation of relative intensity noise within the system as the source lasers propagate through a two step nonlinear conversion is presented and discussed within.