Photonic Waveguides Research Articles

Dispersion-tunable photonic topological waveguides

Dispersion-tunable photonic topological waveguides have recently attracted much attention, due to their promising applications on topological devices with tunable operational frequencies. Since dispersions of topological waveguides traverse the whole bandgaps of bulk structures, tuning the dispersions (especially the bandwidths) requires changing the whole bulk of corresponding photonic topological insulators. A previously reported material-modification approach provided a parallel tuning on such numerous lattices; however, the increased material loss deteriorated transmissions of the topological waveguide. Here, a parallel tuning approach on structures is theoretically proposed and demonstrated, which spawns dispersion-tunable photonic topological waveguides without increasing material loss. Based on the bilayer honeycomb model, a topological valley waveguide by utilizing bilayer designer plasmonic structures is constructed, accomplished with dispersion tunings by altering interlayer distance. Experimental results validate the theoretical model and display a 61%-relative-tuning range of frequency, with a tunable relative bandwidth up to 16%. This approach may promise applications in tunable topological lasers, robust delay lines, and intelligent photonic devices.

Light dynamics around an exceptional point in a 1D photonic bandgap waveguide

Exceptional points (EP) in a system parameter space at which eigenvalues and corresponding eigenvectors coalesce are ubiquitous in non-Hermitian systems. Many unconventional applications have been proposed while encircling around the EPs. One of the unique application is the direction-dependent mode conversion. Here the appearance of an EP has been investigated in a planar 1D Bragg reflection waveguide(BRW) geometry, which has provided an additional degree of freedom to explore EP-based exotic light dynamics. A planar 1D BRW consists of periodic dielectric arrays and a defect dielectric layer forming the core. An inhomogeneous customized gain-loss profile is incorporated in the core region to couple two quasi-guided transverse electric (TE) modes so that the waveguide hosts an EP of order two. Here, we propose a dynamical EP encirclement scheme and corresponding asymmetric mode conversion phenomenon between two photonic bandgap quasi-guided TE modes in a 1D photonic bandgap-guided structure. Our findings will be potentially important to open up a fertile platform using the paradigm of non-Hermitian coupling to meet a wide range of exotic integrated chip-scale applications in the context of mode selectivity for switching and conversion.

Standalone, CMOS-based Faraday rotation in a silicon photonic waveguide.

Nonreciprocity is a fundamental requirement of signal isolation in optical communication systems. However, on chip isolator designs require either post-processing steps or external magnetic biasing, which are impractical for commercial applications. This raises the need for standalone devices which support nonreciprocal functionality using standardized fabrication techniques. Here, we report the first design of an electromagnetic coil surrounding a waveguide which exclusively employed the complementary metal-oxide-semiconductor (CMOS) process flow. The coil supported an electric current up to 14 mA. In simulations, it generated an alternating magnetic flux density up to 1.16 mT inside a strip waveguide and thereby induced a rotation of 50.71 picodegrees for the fundamental transverse-magnetic mode at a wavelength of 1352 nm. Our analysis further revealed methods to increase the rotation by orders of magnitude. It demonstrated the scope of manufacturing processes and serves as a building block for the development of a commercially viable, on-chip optical isolator.

An integrated photonic device for on-chip magneto-optical memory reading.

This study presents the design, fabrication and experimental demonstration of a magneto-photonic device that delivers non-volatile photonic memory functionality. The aim is to overcome the energy and speed bottleneck of back-and-forth signal conversion between the electronic and optical domains when retrieving information from non-volatile memory. The device combines integrated photonic components based on the InP membrane on silicon (IMOS) platform and a non-volatile, built-in memory element (ferromagnetic thin-film multilayers) realized as a top-cladding on the photonic waveguides (a post-processing step). We present a design where the phase of the guided light is engineered via two mechanisms: the polar magneto-optical Kerr effect (MOKE) and the propagation in an asymmetrical cross-section (triangular) waveguide. Thanks to its design, the device yields different mode-specific transmissions depending on the memory state it encodes. We demonstrate the recording of the magnetic hysteresis using the transmitted optical signal, providing direct proof for all optical magnetic memory reading using an integrated photonic chip. Using mathematical model and optical simulations, we support the experimental observations and quantitatively reproduce the Kerr signal amplitudes on-chip. A 1% transmitted power contrast from devices is promising indicating that in a shot noise limited scenario the theoretical bandwidth of memory read-out exceeds 50 Gbits/s.

