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Second-order nonlinear optical processes convert light from one wavelength to another and generate quantum entanglement. Creating chip-scale devices to efficiently control these interactions greatly increases the reach of photonics. Existing silicon-based photonic circuits utilize the third-order optical nonlinearity, but an analogous integrated platform for second-order nonlinear optics remains an outstanding challenge. Here we demonstrate efficient frequency doubling and parametric oscillation with a threshold of tens of micro-watts in an integrated thin-film lithium niobate photonic circuit. We achieve degenerate and non-degenerate operation of the parametric oscillator at room temperature and tune its emission over one terahertz by varying the pump frequency by hundreds of megahertz. Finally, we observe cascaded second-order processes that result in parametric oscillation. These resonant second-order nonlinear circuits will form a crucial part of the emerging nonlinear and quantum photonics platforms. Here, the authors demonstrate a chip-scale device that realizes a comprehensive set of resonant second order nonlinear processes including optical parametric oscillation with a threshold power of 70 microwatts.

Lithium niobate is a promising material for developing quantum acoustic technologies due to its strong piezoelectric effect and availability in the form of crystalline thin films of high quality. However, at radio frequencies and cryogenic temperatures, these resonators are limited by the presence of decoherence and dephasing due to two-level systems. To mitigate these losses and increase device performance, a more detailed picture of the microscopic nature of these loss channels is needed. In this study, we fabricate several lithium niobate acoustic wave resonators and apply different processing steps that modify their surfaces. These treatments include argon ion sputtering, annealing, and acid cleans. We characterize the effects of these treatments using three surface-sensitive measurements: cryogenic microwave spectroscopy measuring density and coupling of TLS to mechanics, X-ray photoelectron spectroscopy and atomic force microscopy. We learn from these studies that, surprisingly, increases of TLS density may accompany apparent improvements in the surface quality as probed by the latter two approaches. Our work outlines the importance that surfaces and fabrication techniques play in altering acoustic resonator coherence, and suggests gaps in our understanding as well as approaches to address them.

Optical parametric oscillators (OPOs) have emerged as highly versatile platforms for signal processing, machine learning, and all-optical computation. In particular, integrated photonic circuits have demonstrated an efficient and scalable route to build OPO networks through time-multiplexing. However, for tasks requiring massive parallelism with low latency, spatial multiplexing with vertical micro-cavities is a more natural approach to overcome the shoreline density limits of edge-emitting photonics. To this end, we propose an approach to realizing vertical micro-cavity OPOs (VCOPOs) leveraging recent developments in micro-optical fabrication techniques. We consider thin film LiNbO3-filled dielectric micro-cavities as a case study, but the approach taken here is readily extended to any χ(2) nonlinear medium. Based on conservative industrial fabrication tolerances, we predict a minimal foot-print of ca. 5×5μm2, while achieving oscillation thresholds in the microwatts range. Advanced fabrication methods open a path toward sub-µW oscillation threshold, with a ratio of single-photon non-linear coupling rate to dissipation rate g/κ>5%. We propose a theoretical framework for the classical and quantum operation of two dimensional arrays of VCOPOs, and discuss potential applications such as surface emitting devices, spatially multi-mode parametric amplifiers and squeezers, as well as optical simulators of classical and quantum Hamiltonians.

This article reviews recent progress in quasi-phasematched χ ( 2 ) nonlinear nanophotonics, with a particular focus on dispersion-engineered nonlinear interactions. Throughout this article, we establish design rules for the bandwidth and interaction lengths of various nonlinear processes, and provide examples for how these processes can be engineered in nanophotonic devices. In particular, we apply these rules towards the design of sources of non-classical light and show that dispersion-engineered devices can outperform their conventional counterparts. Examples include ultra-broadband optical parametric amplification as a resource for measurement-based quantum computation, dispersion-engineered spontaneous parametric downconversion as a source of separable biphotons, and synchronously pumped nonlinear resonators as a potential route towards single-photon nonlinearities.

