Photonic Integration Platform Research Articles

Optical Reading of Nanoscale Magnetic Bits in an Integrated Photonic Platform

In this paper, we propose a compact integrated hybrid plasmonic-photonic device for optical reading of nanoscale magnetic bits with perpendicular magnetic anisotropy in a magnetic racetrack on top of a photonic waveguide on the indium phosphide membrane on silicon platform. The hybrid device is constructed by coupling a doublet of V-shaped gold plasmonic nanoantennas on top of the indium phosphide waveguide. By taking advantage of the localized surface plasmons, our hybrid device can enable detection of the magnetization state in magnetic bits beyond the diffraction limit of light and enhance the polar magneto-optical Kerr effect (PMOKE). We further illustrate how combining the hybrid device with a plasmonic polarization rotator provides magneto-optical read-out by transforming the PMOKE-induced polarization change into an intensity variation of the waveguide mode. According to the simulation results based on a three-dimensional finite-difference time-domain method, the hybrid device can detect the magnetization states in targeted bits in a magnetic racetrack medium down to ~ 100x100 nm2, regardless of the magnetization state of the rest of the racetrack with a relative intensity contrast of greater than 0.5% for a ~ 200x100 nm2 magnetic bit. We believe our hybrid device can be an enabling technology that can connect integrated photonics with nanoscale spintronics, paving the way toward ultrafast and energy efficient advanced on-chip applications.

Experimental Observation of Dissipatively Coupled Bound States in the Continuum on an Integrated Photonic Platform

AbstractBound states in the continuum (BICs) are a special type of waves that coexist with continuous waves without any radiation loss. Owing to eliminated dissipative coupling to the environment, the coupling between BICs is naturally considered to be dispersive. Here, dissipative coupling between BICs in coupled photonic waveguides is exploited on an etchless lithium niobate integrated platform, and for the first time, supermode BICs are experimentally observed on a chip. It is discovered that, under the variation of the waveguide gap, the two supermodes have complementarily varied loss rates and the supermode BIC appears periodically, which verifies respectively the dissipative nature of coupling and periodic variation of the coupling coefficient between the two coupled photonic waveguides. The observed dissipatively coupled BICs can enable many on‐chip applications such as high‐sensitivity sensing, optical communication, and topological physics.

Brillouin and Kerr nonlinearities of a low-index silicon oxynitride platform

Nonlinear optical effects including stimulated Brillouin scattering (SBS) and four-wave mixing (FWM) play an important role in microwave photonics, optical frequency combs, and quantum photonics. Harnessing SBS and FWM in a low-loss and versatile integrated platform would open the path to build large-scale Brillouin/Kerr-based photonic integrated circuits. In this letter, we investigate the Brillouin and Kerr properties of a low-index (n = 1.513 @ 1550 nm) silicon oxynitride (SiON) platform. We observed, for the first time, backward Brillouin scattering in SiON waveguides with a Brillouin gain coefficient of 0.3 m−1 W−1, which can potentially be increased to 0.95 m−1 W−1 by just tailoring the waveguide cross section. We also performed FWM experiments in SiON rings and obtained the nonlinear parameter γ of 0.02 m−1 W−1. Our results point to a low-loss and low-index photonic integrated platform that is both Brillouin and Kerr active.

Individually addressable and spectrally programmable artificial atoms in silicon photonics

A central goal for quantum technologies is to develop platforms for precise and scalable control of individually addressable artificial atoms with efficient optical interfaces. Color centers in silicon, such as the recently-isolated carbon-related G-center, exhibit emission directly into the telecommunications O-band and can leverage the maturity of silicon-on-insulator photonics. We demonstrate the generation, individual addressing, and spectral trimming of G-center artificial atoms in a silicon-on-insulator photonic integrated circuit platform. Focusing on the neutral charge state emission at 1278 nm, we observe waveguide-coupled single photon emission with narrow inhomogeneous distribution with standard deviation of 1.1 nm, excited state lifetime of 8.3 ± 0.7 ns, and no degradation after over a month of operation. In addition, we introduce a technique for optical trimming of spectral transitions up to 300 pm (55 GHz) and local deactivation of single artificial atoms. This non-volatile spectral programming enables alignment of quantum emitters into 25 GHz telecommunication grid channels. Our demonstration opens the path to quantum information processing based on implantable artificial atoms in very large scale integrated photonics.

