In this paper, we will see in more detail how the modal dispersion present in any waveguide can replace or be combined with the birefringence to fulfil the phase-matching condition. Chapter Contents: 8.1 Introduction 8.2 Proton exchange in LiNbO3, 8.2.1 The first discoveries: High δne, low loss 8.2.2 Combination with birefringence phase matching 8.2.3 Cerenkov configuration 8.2.4 Destruction of the X2 and strain-induced losses in channel waveguides 8.3 Periodic poling 8.3.1 Surface domains: Titanium diffusion and heat treatment 8.3.2 E-Field poling 8.3.3 The different PE processes and their impact on the nonlinearity of the crystal 8.3.3.1 Direct exchange 8.3.3.2 Annealing 8.3.3.3 IR absorption 8.3.3.4 Nonlinear optical properties of PE lithium niobate waveguides 8.3.4 PE and periodic poling 8.3.4.1 PE in PPLN crystals 8.3.4.2 E-field poling of PE samples 8.3.5 Components 8.3.5.1 SHG 8.3.5.2 Parametric fluorescence 8.3.5.3 DFG and amplification 8.4 Single photon pair generators 8.5 Quantum photonics integrated circuits on PPLN 8.5.1 Quantum relay 8.5.2 Squeezed states 8.6 Further improvements 8.6.1 Power-resistant materials 8.6.2 Highly confining waveguides 8.6.2.1 High-index soft proton-exchanged (HISoPE) waveguides 8.6.2.2 Thin-film lithium niobate 8.7 Today's issues 8.7.1 Control of the domains 8.7.1.1 E-beam poling of PE samples 8.7.2 Insensitive phase-matching configurations 8.7.2.1 Experimental observation of insensitive phase-matching configurations 8.7.2.2 Numerical studies of the phase mismatch 8.8 Conclusion Acknowledgments References Inspec keywords: ion exchange; optical phase matching; optical dispersion; optical waveguides; lithium compounds Other keywords: classical optics; LiNbO3; modal dispersion; quantum optics; nonlinear integrated optics; proton-exchanged lithium niobate waveguides; birefringence; phase-matching condition Subjects: Optical waveguides; Optical propagation, transmission and absorption; Nonlinear optics; Optical waveguides and couplers; Nonlinear optics and devices
In this paper, we will see in more detail how the modal dispersion present in any waveguide can replace or be combined with the birefringence to fulfil the phase-matching condition. Chapter Contents: 8.1 Introduction 8.2 Proton exchange in LiNbO3, 8.2.1 The first discoveries: High δne, low loss 8.2.2 Combination with birefringence phase matching 8.2.3 Cerenkov configuration 8.2.4 Destruction of the X2 and strain-induced losses in channel waveguides 8.3 Periodic poling 8.3.1 Surface domains: Titanium diffusion and heat treatment 8.3.2 E-Field poling 8.3.3 The different PE processes and their impact on the nonlinearity of the crystal 8.3.3.1 Direct exchange 8.3.3.2 Annealing 8.3.3.3 IR absorption 8.3.3.4 Nonlinear optical properties of PE lithium niobate waveguides 8.3.4 PE and periodic poling 8.3.4.1 PE in PPLN crystals 8.3.4.2 E-field poling of PE samples 8.3.5 Components 8.3.5.1 SHG 8.3.5.2 Parametric fluorescence 8.3.5.3 DFG and amplification 8.4 Single photon pair generators 8.5 Quantum photonics integrated circuits on PPLN 8.5.1 Quantum relay 8.5.2 Squeezed states 8.6 Further improvements 8.6.1 Power-resistant materials 8.6.2 Highly confining waveguides 8.6.2.1 High-index soft proton-exchanged (HISoPE) waveguides 8.6.2.2 Thin-film lithium niobate 8.7 Today's issues 8.7.1 Control of the domains 8.7.1.1 E-beam poling of PE samples 8.7.2 Insensitive phase-matching configurations 8.7.2.1 Experimental observation of insensitive phase-matching configurations 8.7.2.2 Numerical studies of the phase mismatch 8.8 Conclusion Acknowledgments References Inspec keywords: ion exchange; optical phase matching; optical dispersion; optical waveguides; lithium compounds Other keywords: classical optics; LiNbO3; modal dispersion; quantum optics; nonlinear integrated optics; proton-exchanged lithium niobate waveguides; birefringence; phase-matching condition Subjects: Optical waveguides; Optical propagation, transmission and absorption; Nonlinear optics; Optical waveguides and couplers; Nonlinear optics and devices
Second-order optical nonlinearity is widely used for both classical and quantum photonic applications. Due to material dispersion and phase matching requirements, the polarization of optical fields is pre-defined during the fabrication. Only one type of phase matching condition is normally satisfied, and this limits the device flexibility. Here, we demonstrate that phase matching for both type-I and type-II second-order optical nonlinearity can be realized simultaneously in the same waveguide fabricated from thin-film lithium niobate. This is achieved by engineering the geometry dispersion to compensate for the material dispersion and birefringence. The simultaneous realization of both phase matching conditions is verified by the polarization dependence of second-harmonic generation. Correlated photons are also generated through parametric down conversion from the same device. This work provides a novel approach to realize versatile photonic functions with flexible devices.