Integrated 1 × 3 MEMS silicon nitride photonics switch.

We present a 1 × 3 optical switch based on a translational microelectromechanical system (MEMS) platform with integrated silicon nitride (SiN) photonic waveguides. The fabricated devices demonstrate efficient optical signal transmission between fixed and suspended movable waveguides. We report a minimum average insertion loss of 4.64 dB and a maximum average insertion loss of 5.83 dB in different switching positions over a wavelength range of 1530 nm to 1580 nm. The unique gap closing mechanism reduces the average insertion loss across two air gaps by a maximum of 7.89 dB. The optical switch was fabricated using a custom microfabrication process developed by AEPONYX Inc. This microfabrication process integrates SiN waveguides with silicon-on-insulator based MEMS devices with minimal stress related deformation of the MEMS platform.

Photonic neuromorphic computing using vertical cavity semiconductor lasers

Photonic realizations of neural network computing hardware are a promising approach to enable future scalability of neuromorphic computing. The number of special purpose neuromorphic hardware and neuromorphic photonics has accelerated on such a scale that one can now speak of a Cambrian explosion. Work along these lines includes (i) high performance hardware for artificial neurons, (ii) the efficient and scalable implementation of a neural network’s connections, and (iii) strategies to adjust network connections during the learning phase. In this review we provide an overview on vertical-cavity surface-emitting lasers (VCSELs) and how these high-performance electro-optical components either implement or are combined with additional photonic hardware to demonstrate points (i-iii). In the neurmorphic photonics context, VCSELs are of exceptional interest as they are compatible with CMOS fabrication, readily achieve 30% wall-plug efficiency, >30 GHz modulation bandwidth and multiply and accumulate operations at sub-fJ energy. They hence are highly energy efficient and ultra-fast. Crucially, they react nonlinearly to optical injection as well as to electrical modulation, making them highly suitable as all-optical as well as electro-optical photonic neurons. Their optical cavities are wavelength-limited, and standard semiconductor growth and lithography enables non-classical cavity configurations and geometries. This enables excitable VCSELs (i.e. spiking VCSELs) to finely control their temporal and spatial coherence, to unlock terahertz bandwidths through spin-flip effects, and even to leverage cavity quantum electrodynamics to further boost their efficiency. Finally, as VCSEL arrays they are compatible with standard 2D photonic integration, but their emission vertical to the substrate makes them ideally suited for scalable integrated networks leveraging 3D photonic waveguides. Here, we discuss the implementation of spatially as well as temporally multiplexed VCSEL neural networks and reservoirs, computation on the basis of excitable VCSELs as photonic spiking neurons, as well as concepts and advances in the fabrication of VCSELs and microlasers. Finally, we provide an outlook and a roadmap identifying future possibilities and some crucial milestones for the field.

Photonic waveguides shed their cladding

The slimmed-down conduits avoid cross talk between adjacent channels by using materials that support different wave modes.

Observation of topological Anderson phase in laser-written quasi-periodic waveguide arrays.

We report on the experimental observation of the topological Anderson phase in one-dimensional quasi-periodical waveguide arrays produced by femtosecond laser writing. The evanescently coupled waveguides are with alternating coupling constants, constructing photonic lattices analogous to the Su-Schrieffer-Heeger model. Dynamic tuning of the interdimer hopping amplitudes of the waveguide array generates the quasi-periodic disorder of the coupling constants for the model. As light propagates in the corresponding photonic waveguides, it exhibits different modes depending on the magnitude of the disorder. The topological Anderson phase is observed as the disorder is sufficiently strong, which corresponds to the zero-energy mode in its spectrum. The experimental results are consistent with the theoretical simulations, confirming the existence of the disorder-driven topological phase from a trivial band in the photonic lattice.