High-Precision Wavelength Tuning of GeSn Nanobeam Lasers via Dynamically Controlled Strain Engineering.

The technology to develop a large number of identical coherent light sources on an integrated photonics platform holds the key to the realization of scalable optical and quantum photonic circuits. Herein, a scalable technique is presented to produce identical on-chip lasers by dynamically controlled strain engineering. By using localized laser annealing that can control the strain in the laser gain medium, the emission wavelengths of several GeSn one-dimensional photonic crystal nanobeam lasers are precisely matched whose initial emission wavelengths are significantly varied. The method changes the GeSn crystal structure in a region far away from the gain medium by inducing Sn segregation in a dynamically controllable manner, enabling the emission wavelength tuning of more than 10nm without degrading the laser emission properties such as intensity and linewidth. The authors believe that the work presents a new possibility to scale up the number of identical light sources for the realization of large-scale photonic-integrated circuits.

A mid-infrared focusing grating coupler with a single circular arc element based on germanium on silicon.

A mid-infrared (MIR) focusing grating coupler (FGC) with a single circular arc element (CAE) in the front of the gratings based on a germanium-on-silicon (Ge-on-Si) platform is designed and demonstrated. It can be used equivalently to a traditional FGC with all-focusing gratings. By optimizing the structural parameters of the CAE, the combination of a tapered linear grating and the CAE can improve the coupling efficiency to 8.61%, which is twice as large as that of the traditional MIR grating couplers. To the best of our knowledge, it is the highest coupling efficiency in a full-etch grating coupler based on Ge-on-Si. Moreover, the proposed grating coupler can be used for refractive index (RI) sensing, and the maximum sensitivity is 980.7 nm/RIU when the RI changes from 1 to 1.04. By comparing with traditional grating couplers requiring secondary etching, the proposed full-etch grating coupler structure can reduce the complexity of fabrication and can provide a prospective platform for MIR photonic integration and photonic biosensor detection.

Aluminum nitride photonic integrated circuits: from piezo-optomechanics to nonlinear optics

The commercial success of radio-frequency acoustic filters in wireless communication systems has launched aluminum nitride (AlN) as one of the most widely used semiconductors across the globe. Over recent years, AlN has also been investigated as an attractive photonic integrated platform due to its excellent characteristics, such as enormous bandgaps (∼6.2 eV), quadratic and cubic optical nonlinearities, Pockels electro-optic effects, and compatibility with the complementary metal-oxide semiconductor technology. In parallel, AlN possesses outstanding piezoelectric and mechanical performances, which can provide new aspects for controlling phonons and photons at the wavelength scale using nanophotonic architectures. These characteristics pose AlN as a promising candidate to address the drawbacks in conventional silicon and silicon nitride platforms. In this review, we aim to present recent advances achieved in AlN photonic integrated circuits ranging from material processing and passive optical routing to active functionality implementation such as electro-optics, piezo-optomechanics, and all-optical nonlinear frequency conversion. Finally, we highlight the challenges and future prospects existing in AlN nanophotonic chips.

Photonic Dirac cavities with spatially varying mass term.

In recent years, photonics has proven itself as an excellent platform for emulation of relativistic phenomena. Here, we show an example of relativistic-like trapping in photonic system that realizes Dirac-like dispersion with spatially inhomogeneous mass term. The modes trapped by such cavities, their energy levels, and corresponding orbitals are then characterized through optical imaging in real and momentum space. The fabricated cavities host a hierarchy of photonic modes with distinct radiation profiles directly analogous to various atomic orbitals endowed with unique characteristics, such as pseudo-particle-hall symmetry and spin degeneracy, and they carry topological charge which gives rise to radiative profiles with angular momentum. We demonstrate that these modes can be directionally excited by pseudo-spin-polarized boundary states. In addition to the fundamental interest in the structure of these pseudo-relativistic orbitals, the proposed system offers a route for designing new types of nanophotonic devices, spin-full resonators and topological light sources compatible with integrated photonics platforms.

