Silicon MMI-based power splitter for multi-band operation at the 1.55 and 2 µm wave bands

Low Temperature Characteristics of Ge-on-Si Waveguide Photodetectors: A Combined Simulation and Experimental Study

Benefiting from the progress of the germanium (Ge) epitaxy process on silicon (Si) substrates, waveguide-integrated Ge-on-Si photodetectors (PDs) have demonstrated decent performances in short-wave infrared (SWIR) detection. By lowering the operating temperature, theses PDs can meet the stringent signal-to-noise requirements for high-sensitivity detection. We systematically investigated the dark current characteristics and optical response in the 1500-1600 nm wavelength range of the waveguide-integrated Ge-on-Si PDs operated at low temperatures (200 K to 300 K). Under a -3 V bias, the PD exhibits a room-temperature dark current of 4.62 nA and a responsivity of 0.87 A/W at 1550 nm. When the temperature was reduced to 200 K, the dark current decreased to 93.69 pA, and the responsivity dropped to 0.34 A/W. Using finite-difference time-domain (FDTD) and technology computer-aided design (TCAD) simulations, we extracted the absorption coefficients of epitaxial Ge on Si at low temperatures. At room temperature, the absorption coefficient at the wavelength of 1550 nm was approximately 1100 cm-1, while at 200 K, the absorption coefficient decreased to 248 cm-1. The outcomes of this work pave the way for the high-performance low-temperature Si photonic systems in the future.

Inverse design of nanophotonic devices enabled by optimization algorithms and deep learning: recent achievements and future prospects

Nanophotonics, which explores significant light-matter interactions at the nanoscale, has facilitated significant advancements across numerous research fields. A key objective in this area is the design of ultra-compact, high-performance nanophotonic devices to pave the way for next-generation photonics. While conventional brute-force, intuition-based forward design methods have produced successful nanophotonic solutions over the past several decades, recent developments in optimization methods and artificial intelligence offer new potential to expand these capabilities. In this review, we delve into the latest progress in the inverse design of nanophotonic devices, where AI and optimization methods are leveraged to automate and enhance the design process. We discuss representative methods commonly employed in nanophotonic design, including various meta-heuristic algorithms such as trajectory-based, evolutionary, and swarm-based approaches, in addition to adjoint-based optimization. Furthermore, we explore state-of-the-art deep learning techniques, involving discriminative models, generative models, and reinforcement learning. We also introduce and categorize several notable inverse-designed nanophotonic devices and their respective design methodologies. Additionally, we summarize the open-source inverse design tools and commercial foundries. Finally, we provide our perspectives on the current challenges of inverse design, while offering insights into future directions that could further advance this rapidly evolving field.

Ultra-miniaturized Bloch mode metasplitters for one-dimensional grating waveguides

We present, for the first time, to our knowledge, power splitters with multiple channel configurations in one-dimensional grating waveguides (1DGWs) that maintain crystal lattice-sensitive Bloch mode profiles without perturbation across all output channels, all within an ultra-miniaturized footprint of just 2.1 × 2.2 μm2. This novel capability reduces the need for transition regions, simplifies multi-channel configurations of 1DGWs, and maximizes the effective use of chip area. The pixelated metamaterial approach, integrated with a time-domain heuristic algorithm, is utilized to concurrently achieve broadband operation, optimized dispersion control, and minimal loss. We experimentally demonstrate that the 1 × 2 and 1 × 3 metasplitters achieve average minimum losses per channel of 3.80 dB and 5.36 dB, respectively, which are just 0.80 dB and 0.59 dB above ideal splitting. The measurements for both designs demonstrate a 1 dB bandwidth of 15 nm, with excellent uniformity across all output channels. These versatile metasplitter designs can serve as fundamental building blocks for ultrahigh-bandwidth, densely integrated photonic circuits and in scenarios where slow light is essential.

Low loss and ultra-broadband design of an integrated 3 dB power splitter centered at 2 µm

Because chemical gas is sensitive to absorption in the 2 µm band, and 2 µm matches the absorption band of the remote sensing material, many remote sensors and optical sensors are designed to operate in the 2 µm wavelength region. In this paper, we designed an integrated 3 dB power splitter centered at 2 µm. The study of this device is built on a silicon-on-insulator (SOI) platform. We introduced a subwavelength grating (SWG) to improve the performance of the device. We used the three-dimensional finite-difference time-domain (3D FDTD) method to analyze the effect of the structure on the power splitter. The insertion loss (IL) of the fundamental TE mode is only 0.04 dB at 2 µm and its bandwidth of IL <0.45d B is 940 nm (1570-2510 nm). It is suitable for multidomain and all-band photonic integrated circuits at 2 µm.

On-chip Y-junction with adaptive power splitting toward ultrabroad bandwidth

Growing research interests have been directed to the emerging optical communication band at 2-µm wavelengths. The silicon photonic components are highly desired to operate over a broad bandwidth covering both C-band and the emerging 2-µm wave band. However, the dispersions of the silicon waveguides eventually limit the optical bandwidth of the silicon photonic devices. Here, we introduce a topology-optimized Y-junction with a shallow-etched trench and its utility to reverse the detrimental dispersion effect. The shallow trench enables the Y-junction to have an adaptive splitting capability over a broad spectral range. The 0.2-dB bandwidth of the power splitter exceeds 800 nm from 1400 nm to 2200 nm. The device has a compact footprint of 3 µm × 1.64 µm. The device is characterized at the C-band and 2-µm band with a measured excess loss below 0.4 dB for a proof-of-concept demonstration.