Efficient self-imaging grating couplers on a Lithium-Niobate-On-Insulator platform at near-visible and telecom wavelengths

Lithium-Niobate-On-Insulator (LNOI) has emerged as a promising platform in the field of integrated photonics. Nonlinear optical processes and fast electro-optic modulation have been reported with outstanding performance in ultra-low loss waveguides. In order to harness the advantages offered by the LNOI technology, suitable fiber-to-chip interconnects operating at different wavelength ranges are demanded. Here we present easily manufacturable, self-imaging apodized grating couplers, featuring a coupling efficiency of the TE0 mode as high as $\simeq 47.1\%$ at $\lambda$=1550 nm and $\simeq 44.9\%$ at $\lambda$=775 nm. Our approach avoids the use of any metal back-reflector for an improved directivity or multi-layer structures for an enhanced grating strength

High-Index Organic Polymeric Carbon Nitride-Based Photonic Devices for Telecommunication Wavelengths

Because of appealing material properties and ease of fabrication, organic semiconductors have found a variety of applications in integrated photonics, including optical waveguiding in broadband communication systems, use as amplifiers and modulators in signal processing, and for realizing optical detectors and sensors. Polymeric carbon nitride thin films have emerged as a valuable alternative to currently employed inorganic materials in light manipulation and waveguiding owing to their structural flexibility, transparency over a wide wavelength range, and accessible synthesis from sustainable and cost-effective materials. Here, we demonstrate organic polymeric carbon nitride-based nanophotonic devices for telecommunication wavelengths. The high ordinary refractive index of the polymer of 2 or higher, covering both visible and near-infrared wavelength ranges, enables a small device footprint, strong mode confinement, and efficient fiber-to-chip coupling via grating couplers. Proof-of-concept experiments with photonic waveguides and microring resonators show broadband transmission in the visible wavelength range and quality factors exceeding 104 for a wavelength of 1550 nm. The outstanding material properties of polymeric carbon nitride will open new perspectives for polymeric photonic devices for a broad wavelength range

Hybrid Quantum Nanophotonics: Interfacing Color Center in Nanodiamonds with Si3N4-Photonics

This chapter covers recent developments in the field of hybrid quantum photonics based on color centers in nanodiamonds and Si3N4-photonics towards a technology platform with applications in quantum information processing and quantum information distribution. The methodological approach can be divided in three main tasks. First, the fabrication and optimization of Si3N4-photonics. Second, the creation, characterization and control of color centers in nanodiamonds. Third, the assembly of hybrid quantum photonics by integrating the nanodiamonds into the photonic structures. One focus will be the efficient interfacing of the color centers done by optimizing the optical coupling. The chapter describes recent progress in all three steps and summarizes the established hybrid platform. We believe, that the hybrid approach provides a promising path to realize quantum photonic applications, such as quantum networks or quantum repeaters, in the near future

Efficient Coupling of an Ensemble of Nitrogen Vacancy Center to the Mode of a High-Q, Si₃N₄ Photonic Crystal Cavity

Integrated nanophotonics is an emerging field with high potential for quantum technology applications such as quantum sensing or quantum networks. A desired photonics platform is Si3N4 due to low-photon loss and well-established fabrication techniques. However, quantum optics applications are not yet established. Here, we investigate an approach toward Si3N4-based quantum photonics utilizing a crossed waveguide, pump–probe design. The platform enables efficient, on-chip excitation, strong background suppression, and at the same time, efficient coupling to the mode of a high-Q photonic crystal cavity. The freestanding photonic crystal cavities reach high Q-factors up to 47 × 103. To test our platform, we positioned an ensemble of negatively charged nitrogen vacancy centers located in a nanodiamond within the interaction zone of the photonic crystal cavity. We quantify the efficiency of the coupling with the βλ-factor reaching values as large as 0.71. We further demonstrate on-chip excitation of the quantum emitter with strong suppression (∼20 dB) of the background fluorescence. Our results unfold the potential to utilize negatively charged nitrogen vacancy centers in nanodiamonds and Si3N4 platforms as an efficient, on-chip spin-photon interface in quantum photonics experiments

Non-volatile silicon photonic memory with more than 4-bit per cell capability

We present the first demonstration of an integrated photonic phase-change memory using GeSbTe-225 on silicon-on-insulator and demonstrate reliable multilevel operation with a single programming pulse. We also compare our results on silicon with previous demonstrations on silicon nitride. Crucially, achieving this on silicon enables tighter integration of traditional electronics with photonic memories in future, making phase-change photonic memory a viable and integrable technology

Scalable and efficient grating couplers on low-index photonic platforms enabled by cryogenic deep silicon etching