Optical frequency combs have revolutionized precision measurement, time-keeping and molecular spectroscopy 1 – 7 . A substantial effort has developed around ‘microcombs’: integrating comb-generating technologies into compact photonic platforms 5 , 7 – 9 . Current approaches for generating these microcombs involve either the electro-optic 10 or Kerr mechanisms 11 . Despite rapid progress, maintaining high efficiency and wide bandwidth remains challenging. Here we introduce a previously unknown class of microcomb—an integrated device that combines electro-optics and parametric amplification to yield a frequency-modulated optical parametric oscillator (FM-OPO). In contrast to the other solutions, it does not form pulses but maintains operational simplicity and highly efficient pump power use with an output resembling a frequency-modulated laser 12 . We outline the working principles of our device and demonstrate it by fabricating the complete optical system in thin-film lithium niobate. We measure pump-to-comb internal conversion efficiency exceeding 93% (34% out-coupled) over a nearly flat-top spectral distribution spanning about 200 modes (over 1 THz). Compared with an electro-optic comb, the cavity dispersion rather than loss determines the FM-OPO bandwidth, enabling broadband combs with a smaller radio-frequency modulation power. The FM-OPO microcomb offers robust operational dynamics, high efficiency and broad bandwidth, promising compact precision tools for metrology, spectroscopy, telecommunications, sensing and computing. An integrated device that combines optical parametric oscillation and electro-optic modulation in lithium niobate creates a flat-top frequency-comb-like output with low power requirements.

This article reviews recent progress in quasi-phasematched \\(\\chi^{(2)}\\) nonlinear nanophotonics, with a particular focus on dispersion-engineered nonlinear interactions. Throughout this article, we establish design rules for the bandwidth and interaction lengths of various nonlinear processes, and provide examples for how these processes can be engineered in nanophotonic devices. In particular, we apply these rules towards the design of sources of non-classical light and show that dispersion-engineered devices can outperform their conventional counterparts. Examples include ultra-broadband optical parametric amplification as a resource for measurement-based quantum computation, dispersion-engineered spontaneous parametric downconversion as a source of separable biphotons, and synchronously pumped nonlinear resonators as a potential route towards single-photon nonlinearities.

Recent advances in nonlinear photonics have enabled a new class of broadband ultra-stable light sources known as optical frequency combs. These light sources have given rise to an array of new optical devices and systems, spanning applications such as spectroscopy, astronomy, remote sensing, frequency synthesis, attoscience, telecommunications, and optical clockwork. At this time, there are a number of unsolved problems within the field. Optical frequency combs are often constrained to wavelengths within the near-infrared (NIR) due to the limited variety of in mature laser gain media and host glasses, and many applications such as spectroscopy, sensing, and attoscience would benefit from the development of optical frequency combs at longer wavelength ranges such as the mid-infrared (MIR). Furthermore, the generation and stabilization of frequency combs often requires rather complicated nonlinear optical systems, which have prevented these light sources from being used outside of dedicated optics labs.This dissertation considers new approaches to frequency comb generation based on recently discovered nonlinear dynamical processes that occur in quasi-phasematched (QPM) devices with quadratic nonlinearities. A recurring theme is that the interplay of nonlinear optical effects, such as optical parametric amplification and self-phase modulation, with linear optical effects, such as dispersion, can produce qualitatively new dynamical regimes. In many cases, these dynamical regimes exhibit favorable features that potentially solve the problems discussed above. The first half of this thesis considers the pulse formation mechanisms present in optical parametric oscillators (OPOs), and discusses new operating regimes that enable the generation of MIR combs with substantially more bandwidth than the NIR comb used to drive the OPO. These devices can produce few-cycle pulses with conversion efficiencies exceeding 50% while also preserving the coherence of the frequency comb.The latter portion of this thesis studies the dynamics of femtosecond pulses in nanophotonic waveguides. Here, the geometric dispersion associated with sub-wavelength confinement be used to achieve long interaction lengths with femtosecond pulses. Using these effects we are able to achieve saturated SHG with femtojoules of pulse energy, where state-of-the-art devices previously used picojoules. In the limit of phase-mismatched SHG driven with picojoules of pulse energy we observe the formation of a coherent multi-octave supercontinuum comprised of multiple spectrally broadened harmonics. The mechanisms of spectral broadening in this system are shown to be completely unique to dispersion-engineered nanophotonic QPM devices and exhibit a number of desirable features including i) low power requirements, ii) fewer decoherence mechanisms than traditional approaches, and iii) the formation of carrier-envelope-offset beatnotes in the regions of spectral overlap between the harmonics.