Mode-selective single-dipole excitation and controlled routing of guided waves in a multi-mode topological waveguide

Topology-linked binary degrees of freedom of guided waves have been used to expand the channel capacity of and to ensure robust transmission through photonic waveguides. However, selectively exciting optical modes associated with the desired degree of freedom is challenging and typically requires spatially extended sources or filters. Both approaches are incompatible with the ultimate objective of developing compact mode-selective sources powered by single emitters. In addition, the implementation of highly desirable functionalities, such as controllable distribution of guided modes between multiple detectors, becomes challenging in highly compact devices due to photon loss to reflections. Here, we demonstrate that a linearly polarized dipole-like source can selectively excite a topologically robust edge mode with the desired valley degree of freedom. Reflection-free routing of valley-polarized edge modes into two spatially separated detectors with reconfigurable splitting ratios is also presented. An optical implementation of such a source will have the potential to broaden the applications of topological photonic devices.

Design and Bulk Sensitivity Analysis of a Silicon Nitride Photonic Biosensor for Cancer Cell Detection

Bulk sensitivity is an important parameter to validate the efficiency of the photonic waveguide sensor. Due to recent advancements in point-of-care silicon photonic biosensing, the focus is to identify the effective way to improve sensitivity. Integrating polydimethylsiloxane (PDMS) microfluidic channel in sensor architecture decreases the sensitivity due to leakage of molecules at edges. The silicon nitride (SiN4) Mach–Zehnder interferometer utilizes the refractive index of different cancer cells (1.39–1.401) to determine the bulk sensitivity. The proposed gradient step rib-slot structure of 970 nm wide and 400 nm thickness is designed to hold the liquid sample without any PDMS material. This novel waveguide exhibits high waveguide bulk sensitivity S w , bulk and device bulk sensitivity S d compared with the gradient rib waveguide. We achieved a waveguide bulk sensitivity S w , bulk of 2.0699 RIU/RIU and device sensitivity S d of 568 nm/RIU through finite-difference time-domain (FDTD) analysis.

Neural network-based surrogate model for inverse design of metasurfaces

Metasurfaces composed of spatially arranged ultrathin subwavelength elements are promising photonic devices for manipulating optical wavefronts, with potential applications in holography, metalens, and multiplexing communications. Finding microstructures that meet light modulation requirements is always a challenge in designing metasurfaces, where parameter sweep, gradient-based inverse design, and topology optimization are the most commonly used design methods in which the massive electromagnetic iterations require the design computational cost and are sometimes prohibitive. Herein, we propose a fast inverse design method that combines a physics-based neural network surrogate model (NNSM) with an optimization algorithm. The NNSM, which can generate an accurate electromagnetic response from the geometric topologies of the meta-atoms, is constructed for electromagnetic iterations, and the optimization algorithm is used to search for the on-demand meta-atoms from the phase library established by the NNSM to realize an inverse design. This method addresses two important problems in metasurface design: fast and accurate electromagnetic wave phase prediction and inverse design through a single phase-shift value. As a proof-of-concept, we designed an orbital angular momentum (de)multiplexer based on a phase-type metasurface, and 200 Gbit/s quadrature-phase shift-keying signals were successfully transmitted with a bit error rate approaching 1.67 × 10 − 6 . Because the design is mainly based on an optimization algorithm, it can address the “one-to-many” inverse problem in other micro/nano devices such as integrated photonic circuits, waveguides, and nano-antennas.

Higher-order topological pumping and its observation in photonic lattices

The discovery of the quantization of particle transport in adiabatic pumping cycles of periodic structures by Thouless [Phys. Rev. B 27, 6083 (1983)] linked the Chern number, a topological invariant characterizing the quantum Hall effect in two-dimensional electron gases, with the topology of dynamical periodic systems in one dimension. Here, we demonstrate its counterpart for higher-order topology. Specifically, we show that adiabatic cycles in two-dimensional crystals with vanishing dipole moments (and therefore zero overall particle transport) can nevertheless be topologically nontrivial. These cycles are associated with higher-order topology and can be diagnosed by their ability to produce corner-to-corner transport in certain metamaterial platforms. We experimentally verify the corner to corner transport associated with this topological pump by using an array of photonic waveguides adiabatically modulated in their separations and refractive indices. By mapping the dynamical phenomenon demonstrated here from two spatial and one temporal dimensions to three spatial dimensions, our observations are equivalent to the observation of chiral hinge states in a three-dimensional second-order topological insulator.