Measurement of the Kerr Nonlinear Refractive Index and its Variation Among 4H - SiC Wafers

The unique material property of silicon carbide ($\mathrm{Si}\mathrm{C}$) and the recent demonstration of low-loss $\mathrm{Si}\mathrm{C}$-on-insulator integrated photonics platform have attracted considerable research interests for chip-scale photonic and quantum applications. Despite the impressive progresses made in $\mathrm{Si}\mathrm{C}$ photonics, some of its beneficial photonic properties are yet to be fully explored. Here, we carry out a thorough investigation of the Kerr nonlinearity among $4H$-$\mathrm{Si}\mathrm{C}$ wafers from several major wafer manufacturers, and reveal that their Kerr nonlinear refractive index can be significantly different. By eliminating various measurement uncertainties in the four-wave mixing experiment, the best Kerr nonlinear refractive index of $4H$-$\mathrm{Si}\mathrm{C}$ wafers is estimated to be approximately 4 times, instead of the prior estimate of 2 to 3 times, of that of stoichiometric silicon nitride in the telecommunication band. In addition, experimental evidence is developed that the Kerr nonlinearity in $4H$-$\mathrm{Si}\mathrm{C}$ wafers can be stronger along the $c$ axis than that in the orthogonal direction. Our examination of the Kerr nonlinear refractive index also compels a useful correction to the existing model in high-index-contrast waveguides; otherwise, considerable errors can be introduced.

High-Performance Mach-Zehnder Modulator Based on Thin-Film Lithium Niobate with Low Voltage-Length Product.

Electro-optic modulators (EOMs) based on a thin-film lithium niobate (TFLN) photonic integration platform play a crucial role in loading electrical signals onto optical signals. In this paper, we proposed on-chip EOMs operating at two commercially available wavelengths of 850 and 1550 nm and successfully demonstrated rather low voltage-length products (V π ·Ls) of 0.78 V·cm and 1.29 V·cm, respectively. Additionally, the EOM working at 1550 nm exhibits the capability of 3-dB electro-optic (E-O) bandwidth beyond 40 GHz due to the limitation of our test conditions. This study is quite helpful for understanding EOM structures in a TFLN platform, as well as the fabrication of high-performance and multifunctional EOM devices.

Widely tunable 2 µm hybrid laser using GaSb semiconductor optical amplifiers and a Si3N4 photonics integrated reflector

Tunable lasers emitting in the 2-3 µm wavelength range that are compatible with photonic integration platforms are of great interest for sensing applications. To this end, combining GaSb-based semiconductor gain chips with Si3N4 photonic integrated circuits offers an attractive platform. Herein, we utilize the low-loss features of Si3N4 waveguides and demonstrate a hybrid laser comprising a GaSb gain chip with an integrated tunable Si3N4 Vernier mirror. At room temperature, the laser exhibited a maximum output power of 15 mW and a tuning range of ∼90 nm (1937-2026 nm). The low-loss performance of several fundamental Si3N4 building blocks for photonic integrated circuits is also validated. More specifically, the single-mode waveguide exhibits a transmission loss as low as 0.15 dB/cm, the 90° bend has 0.008 dB loss, and the 50/50 Y-branch has an insertion loss of 0.075 dB.

Heterogeneous integration of III–V semiconductor lasers on thin-film lithium niobite platform by wafer bonding

Thin-film lithium niobate (TFLN) photonic integrated circuits (PICs) have emerged as a promising integrated photonics platform for the optical communication, microwave photonics, and sensing applications. In recent years, rapid progress has been made on the development of low-loss TFLN waveguides, high-speed modulators, and various passive components. However, the integration of laser sources on the TFLN photonics platform is still one of the main hurdles in the path toward fully integrated TFLN PICs. Here, we present the heterogeneous integration of InP-based semiconductor lasers on a TFLN PIC. The III–V epitaxial layer stack is adhesively bonded to a TFLN waveguide circuit. In the laser device, the light is coupled from the III–V gain section to the TFLN waveguide via a multi-section spot size converter. A waveguide-coupled output power above 1 mW is achieved for the device operating at room temperature. This heterogeneous integration approach can also be used to realize on-chip photodetectors based on the same epitaxial layer stack and the same process flow, thereby enabling large-volume, low-cost manufacturing of fully integrated III–V-on-lithium niobate systems for next-generation high-capacity communication applications.

Low Power Coherent Ising Machine Based on Mechanical Kerr Nonlinearity.