High-gain optical parametric amplification is an important nonlinear process used both as a source of coherent infrared light and as a source of nonclassical light. In this work, we experimentally demonstrate an approach to optical parametric amplification that enables extremely large parametric gains with low energy requirements. In conventional nonlinear media driven by femtosecond pulses, multiple dispersion orders limit the effective interaction length available for parametric amplification. Here, we use the dispersion engineering available in periodically poled thin-film lithium niobate nanowaveguides to eliminate several dispersion orders at once. The result is a quasi-static process; the large peak intensity associated with a short pump pulse can provide gain to signal photons without undergoing pulse distortion or temporal walk-off. We characterize the parametric gain available in these waveguides using optical parametric generation, where vacuum fluctuations are amplified to macroscopic intensities. When driven with pulse energies in excess of 10 pJ, we observe saturated parametric gains as large as 88 dB (146 dB/cm). The devices shown here achieve saturated optical parametric generation with orders of magnitude less pulse energy than previous techniques.

Over the last few decades, nonlinear optics has become significantly more nonlinear, traversing nearly a billionfold improvement in energy efficiency, with ultrafast nonlinear nanophotonics in particular emerging as a frontier for combining both spatial and temporal engineering. At present, cutting-edge experiments in nonlinear nanophotonics place us just above the mesoscopic regime, where a few hundred photons suffice to trigger nonlinear saturation. In contrast to classical or deep-quantum optics, the mesoscale is characterized by dynamical interactions between mean-field, Gaussian, and non-Gaussian quantum features, all within a close hierarchy of scales. When combined with the inherent multimode complexity of optical fields, such hybrid quantum-classical dynamics present theoretical, experimental, and engineering challenges to the contemporary framework of quantum optics. In this review, we highlight the unique physics that emerges in multimode nonlinear optics at the mesoscale and outline key principles for exploiting both classical and quantum features to engineer novel functionalities. We briefly survey the experimental landscape and draw attention to outstanding technical challenges in materials, dispersion engineering, and device design for accessing mesoscopic operation. Finally, we speculate on how these capabilities might usher in some new paradigms in quantum photonics, from quantum-augmented information processing to nonclassical-light-driven dynamics and phenomena to all-optical non-Gaussian measurement and sensing. The physics unlocked at the mesoscale present significant challenges and opportunities in theory and experiment alike, and this review is intended to serve as a guidepost as we begin to navigate this new frontier in ultrafast quantum nonlinear optics.

On-chip photonic processors for neural networks have potential benefits in both speed and energy efficiency but have not yet reached the scale at which they can outperform electronic processors. The dominant paradigm for designing on-chip photonics is to make networks of relatively bulky discrete components connected by one-dimensional waveguides. A far more compact alternative is to avoid explicitly defining any components and instead sculpt the continuous substrate of the photonic processor to directly perform the computation using waves freely propagating in two dimensions. We propose and demonstrate a device whose refractive index as a function of space, \\(n(x,z)\\), can be rapidly reprogrammed, allowing arbitrary control over the wave propagation in the device. Our device, a 2D-programmable waveguide, combines photoconductive gain with the electro-optic effect to achieve massively parallel modulation of the refractive index of a slab waveguide, with an index modulation depth of \\(10^{-3}\\) and approximately \\(10^4\\) programmable degrees of freedom. We used a prototype device with a functional area of \\(12\\,\\text{mm}^2\\) to perform neural-network inference with up to 49-dimensional input vectors in a single pass, achieving 96% accuracy on vowel classification and 86% accuracy on \\(7 \\times 7\\)-pixel MNIST handwritten-digit classification. This is a scale beyond that of previous photonic chips relying on discrete components, illustrating the benefit of the continuous-waves paradigm. In principle, with large enough chip area, the reprogrammability of the device's refractive index distribution enables the reconfigurable realization of any passive, linear photonic circuit or device. This promises the development of more compact and versatile photonic systems for a wide range of applications, including optical processing, smart sensing, spectroscopy, and optical communications.