Reconfigurable Photonic Circuits Research Articles

Heterogeneous integration of amorphous silicon carbide on thin film lithium niobate

In the past decade, lithium niobate (LiNbO3 or LN) photonics, thanks to its heat-free and fast electro-optical modulation, second-order non-linearities, and low-loss, has been extensively investigated. Despite numerous demonstrations of high-performance LN photonics, processing lithium niobate remains challenging and suffers from incompatibilities with standard complementary metal–oxide–semiconductor (CMOS) fabrication lines, limiting its scalability. Silicon carbide (SiC) is an emerging material platform with a high refractive index, a large non-linear Kerr coefficient, and a promising candidate for heterogeneous integration with LN photonics. Current approaches of SiC/LN integration require transfer-bonding techniques, which are time-consuming, expensive, and lack precision in layer thickness. Here, we show that amorphous silicon carbide (a-SiC), deposited using inductively coupled plasma enhanced chemical vapor deposition at low temperatures (<165 °C), can be conveniently integrated with LiNbO3 and processed to form high-performance photonics. Most importantly, the fabrication only involves a standard, silicon-compatible, reactive ion etching step and leaves the LiNbO3 intact, hence its compatibility with standard foundry processes. As a proof-of-principle, we fabricated waveguides and ring resonators on the developed a-SiC/LN platform and achieved intrinsic quality factors higher than 1.06 × 105 and a resonance electro-optic tunability of 3.4 pm/V with a 3 mm tuning length. We showcase the possibility of dense integration by fabricating and testing ring resonators with a 40 μm radius without a noticeable loss penalty. Our platform offers a CMOS-compatible and scalable approach for the implementation of future fast electro-optic modulators and reconfigurable photonic circuits, as well as nonlinear processes that can benefit from involving both second- and third-order nonlinearities.

Ultra-precise, sub-picometer tunable free spectral range in a parabolic microresonator induced by optical fiber bending.

Surface nanoscale axial photonic (SNAP) microresonators are fabricated on silica optical fibers, leveraging silica's outstanding material and mechanical properties. These properties allow for precise control over the microresonators' dimension, shape, and mode structure, a key feature for reconfigurable photonic circuits. Such circuits find applications in high-speed communications, optical computing, and optical frequency combs (OFCs). However, consistently producing SNAP microresonators with equally spaced eigenmodes has remained challenging. In this study, we introduce a method to induce a SNAP microresonator with a parabolic profile. We accomplish this by bending a silica optical fiber in a controlled manner using two linear stages. This approach achieves a uniform free spectral range (FSR) as narrow as 1 pm across more than 45 modes. We further demonstrate that the FSR of the SNAP microresonator can be continuously adjusted over a range nearly as wide as one FSR itself, specifically from 1.09 to 1.72 pm, with a precision of ±0.01 pm and high repeatability. Given its compact size and tuning capability, this SNAP microresonator is highly promising for various applications, including the generation of tunable low-repetition-rate OFC and delay lines.

QUICK3$^3$ ‐ Design of a Satellite‐Based Quantum Light Source for Quantum Communication and Extended Physical Theory Tests in Space

AbstractModern quantum technologies have matured such that they can now be used in space applications, e.g., long‐distance quantum communication. Here, the design of a compact true single photon source is presented that can enhance the secure data rates in satellite‐based quantum key distribution scenarios compared to conventional laser‐based light sources. The quantum light source is a fluorescent color center in hexagonal boron nitride. The emitter is off‐resonantly excited by a diode laser and directly coupled to an integrated photonic processor that routes the photons to different experiments performed directly on‐chip: i) the characterization of the single photon source and ii) testing a fundamental postulate of quantum mechanics, namely the relation of the probability density and the wave function (known as Born's rule). The described payload is currently being integrated into a 3U CubeSat and scheduled to launch in 2024 into low Earth orbit. Therefore the feasibility of true single photon sources and reconfigurable photonic circuits in space can be evaluated. This provides a promising route toward a high‐speed quantum network.

