2 μm wavelength range InP-based type-II quantum well photodiodes heterogeneously integrated on silicon photonic integrated circuits

On-chip germanium photodetector with interleaved junctions for the 2-µm wave band

Recently, the 2-µm wave band has gained increased interest due to its potential application for the next-generation optical communication. As a proven integration platform, silicon photonics also benefit from the lower nonlinear absorption and larger electro-optic coefficient. However, this spectral range is far beyond the photodetection range of germanium, which places an ultimate limit for on-chip applications. In this work, we demonstrate a waveguide-coupled photodetector enabled by a tensile strain-induced absorption in germanium. Responsivity is greatly enhanced by the proposed interleaved junction structure. The device is designed on a 220-nm silicon-on-insulator and is fabricated via a standard silicon photonic foundry process. By utilizing different interleaved PN junction spacing configurations, we were able to measure a responsivity of 0.107 A/W at 1950 nm with a low bias voltage of -6.4 V for the 500-μm-long device. Additionally, the 3-dB bandwidth of the device was measured to be up to 7.1 GHz. Furthermore, we successfully achieved data transmission at a rate of 20 Gb/s using non-return-to-zero on-off keying modulation.

Breakthrough in Silicon Photonics Technology in Telecommunications, Biosensing, and Gas Sensing

Silicon photonics has been an area of active research and development. Researchers have been working on enhancing the integration density and intricacy of silicon photonic circuits. This involves the development of advanced fabrication techniques and novel designs to enable more functionalities on a single chip, leading to higher performance and more efficient systems. In this review, we aim to provide a brief overview of the recent advancements in silicon photonic devices employed for telecommunication and sensing (biosensing and gas sensing) applications.

Heterogeneously Integrated Graphene/Silicon/Halide Waveguide Photodetectors toward Chip-Scale Zero-Bias Long-Wave Infrared Spectroscopic Sensing

Mid-infrared absorption spectroscopy plays an important role in molecule identification and quantification for widespread applications. Integrated photonics provides opportunities to perform spectroscopic sensing on-chip for the minimization of device size, cost, and power consumption. The integration of waveguides and photodetectors is an indispensable step toward the realization of these on-chip sensing systems. It is desired to extend the operating wavelengths of these on-chip sensing systems to the long-wave infrared (LWIR) range to utilize more molecular absorption fingerprints. However, the development of LWIR waveguide-integrated photodetectors faces challenges from both waveguide platforms due to the bottom cladding material absorption and photodetection technologies due to the low LWIR photon energy. Here, we demonstrate LWIR waveguide-integrated photodetectors through heterogeneous integration of graphene photodetectors and Si waveguides on CaF₂ substrates. A high-yield transfer printing method is developed for flexibly integrating the waveguide and substrate materials to solve the bottom cladding material absorption issue. The fabricated Si-on-CaF₂ waveguides show low losses in the broad LWIR wavelength range of 6.3-7.1 μm. The graphene photodetector achieves a broadband responsivity of ∼8 mA/W in these low-photon-energy LWIR wavelengths under zero-bias operation with the help of waveguide integration and plasmonic enhancement. We further integrate the graphene photodetector with a Si-on-CaF₂ folded waveguide and demonstrate on-chip absorption sensing using toluene as an example. These results reveal the potential of our technology for the realization of chip-scale, low-cost, and low-power-consumption LWIR spectroscopic sensing systems.

Mid-infrared polarization rotator based on a Si3N4-CaF2 hybrid plasmonic waveguide with asymmetric metal claddings

In this paper, a Si₃N₄-CaF₂ hybrid plasmonic waveguide (HPW) with an asymmetric metal cladding is designed for the mid-infrared polarization rotator (PR). The mode characteristics and polarization rotation performances of the Si₃N₄-CaF₂ HPW-based PR are simulated by using the finite element method. Operating at the wavelength of 3.5 µm, the polarization conversion efficiency between two polarization modes (PM 1 and PM 2) is larger than 99% at a Si₃N₄-CaF₂ HPW length of 104 µm. The Si₃N₄-CaF₂ HPW-based PR maintains good polarization rotation performances within fabrication tolerances from -10 to 10 nm. The polarization rotator based on the Si₃N₄-CaF₂ HPW paves the way to achieve integrated waveplates, driving many important optical functions from free space onto a chip.