This thesis describes the establishment of a new fabrication facility for the realization of annealed and reverse proton exchanged waveguides in congruent lithium niobate at Grith University{Centre for Quantum Dynamics, and their application in dierent contexts of integrated quantum photonics technologies. We demonstrated the development of an accurate numerical model for the design of waveguides in X-cut and Z-cut substrates and high quality fabrication of both quasi-phase matched quadratic nonlinear waveguides and fast electro-optic waveguide modulators. Within the context of nonlinear optical processes, we demonstrated a newly proposed method which eciently reconstruct the biphoton state produced through spontaneous parametric down-conversion by the use of only classical measurements. The validity of the method is tested by a direct comparison with photon-coincidence counting measurements in a multi-channel integrated nonlinear device fabricated in our facility. We also implemented the rst frequency conversion of 369:5 nm light {resonant with the transition of Yb+ trapped ions{ to the 1550 nm telecom wavelength range with a periodically poled waveguide fabricated by the reverse proton exchange technique. Within the context of electro-optical processes, we showed the rst active demultiplexing of single-photons emitted from a quantum dot source with a single integrated device made of a network of electro-optically tunable directional couplers.
Ion-sliced thin-film lithium niobate (LN) compact waveguide technology has facilitated the resurgence of integrated photonics based on lithium niobate. These thin-film LN waveguides offer over an order of magnitude improvement in optical confinement, and about two orders of magnitude reduction in waveguide bending radius, compared to conventional LN waveguides. Harnessing the improved confinement, a variety of miniaturized and efficient photonic devices are demonstrated in this work. First, two types of compact electrooptic modulators are presented – microring modulators, and Mach-Zehnder modulators. Next, two distinct approaches to nonlinear optical frequency converters are implemented – periodically poled lithium niobate, and mode shape modulation (grating assisted quasi-phase matching). Following this, stochastic variations are added to the mode shape modulation approach to demonstrate random quasi-phase matching. Afterward, broadband photon-pair generation is demonstrated in the miniaturized periodically poled lithium niobate, and spectral correlations of the biphoton spectrum are reported. Finally, extensions of the aforementioned results suitable for future work are discussed.
The quantum nature of light at the single photon level allows for unique applications that classical physics neither predicts nor describes. Most notably, appropriate conditions may cause the states of photons with indistinguishable properties to become entangled, enabling novel approaches to quantum computation, secure communications, and metrology. \n \nAny operational quantum information network transporting entangled-photon states must establish a high-fidelity link between its distant nodes despite the inherent fragility of entangled states. However, the lack of a universal operating wavelength for all optical devices makes this a substantial challenge. Telecommunication fibers, entangled photon pair sources, quantum optical memories, and quantum repeaters all function in disparate spectral windows dictated by the properties of the materials used to fabricate them. Quantum frequency conversion (QFC), the single-photon limit of nonlinear parametric sum and difference frequency generation in optics, offers a bridge between spectral regions with full preservation of quantum state character, including entanglement. \n \nPeriodically poled optical waveguides in ferroelectric crystals are versatile tools in nonlinear optics. Confining the nonlinear interaction to a waveguide greatly enhances its efficiency compared to bulk optics. Periodic poling, the process of periodically inverting the domains of a ferroelectric medium using a strong electric field, enables a variety of quasi-phase matching configurations to engineer a desired nonlinear interaction. \n \nThis work concentrates on the design, development, fabrication, and characterization of a titanium-diffused periodically poled lithium niobate (Ti:PPLN) waveguide device. This device is designed to execute single-step quantum frequency conversion from 369.5nm to 1550nm in order to facilitate a quantum state transfer between standard fiber telecommunication wavelengths and an atomic quantum memory system employing trapped ytterbium ions. \n \nThe creation of phase matching conditions for such an extreme distance in frequency demands precise control of high magnitude electric fields on the single-micron scale. As the first demonstration of its kind in Ti:PPLN, the development of these devices included novel improvements to existing fabrication methods, improving the state-of-the-art of precision, quality, and yield for poling of ferroelectric crystals.
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.
Lithium niobate is a versatile material for both classical and quantum photonics, recognized for its outstanding electro-optic and nonlinear optical properties. Through a process known as poling, periodic ferroelectric crystal domains can be engineered to enable quasi-phase-matched frequency conversion, efficient modulation, and the generation of quantum light sources. The emergence of lithium niobate on insulator technology has further enhanced its suitability for scalable integrated photonics, offering ultra-low optical losses and strong light confinement while retaining the material's inherent advantages. Here, the techniques used to fabricate and characterize periodically poled lithium niobate are reviewed. Key developments are discussed, offering insights into the future of domain engineering of lithium niobate.