Physics to system-level modeling of silicon-organic-hybrid nanophotonic devices.

The continuous growth in data volume has sparked interest in silicon-organic-hybrid (SOH) nanophotonic devices integrated into silicon photonic integrated circuits (PICs). SOH devices offer improved speed and energy efficiency compared to silicon photonics devices. However, a comprehensive and accurate modeling methodology of SOH devices, such as modulators corroborating experimental results, is lacking. While some preliminary modeling approaches for SOH devices exist, their reliance on theoretical and numerical methodologies, along with a lack of compatibility with electronic design automation (EDA), hinders their seamless and rapid integration with silicon PICs. Here, we develop a phenomenological, building-block-based SOH PICs simulation methodology that spans from the physics to the system level, offering high accuracy, comprehensiveness, and EDA-style compatibility. Our model is also readily integrable and scalable, lending itself to the design of large-scale silicon PICs. Our proposed modeling methodology is agnostic and compatible with any photonics-electronics co-simulation software. We validate this methodology by comparing the characteristics of experimentally demonstrated SOH microring modulators (MRMs) and Mach Zehnder modulators with those obtained through simulation, demonstrating its ability to model various modulator topologies. We also show our methodology's ease and speed in modeling large-scale systems. As an illustrative example, we use our methodology to design and study a 3-channel SOH MRM-based wavelength-division (de)multiplexer, a widely used component in various applications, including neuromorphic computing, data center interconnects, communications, sensing, and switching networks. Our modeling approach is also compatible with other materials exhibiting the Pockels and Kerr effects. To our knowledge, this represents the first comprehensive physics-to-system-level EDA-compatible simulation methodology for SOH modulators.

Monolithically integrated 940†nm VCSELs on bulk Ge substrates.

This research successfully developed an independent Ge-based VCSEL epitaxy and fabrication technology route, which set the stage for integrating AlGaAs-based semiconductor devices on bulk Ge substrates. This is the second successful Ge-based VCSEL technology reported worldwide and the first Ge-based VCSEL technology with key details disclosed, including Ge substrate specification, transition layer structure and composition, and fabrication process. Compared with the GaAs counterparts, after epitaxy optimization, the Ge-based VCSEL wafer has a 40% lower surface root-mean-square roughness and 72% lower average bow-warp. After device fabrication, the Ge-based VCSEL has a 10% lower threshold current density and 19% higher maximum optical differential efficiency than the GaAs-based VCSEL.

On-chip hybrid integration of swept frequency distributed-feedback laser with silicon photonic circuits using photonic wire bonding.

This paper presents a novel co-packaging approach through on-chip hybrid laser integration with photonic circuits using photonic wire bonding. The process involves die-bonding a low-cost semiconductor distributed-feedback (DFB) laser into a deep trench on a silicon-on-insulator (SOI) chip and coupling it to the silicon circuitry through photonic wire bonding (PWB). After characterizing the power-current-voltage (LIV) and optical spectrum of the laser, a wavelength-current relationship utilizing its tunability through self-heating a swept-frequency laser (SFL) is developed. Photonic integrated circuit (PIC) resonators are successfully characterized using the SFL method, demonstrating signal detection with a quality factor comparable to measurements conducted with an off-chip benchtop laser.

Roadmapping the next generation of silicon photonics.

Silicon photonics has developed into a mainstream technology driven by advances in optical communications. The current generation has led to a proliferation of integrated photonic devices from thousands to millions-mainly in the form of communication transceivers for data centers. Products in many exciting applications, such as sensing and computing, are around the corner. What will it take to increase the proliferation of silicon photonics from millions to billions of units shipped? What will the next generation of silicon photonics look like? What are the common threads in the integration and fabrication bottlenecks that silicon photonic applications face, and which emerging technologies can solve them? This perspective article is an attempt to answer such questions. We chart the generational trends in silicon photonics technology, drawing parallels from the generational definitions of CMOS technology. We identify the crucial challenges that must be solved to make giant strides in CMOS-foundry-compatible devices, circuits, integration, and packaging. We identify challenges critical to the next generation of systems and applications-in communication, signal processing, and sensing. By identifying and summarizing such challenges and opportunities, we aim to stimulate further research on devices, circuits, and systems for the silicon photonics ecosystem.