Transfer-print integration of GaAs p-i-n photodiodes onto silicon nitride waveguides for near-infrared applications

We demonstrate waveguide-detector coupling through the integration of GaAs p-i-n photodiodes (PDs) on top of silicon nitride grating couplers (GCs) by means of transfer-printing. Both single device and arrayed printing is demonstrated. The photodiodes exhibit dark currents below 20 pA and waveguide-referred responsivities of up to 0.30 A/W at 2V reverse bias, corresponding to an external quantum efficiency of 47% at 860 nm. We have integrated the detectors on top of a 10-channel on-chip arrayed waveguide grating (AWG) spectrometer, made in the commercially available imec BioPIX-300 nm platform.

High-performance silicon-graphene hybrid plasmonic waveguide photodetectors beyond 1.55 μm

Graphene has attracted much attention for the realization of high-speed photodetection for silicon photonics over a wide wavelength range. However, the reported fast graphene photodetectors mainly operate in the 1.55 μm wavelength band. In this work, we propose and realize high-performance waveguide photodetectors based on bolometric/photoconductive effects by introducing an ultrathin wide silicon-graphene hybrid plasmonic waveguide, which enables efficient light absorption in graphene at 1.55 μm and beyond. When operating at 2 μm, the present photodetector has a responsivity of ~70 mA/W and a setup-limited 3 dB bandwidth of >20 GHz. When operating at 1.55 μm, the present photodetector also works very well with a broad 3 dB bandwidth of >40 GHz (setup-limited) and a high responsivity of ~0.4 A/W even with a low bias voltage of -0.3 V. This work paves the way for achieving high-responsivity and high-speed silicon-graphene waveguide photodetection in the near/mid-infrared ranges, which has applications in optical communications, nonlinear photonics, and on-chip sensing.

Waveguide-Integrated Black Phosphorus Photodetector for Mid-Infrared Applications

Midinfrared (MIR), which covers numerous molecular vibrational fingerprints, has attracted enormous research interest due to its promising potential for label-free and damage-free sensing. Despite intense development efforts, the realization of waveguide-integrated on-chip sensing system has seen very limited success to date. The huge lattice mismatch between silicon and the commonly used detection materials such as HgCdTe, III-V, or II-VI compounds has been the key bottleneck that hinders their integration. Here, we realize an integration of silicon-on-insulator (SOI) waveguides with black phosphorus (BP) photodetectors. When operating near BP's cutoff wavelength where absorption is weak, the light-BP interaction is enhanced by exploiting the optical confinement in the Si waveguide and grating structure to overcome the limitation of absorption length constrained by the BP thickness. Devices with different BP crystal orientation and thickness are compared in terms of their responsivity and noise equivalent power (NEP). Spectral photoresponse from 3.68 to 4.03 μm was investigated. Additionally, power-dependent responsivity and gate-tunable photocurrent were also studied. At a bias of 1 V, the BP photodetector achieved a responsivity of 23 A/W at 3.68 μm and 2 A/W at 4 μm and a NEP less than 1 nW/Hz^1/2 at room temperature. The integration of passive Si photonics and active BP photodetector is envisaged to offer a potential pathway toward the realization of integrated on-chip systems for MIR sensing applications.

Key enabling technologies for optical communications at 2000 nm

This paper discusses the potential for opening a new wavelength window at the 2 μm waveband for optical communications, showing current limitations of the system's performance. It focuses on novel results for key enabling technologies, including the analysis of laser injection locking at this waveband, an improved responsivity for bulk and strained InGaAs edge-couple detectors, and also an increased gain profile for thulium-doped fiber amplifiers.

