Nanophotonic waveguides that implement long optical pathlengths on chips are promising to enable chip-scale gas sensors. Nevertheless, current absorption-based waveguide sensors suffer from weak interactions with analytes, limiting their adoptions in most demanding applications such as exhaled breath analysis and trace-gas monitoring. Here, we propose an all-dielectric metamaterial-assisted comb (ADMAC) waveguide to greatly boost the sensing capability. By leveraging large longitudinal electric field discontinuity at periodic high-index-contrast interfaces in the subwavelength grating metamaterial and its unique features in refractive index engineering, the ADMAC waveguide features strong field delocalization into the air, pushing the external optical field confinement factor up to 113% with low propagation loss. Our sensor operates in the important but underdeveloped long-wave infrared spectral region, where absorption fingerprints of plentiful chemical bonds are located. Acetone absorption spectroscopy is demonstrated using our sensor around 7.33 μm, showing a detection limit of 2.5 ppm with a waveguide length of only 10 mm.

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As miniaturized solutions, mid-infrared (MIR) waveguide sensors are promising for label-free compositional detection of mixtures leveraging plentiful absorption fingerprints. However, the quantitative analysis of liquid mixtures is still challenging using MIR waveguide sensors, as the absorption spectrum overlaps for multiple organic components accompanied by strong water absorption background. Here, we present an artificial-intelligence-enhanced metamaterial waveguide sensing platform (AIMWSP) for aqueous mixture analysis in the MIR. With the sensitivity-improved metamaterial waveguide and assistance of machine learning, the MIR absorption spectra of a ternary mixture in water can be successfully distinguished and decomposed to single-component spectra for predicting concentration. A classification accuracy of 98.88% for 64 mixing ratios and 92.86% for four concentrations below the limit of detection (972 ppm, based on 3σ) with steps of 300 ppm are realized. Besides, the mixture concentration prediction with root-mean-squared error varying from 0.107 vol % to 1.436 vol % is also achieved. Our work indicates the potential of further extending this sensing platform to MIR spectrometer-on-chip aiming for the data analytics of multiple organic components in aqueous environments.

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their label-free and nondestructive analysis capabilities benefit from unique characteristic spectra of molecules under test. [1][2][3][4] Tremendous efforts have been made in free-space MIR spectroscopy for applications ranging from personal healthcare to environmental monitoring. [5][6][7][8][9][10] Toward future miniaturized chip-scale sensing systems, nanophotonic waveguide platforms provide an attractive approach for costeffective dense integration. [11,12] In the past decades, silicon photonics becomes a viable solution to miniaturize optical systems due to its compatibility with the mature complementary-metal-oxide-semiconductor fabrication infrastructure. [13][14][15][16] Despite the conventional silicon-on-insulator (SOI) platform has been widely adopted in the near-infrared (NIR) telecommunication band, the presence of the buried oxide (BOX) limits its effectiveness in MIR photonics due to the severe absorption issue. In addition to transparency considerations, the optical properties of intrinsic silicon (Si) might be incapable of simultaneously providing all the functionalities desired for photonic integrated circuits (PICs) such as on-chip light modulation and nonlinear optics application [17] due to the centrosymmetric crystal structure. Especially, optical phase shifters are important for phased array beam-steering in free-space communication, light detection and ranging (LIDAR) applications, [18,19] on-chip Fourier transform infrared (FTIR) spectrometers, [20,21] and photonic neural network. [22] State-ofthe-art Si modulators mainly realize phase modulation through the plasma dispersion effect of free carriers in doped Si. [23][24][25] While relatively efficient, these devices suffer from considerable insertion loss and spurious amplitude modulation. The concurrent modulation of real and imaginary part of the refractive index becomes a major restriction when proliferating them in a coherent photonic network in which adjustments of purely optical phase are required. Phase shifting has been also achieved through the thermo-optic effect in Si, but they are inherently slow and constrained by stringent tradeoffs between modulation speed and power dissipation. [26,27] Alternately, by building mature Si photonic devices on MIR-transparent materials with strong Pockels effect (otherwise known as the linear electrooptic effect), phase modulation can be also performed in a lowloss, high compact, and high energy-efficient manner. Mid-infrared spectroscopy is an emerging technique in various applicationssuch as molecule identification and label-free chemical sensing. Integrated photonic platforms have a promising potential to perform miniaturized spectroscopic sensing with the advantage of compact footprint, low cost, and low power consumption. As an essential building block for integrated photonics, on-chip phase shifter plays an important role in mid-infrared spectroscopic systems, enabling signal processing and spectrum analysis. However, the implementation of effective pure phase modulation in mid-infrared...