Germanium (Ge) has played a key role in silicon photonics as an enabling material for datacom applications. Indeed, the unique properties of Ge have been leveraged to develop high performance integrated photodectors, which are now mature devices. Ge is also very useful for the achievement of compact modulators and monolithically integrated laser sources on silicon. Interestingly, research efforts in these domains also put forward the current revolution of mid-IR photonics. Ge and Ge-based alloys also present strong advantages for mid-infrared photonic platform such as the extension of the transparency window for these materials, which can operate at wavelengths beyond 8 μm. Different platforms have been proposed to take benefit from the broad transparency of Ge up to 15 μm, and the main passive building blocks are now being developed. In this review, we will present the most relevant Ge-based platforms reported so far that have led to the demonstration of several passive and active building blocks for mid-IR photonics. Seminal works on mid-IR optical sensing using integrated platforms will also be reviewed.

Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at λ~3.6 µm and at λ ~8.5 µm respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (λ ~2-20 µm). Here, we propose using a compact Ge-rich graded-index SiGe platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from λ = 5.5 µm to 8.5 µm is demonstrated in Ge-rich SiGe waveguides of only 6 µm thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich SiGe platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to λ ~15 µm.

The popular tracking-by-detection paradigm for multi-object tracking (MOT) focuses on solving data association problem, of which a robust similarity model lies in the heart. Most previous works make effort to improve feature representation for individual object while leaving the relations among objects less explored, which may be problematic in some complex scenarios. In this paper, we focus on leveraging the relations among objects to improve robustness of the similarity model. To this end, we propose a novel graph representation that takes both the feature of individual object and the relations among objects into consideration. Besides, a graph matching module is specially designed for the proposed graph representation to alleviate the impact of unreliable relations. With the help of the graph representation and the graph matching module, the proposed graph similarity model, named GSM, is more robust to the occlusion and the targets sharing similar appearance. We conduct extensive experiments on challenging MOT benchmarks and the experimental results demonstrate the effectiveness of the proposed method.

Midinfrared spectroscopy is a universal way to identify chemical and biological substances. Indeed, when interacting with a light beam, most molecules are responsible for absorption at specific wavelengths in the mid-IR spectrum, allowing to detect and quantify small traces of substances. On-chip broadband light sources in the mid-infrared are thus of significant interest for compact sensing devices. In that regard, supercontinuum generation offers a mean to efficiently perform coherent light conversion over an ultrawide spectral range, in a single and compact device. This work reports the experimental demonstration of on-chip two-octave supercontinuum generation in the mid-infrared wavelength, ranging from 3 to 13 μm (that is larger than 2500 cm –1 ) and covering almost the full transparency window of germanium. Such an ultrawide spectrum is achieved thanks to the unique features of Ge-rich graded SiGe waveguides, which allow second-order dispersion tailoring and low propagation losses over a wide wavelength range. The influence of the pump wavelength and power on the supercontinuum spectra has been studied. A good agreement between the numerical simulations and the experimental results is reported. Furthermore, a very high coherence is predicted in the entire spectrum. These results pave the way for wideband, coherent, and compact mid-infrared light sources by using a single device and compatible with large-scale fabrication processes.

Mid-infrared (mid-IR) integrated photonics are expected to provide key advances for the demonstration of chip-scale spectroscopic systems. It has been recently reported that Ge-rich SiGe alloy-based photonic structures can provide broadband operation for a wavelength range spanning from 5.5 to 8.5 µm, thus holding great potential for mid-IR applications. In this paper, the Ge-rich SiGe platform is considered for a mid-IR photonic chip-scale sensor, based on the use of the evanescent component of the guided optical mode to probe specific molecular absorption features of the surrounding cladding environment. As a proof of concept, we monitored the absorption spectral patterns of a standalone photoresist spin-coated onto spiral Ge-rich SiGe waveguides. A significant increase of the waveguide optical loss at the spectral window of 5.8-6.2 µm is identified and correlated with the inherent photoresist absorption. The ability of this platform to sense small concentrations of methane gas is also discussed. These results pave the way towards the demonstration of compact, portable, label-free and highly sensitive photonic integrated sensors based on Ge-rich SiGe circuits. , "On-chip midinfrared gas detection using chalcogenide glass waveguide," Appl.

This Letter explores the use of Ge-rich SiGe waveguides on graded SiGe substrate for the demonstration of ultra-wideband photonic integrated circuits in the mid-infrared (mid-IR) wavelength range. We designed, fabricated, and characterized broadband Mach-Zehnder interferometers fully covering a range of 3 μm in the mid-IR band. The fabricated devices operate indistinctly in quasi-TE and quasi-TM polarizations, and have an extinction ratio higher than 10 dB over the entire operating wavelength range. The obtained results are in good correlation with theoretical predictions, while numerical simulations indicate that the device bandwidth can reach one octave with low additional losses. This Letter paves the way for further realization of mid-IR integrated spectrometers using low-index-contrast SiGe waveguides with high germanium concentration.

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A novel Ni/Mg–Al–O catalyst has been successfully synthesized by the cation–anion double hydrolysis (CADH) method for carbon dioxide (CO2) reforming of methane. Compared with the Ni/MgO–Al2O3 catalyst prepared by the conventional co-impregnation method, the Ni/Mg–Al–O catalyst exhibited enhanced initial activity and improved long-term stability. Structural characterizations revealed that higher metal dispersion (2.8 times) and better intrinsic activity (1.8 times in terms of turnover frequency at 700 °C) were achieved by ultrasmall Ni nanoparticles (2.1 nm) on the Ni/Mg–Al–O catalyst, leading to the enhanced initial activity. The improved long-term stability of the Ni/Mg–Al–O catalyst was attributed to the superior anti-coke property, which was rationalized by the ultrasmall Ni nanoparticles and abundant basic sites. The present work provides a facile and green method for the synthesis of efficient reforming catalysts and elucidates the mechanisms underlying excellent catalytic performances.