Silicon arrayed waveguide gratings at 2.0-μm wavelength characterized with an on-chip resonator

Low-loss arrayed waveguide gratings (AWGs) are demonstrated at a 2.0-μm wavelength. These devices promote rapidly developing photonic applications, supported by the recent development of mid-infrared lasers integrated on silicon (Si). Multi-spectral photonic integrated circuits at 2.0-μm are envisioned since the AWGs are fabricated with the 500-nm-thick Si-on-insulator platform compatible with recently demonstrated lasers and semiconductor optical amplifiers on Si. Characterization with the AWG-ring method improves the on-chip transmission uncertainty to ∼6% compared to the conventional method with an uncertainty of ∼53%. Channel losses of ∼2.4  dB are found, with -31  dB crosstalk per channel. Fully integrated multi-spectral sources at 2.0 μm with pump lasers, low-loss multiplexers, and an output amplifier are now feasible.

III-V-on-Silicon Photonic Integrated Circuits for Spectroscopic Sensing in the 2-4 μm Wavelength Range

The availability of silicon photonic integrated circuits (ICs) in the 2-4 μm wavelength range enables miniature optical sensors for trace gas and bio-molecule detection. In this paper, we review our recent work on III-V-on-silicon waveguide circuits for spectroscopic sensing in this wavelength range. We first present results on the heterogeneous integration of 2.3 μm wavelength III-V laser sources and photodetectors on silicon photonic ICs for fully integrated optical sensors. Then a compact 2 μm wavelength widely tunable external cavity laser using a silicon photonic IC for the wavelength selective feedback is shown. High-performance silicon arrayed waveguide grating spectrometers are also presented. Further we show an on-chip photothermal transducer using a suspended silicon-on-insulator microring resonator used for mid-infrared photothermal spectroscopy.

2.3 µm range InP-based type-II quantum well Fabry-Perot lasers heterogeneously integrated on a silicon photonic integrated circuit

Heterogeneously integrated InP-based type-II quantum well Fabry-Perot lasers on a silicon waveguide circuit emitting in the 2.3 µm wavelength range are demonstrated. The devices consist of a "W"-shaped InGaAs/GaAsSb multi-quantum-well gain section, III-V/silicon spot size converters and two silicon Bragg grating reflectors to form the laser cavity. In continuous-wave (CW) operation, we obtain a threshold current density of 2.7 kA/cm² and output power of 1.3 mW at 5 °C for 2.35 μm lasers. The lasers emit over 3.7 mW of peak power with a threshold current density of 1.6 kA/cm² in pulsed regime at room temperature. This demonstration of heterogeneously integrated lasers indicates that the material system and heterogeneous integration method are promising to realize fully integrated III-V/silicon photonics spectroscopic sensors in the 2 µm wavelength range.

III-V-on-silicon 2-µm-wavelength-range wavelength demultiplexers with heterogeneously integrated InP-based type-II photodetectors

2-µm-wavelength-range silicon-on-insulator (SOI) arrayed waveguide gratings (AWGs) with heterogeneously integrated InP-based type-II quantum well photodetectors are presented. Low insertion loss (2.5-3 dB) and low crosstalk (-30 to -25 dB) AWGs are realized. The InP-based type-II photodetectors are integrated with the AWGs using two different coupling approaches. Adiabatic-taper-based photodetectors show a responsivity of 1.6 A/W at 2.35 µm wavelength and dark current of 10 nA at -0.5 V, while photodetectors using grating-assisted coupling have a responsivity of 0.1 A/W and dark current of 5 nA at -0.5 V. The integration of the photodetector array does not degrade the insertion loss and crosstalk of the device. The photodetector epitaxial stack can also be used to realize the integration of a broadband light source, thereby enabling fully integrated spectroscopic systems.