Fully-integrated photonic tensor core for image convolutions.

Convolutions are one of the most critical signal and image processing operations. From spectral analysis to computer vision, convolutional filtering is often related to spatial information processing involving neighbourhood operations. As convolution operations are based around the product of two functions, vectors or matrices, dot products play a key role in the performance of such operations; for example, advanced image processing techniques require fast, dense matrix multiplications that typically take more than 90% of the computational capacity dedicated to solving convolutional neural networks. Silicon photonics has been demonstrated to be an ideal candidate to accelerate information processing involving parallel matrix multiplications. In this work, we experimentally demonstrate a multiwavelength approach with fully integrated modulators, tunable filters as microring resonator weight banks, and a balanced detector to perform matrix multiplications for image convolution operations. We develop a scattering matrix model that matches the experiment to simulate large-scale versions of these photonic systems with which we predict performance and physical constraints, including inter-channel cross-talk and bit resolution.

Cost-effective silicon-photonic biosensors using doped silicon detectors and a broadband source.

We propose and demonstrate a cost-effective, microring-based, silicon photonic sensor that uses doped silicon detectors and a broadband source. Shifts in the sensing microring resonances are electrically tracked by a doped second microring, which acts as both a tracking element and a photodetector. By tracking the power supplied to this second ring, as the sensing ring's resonance shifts, the effective refractive index change caused by the analyte is determined. This design reduces the cost of the system by eliminating high-cost, high-resolution tunable lasers, and is fully compatible with high-temperature fabrication processes. We report a bulk sensitivity of 61.8 nm/RIU and a system limit of detection of 9.8x10-4 RIU.

Emulation of deep-ultraviolet lithography using rapid-prototyping, electron-beam lithography for silicon photonics design.

We demonstrate a method to emulate the optical performance of silicon photonic devices fabricated using advanced deep-ultraviolet lithography (DUV) processes on a rapid-prototyping electron-beam lithography process. The method is enabled by a computational lithography predictive model generated by processing SEM image data of the DUV lithography process. We experimentally demonstrate the emulation method's accuracy on integrated silicon Bragg grating waveguides and grating-based, add-drop filter devices, two devices that are particularly susceptible to DUV lithography effects. The emulation method allows silicon photonic device and system designers to experimentally observe the effects of DUV lithography on device performance in a low-cost, rapid-prototyping, electron-beam lithography process to enable a first-time-right design flow.

Towards a high-density photonic tensor core enabled by intensity-modulated microrings and photonic wire bonding.

We propose a photonic processing unit for high-density analog computation using intensity-modulation-based microring modulators (IM-MRMs). The output signal at the fixed resonance wavelength is directly intensity modulated by changing the extinction ratio (ER) of the IM-MRM . Thanks to the intensity-modulated approach, the proposed photonic processing unit is less sensitive to the inter-channel crosstalk. Simulation results reveal that the proposed design offers a maximum of 17-fold increase in wavelength channel density compared to its wavelength-modulated counterpart. Therefore, a photonic tensor core of size 512 [Formula: see text] 512 can be realized by current foundry lines. A convolutional neural network (CNN) simulator with 6-bit precision is built for handwritten digit recognition task using the proposed modulator. Simulation results show an overall accuracy of 96.76%, when the wavelength channel spacing suffers a 3-dB power penalty. To experimentally validate the system, 1000 dot product operations are carried out with a 4-bit signed system on a co-packaged photonic chip, where optical and electrical I/Os are realized using photonic and electrical wire bonding techniques. Study of the measurement results show a mean squared error (MSE) of 3.09[Formula: see text]10[Formula: see text] for dot product calculations. The proposed IM-MRM, therefore, renders the crosstalk issue tractable and provides a solution for the development of large-scale optical information processing systems with multiple wavelengths.