Efficient fiber-to-chip couplers for multi-port access to photonic integrated circuits are paramount for a broad class of applications, ranging, e.g., from telecommunication to photonic computing and quantum technologies. While grating-based approaches are convenient for out-of-plane access and often desirable from a packaging point of view, on low-index photonic platforms, such as silicon nitride or thin-film lithium niobate, the limited grating strength has thus far hindered the achievement of coupling efficiencies comparable to the ones attainable in silicon photonics. Here we present a flexible strategy for the realization of highly efficient grating couplers on low-index photonic platforms. To simultaneously reach a high scattering efficiency and a near-unitary modal overlap with optical fibers, we make use of self-imaging gratings designed with a negative diffraction angle. To ensure high directionality of the diffracted light, we take advantage of a metal back-reflector patterned underneath the grating structure by cryogenic deep reactive ion etching of the silicon handle. Using silicon nitride as a testbed material, we experimentally demonstrate coupling efficiency up to -0.55 dB in the telecom C-band with near unity chip-scale device yield

Emergent self-adaptation in an integrated photonic neural network for backpropagation-free learning

Plastic self-adaptation, nonlinear recurrent dynamics and multi-scale memory are desired features in hardware implementations of neural networks, because they enable them to learn, adapt and process information similarly to the way biological brains do. In this work, we experimentally demonstrate these properties occurring in arrays of photonic neurons. Importantly, this is realised autonomously in an emergent fashion, without the need for an external controller setting weights and without explicit feedback of a global reward signal. Using a hierarchy of such arrays coupled to a backpropagation-free training algorithm based on simple logistic regression, we are able to achieve a performance of 98.2% on the MNIST task, a popular benchmark task looking at classification of written digits. The plastic nodes consist of silicon photonics microring resonators covered by a patch of phase-change material that implements nonvolatile memory. The system is compact, robust, and straightforward to scale up through the use of multiple wavelengths. Moreover, it constitutes a unique platform to test and efficiently implement biologically plausible learning schemes at a high processing speed

Single-photon detection and cryogenic reconfigurability in Lithium Niobate nanophotonic circuits

Lithium-Niobate-On-Insulator (LNOI) is emerging as a promising platform for integrated quantum photonic technologies because of its high second-order nonlinearity and compact waveguide footprint. Importantly, LNOI allows for creating electro-optically reconfigurable circuits, which can be efficiently operated at cryogenic temperature. Their integration with superconducting nanowire single-photon detectors (SNSPDs) paves the way for realizing scalable photonic devices for active manipulation and detection of quantum states of light. Here we report the first demonstration of these two key components integrated in a low loss (0.2 dB/cm) LNOI waveguide network. As an experimental showcase of our technology, we demonstrate the combined operation of an electrically tunable Mach-Zehnder interferometer and two waveguide-integrated SNSPDs at its outputs. We show static reconfigurability of our system with a bias-drift-free operation over a time of 12 hours, as well as high-speed modulation at a frequency up to 1 GHz. Our results provide blueprints for implementing complex quantum photonic devices on the LNOI platform

Photonics for artificial intelligence and neuromorphic computing

Research in photonic computing has flourished due to the proliferation of optoelectronic components on photonic integration platforms. Photonic integrated circuits have enabled ultrafast artificial neural networks, providing a framework for a new class of information processing machines. Algorithms running on such hardware have the potential to address the growing demand for machine learning and artificial intelligence, in areas such as medical diagnosis, telecommunications, and high-performance and scientific computing. In parallel, the development of neuromorphic electronics has highlighted challenges in that domain, in particular, related to processor latency. Neuromorphic photonics offers sub-nanosecond latencies, providing a complementary opportunity to extend the domain of artificial intelligence. Here, we review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges

Controlling All Degrees of Freedom of the Optical Coupling in Hybrid Quantum Photonics

Nanophotonic quantum devices can significantly boost light–matter interaction, which is important for applications such as quantum networks. Reaching a high interaction strength between an optical transition of a spin system and a single mode of light is an essential step that demands precise control over all degrees of freedom of the optical coupling. While current devices have reached a high accuracy of emitter positioning, the placement process remains overall statistically, reducing the device fabrication yield. Furthermore, not all degrees of freedom of the optical coupling can be controlled, limiting the device performance. Here, we develop a hybrid approach based on negatively charged silicon vacancy center in nanodiamonds coupled to a mode of a Si3N4-photonic crystal cavity, where all terms of the coupling strength can be controlled individually. We used the frequency of coherent Rabi oscillations and line-broadening as a measure of the device performance. This allows for iterative optimization of the position and rotation of the dipole with respect to individual preselected modes of light. Therefore, our work marks an important step for optimization of hybrid quantum photonics and enables us to align device simulations with real device performance