N. Barua, L. Menges, T. Hutter
The University of Texas at Austin,
United States
Keywords: waveguide, photonics, photonics sensor, porous waveguide, gas sensing
Summary:
We report a novel optical porous waveguide for near-infrared (NIR) spectroscopy for quantitative detection of volatile organic compounds (VOCs), enabling sensitive and selective measurements of multiple VOCs simultaneously. The multimodal nanoporous silica waveguide is constructed of porous core and cladding. The waveguide is 200 µm wide, 10 mm long, and the core and cladding are 200 µm and 30 µm thick, respectively. The porosity of the cladding is fabricated to be higher compared to the core so that the refractive index of the core is higher than that of the cladding, which enables light propagation through the nanoporous core. The features of the porous matrix (~7.5 nm average pore size) are much smaller than the wavelength of light, thus light propagation is the same as for any dielectric waveguide. Since the core and the cladding of the waveguide are made from porous silica, the waveguide is transparent from 1.2 µm to 2.4 µm wavelength, allowing one to either use a broadband light source or select a desired set of discrete wavelengths based on the unique spectral peak of the gases, thus offering a high degree of selectivity. The nanoporous core acts both as a medium for light propagation as well as a preconcentrator where gas molecules are adsorbed into. Here, all the light that propagates in the core directly interacts with the molecules, resulting in highly sensitive detection. In our previous work, we demonstrated that the nanoporous silica waveguide can measure different volatile organic gases and compared the NIR spectral features of the gases absorbed inside the waveguide-pores with their respective gaseous and liquid spectra. In this work, we explore the gas-sensing capability of the waveguide using toluene at different concentrations and waveguide temperatures. Toluene is commonly used in industrial processes and due to its harmful impact on human health, its monitoring is crucial to ensure occupational safety. The adsorption of the VOC molecules inside the nanopores allows detection at a much lower concentration compared to traditional gas-cell detection where infrared light propagates through the gaseous molecules in the free space. Our results show that a 10 mm long porous core waveguide offers a significant increase in sensitivity of 1800x per millimeter pathlength compared to a 400 mm long conventional gas-cell. With the waveguide we were able to detect toluene concentrations as low as 5 ppm. Moreover, controlling and modulating the temperature of the waveguide has a significant effect on the adsorption of gaseous molecules in the waveguide core, thus adding further capability to improve the selectivity and sensitivity of detection. An increase in signal by a factor of 10 is observed as the temperature decreases from 30°C to 5°C for 50 ppm toluene. The combined effect of nanopore adsorption and temperature-assisted condensation allows for selective and sensitive on-chip detection of volatile organics. The proposed waveguide sensor technology has promising potential for industrial gas leak detection, air quality monitoring, and medical applications.