Pump-probe-alternating photothermal interferometry for two-component gas sensing.

We demonstrate a high-sensitivity acetylene/methane gas sensor based on hollow-core fiber photothermal interferometry (PTI) with a pump-probe-alternating technique. This technique utilizes two distributed-feedback lasers as pump and probe beams alternatively for two gas components to facilitate photothermal phase modulation and detection through time-division multiplexing. With a 2.5-cm-long hollow-core conjoint-tube fiber, noise-equivalent concentrations of 370 ppb and 130 ppb are demonstrated for methane and acetylene, respectively. Noise characteristics of the PTI system are analyzed and experimentally tested. The proposed technique eliminates the need for an additional laser in the traditional PTI setup, enabling the construction of a sensitive yet more cost-effective multi-gas component detection system.

Geochemical characteristics and socioeconomic associations of carbonaceous aerosols in coal-fueled cities with significant seasonal pollution pattern.

Carbonaceous aerosols, comprising organic carbon (OC) and elemental carbon (EC), are critical component of fine particulate matter (PM2.5), with diverse impacts on air quality and human health. This study investigated the concentrations and seasonal patterns of carbonaceous species in PM2.5 during both the heating season (January 2021) and non-heating season (July 2021) in three coal-fueled cities in northern China, as well as the differences in carbonaceous aerosols and their associations with socioeconomic parameters in cities situated on either side of the "Hu Line" in China. The results showed that, owing to intensified coal combustion and unfavorable meteorological conditions, levels of OC, EC, and OC/EC ratios were higher in winter compared to summer. Moreover, the presence of dust (DU) and light pollution (LP) days resulted in elevated OC levels but decreased EC levels. The Char-EC/Soot-EC ratios were highest during LP, followed by CL and DU. A source apportionment analysis demonstrated that coal burning, vehicle exhaust, road dust, and biomass burning were the primary contributors to carbonaceous aerosols, as confirmed by diagnostic ratios, Char-EC/Soot-EC ratios, and PCA analysis. Furthermore, our study found that carbonaceous aerosols concentrations and source apportionment primarily varied with diurnal and seasonal trends and different pollution types. Additionally, at the national scale, population density and urban green space exhibited a positive correlation with OC/EC ratios (p < 0.05), while energy consumption per unit of GDP showed a negative correlation (p < 0.05). The observation that OC/EC ratios were lower in coal-fueled cities than in economy-based cities suggests a more severe pollution scenario. These findings highlight the importance of comprehending of the seasonal variation and chemical characteristics of carbonaceous aerosol for understanding air pollution sources and characteristics, which is essential for both air quality management and human health.

Parameter optimization of hollow-core optical fiber phase modulators.

We studied the effect of varying gas concentration, buffer gas, length, and type of fibers on the performance of optical fiber photothermal phase modulators based on C2H2-filled hollow-core fibers. For the same control power level, the phase modulator with Ar as the buffer gas achieves the largest phase modulation. For a fixed length of hollow-core fiber, there exists an optimal C2H2 concentration that achieves the largest phase modulation. With a 23-cm-long anti-resonant hollow-core fiber filled with 12.5% C2H2 balanced with Ar, phase modulation of π-rad at 100 kHz is achieved with a control power of 200 mW. The modulation bandwidth of the phase modulator is 150 kHz. The modulation bandwidth is extended to ∼1.1 MHz with a photonic bandgap hollow-core fiber of the same length filled with the same gas mixture. The measured rise and fall time of the photonic bandgap hollow-core fiber phase modulator are 0.57 µs and 0.55 µs, respectively.

Theoretical Analysis of Lattice-Mediated Plasmon Resonance Using Finite-Difference Time-Domain Method.

Although much progress has been made on lattice plasmon mode (LPM), there is still a lack of systematic studies on LPM generation; questions remain unanswered on topics such as high-order LPM generation, LPM generation from near-coupling complex elements, and modulation of incidence energy. Here, we systematically evaluated the properties of multiple high-order LPM, energy flow modulation of incident polarization, element, angle of incidence, and hybrid of dual lattice using the finite-difference time-domain method. This study presents a clear illustration of LPM and will help on further development of LPM and plasmonics-based fields.