In this work, a photoacoustic (PA) gas sensor with a micro-embedded acoustic resonator for gas leakage detection was demonstrated. The micro-embedded acoustic resonator was fabricated by putting a leaky hollow-core fiber (L-HCF) into a cylindrical buffer chamber. The L-HCF was utilized as the PA cavity and the light transmission media simultaneously. The optimal inner diameter of the L-HCF was 1.7 mm. The embedded acoustic resonator was experimentally proven to be equivalent to a T-type half-open acoustic resonator, but the structure became much more compact. The volume of the amount of gas in the cell was only ∼0.3 mL, and the gas diffusion time to fill the sensor under room temperature (25°C) and ambient pressure (101 kPa) was ∼44 s. Trace acetylene (C₂H₂) in pure nitrogen (N₂) was chosen as the target gas, and the minimum detectable limit (MDL) reached 29 ppb when the lock-in integration time was 1 s. The normalized noise equivalent absorption (NNEA) coefficient was calculated to be 3.0 × 10⁻⁹ W·cm^-1·Hz^-1/2. The micro-resonant PA gas sensor, with merits of compactness, low gas consumption, and low cost, has the potential to be a remote gas sensing scheme in fields of environmental protection, industrial process monitoring, and so on.

Dual-parameter measurements of refractive index and methane concentration based on electromagnetic Fano resonance are proposed. Two independent Fano resonances can be produced through electric dipole and toroidal dipole resonance in an all-dielectric metasurface separately. The linear relationship between the spectral peak-shifts and the parameters to be measured will be obtained directly. The refractive index (RI) sensitivity and gas sensitivity are 1305.6 nm/refractive index unit (RIU), −0.295 nm/% for one resonance peak (dip1), and 456.6 nm/RIU, −0.61 nm/% for another resonance peak (dip2). Such a metasurface has simpler structure and higher sensitivity, which is beneficial for environmental gas monitoring or multi-parameter measurements.

This study presents a multi-gas detection system based on low-loss hollow waveguide, demonstrating significant improvements in both detection sensitivity and accuracy. The system utilizes 4 hollow waveguides optimized for different mid-infrared bands, coupled with a broadband thermal infrared light source and tunable Fabry-Pérot detectors. The hollow waveguides, which are inner-coated with silver iodide and silver films, exhibit low transmission loss and high flexibility, enabling efficient gas sensing. Experiments were conducted to detect greenhouse gases (CH₄, CO₂), toxic gases (CO, NO, SO₂), and atmospheric pollutants (CCl₂F₂) at various concentrations. The system achieved detection limits of 58.7 ppm for CH₄, 5.18 ppm for CO₂, 85.1 ppm for CO, 70.1 ppm for NO, 5.79 ppm for SO₂, and 30.9 ppm for CCl₂F₂. The system demonstrates high sensitivity and selectivity, with potential for further improvement through noise reduction and data fusion techniques. This cost-effective and compact system offers a robust solution for real-time multi-gas detection in environmental monitoring, industrial safety, and medical diagnostics.

A high-sensitive multi-parameter gas sensor composed of symmetric dual side-coupled ring resonators in a metallic-dielectric-metallic (MDM) structure is investigated by the finite difference time domain (FDTD) method. The dual side-coupled resonators have lower transmission dip and faster response due to the superposition of the outgoing fields’ amplitude of the symmetric dual resonators, which is very beneficial for detection. Two independently tunable transmission dips are achieved by expanding the ring resonator in ${x}$ -direction, making it possible for dual-parameter sensing. The electric and magnetic distribution characteristics at the two resonant wavelengths are analyzed. Another plasmonic structure expanded along both sides in a central resonator-straight waveguide way is also investigated, and the former structure expanded in ${x}$ -direction works better in detection difficulty and structural utilization. After optimizing the radius and position of ring resonators, the refractive index sensitivities of cavity A and B can reach 2427 nm/RIU and 2431.9 nm/RIU, respectively. The methane and hydrogen sensitivities are −9.213 nm/% and −1.65 nm/% with an extremely high linearity, after coating the methane and hydrogen gas-sensitive film in cavity A and B respectively. This investigation provides an effective way for the design of multi-parameter high-sensitive independently tunable sensors.

In this study, the transmission properties of flexible dielectric coated metallic hollow waveguides at 4.28 THz were studied. Waveguide samples was fabricated, characterized, and quantitatively analyzed including loss and mode structure. High-reflective silver and low-absorptive cyclic olefin polymer (COP) were selected and plated on the inner wall of flexible manufactured polycarbonate (PC) base tubes through dynamic liquid-phase coating method. Waveguides with a theoretically optimized dielectric film thickness and different inner diameters were characterized by using terahertz quantum cascade laser. Their mode structure and energy coupling efficiency with fundamental mode were analyzed. And distinctive results were obtained. The waveguide with 2mm inner has a quasi-single-mode property with coupling efficiency of 90.2%, corresponding to a gaussian field distribution. Moreover, the efficiency is above 89% even as the waveguide is bent to 45°. The transmission loss are 2.5 and 1.7 dB/m for the waveguides with 1mm and 1.5mm inner radius, respectively.