Enhanced poling and infiltration for highly efficient electro-optic polymer-based Mach-Zehnder modulators.

An ultra-narrow 40-nm slotted waveguide is fabricated to enable highly efficient, electro-optic polymer modulators. Our measurement results indicate that VπL's below ∼ 1.19 V.mm are possible for the balanced Mach-Zehnder modulators using this ultra-narrow slotted waveguide on a hybrid silicon-organic hybrid platform. Our simulations suggest that VπL's can be further reduced to ∼ 0.35 V.mm if appropriate doping is utilized. In addition to adapting standard recipes, we developed two novel fabrication processes to achieve miniaturized devices with high modulation sensitivity. To boost compactness and decrease the overall footprint, we use a fabrication approach based on air bridge interconnects on thick, thermally-reflowed, MaN 2410 E-beam resist protected by an alumina layer. To overcome the challenges of high currents and imperfect infiltration of polymers into ultra-narrow slots, we use a carefully designed, atomically-thin layer of TiO2 as a carrier barrier to enhance the efficiency of our electro-optic polymers. The anticipated increase in total capacitance due to the TiO2 layer is negligible. Applying our TiO2 surface treatment to the ultra-narrow slot allows us to obtain an improved index change efficiency (∂n/∂V) of ∼ 22% for a 5 nm TiO2 layer. Furthermore, compared to non-optimized cases, our peak measured current during poling is reduced by a factor of ∼ 3.

An Optimization Framework for Silicon Photonic Evanescent-Field Biosensors Using Sub-Wavelength Gratings.

Silicon photonic (SiP) evanescent-field biosensors aim to combine the information-rich readouts offered by lab-scale diagnostics, at a significantly lower cost, and with the portability and rapid time to result offered by paper-based assays. While SiP biosensors fabricated with conventional strip waveguides can offer good sensitivity for label-free detection in some applications, there is still opportunity for improvement. Efforts have been made to design higher-sensitivity SiP sensors with alternative waveguide geometries, including sub-wavelength gratings (SWGs). However, SWG-based devices are fragile and prone to damage, limiting their suitability for scalable and portable sensing. Here, we investigate SiP microring resonator sensors designed with SWG waveguides that contain a "fishbone" and highlight the improved robustness offered by this design. We present a framework for optimizing fishbone-style SWG waveguide geometries based on numerical simulations, then experimentally measure the performance of ring resonator sensors fabricated with the optimized waveguides, targeting operation in the O-band and C-band. For the O-band and C-band devices, we report bulk sensitivities up to 349 nm/RIU and 438 nm/RIU, respectively, and intrinsic limits of detection as low as 5.1 × 10-4 RIU and 7.1 × 10-4 RIU, respectively. This performance is comparable to the state of the art in SWG-based sensors, positioning fishbone SWG resonators as an attractive, more robust, alternative to conventional SWG designs.

Biofunctionalization of Multiplexed Silicon Photonic Biosensors.

Silicon photonic (SiP) sensors offer a promising platform for robust and low-cost decentralized diagnostics due to their high scalability, low limit of detection, and ability to integrate multiple sensors for multiplexed analyte detection. Their CMOS-compatible fabrication enables chip-scale miniaturization, high scalability, and low-cost mass production. Sensitive, specific detection with silicon photonic sensors is afforded through biofunctionalization of the sensor surface; consequently, this functionalization chemistry is inextricably linked to sensor performance. In this review, we first highlight the biofunctionalization needs for SiP biosensors, including sensitivity, specificity, cost, shelf-stability, and replicability and establish a set of performance criteria. We then benchmark biofunctionalization strategies for SiP biosensors against these criteria, organizing the review around three key aspects: bioreceptor selection, immobilization strategies, and patterning techniques. First, we evaluate bioreceptors, including antibodies, aptamers, nucleic acid probes, molecularly imprinted polymers, peptides, glycans, and lectins. We then compare adsorption, bioaffinity, and covalent chemistries for immobilizing bioreceptors on SiP surfaces. Finally, we compare biopatterning techniques for spatially controlling and multiplexing the biofunctionalization of SiP sensors, including microcontact printing, pin- and pipette-based spotting, microfluidic patterning in channels, inkjet printing, and microfluidic probes.

