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.

An Integrated Optical Circuit Architecture for Inverse-Designed Silicon Photonic Components.

In this work, we demonstrate a compact toolkit of inverse-designed, topologically optimized silicon photonic devices that are arranged in a "plug-and-play" fashion to realize many different photonic integrated circuits, both passive and active, each with a small footprint. The silicon-on-insulator 1550-nm toolkit contains a 2 × 2 3-dB splitter/combiner, a 2 × 2 waveguide crossover, and a 2 × 2 all-forward add-drop resonator. The resonator can become a 2 × 2 electro-optical crossbar switch by means of the thermo-optical effect, phase-change cladding, or free-carrier injection. For each of the ten circuits demonstrated in this work, the toolkit of photonic devices enables the compact circuit to achieve low insertion loss and low crosstalk. By adopting the sophisticated inverse-design approach, the design structure, shape, and sizing of each individual device can be made more flexible to better suit the architecture of the greater circuit. For a compact architecture, we present a unified, parallel waveguide circuit framework into which the devices are designed to fit seamlessly, thus enabling low-complexity circuit design.

High-Performance Pockels Effect Modulation and Switching in Silicon-Based GaP/Si, AlP/Si, ZnS/Si, AlN/3C-SiC, GaAs/Ge, ZnSe/GaAs, and ZnSe/Ge Superlattice-On-Insulator Integrated Circuits.

We propose new a Si-based waveguided Superlattice-on-Insulator (SLOI) platforms for high-performance electro-optical (EO) 2 × 2 and N × M switching and 1 × 1 modulation, including broad spectrum and resonant. We present a theoretical investigation based on the tight-binding Hamiltonian of the Pockels EO effect in the lattice-matched undoped (GaP)N/(Si2)M, (AlP)N/(Si2)M, (ZnS)N/(Si2)M, (AlN)N/(3C-SiC)M, (GaAs)N/(Ge2)M, (ZnSe)N/(GaAs)M, and (ZnSe)N/(Ge2)M wafer-scale short-period superlattices that are etched into waveguided networks of small-footprint Mach-Zehnder interferometers and micro-ring resonators to yield opto-electronic chips. The spectra of the Pockels r33 coefficient have been simulated as a function of the number of the atomic monolayers for "non-relaxed" heterointerfaces. The large obtained r33 values enable the SLOI circuit platforms to offer a very favorable combination of monolithic construction, cost-effective manufacturability, high modulation/switching speed, high information bandwidth, tiny footprint, low energy per bit, low switching voltage, near-IR-and-telecom wavelength coverage, and push-pull operation. By optimizing waveguide, clad, and electrode dimensions, we obtained very desirable values of the VπL performance metric, in the range of 0.062 to 0.275 V·cm, portending a bright future for a variety of applications, such as sensor networks or Internet of Things (IoT).

Design and Optimization of GeSn Waveguide Photodetectors for 2-µm Band Silicon Photonics.

Silicon photonics is emerging as a competitive platform for electronic-photonic integrated circuits (EPICs) in the 2 µm wavelength band where GeSn photodetectors (PDs) have proven to be efficient PDs. In this paper, we present a comprehensive theoretical study of GeSn vertical p-i-n homojunction waveguide photodetectors (WGPDs) that have a strain-free and defect-free GeSn active layer for 2 µm Si-based EPICs. The use of a narrow-gap GeSn alloy as the active layer can fully cover entire the 2 µm wavelength band. The waveguide structure allows for decoupling the photon-absorbing path and the carrier collection path, thereby allowing for the simultaneous achievement of high-responsivity and high-bandwidth (BW) operation at the 2 µm wavelength band. We present the theoretical models to calculate the carrier saturation velocities, optical absorption coefficient, responsivity, 3-dB bandwidth, zero-bias resistance, and detectivity, and optimize this device structure to achieve highest performance at the 2 µm wavelength band. The results indicate that the performance of the GeSn WGPD has a strong dependence on the Sn composition and geometric parameters. The optimally designed GeSn WGPD with a 10% Sn concentration can give responsivity of 1.55 A/W, detectivity of 6.12 × 1010 cmHz½W-1 at 2 µm wavelength, and ~97 GHz BW. Therefore, this optimally designed GeSn WGPD is a potential candidate for silicon photonic EPICs offering high-speed optical communications.