Tunable non-Markovian dynamics with a three-level atom mediated by the classical laser in a semi-infinite photonic waveguide

In this work, we investigate the non-Markovian (NM) dynamical evolution of a three-level atom coupled to a semi-infinite one-dimensional photonic waveguide and driven by a classical driving field. The single-end of waveguide behaves as a perfect mirror while light can pass through the opposite end with no backreflection. We derive the exact analytical expressions for the wave function of the driven three-level atom by solving a set of delay differential equations and give the conditions of atom-photon bound state formation that can inhibit the dissipation, which appears in the atom-mirror interspace in order to trap a considerable amount of initial atomic excitation. We show that when the evolution time of the system is shorter than the time delay taken by a photon to perform a round trip between the atom and mirror, the atomic dynamics exhibits Markovian properties, which depend on the driving strength and rescaled parameters. If the evolution time of the system exceeds the time delay, the photon can be reabsorbed by the atom with the photon emitted by the atom having completed the round trip, which exhibits NM memory effects, where the population eventually convergences to the finite-steady values. We also study the influence of the loss and dephasing on the formation of a bound state. Finally, the above results are extended to a more general quantum system involving single-end photonic waveguide coupled to an arbitrary number of noninteracting three-level atoms driven by the driving fields. The presented formalism might open a way to better understand exactly the NM dynamics of driven multilevel atoms coupled to a semi-infinite waveguide.

Photonic waveguides shed their cladding

Photonic waveguides shed their cladding

Topological Corner States in Non-Unitary Coinless Discrete-Time Quantum Walks

The discrete-time quantum walk provides a versatile platform for exploring abundant topological phenomena due to its intrinsic spin-orbit coupling. In this work, we study the non-Hermitian second-order topology in a two-dimensional non-unitary coinless discrete-time quantum walk, which is realizable in the three-dimensional photonic waveguides. By adding the non-unitary gain-loss substep operators into the one-step operator of the coinless discrete-time quantum walk, we find the appearance of the four-degenerate zero-dimensional corner states at ReE = 0 when the gain-loss parameter of the system is larger than a critical value. This intriguing phenomenon originates from the nontrivial second-order topology of the system, which can be characterized by a second-order topological invariant of polarizations. Finally, we show that the exotic corner states can be observed experimentally through the probability distributions during the multistep non-unitary coinless discrete-time quantum walks. Our work potentially pave the way for exploring exotic non-Hermitian higher-order topological states of matter in coinless discrete-time quantum walks.

High-bandwidth Si/In2O3 hybrid plasmonic waveguide modulator

A novel Si/In2O3 hybrid plasmonic waveguide modulator was experimentally realized by using an asymmetric directional coupler (ADC), which consists of a silicon photonic waveguide and a Si/In2O3 hybrid plasmonic waveguide. All the silicon cores are covered with a silica layer, above which there is a metal–oxide–semiconductor (MOS) capacitor consisting of the In2O3/HfO2/Au layers. The Au layer sitting on the top of the MOS capacitor works as the top-electrode, while the In2O3 thin film covers the sidewall and contacts with the Au bottom-electrode. When the bias voltage is not applied, light launched from the silicon photonic waveguide is weakly coupled into the Si/In2O3 hybrid plasmonic waveguide, and thus, one has a high transmission at the through port of the ADC. On the other hand, when the bias voltage is applied, the carrier density in the In2O3 layer is changed, which introduces some modification to the refractive index of the In2O3 thin film. As a result, light is strongly cross-coupled from the silicon photonic waveguide to a Si/In2O3 hybrid plasmonic waveguide, and one has low transmission at the through port. In this Letter, an ultra-compact Si/In2O3 hybrid plasmonic waveguide modulator is realized with a 3.5-μm-long ADC. In the experiments, the fabricated waveguide modulator works well and exhibits a high modulation bandwidth of >40 GHz for the first time.

