1 Introduction

Mid-infrared (2 to 20 μm) photonics has attracted great attention in the past fewdecades due to its tremendous application scenarios in information science, indus-try safety, and biomedicine, as shown in Fig. 1. In the 3- to 5-μm wavelengthregion, the so-called atmospheric transmission window, the transmission of mid-infrared light is much higher than that of visible and near-infrared light; it thushas been widely studied for free-space optical communication,1 remote sensing,2

as well as light detection and ranging (LiDAR).3,4 Moreover, due to distinguish-able fundamental vibrational transitions of molecules in the mid-infrared spectralregion, chemical molecules can be identified by measuring the absorption spectraof molecules in the functional group region of the mid-infrared spectrum (around2 to 8 μm).5 Thus mid-infrared spectroscopy has been widely used in industry forenvironmental monitoring,6 toxic gas detection,7 and pesticide residue testing.8 Atlonger wavelengths, namely, the fingerprint region of the mid-infrared spectrum(around 8 to 14 μm), molecular spectra reflect not only bending and stretchingmotions in molecules but also the coupling between different vibrational modes.As a result, this part of the spectrum usually contains a series of complicatedabsorption peaks, which are provided with extremely high specificity to mole-cules, especially macromolecules, thus it is known as a “molecular fingerprint.”This feature brings us broad applications in biomedicine from purely scientificuses to clinical uses, such as breath analysis,9 glucose monitoring,10 and cancerdiagnosis.11 It is worthwhile to note that current mid-infrared applications aremainly developed based on benchtop optical instruments (e.g., Fourier-transforminfrared spectroscopy spectrometers), which inevitably suffer from expensive,heavy, and bulky setups. To overcome this limitation, mid-infrared integrated

Figure 1 Promising applications of mid-infrared photonics in the functional group region (thewhite area) and the fingerprint region (the gray area). The absorption spectra of gas mole-cules (H2O, CH4, CO2, CO, NO, NO2, and CH4) come from the HITRAN2016 molecularspectroscopic database HITRAN.12

Cheng, Chen, and Liu: Mid-Infrared Germanium Photonics 1

After developing mid-infrared germanium fiber devices,62,63 mid-infraredgermanium photonics has been studied for chip-integrated applications based onvarious types of wafers, including germanium-on-silicon (GOS) wafers,germanium-on-SOI (GOSOI) wafers, germanium-on-insulator (GOI) wafers, andgermanium-on-silicon nitride (GOSiN) wafers, as shown in Fig. 2 and summarizedin Table 2. The first mid-infrared germanium waveguide was demonstrated byHerzig et al.53 based on a GOS wafer in 2012. Based on such a platform, the lowestoptical loss reached 2.5 dB/cm at a wavelength of 5.8 μm, whereas the bending losswas measured to be 0.12 dB for a 90-deg bend with a radius of 115 μm. Later, mid-infrared germanium waveguides based on GOSOI wafers were demonstrated byRoelkens et al.64 in 2014 and Ang et al.65 in 2016. The minimum optical loss ofthe GOSOI-based waveguide was reported as 2.5 dB/cm at a wavelength of3.7 μm.66 It is worthwhile to note that due to the moderate RI contrast betweenthe top germanium layer and the silicon substrate, it is still challenging to developcompact photonic devices based on the GOS and GOSOI wafers. To reduce themid-infrared germanium device footprint, GOI and GOSiN wafers were

Figure 2 Schematic of the cross sections of four types of demonstrated wafers for mid-infra-red germanium photonics.

Table 2 Comparison of state-of-the-art performance of the reported mid-infrared germaniumwaveguides.

GOS-basedwaveguide

GOSOI-basedwaveguide

GOI-basedwaveguide

GOSiN-basedwaveguide

Optical loss(dB/cm)

2.5 2.5 14.0 3.35

Wavelength (μm) 5.8 3.7 2.0 3.7

Bending loss(dB/90-deg bend)

0.12 at 115 μmbending radius

N. A. 0.2 at 5 μmbending radius

0.14 at 5 μmbending radius

Reference 53 66 70 69

Cheng, Chen, and Liu: Mid-Infrared Germanium Photonics 5

the wet etching process, as illustrated in Fig. 3. As shown in Fig. 6(b), the bestcoupling efficiency was measured to be −11 dB with 1-dB bandwidth of∼58 nm for quasi-TE mode coupling. Compared with the three-dimensional(3-D) finite-difference time-domain (FDTD) simulation results, the large discrep-ancy in the coupling efficiency is mainly because the light emitted from the inputoptical fiber was elliptically polarized.

2.3 Microresonator

As a basic component in photonic integrated circuits, microring or microracetrackresonators play an important role in the success of silicon photonics in opticalcommunication, optical sensing, nonlinear optics, optomechanics, and so forth.84

In the mid-infrared spectral region, silicon-based microresonators have beenwidely demonstrated23 and used for applications in on-chip sensors,85 optical fre-quency combs,45 electro-optical modulators,41 and so on. Also mid-infraredmicroresonators have been demonstrated on germanium platforms. Takenakaet al., Mashanovich et al., and Roelkens et al. demonstrated microresonators basedon GOI and GOS platforms, respectively. In 2016, based on a microresonator on aGOI wafer, Takenaka et al.80 demonstrated that the germanium’s TO coefficient is3 times larger than that of silicon. Later, the same authors demonstrated a tunableVernier filter based on a GOI-based microring resonator. Due to the high TOcoefficient of germanium, the tunability of the germanium microring resonator isapproximately twofold higher than that of silicon.86 In 2016, Mashanovich et al.designed and demonstrated Vernier-effect microracetrack resonators based on aGOS wafer. A maximum quality (Q) factor of higher than 5000 with an insertionloss of ∼5 dB and extinction ratio (ER) of ∼23 dB was experimentally achievedat a wavelength of 3.8 μm.87 In 2018, Roelkens et al.88 demonstrated a

Figure 6 Experimental results of the germanium focusing subwavelength grating coupler.(a) SEM image of the top view of the grating coupler. (b) 3-D FDTD simulation and meas-urement results. Reproduced with permission from Ref. 71. Copyright © 2017 by theOptical Society of America.

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Figure 10 Schematic of the monolithically chip-integrated germanium photonic crystal devi-ces. Focusing subwavelength grating couplers, suspended membrane waveguides, free-standing waveguides, photonic crystal waveguides, and a photonic crystal cavity withFano resonance were integrated on a chip, while the mid-infrared light was coupled fromthe mid-infrared optical fibers. The inset shows that the H0 photonic crystal cavity wasdesigned by adjusting the locations of four nanoholes.

Figure 11 Simulation and measurement results of the germanium photonic crystal cavitywith Fano resonance. (a) Simulation of transmission profile. Insets: top view of theelectric-field distributions of excited modes in the germanium photonic crystal cavity.(b) Experimental measurements of the device transmission (blue curve) and the couplingprofile of the focusing subwavelength grating coupler (red curve). Reproduced with permis-sion from Ref. 90. Copyright © 2017 by the Optical Society of America.

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