Fast Cycling Speed with Multimillion Cycling Endurance of Ultra‐Low Loss Phase Change Material (Sb2Se3) by Engineered Laser Pulse Irradiation

AbstractProgrammable and reconfigurable photonics is revolutionized the next generation of Si/SiN‐based photonics processors, optical signal processing, and neural and quantum networks. Ultra‐low loss phase‐change technology has the enormous potential to offer energy‐efficient, extra large‐scale integrated (ELSI) nonvolatile programmable photonics that can overcome the limitations of commonly used volatile and energy‐hungry programmable photonic systems. So far, most of the phase change materials (PCM) devices have already shown to be suitable for programmable and reconfigurable photonic circuit applications that do not require extreme endurance or ultra‐fast switching speeds. However, applications with ultra‐fast speed and multi‐million cycling requirements, such as memory cells, quantum computing, optical displays, and optical modulators necessitate enormous improvements in the existing cycling speed and endurance of PCMs, for high‐performance device configuration. Here, optical switching of an ultra‐low loss PCM (Sb2Se3) is demonstrated with an unprecedented combination of rapid cycling speed (0.6 × 105 cycles s−1) and extreme endurance (> 2 × 106 cycles) that pave the way for ultrafast, large‐scale, programmable, and reconfigurable integrated photonic circuits and devices.

Time-Resolved Temperature Mapping Leveraging the Strong Thermo-Optic Effect in Phase-Change Materials.

Optical phase-change materials are highly promising for emerging applications such as tunable metasurfaces, reconfigurable photonic circuits, and non-von Neumann computing. However, these materials typically require both high melting temperatures and fast quenching rates to reversibly switch between their crystalline and amorphous phases: a significant challenge for large-scale integration. In this work, we use temperature-dependent ellipsometry to study the thermo-optic effect in GST and use these results to demonstrate an experimental technique that leverages the thermo-optic effect in GST to enable both spatial and temporal thermal measurements of two common electro-thermal microheater designs currently used by the phase-change community. Our approach shows excellent agreement between experimental results and numerical simulations and provides a noninvasive method for rapid characterization of electrically programmable phase-change devices.

Integrated Bragg grating filters based on silicon-Sb2Se3 with non-volatile bandgap engineering capability.

Integrated optical filters show outstanding capability in integrated reconfigurable photonic applications, including wavelength division multiplexing (WDM), programmable photonic processors, and on-chip quantum photonic networks. Present schemes for reconfigurable filters either have a large footprint or suffer from high static power consumption, hindering the development of reconfigurable photonic integrated systems. Here, a reconfigurable hybrid Bragg grating filter is elaborately designed through a precise, modified coupling mode theory. It is also experimentally presented by integrating non-volatile phase change material (PCM) Sb2Se3 on silicon to realize compact, low-loss, and broadband engineering operations. The fabricated filter holds a compact footprint of 0.5 µm × 43.5 µm and maintains a low insertion loss of < 0.5 dB after multiple levels of engineering to achieve crystallization. The filter is able to switch from a low-loss transmission state to the Bragg reflection state, making it a favorable solution for large-scale reconfigurable photonic circuits. With a switching extinction ratio over 30 dB at 1504.85 nm, this hybrid filter breaks the tradeoff between insertion loss and tuning range. These results reveal its potential as a new candidate for a basic element in large-scale non-volatile reconfigurable systems.

Scalable simulation of programmable photonic integrated waveguide meshs

Programmable photonic integrated waveguide mesh (PPIWM), which is similar to the field-programmable gate array (FPGA) in electronics, can be reconfigured to form a variety of mesh topologies to provide different signal processing functions flexibly. With the development of large-scale PPIWM, multipath routing is becoming more and more complex. Automatic and efficient calculation of the transmission characteristics between arbitrary input and output ports is very essential to realize the simulation and theoretical analysis task for the PPIWMs. In this paper, a simple and effective calculation method for the transmission characteristics of PPIWMs with any scale is proposed. Comparedwiththeavailabletechnologies, this method can greatly reduce the time of solving the total scattering matrix in the large-scale PPIWM and has the advantage of easy programming. Typical simulation examples are given to verify the effectiveness of this method. The generalityandhighefficiency of this method can help us analyze increasingly large and complex reconfigurable photonic circuits with a wide variety of functions.