Contra-directional pump reject filters integrated with a micro-ring resonator photon-pair source in silicon.

High coincidence-to-accidental ratio (CAR) is crucial for photon-pair sources (PPSs) integrated with pump reject filters (PRFs) in silicon, but CAR values currently reported for integrated PPS/PRF chips still fall short of those achieved using stand-alone sources with external PRFs. Here we report measured and modelled CAR values for a micro-ring resonator PPS integrated with a PRF consisting of a three-stage, cascaded (via their through ports), contra-directional coupler (CDC) that compare favorably even with some stand-alone sources. CDC-based PRFs provide the benefits of compact area and wide reject bands without a need for tuning, in comparison to prior-art implementations.

Broadband, silicon photonic, optical add-drop filters with 3 dB bandwidths up to 11 THz: publisher's note.

This publisher's note contains corrections to Opt. Lett.46, 2738 (2021)OPLEDP0146-959210.1364/OL.423745.

Broadband, silicon photonic, optical add-drop filters with 3 dB bandwidths up to 11 THz.

We present the designs, theory, and experimental demonstrations of ultra-broadband, optical add-drop filters on the silicon-on-insulator platform, realized using period-chirped contra-directional couplers. Our fabricated devices have ultra-broad 3 dB bandwidths of up to 11 THz (88.1 nm), with flat-top responses at their drop ports. All of our devices were fabricated using a commercial, CMOS-compatible, 193 nm deep-ultraviolet lithography process. By using lithography-prediction models, the measured bandwidths, insertion losses, central wavelengths, and extinction ratios of our devices are all in good agreement with our predicted, simulated results. Such filters are necessary for photonic integrated circuits to operate over multiple optical bands.

Sub-wavelength grating-assisted polarization splitter-rotators for silicon-on-insulator platforms: erratum.

An erratum is presented to correct the caption of Fig. 1 and the citation number in Fig. 7(d) in the original article [Opt. Express 27, 17581 (2019)].

Efficient layout-aware statistical analysis for photonic integrated circuits.

Fabrication variability significantly impacts the performance of photonic integrated circuits (PICs), which makes it crucial to quantify the impact of fabrication variations before the final fabrication. Such analysis enables circuit and system designers to optimize their designs to be more robust and obtain maximum yield when designing for manufacturing. This work presents a simulation methodology, Reduced Spatial Correlation Matrix-based Monte-Carlo (RSCM-MC), to efficiently study the impact of spatially correlated fabrication variations on the performance of PICs. First, a simple and reliable method to extract physical correlation lengths, variability parameters that define the inverse of the spatial frequencies of width and height variations over a wafer, is presented. Then, the process of generating correlated variations for MC simulations using RSCM-MC methodology is presented. The methodology generates correlated variations by first creating a reduced correlation matrix containing spatial correlations between all the circuit components, and then processing it using Cholesky decomposition to obtain correlated variations for all circuit components. These variations are then used to conduct MC simulations. The accuracy and the computation performance of the proposed methodology are compared with other layout-dependent Monte-Carlo simulation methodologies, such as Virtual wafer-based Monte-Carlo (VW-MC). A Mach-Zehnder lattice filter is used to study the accuracy, and a second-order Mach-Zehnder filter and a 16x16 optical switch matrix system are used to compare the computational performance.

