signrificarnee Mid-infrared (2-20 mu m wavelength) photonics has extensive applications in spectroscopic analysis, environmental monitoring, medical diagnosis, free-space optical communication, and ranging, due to the distinguishable fundamental vibrational transitions of molecules and the atmospheric transmission windows (e. g. 2-2.5 mu m, 3-5 mu m, and 8-13 mu m wavelengths) in the mid-infrared spectral region. Previously, mid-infrared applications have been mainly developed based on benchtop free-space optical instruments (e. g. Fourier-transform infrared spectrometers), which inevitably suffer from expensive, heavy, and bulky setups. To overcome this limitation, mid-infrared integrated optics has been proposed and quickly developed in the past few decades. By using the nanofabrication technology, on-chip mid-infrared devices not only significantly reduce footprints, weights, and costs of mid-infrared photonic systems, but also open an avenue to explore the light-matter interaction at the nanoscale level. Nowadays, numerous optical materials have been investigated to develop mid-infrared integrated optics, namely, noble metals, low-dimensional semiconductors, chalcogenide glasses, and group-IV semiconductors. As for noble metals and low-dimensional semiconductors, high optical losses of the developed waveguides hinder the potential large-scale integration of on-chip systems. While chalcogenide-glass-waveguides have attracted a great attention in many mid-infrared applications due to their ultra-low optical losses. However, the fabrication of the chalcogenide-glass-waveguides is not fully compatible with the complementary metal-oxide-semiconductor (CMOS) technology. On the other hand, photonic devices based on group-IV semiconductors, namely, silicon, germanium, tin, have the notable advantages of low optical loss, excellent physiochemical stability, and full CMOS compatibility, which are critical for practical applications with low-cost and high-volume production requirements. Consequently, mid-infrared group-IV photonics has been a hot topic in the past few years. As for the most commonly used group-IV semiconductors, silicon is first used to explore mid-infrared photonic integrated circuits. As early as 2006, Soref et al. published a paper to discuss the prospects of mid-infrared silicon photonics. Compared with the near-infrared band, silicon dioxide has huge optical absorption to the mid-infrared light, thus the silicon photonic devices utilized for the near-infrared band cannot be directly used in the mid-infrared band. Numerous novel silicon waveguide configurations, namely, suspended membrane waveguides, subwavelength-cladding waveguides, and silicon-on-sapphire waveguides, have been demonstrated. However, due to the strong multi-phonon absorption of silicon, the low-optical-loss spectral region of silicon photonic devices can only reach the functional group region (wavelengths below 8. 0 mu m). For silicon-germanium alloys, the photonic devices can be operated up to at least 8. 5 mu m wavelength. In contrast, for undoped crystal germanium material, optical absorption can be as low as 1 dB/cm within a spectral range from 1.9 mu m to 16.7 mu m at room temperature. Therefore, it is extremely promising to develop mid-infrared waveguides for long wavelengths based on a germanium platform. Progress Germanium possesses advantages of wide transparency window (2-14 mu m wavelength), high refractive index (similar to 4. 0), an excellent thermal optic coefficient (> 10(-4) K-1), large third-order nonlinear susceptibility (similar to 10(-18) m(2). V-2), and low cost for high-quality and high-density device fabrication. Therefore, germanium devices could be an excellent candidate to develop mid-infrared applications, especially in the fingerprint region. Since the first germanium waveguide was developed in 2012, mid-infrared germanium photonics has been attracting increasing research attention. Currently, germanium waveguides are mainly demonstrated based on four types of integration platforms, namely, germanium-on-silicon wafer, germanium-on-silicon-on-insulator wafer, germanium-on-insulator wafer, and germanium-on-silicon nitride wafer. Based on the above germanium platforms, researchers have not only developed state-of-the-art passive optical components on a chip, such as low optical loss waveguides, grating couplers, high quality-factor microring resonators, and photonic crystal nanocavities, but also demonstrated mid-infrared waveguide-integrated lasers and electro-optical modulators. Moreover, to extend the spectral range of onchip sensing applications to the fingerprint region, researchers have developed diverse chip-integrated gas and protein sensors by using the germanium waveguide devices. Besides, nonlinear optical phenomena, namely Kerr frequency combs and supercontinuum generation, have also been theoretically explored in the germanium devices to overcome the spectral bandwidth limitation of mid-infrared on-chip lasers. Conclusion and Prospect In this paper, we briefly review the historical progress of mid-infrared group-IV photonics, and comprehensively summarize the development of recently emerging germanium photonics integrated circuits and their applications. In addition, the prospect of mid-infrared integrated optics is discussed. We hope this paper can not only serve as a reference for researchers specialized in mid-infrared photonics, silicon photonics, germanium photonics, optoelectronic materials, optical sensing, and spectroscopy, but also arouse attentions of researchers to mid-infrared integrated optoelectronics.