Realization of Mechanically Maneuverable Circuit Ports in Organic Hybrid Photonic Chip for 360° Steering of Bandwidth‐Engineered Signals

AbstractPhoton‐based data communication technologies provide safe, efficient, and speedy data transfer. The conventional silicon‐based photonic circuits possess several inherent shortcomings. One of them is the mechanical rigidity of circuits. Realizing a flexible organic photonic integrated circuit (FOPIC) or organic photonic chip with functional complexity requires innovative materials and a hybrid materials platform. Mechanically flexible organic acicular crystals with fewer imperfections, high refractive index, and high quantum yield are promising materials for FOPIC fabrication. Here, the fabrication of a hybrid FOPIC using (E)‐1‐(4‐(N,N‐dimethyl)phenyl)iminomethyl‐2‐hydroxyl‐naphthalene (DPIN) and (E)‐1‐(4‐(chloro)phenyl) iminomethyl‐2‐hydroxyl‐naphthalene (CPIN) crystals for precise steering of output light at different angles is presented. The crystals of DPIN and CPIN are mechanically compliant and optically dissimilar. Mechanical micromanipulation of DPIN and CPIN flexible crystals provides a four‐port FOPIC that comprises waveguide integrated ring resonator. Mechanical maneuverability of the circuit ports enables selective routing of six bandwidth‐engineered signals at desirable angles (0–360°) in 2D. The circuit applies active/passive light‐guiding principles, reabsorbance, and energy‐transfer mechanisms for its operation. The presented proof‐of‐principle reconfigurable photonic circuit concept is advantageous in programmable circuits, navigable detectors, and intelligent sensors.

Zero-Power Calibration of Photonic Circuits at Cryogenic Temperatures

The continual success of superconducting photon-detection technologies in quantum photonics asserts cryogenic-compatible systems as a cornerstone of full quantum photonic integration. Here, we present a way to reversibly fine-tune the optical properties of individual waveguide structures through local changes to their geometry using solidified xenon. Essentially, we remove the need for additional on-chip calibration elements, effectively zeroing the power consumption tied to reconfigurable elements, with virtually no detriment to photonic device performance. We enable passive circuit tuning in pressure-controlled environments, locally manipulating the cladding thickness over portions of optical waveguides. We realize this in a cryogenic environment, through controlled deposition of xenon gas and precise tuning of its thickness using sublimation, triggered by on-chip resistive heaters. $\pi$ phase shifts occur over a calculated length of just $L_{\pi}$ = 12.3$\pm$0.3 $\mu m$. This work paves the way towards the integration of compact, reconfigurable photonic circuits alongside superconducting detectors, devices, or otherwise.

Nanoimaging of Low-Loss Plasmonic Waveguide Modes in a Graphene Nanoribbon.

Graphene nanoribbons are predicted to support low-loss and tunable plasmonic waveguide modes with an ultrasmall mode area. Experimental observation of the plasmonic waveguide modes in graphene nanoribbons, however, is challenging because conventional wet lithography has difficulty creating a clean graphene nanoribbon with a low edge roughness. Here, we use a dry lithography method to fabricate ultraclean and low-roughness graphene nanoribbons, which are then encapsulated in hexagonal boron nitride (hBN). We demonstrate low-loss plasmon propagation with a quality factor up to 35 in the ultraclean nanoribbon waveguide using cryogenic infrared nanoscopy. In addition, we observe both the fundamental and the higher-order plasmonic waveguide modes for the first time. All the plasmon waveguide modes can be tuned through electrostatic gating. The observed tunable plasmon waveguide modes in ultraclean graphene nanoribbons agree well with the finite-difference time-domain (FDTD) simulation results. They are promising for reconfigurable photonic circuits and devices at a subwavelength scale.

