As photonic technologies grow in multidimensional aspects, integrated photonics holds a unique position and continuously presents enormous possibilities for research communities. Applications include data centers, environmental monitoring, medical diagnosis, and highly compact communication components, with further possibilities continuously growing. Herein, we review state-of-the-art integrated photonic on-chip sensors that operate in the visible to mid-infrared wavelength region on various material platforms. Among the different materials, architectures, and technologies leading the way for on-chip sensors, we discuss the optical sensing principles that are commonly applied to biochemical and gas sensing. Our focus is on passive optical waveguides, including dispersion-engineered metamaterial-based structures, which are essential for enhancing the interaction between light and analytes in chip-scale sensors. We harness a diverse array of cutting-edge sensing technologies, heralding a revolutionary on-chip sensing paradigm. Our arsenal includes refractive-index-based sensing, plasmonics, and spectroscopy, which forge an unparalleled foundation for innovation and precision. Furthermore, we include a brief discussion of recent trends and computational concepts, incorporating Artificial Intelligence & Machine Learning (AI/ML) and deep learning approaches over the past few years to improve the qualitative and quantitative analysis of sensor measurements.

This paper presents a novel silicon-based hollow-core surface-normal photonic crystal waveguide with an air-core even mode and high group index in the meta structure. This mode is proposed for gas detection in the mid-infrared range. The structure’s bandgap map and guided modes were calculated through numerical simulations, allowing for design optimization to enable slow-light propagation of the guided modes. The design enhances light–matter interaction by leveraging large group indices and dispersion control. In experimental simulations of photonic crystal waveguide structures, an ultra-high group index of 1424 was achieved at a wavelength of 3.31 μm, matching with methane absorption. This high group index—i.e., the realization of extremely slow light—facilitates enhanced interaction with analytes of interest. The result reveals that 60.1% of the energy of the fundamental mode resides within the center air hole, providing a high overlap with analytes. The device geometry can be customized to introduce the absorption wavelengths of various analytes, enabling arrays of different device geometries on the same chip for simultaneous detection of multiple gas species.

This paper investigates the impact of the quantum cascade laser’s frequency modulation response on its tuning rate and tunability. We show a significant disparity in laser tuning rates and tunability between single and dual-frequency modulation schemes frequently used in typical direct absorption and wavelength modulation spectroscopy (WMS) techniques. We show that the DC-characterized tuning rate of a laser can be reduced significantly under a specific set of modulation frequencies of the laser injection current. We characterize these effects by simultaneous measurements of higher harmonic WMS of methane and nitrous oxide in the 7.8 µm spectral regions. We further show that WMS signal modulation broadening mechanisms and spectral structure, i.e., its zero-crossings and turning points, can be used to quantify such laser-modulation effects and validate laser frequency response under dual modulation schemes.

Two-dimensional photonic crystal waveguides (PCWs) enhance light–matter interactions through slow-light effects and resonant field confinement. We report the design, simulation, and experimental realization of silicon VPCW arrays exhibiting dual mid-infrared (mid-IR) resonances: a symmetric Lorentzian-type and an asymmetric Fano-type resonance. Each VPCW is formed by introducing a hollow air-core defect into a hexagonal lattice of deeply etched air holes in silicon. By tuning the defect radius and lattice constant, these resonances were aligned with the absorption lines of acetaldehyde (C₂H₄O) (λ ≈ 5.88 μm) and water vapor (H₂O) (λ ≈ 6.05 μm), enabling multiplexed gas detection. Simulations predicted large group indices (n_g ≈ 1064 for C₂H₄O and n_g ≈ 204 for H₂O), consistent with slow-light features. Experimentally, Lorentzian and Fano resonances were observed, with linewidth broadening attributed to fabrication imperfections, at photonic band-edge wavelengths where slow-light modes are expected. This work demonstrates the first realization of dual Lorentzian–Fano resonances in VPCWs under normal-incidence light excitation, establishing a CMOS-compatible platform for compact mid-IR absorption spectroscopy and multiplexed gas sensing.