Non-Hermitian Sensing in Photonics and Electronics: A Review.

Recently, non-Hermitian Hamiltonians have gained a lot of interest, especially in optics and electronics. In particular, the existence of real eigenvalues of non-Hermitian systems has opened a wide set of possibilities, especially, but not only, for sensing applications, exploiting the physics of exceptional points. In particular, the square root dependence of the eigenvalue splitting on different design parameters, exhibited by 2 × 2 non-Hermitian Hamiltonian matrices at the exceptional point, paved the way to the integration of high-performance sensors. The square root dependence of the eigenfrequencies on the design parameters is the reason for a theoretically infinite sensitivity in the proximity of the exceptional point. Recently, higher-order exceptional points have demonstrated the possibility of achieving the nth root dependence of the eigenfrequency splitting on perturbations. However, the exceptional sensitivity to external parameters is, at the same time, the major drawback of non-Hermitian configurations, leading to the high influence of noise. In this review, the basic principles of PT-symmetric and anti-PT-symmetric Hamiltonians will be shown, both in photonics and in electronics. The influence of noise on non-Hermitian configurations will be investigated and the newest solutions to overcome these problems will be illustrated. Finally, an overview of the newest outstanding results in sensing applications of non-Hermitian photonics and electronics will be provided.

Power-Dependent Investigation of Photo-Response from GeSn-Based p-i-n Photodetector Operating at High Power Density.

We report an investigation on the photo-response from a GeSn-based photodetector using a tunable laser with a range of incident light power. An exponential increase in photocurrent and an exponential decay of responsivity with increase in incident optical power intensity were observed at higher optical power range. Time-resolved measurement provided evidence that indicated monomolecular and bimolecular recombination mechanisms for the photo-generated carriers for different incident optical power intensities. This investigation establishes the appropriate range of optical power intensity for GeSn-based photodetector operation.

Impact of nonlinear effects in Si towards integrated microwave-photonic applications.

As one of major integrated microwave photonics (IMWP) platforms, Si photonics exhibits the intensity-dependent Kerr effect and two-photon absorption (TPA) with associated free carrier effects (FCE). At the commonly used 1.55 µm, TPA losses and the associated FCE would eventually limit the dynamic range of Si photonic links. Resonating structures such as ring resonators (RRs) experience enhanced nonlinear effects due to significant intensity buildup. According to the bandgap characteristics of Si, TPA can be eliminated at and beyond 2.2 µm. In this work, a systemic simulation of straight waveguides and RRs is performed at wavelengths from 1.55 to 2.2 µm where the wavelength-dependent TPA loss is investigated. Moreover, the Kerr effect leads to unwanted change of refractive index, which shifts the RR resonant wavelength at both 1.55 and 2.2 µm, thus needing shift compensation. Compensated RRs operating at 2.2 µm could open a new venue for Si photonics towards IMWP applications.

Planar GeSn lateral p-i-n resonant-cavity-enhanced photodetectors for short-wave infrared integrated photonics.

We report normal-incidence planar GeSn resonant-cavity-enhanced photodetectors (RCE-PDs) with a lateral p-i-n homojunction configuration on a silicon-on-insulator (SOI) platform for short-wave infrared (SWIR) integrated photonics. The buried oxide of the SOI platform and the deposited SiO2 layer serve as the bottom and top reflectors, respectively, creating a vertical cavity for enhancing the optical responsivity. The planar p-i-n diode structure is favorable for complementary-metal-oxide-semiconductor-compatible, large-scale integration. With the bandgap reduction enabled by the 4.2% Sn incorporation into the GeSn active layer, the photodetection range extends to 1960 nm. The promising results demonstrate that the developed planar GeSn RCE-PDs are potential candidates for SWIR integrated photonics.