Reconfigurable photonics with on-chip single-photon detectors

Integrated quantum photonics offers a promising path to scale up quantum optics experiments by miniaturizing and stabilizing complex laboratory setups. Central elements of quantum integrated photonics are quantum emitters, memories, detectors, and reconfigurable photonic circuits. In particular, integrated detectors not only offer optical readout but, when interfaced with reconfigurable circuits, allow feedback and adaptive control, crucial for deterministic quantum teleportation, training of neural networks, and stabilization of complex circuits. However, the heat generated by thermally reconfigurable photonics is incompatible with heat-sensitive superconducting single-photon detectors, and thus their on-chip co-integration remains elusive. Here we show low-power microelectromechanical reconfiguration of integrated photonic circuits interfaced with superconducting single-photon detectors on the same chip. We demonstrate three key functionalities for photonic quantum technologies: 28 dB high-extinction routing of classical and quantum light, 90 dB high-dynamic range single-photon detection, and stabilization of optical excitation over 12 dB power variation. Our platform enables heat-load free reconfigurable linear optics and adaptive control, critical for quantum state preparation and quantum logic in large-scale quantum photonics applications.

Tantalum pentoxide nanophotonic circuits for integrated quantum technology.

Nanophotonics holds great promise for integrated quantum technologies, but realizing all functionalities for processing quantum states of light in optical waveguides poses an outstanding challenge. Here we show that tantalum pentoxide-on-insulator offers significant advantages for such purpose and experimentally demonstrate crucial photonic integrated circuit components. Exploiting advanced nanophotonic design and state-of-the-art nanofabrication processes, we realize low-loss waveguiding with 1 dB/cm propagation loss, efficient optical fiber-chip interfaces with more than 100 nm bandwidth, micro-ring resonators with quality factors of 357,200 and tunable directional couplers. We further achieve active functionality with nano-electromechanical phase-shifters. Our work enables reconfigurable photonic circuit configurations in the Ta2O5 material system with highly favorable optical properties for integrated quantum photonics.

Graph Representations for Programmable Photonic Circuits

We propose graph representations for reconfigurable photonic mesh circuits. Waveguide mesh circuits are abstracted into a graph to highlight the connectivity and topology. We model the optical ports as graph nodes. Performance metrics for each connection are incorporated into the edge attributes in categories such as propagation loss, crosstalk penalty, power consumption, phase accumulation, and so on. We use three types of graph representations for tunable couplers to model the flow of light and create a circuit graph representation to an example hexagonal mesh. The representation should respect the physics of waveguide circuits (e.g. directional flow of light). Of the three types, the directed graph with eight artificial nodes performs best for solving light distributions with feedback paths. This graph representation is demonstrated in four distribution cases: a single pair input-output, multi-pair inputs and outputs without collisions, a single input to multiple outputs (distribution), and multiple distributions without collisions. The programming tolerance against malfunctioning tunable elements is also demonstrated. With this circuit representation, we can reduce all these distribution cases to different graph problems and leverage a wealth of existing algorithms developed in graph theory to program the photonic mesh. Thus we provide a systematical strategy to design and program complex reconfigurable photonic circuits, especially in photonic meshes with feedback paths.

Modeling and control of a reconfigurable photonic circuit using deep learning

The complexity of experimental quantum information processing devices is increasing rapidly, requiring new approaches to control them. In this paper, we address the problems of practically modeling and controlling an integrated optical waveguide array chip—a technology expected to have many applications in telecommunications and optical quantum information processing. This photonic circuit can be electrically reconfigured, but only the output optical signal can be monitored. As a result, the conventional control methods cannot be naively applied. Characterizing such a chip is challenging for three reasons. First, there are uncertainties associated with the Hamiltonian model describing the chip. Second, we expect distortions of the control voltages caused by the chip’s electrical response, which cannot be directly observed. And third, there are imperfections in the measurements caused by losses from coupling the chip externally to optical fibers. We have developed a deep neural network (NN) approach to solve these problems. The architecture is designed specifically to overcome the aforementioned challenges using a gated recurrent unit -based network as the central component. The Hamiltonian is estimated as a blackbox, while the rules of quantum mechanics such as state evolution is embedded in the structure as a whitebox. The resulting overall graybox model of the chip shows good performance both quantitatively in terms of the mean square error and qualitatively in terms of the shape of the predicted waveforms We use this NN to solve a classical and a quantum control problem. In the classical application we find a control sequence to approximately realize a time-dependent output power distribution. For the quantum application we obtain the control voltages to realize a target set of quantum gates. The method we propose is generic and can be applied to other systems that can only be probed indirectly.