Measuring on-chip waveguide losses using a single, two-point coupled microring resonator.

We demonstrate a method for measuring on-chip waveguide losses using a single microring resonator with a tunable coupler. By tuning the power coupling to the microring and measuring the microring's through-port transmission at each power coupling, one can separate the waveguide propagation loss and the effects of the coupling to the microring. This method is tolerant of fiber-chip coupling/alignment errors and does not require the use of expensive instruments for phase response measurements. In addition, this method offers a compact solution for measuring waveguide propagation losses, only using a single microring (230 µm×190 µm, including the metal pads). We demonstrate this method by measuring the propagation losses of silicon-on-insulator rib waveguides, yielding propagation losses of 3.1-1.3 dB/cm for core widths varying from 400-600 nm.

Automated control algorithms for silicon photonic polarization receiver.

We demonstrate greedy linear descent-based, basic gradient descent-based, two-point step size gradient descent-based, and two-stage optimization method-based automated control algorithms and examine their performance for use with a silicon photonic polarization receiver. With an active feedback loop control process, time-varying arbitrary polarization states from an optical fiber can be automatically adapted and stabilized to the transverse-electric (TE) mode of a single-mode silicon waveguide. Using the proposed control algorithms, we successfully realize automated adaptations for a 10 Gb/s on-off keying signal in the polarization receiver. Based on the large-signal measurement results, the control algorithms are examined and compared with regard to the iteration number and the output response. In addition, we implemented a long-duration experiment to track, adapt, and stabilize arbitrary input polarization states using the two-point step size gradient descent-based and two-stage optimization method-based control algorithms. The experimental results show that these control algorithms enable the polarization receiver to achieve real-time and continuous polarization management.

Long-term monitoring in a microfluidic system to study tumour spheroid response to chronic and cycling hypoxia.

We demonstrate the application of a microfluidic platform combining spatiotemporal oxygen control and long-term microscopy monitoring to observe tumour spheroid response to hypoxia. The platform is capable of recreating physiologically-relevant low and cycling oxygen levels not attainable in traditional cell culture environments, while image-based monitoring visualizes cell response to these physiologically-relevant conditions. Monitoring spheroid cultures during hypoxic exposure allows us to observe, for the first time, that spheroids swell and shrink in response to time-varying oxygen profiles switching between 0% and 10% O2; this swelling-shrinkage behaviour appears to be driven by swelling of individual cells within the spheroids. We also apply the system to monitoring tumour models during anticancer treatment under varying oxygen conditions. We observe higher uptake of the anticancer agent doxorubicin under a cycling hypoxia profile than under either chronic hypoxia or in vitro normoxia, and the two-photon microscopy monitoring facilitated by our system also allows us to observe heterogeneity in doxorubicin uptake within spheroids at the single-cell level. Combining optical sectioning microscopy with precise spatiotemporal oxygen control and 3D culture opens the door for a wide range of future studies on microenvironmental mechanisms driving cancer progression and resistance to anticancer therapy. These types of studies could facilitate future improvements in cancer diagnostics and treatment.

Compact wavelength- and bandwidth-tunable microring modulator.

Fabrication errors currently hold back the large-scale adoption of silicon micro-ring modulators (MRMs). The ability to correct their spectral features post-fabrication is required to enable their commercialization. Here, we report and demonstrate an MRM that uses a tunable two-point coupling scheme, which maintains the MRM's compact footprint (60 µm×45 µm) and allows one to tune the MRM's operating wavelength and adjust the optical bandwidth (and/or extinction ratio). This means that one can compensate for fabrication errors and thereby improve the yields. We confirm the modulator's operation by showing NRZ and PAM-4 modulation, up to 28 Gb/s and 19.9 Gb/s, respectively. Also, the proposed tunable MRM maintains the microring's free-spectral range (FSR), which proves its compatibility for configurable and high-bandwidth DWDM applications.