Electro-optical logic using dual-nanobeam Mach-Zehnder interferometer switches.

The maturity of integrated photonics enables many applications including high-performance computing. Digital photonic computing always considers resonator-based modulators as the key active components due to their compactness as compared to broad-spectrum Mach-Zehnder interferometers (MZIs). In this paper, we investigate the dual-nanobeam (NB) based MZI 2 × 2 switches with much smaller footprint for realizing electro-optical logic circuits. New logic gates and scalable circuits assisted by multiplexing techniques are proposed. Results show that the NB MZI is another promising candidate for electronic-photonic digital computing.

Compact resonant 2 × 2 crossbar switch using three coupled waveguides with a central nanobeam.

This theoretical simulation paper presents designs and projected performance of ∼1550-nm silicon-on-insulator (SOI) and ∼2000-nm Ge-on-Si-on-nitride and Ge-on-nitride 2×2 optical crossbar switches based upon a three-waveguide coupler in which the central waveguide is a nanobeam actuated by the thermo-optical (TO) effect. A TO heater stripe is located atop the central nanobeam. To implement accurate and realistic designs, the 3D finite difference time domain approach was employed. The metrics of crossbar switching, insertion loss (IL) and crosstalk (CT) were evaluated for choices of 3-waveguide structure parameters and TO-induced index changes. The predicted ILs and CTs were excellent, enabling the designed devices to be considered as fundamental building blocks in wavelength-division-multiplexed cross-connect (WXC) applications. Proposed here are compact, nonblocking space-and-wavelength routing switches to be constructed in a monolithic, industry-standard SOI chip (and in Ge-on-SON and GON chips). Specifics are given for realizing 16 × 16 × Mλ WXCs as well as reconfigurable, multi-resonant, programmable hexagonal and diamond meshes.

Design of an on-Chip Room Temperature Group-IV Quantum Photonic Chem/Bio Interferometric Sensor Based on Parity Detection.

We propose and analyze three Si-based room-temperature strip-guided "manufacturable" integrated quantum photonic chem/bio sensor chips operating at wavelengths of 1550 nm, 1330 nm, and 640 nm, respectively. We propose design rules that will achieve super-sensitivity (above the classical limit) by means of mixing between states of coherent light and single-mode squeezed-light. The silicon-on-insulator (SOI), silicon-on-sapphire (SOS), and silicon nitride-on-SiO2-on Si (SiN) platforms have been investigated. Each chip is comprised of photonic building blocks: a race-track resonator, a pump filter, an integrated Mach-Zehnder interferometric chem/bio sensor, and a photonic circuit to perform parity measurements, where our homodyne measurement circuit avoids the use of single-photon-counting detectors and utilizes instead conventional photodetectors. A combination of super-sensitivity with super-resolution is predicted for all three platforms to be used for chem/bio sensing applications.

Ultrafast electro-optical disk modulators for logic, communications, optical repeaters, and wavelength converters.

We propose a U-shaped pn junction in a silicon-on-insulator microdisk resonator to effectively double the junction-mode overlap in the state-of-the-art, vertical pn junction microdisk electro-optical (EO) modulators. The U-shaped pn junction promotes the maximum overlap between the junction depletion zone and the whispering gallery optical mode in the microdisk. By fully depleting the p region of the npn-sequenced U-junction, the capacitance is reduced below 3 fF, which significantly improves the speed and power performance. In this work, we implement the high-efficiency, depleted U-junction design to maximize the operating bandwidth of EO modulators, EO logic elements, EO 2 × 2 switches for wavelength-division cross-connects, 2 × 2 reconfigurable optical add-drop multiplexers, optical-to-electrical-to-optical (OEO) repeaters-with-gain, OEO wavelength converters, and 2 × 2 optical-optical logic gates. These devices all operate in the 7.6-to-50 GHz bandwidth range with ultralow energy consumption between 0.4 and 9.8 fJ/bit. By using CMOS-compatible materials and fabrication-feasible design dimensions, our proposed high-performance devices offer a promising potential in next-generation, high-volume electro-optical communications and computing circuits.