Interfacing scalable photonic platforms: solid-state based multi-photon interference in a reconfigurable glass chip

Scaling-up optical quantum technologies requires a combination of highly efficient multi-photon sources and integrated waveguide components. Here, we interface these scalable platforms, demonstrating high-rate three-photon interference with a quantum dot based multi-photon source and a reconfigurable photonic chip on glass. We actively demultiplex the temporal train of single photons obtained from a quantum emitter to generate a 3.8×103 s−1three-photon source, which is then sent to the input of a tunable tritter circuit, demonstrating the on-chip quantum interference of three indistinguishable single photons. We show via pseudo number-resolving photon detection characterizing the output distribution that this first combination of scalable sources and reconfigurable photonic circuits compares favorably in performance with respect to previous implementations. Our detailed loss-budget shows that merging solid-state multi-photon sources and reconfigurable photonic chips could allow 10-photon experiments on chip at ∼40 s−1 rate in a foreseeable future.

Free spectral range electrical tuning of a high quality on-chip microcavity.

Reconfigurable photonic circuits have applications ranging from next-generation computer architectures to quantum networks, coherent radar and optical metamaterials. Here, we demonstrate an on-chip high quality microcavity with resonances that can be electrically tuned across a full free spectral range (FSR). FSR tuning allows resonance with any source or emitter, or between any number of networked microcavities. We achieve it by integrating nanoelectronic actuation with strong optomechanical interactions that create a highly geometry-dependent effective refractive index. This allows low voltages and sub-nanowatt power consumption. We demonstrate a basic reconfigurable photonic network, bringing the microcavity into resonance with an arbitrary mode of a microtoroidal optical cavity across a telecommunications fibre link. Our results have applications beyond photonic circuits, including widely tuneable integrated lasers, reconfigurable optical filters for telecommunications and astronomy, and on-chip sensor networks.

A silicon microring resonator with a nanolatch mechanism

A microring resonator with a waveguide latch mechanism is designed and fabricated on silicon-on-insulator wafer as switchable notch filter. The waveguide is 400 nm in width and 260 nm in thickness. The latch mechanism consists of three ultrasmall electrostatic comb-drive actuators with nano hooks. One of the actuators is used for moving busline waveguide to the microring and the other two actuators are used for latching the busline waveguide. The busline waveguide is decoupled from the microring in the switch-off state, and it is coupled to the microring in the switch-on state to filter at the resonant wavelength. The position of the busline waveguide is hold by the hooks of latches without applying voltage. The gap between the busline waveguide and the microring was approximately 108 nm after latching, which corresponded to an undercoupling condition of the microring resonator. Applying another voltage to the two latch actuators, the busline waveguide was released and returned to the initial switch-off state. The proposed nanolatch mechanism is useful for reconfigurable photonic circuits using microring resonators.

Generating, manipulating and measuring entanglement and mixture with a reconfigurable photonic circuit

Entanglement is the quintessential quantum mechanical phenomenon understood to lie at the heart of future quantum technologies and the subject of fundamental scientific investigations. Mixture, resulting from noise, is often an unwanted result of interaction with an environment, but is also of fundamental interest, and is proposed to play a role in some biological processes. Here we report an integrated waveguide device that can generate and completely characterize pure two-photon states with any amount of entanglement and arbitrary single-photon states with any amount of mixture. The device consists of a reconfigurable integrated quantum photonic circuit with eight voltage controlled phase shifters. We demonstrate that for thousands of randomly chosen configurations the device performs with high fidelity. We generate maximally and non-maximally entangled states, violate a Bell-type inequality with a continuum of partially entangled states, and demonstrate generation of arbitrary one-qubit mixed states.

Quantum optics on a chip

Researchers have demonstrated a reconfigurable photonic circuit on a chip that can create a four-photon entangled state. The scalability and compactness of the device opens the door towards practical quantum computation.