High-efficiency four-wave mixing in low-loss silicon photonic spiral waveguides beyond the singlemode regime

Low-loss optical waveguides are highly desired for nonlinear photonics such as four-wave mixing (FWM), optical parametric amplification, and pulse shaping. In this work, low-loss silicon photonic spiral waveguides beyond the single-mode regime are proposed and demonstrated for realizing an enhanced FWM process. In particular, the designed 2-µm-wide silicon photonic waveguides are fabricated with standard foundry processes and have a propagation loss as low as ∼0.28 dB/cm due to the reduced light-matter interaction at the waveguide sidewalls. In the experiments, strong FWM effect is achieved with a high conversion efficiency of -8.52 dB in a 2-µm-wide and 20-cm-long silicon photonic waveguide spiral, and eight new wavelengths are generated with the pump power of ∼80 mW (corresponding to a low power density of ∼195 mW/µm2). In contrast, the FWM efficiency for the 0.45-µm-wide waveguide spiral is around -15.4 dB, which is much lower than that for the 2-µm-wide waveguide spiral. It can be seen that silicon photonics beyond the singlemode regime opens a new avenue for on-chip nonlinear photonics and will bring new opportunities for nonlinear photonic applications.

Bimorphic Floquet topological insulators.

Topological theories have established a unique set of rules that govern the transport properties in a wide variety of wave-mechanical settings. In a marked departure from the established approaches that induce Floquet topological phases by specifically tailored discrete coupling protocols or helical lattice motions, we introduce a class of bimorphic Floquet topological insulators that leverage connective chains with periodically modulated on-site potentials to reveal rich topological features in the system. In exploring a 'chain-driven' generalization of the archetypical Floquet honeycomb lattice, we identify a rich phase structure that can host multiple non-trivial topological phases associated simultaneously with both Chern-type and anomalous chiral states. Experiments carried out in photonic waveguide lattices reveal a strongly confined helical edge state that, owing to its origin in bulk flat bands, can be set into motion in a topologically protected fashion, or halted at will, without compromising its adherence to individual lattice sites.

Analytical and Numerical Analyses of Multilayer Photonic Metamaterial Slab Optical Waveguide Structures with Kerr-Type Nonlinear Cladding and Substrate

In this paper, we propose the analytical and numerical analyses of multilayer photonic metamaterial slab optical waveguide structures with Kerr-type nonlinear cladding and substrate. The multiple-quantum-well (MQW) photonic metamaterial optical waveguide structure with Kerr-type nonlinear cladding and substrate was also analyzed. We can use the proposed method to study the multilayer optical metamaterial slab optical waveguide structure with the linear cladding and substrate.

Artificial Biphasic Synapses Based on Nonvolatile Phase‐Change Photonic Memory Cells

Nonvolatile photonic memory cells are basic building blocks for neuromorphic hardware enabling the realization of all‐optical synapses and artificial neurons. These devices commonly exploit chalcogenide phase‐change materials, which are evanescently coupled to photonic waveguides, and provide fast write/erase speeds and large storage capacity. Here, we report for the first time the programming of a nonvolatile photonic memory cell based on Ag3In4Sb76Te17 (AIST) which is capable of mimicking biphasic synapses. We evaluate the underlying mechanism of biphasic behavior of AIST cells based on numerical simulations and correlate to experimental findings. Switching dynamics demonstrate enhanced performance with a post‐excitation dead time as short as 12.8 ns. Based on AIST double cells, we demonstrate reversible multilevel switching between 45 unique synaptic weights for long‐term depression (LTD) and long‐term potentiation (LTP). The observed biphasic programming and excellent switching performance render AIST‐based photonic memory cells promising for artificial neural networks and neuromorphic photonic computing hardware.