N x N x Mλ electro-optical nanobeam wavelength-multiplexed cross-connect switches using push-push addressing.

In this theoretical work, we design elemental and higher-order wavelength-division-multiplexed cross-connects (WXCs): nonblocking space-and-wavelength routing switches to be constructed in a monolithic, industry-standard, silicon-on-insulator (SOI) chip operating at a center wavelength of 1550 nm. Each multi-spectral multi-crossbar 2×2 x Mλ "element" of the network switch is an M-fold cascade connection of λ-diverse SOI Mach-Zehnder interferometers (MZIs), each of which utilizes a nanobeam cavity in each MZI arm. Within the element, each MZI has an electro-optically (EO) controlled local PN-junction "depleter" embedded in each cavity. The cavity voltage commands are (0,0) or (V,V) where V is a "small" reverse bias. Each element can be reconfigured in 2 to 5 ps, depending on Q, with few-fJ/bit switching energy. For the M = 3 case, a compact 6-element 4×4×3λ WXC is presented. In addition, compact new designs are given for a 12-element 8×8×3λ WXC and for 16×16×8λ WXCs employing either 56 or 72 elements.

On-Chip Group-IV Heisenberg-Limited Sagnac Interferometric Gyroscope at Room Temperature.

A room-temperature strip-guided "manufacturable" Silicon-on-Insulator (SOI)/GeSn integrated-photonics quantum-gyroscope chip operating at 1550 nm is proposed and analysed. We demonstrate how the entangled photons generated in Si Spontaneous Four Wave Mixing (SFWM) can be used to improve the resolution of a Sagnac interferometric gyroscope. We propose different integrated architectures based on degenerate and non-degenerate SFWM. The chip comprises several beam splitters, two SFWM entangled photon sources, a pump filter, integrated Mach-Zehnder interferometric gyro, and an array of waveguide coupled GeSn/Ge/Si single-photon avalanche detectors. The laser pumped SWFM sources generate the signal-idler pairs, which, in turn, are used to measure the two-photon, four-photon, and higher order coincidences, resulting in an increasing of the gyro resolution by a factor of two and four, with respect to the classical approach.

Electronic-photonic arithmetic logic unit for high-speed computing.

The past two decades have witnessed the stagnation of the clock speed of microprocessors followed by the recent faltering of Moore's law as nanofabrication technology approaches its unavoidable physical limit. Vigorous efforts from various research areas have been made to develop power-efficient and ultrafast computing machines in this post-Moore's law era. With its unique capacity to integrate complex electro-optic circuits on a single chip, integrated photonics has revolutionized the interconnects and has shown its striking potential in optical computing. Here, we propose an electronic-photonic computing architecture for a wavelength division multiplexing-based electronic-photonic arithmetic logic unit, which disentangles the exponential relationship between power and clock rate, leading to an enhancement in computation speed and power efficiency as compared to the state-of-the-art transistors-based circuits. We experimentally demonstrate its practicality by implementing a 4-bit arithmetic logic unit consisting of 8 high-speed microdisk modulators and operating at 20 GHz. This approach paves the way to future power-saving and high-speed electronic-photonic computing circuits.

GeSn resonant-cavity-enhanced photodetectors for efficient photodetection at the 2 µm wavelength band.