Finding a reliable Ising machine for solving nondeterministic polynomial-class problems has attracted great attention in recent years, where an authentic system can be expanded with polynomial-scaled resources to find the ground state Ising Hamiltonian. In this Letter, we propose an extremely low power optomechanical coherent Ising machine based on a new enhanced symmetry breaking mechanism and highly nonlinear mechanical Kerr effect. The mechanical movement of an optomechanical actuator induced by the optical gradient force greatly increases the nonlinearity by a few orders and significantly reduces the power threshold using conventional structures capable of fabrication via photonic integrated circuit platforms. With the simple but strong bifurcation mechanism and remarkably low power requirement, our optomechanical spin model opens a path for chip-scale integration of large-size Ising machine implementations with great stability.

Broadband second-harmonic generation in an angle-cut lithium niobate-on-insulator waveguide by a temperature gradient.

Frequency conversion via nonlinear wave mixing is an important technology to broaden the spectral range of lasers, propelling their applications in optical communication, spectroscopy, signal processing, and quantum information. Many applications require not only a high conversion efficiency but also a broad phase matching bandwidth. Here, we demonstrate broadband birefringence phase matching (BPM) second-harmonic generation (SHG) in angle-cut lithium niobate-on-insulator (LNOI) ridge waveguides based on a temperature gradient scheme. The bandwidth and shift of the phase matching spectrum can be effectively tuned by controlling the temperature gradient of the waveguide. Broadband SHG of a telecom C-band femtosecond laser is also demonstrated. The approach may open a new avenue for tunable broadband nonlinear frequency conversion in various integrated photonics platforms.

Silica-Titania Integrated Photonics Platform-Based 1 × 2 Demultiplexer Utilizing Two Serially Cascaded Racetrack Microrings for 1310 nm and 1550 nm Telecommunication Wavelengths

Herein, a numerical analysis of a 1 × 2 demultiplexer based on a silica-titania integrated photonics platform is conducted via the finite element method. The structure is composed of two coupled racetrack microrings (RTMRs) and a subwavelength grating (SWG) structure for the demultiplexing of 1310 nm and 1550 nm telecommunication wavelengths. The material platform selected for this design is highly attractive due to its refined optical, physical, and chemical properties. Moreover, silica-titania sol-gel thin-films can be deposited on glass substrates with the dip-coating method. The proposed device has a small footprint of 84 × 125 μm2 and offers crosstalk as low as ~−6.6 dB and ~−9.04 dB for 1550 nm and 1310 nm, respectively. We are convinced that this study promotes the use of the silica-titania platform for the development of low-cost on-chip optical communication devices for signal multiplexing and demultiplexing.

A 3C-SiC-on-Insulator-Based Integrated Photonic Platform Using an Anodic Bonding Process with Glass Substrates.

Various crystalline silicon carbide (SiC) polytypes are emerging as promising photonic materials due to their wide bandgap energies and nonlinear optical properties. However, their wafer forms cannot readily provide a refractive index contrast for optical confinement in the SiC layer, which makes it difficult to realize a SiC-based integrated photonic platform. In this paper, we demonstrate a 3C-SiC-on-insulator (3C-SiCoI)-based integrated photonic platform by transferring the epitaxial 3C-SiC layer from a silicon die to a borosilicate glass substrate using anodic bonding. By fine-tuning the fabrication process, we demonstrated nearly 100% area transferring die-to-wafer bonding. We fabricated waveguide-coupled microring resonators using sulfur hexafluoride (SF6)-based dry etching and demonstrated a moderate loaded quality (Q) factor of 1.4 × 105. We experimentally excluded the existence of the photorefractive effect in this platform at sub-milliwatt on-chip input optical power levels. This 3C-SiCoI platform is promising for applications, including large-scale integration of linear, nonlinear and quantum photonics.

SiN foundry platform for high performance visible light integrated photonics

We present a high performance silicon nitride photonic integrated circuit platform operating at visible wavelengths, accessible through the commercial foundry, LIGENTEC. Propagation losses were measured across the visible spectrum from 450 nm to 850 nm. For wavelengths above 630 nm, losses were <1 dB/cm in TE and <0.5 dB/cm in TM. Additionally, sets of single mode waveguide-coupled ring resonators across three separate chips were tested and analysed. A peak intrinsic Q factor of 3.69 × 106 was measured for a single resonance at ∼635.3 nm, with an average value of 2.28 × 106 recorded over 10 peaks in a 3 nm tuning range. Analyses of the loss and coupling, as functions of bus-ring coupling gap and waveguide width, are also presented. High confinement, low loss devices realised on the chip-scale in a wide-bandgap material like silicon nitride are increasingly important for the next generation of integrated optical devices operating at visible wavelengths.

