Previous laboratory results have suggested that unheated TiO₂ nanowire arrays provide a means to detect trace level concentrations of nitro-group containing compounds in the gas phase with fast response and high chemical selectivity. The detection method is based on the chemiresistive effect, where an electron-deficient compound adsorbed to the surface of an n-type semiconductor causes a surface charge deficit on the semiconductor leading to an increase in its resistance. However, a major issue with this method is that chemiresistive sensors based on TiO₂ are also sensitive to the presence of water vapor, and this cross-sensitivity could lead to artifacts for sensors used under environmental conditions. Results are presented here, where thin planar arrays of TiO₂ nanowires were tested to determine the sensitivity towards water vapor, along with the detection limits and response times towards several nitrocontaining organic molecules as a function of gas-phase water vapor concentration. When water vapor concentrations were carefully controlled to ensure they remain constant throughout the testing cycle, it was found that TiO₂ nanowire devices could detect 2,4,6-trinitrotoluene at a concentration as low as a few tens of part per trillion by volume with a response time in the range of 10² – 10³ s, depending on experimental conditions.

We present results from a fiber Bragg grating-based microwave energy sensor. The sensor relies on a soft ferromagnetic glass-coated microwire that is bonded to the cladding of the grating. The microwire absorbs microwave energy and heats up thus raising the temperature of the fiber Bragg grating. Compared to a similar sensor that uses gold to absorb electromagnetic radiation, the microwire yields a sensor with greater sensitivity (~10 times at f = 3.25 GHz) relative to the perturbation of the microwave field. With the sensor reported here, the best sensitivity to electromagnetic radiation corresponds to AC electric fields that have an an average electric energy density of approximately 1.3 mJ/m³.

A comparative study of the magnetic softness, giant magnetoimpedance (GMI) effect, and microwave absorption capacity of glass-coated amorphous Co_64.63Fe_4.97B₁₆Si₁₁Cr_3.4Ni_0.02 and Co₆₈B₁₅Si₁₀Mn₇ microwires has been performed. We find that the Mn-containing sample exhibits a softer magnetic property, a larger GMI ratio, and a higher microwave absorption capacity as compared to those with a Fe-Cr-Ni composition. As a result, the Mn-containing microwire is shown to be a better microwave absorber for fabrication of a new class of microwave energy sensor based on Fiber Bragg Grating. Our study emphasizes a correlation between the magnetic softness, GMI, and microwave absorption in the microwires and paves the way to improving the performance of such sensors by tailoring their soft magnetic properties.