The 2 µm wavelength band has recently gained increased attention for potential applications in next-generation optical communication. However, it is still challenging to achieve effective photodetection in the 2 µm wavelength band using group-IV-based semiconductors. Here we present an investigation of GeSn resonant-cavity-enhanced photodetectors (RCEPDs) on silicon-on-insulator substrates for efficient photodetection in the 2 µm wavelength band. Narrow-bandgap GeSn alloys are used as the active layer to extend the photodetection range to cover the 2 µm wavelength band, and the optical responsivity is significantly enhanced by the resonant cavity effect as compared to a reference GeSn photodetector. Temperature-dependent experiments demonstrate that the GeSn RCEPDs can have a wider photodetection range and higher responsivity in the 2 µm wavelength band at higher temperatures because of the bandgap shrinkage. These results suggest that our GeSn RCEPDs are promising for complementary metal-oxide-semiconductor-compatible, efficient, uncooled optical receivers in the 2 µm wavelength band for a wide range of applications.

Compact polarization splitter based on a silicon angled multimode interferometer structure.

The difference between transverse electric (TE) and transverse magnetic (TM) mode-effective indices in a wave-guided angled multimode interferometer structure is found to produce practical polarization splitting (PS) in the silicon-on-insulator platform at 1550 nm. Simulations show that this PS offers competitive performance in low insertion loss (0.4 dB for TE and 0.8 dB for TM), high extinction ratio (ER) (27.6 dB for TE and 26.5 dB for TM), low cross talk (-27.3 dB for TE and -28.0 dB for TM), and a 53-nm bandwidth for ER>20 dB. The compact footprint (∼25 µm2), the identical single-mode input/output waveguides for integration without altering the cross section, and the simplicity in implementation are prominent advantages compared with prior art designs.

Automated logic synthesis for electro-optic logic-based integrated optical computing.

Integrated optical computing attracts increasing interest recently as Moore's law approaches the physical limitation. Among all the approaches of integrated optical computing, directed logic that takes the full advantage of integrated photonics and electronics has received lots of investigation since its first introduction in 2007. Meanwhile, as integrated photonics matures, it has become critical to develop automated methods for synthesizing optical devices for large-scale optical designs. In this paper, we propose a general electro-optic (EO) logic in a higher level to explore its potential in integrated computing. Compared to the directed logic, the EO logic leads to a briefer design with shorter optical paths and fewer components. Then a comprehensive gate library based on EO logic is summarized. At last, an And-Inverter Graphs (AIGs) based automated logic synthesis algorithm is described as an example to implement the EO logic, which offers an instruction for the design automation of high-speed integrated optical computing circuits.

Stimulated Brillouin Scattering in an AlGaN Photonics Platform Operating in the Visible Spectral Range.

We present Stimulated Brillouin Scattering (SBS) process in AlGaN integrated photonic waveguides. The wide bandgap of this III-Nitride material platform allows operating at visible wavelengths enabling large Stokes shifts. For this study, we employ a multiphysics approach that includes electric-photoelastic, magnetic-photoelastic, material interface displacement effects, and for optimal waveguide dimensions to find the Brillouin-active acoustic modes involved in the SBS process. The SBS power gain and the Stokes frequency shift are investigated for both backward and forward scattering processes, and it is shown that stokes shift larger than 50 GHz with high gain are achievable. Moreover, a parametric analysis is presented in order to demonstrate the possibility of realizing Brillouin lasers operating at blue wavelengths.

Optically pumped lasing at 3 µm from compositionally graded GeSn with tin up to 22.3.

The recent demonstration of the GeSn laser opened a promising route towards the monolithic integration of light sources on the Si platform. A GeSn laser with higher Sn content is highly desirable to enhance the emission efficiency and to cover longer wavelength. This Letter reports optically pumped edge-emitting GeSn lasers operating at 3 µm, whose device structure featured Sn compositionally graded with a maximum Sn content of 22.3%. By using a 1950-nm laser pumping in comparison with a 1064-nm pumping, the local heating and quantum defect were effectively reduced, which improved laser performance in terms of higher maximum lasing temperature and lower threshold.