The amazing world of self-organized Ge quantum dots for Si photonics on SiN platforms

Beginning with our exciting discovery of germanium (Ge) spherical quantum-dot (QD) formation via the peculiar and symbiotic interactions of Si, Ge, and O interstitials, we have embarked on a journey of vigorous exploration, creating unique configurations of self-organized Ge-QDs/Si-containing layers. Our aim is to generate advanced Ge-QD photonic devices, while using standard, mainstream Si processing techniques. This paper summarizes our portfolio of innovative Ge-QD configurations. With emphasis on both controllability and repeatability, we have fabricated size-tunable, spherical Ge-QDs that are placed at predetermined spatial locations within Si-containing layers (SiO2, Si3N4, and Si) using a coordinated combination of lithographic patterning and self-assembled growth. We have successfully exploited the multi-dimensional, parameter spaces of process conditions in combination with layout designs to achieve exquisite control available through the thermal oxidation of lithographically patterned, poly-Si1 − xGex structures in close proximity with Si3N4/Si layers. In so doing, we have gained insight into the growth kinetics and formation mechanisms of self-organized, Ge spherical QDs embedded within SiO2, Si3N4, and Si layers, respectively. Our Ge-QD configurations have opened up a myriad of process/integration possibilities including top-to-bottom evanescent-wave coupling structures for SiN-waveguided Ge-QD photodetectors and Ge-QD light emitters for Si photonics within Si3N4 integrated photonics platforms for on-chip interconnects and sensing.

Unconventional Integrated Photonic Accelerators for High-Throughput Convolutional Neural Networks

We provide an overview of the rapidly evolving landscape of integrated photonic neuromorphic architectures, specifically targeting the implementation of convolutional neural networks. The exploding research momentum stems from the well-known advantages of photonic circuits compared to digital electronics, and at the same time, it is driven by the massive need for cognitive image/video processing. In this context, we provide a detailed literature review on photonic cores operating as convolutional neural networks, covering either the functionality of a conventional neural network or its spiking counterpart. Moreover, we propose 2 alternative photonic approaches that refrain from simply transferring neural network concepts directly into the optical domain; instead, they focus on fusing photonic, digital electronic, and event-based bioinspired processing to optimally exploit the virtues of each scheme. These approaches can offer beyond state-of-the-art performance while relying on realistic, scalable technology. The first approach is based on a photonic integrated platform and a bioinspired spectrum-slicing technique. The photonic chip allows feature extraction through optical filtering with low power consumption and an equivalent computational efficiency of 72 femtojoules per multiply-and-accumulate operation for 5-bit precision. When combined with typical digital neural networks, an almost 5-fold reduction in the number of parameters was achieved with a minor loss of accuracy compared to established convolutional neural networks. The second approach follows a bioisomorphic route in which miniaturized spiking laser neurons and unsupervised bioinspired training are unified in a deep architecture, revealing a noise-resilient and power-efficient proposition.

Heterogeneous interconnection of low-loss and dense material platforms using adiabatic tapering coupler

Recently, we successfully realized amorphous silicon carbide (a-SiC) integrated photonics with optical losses as low as 0.78 dB/cm. Moreover, the deposition of a-SiC was done at 150 ℃, which enables successful lift of a-SiC as an additive step to existing photonics circuits. In this work, we present an adiabatic taper coupler which provides bidirectional lossless connection between two integrated photonics platforms: thin-film silicon nitride (Si3N4) and a-SiC. Normalized power transmission of 96.61% is presented, and the coupler enables strong confinement when coupling from weakly confined thin-film device to normal thickness device. By utilizing such a coupler as bridge, switching back and forth between Si3N4 and a-SiC platforms can be easily realized. This allow us to carry out applications including quantum interference and digital Fourier spectroscopy, in which long optical delay lines are constructed on Si3N4 and highly integrated circuits are built on a-SiC.