Acknowledgements

The 6-layered Uniform-45 MPS can thus be used for identifying toxic methanol adulterated in alcoholic solutions. Adulterated alcoholic solutions are prepared by mixing different volume ratios of methanol with ethanol, including 1:0, 7:3, 5:5, 3:7, and 0:1 (ethanol/methanol). The adulterated alcoholic solution is injected into the microfluidic channel of the sealed microfluidic chamber, as shown in Figure 3c, and becomes the vapor molecules via natural evaporation to be detected by THz waves. Figure 11a illustrates the THz transmission spectrum of Uniform-45 MPS exposed to the vaporized mixtures, which are generated from various concentrations of adulterated alcoholic solutions. THz transmission power apparently decreases in the frequency range of 0.25–0.45 THz as the volume ratio of methanol increases. The THz absorption coefficient spectra for the different concentrations of adulterated alcoholic vapors can be estimated and shown in the inset of Figure 11a. The measured THz absorption coefficients for each concentration of alcoholic vapor are almost constant in the frequency range of 0.25–0.45 THz. The relatively high absorption coefficients are resulted from the increment of the adulterated methanol concentration. The refractive index variation before and after exposure to different concentrations of alcoholic vapors can also be calculated. Figure 11b plots the relations of the αeff and Δneff at 0.4 THz against different concentrations of alcoholic aqueous solutions. The αeff and Δneff increase with the methanol concentration adulterated in the alcoholic solution. The proportional relation of αeff and ρ is linearly fitted as αeff = 1.2 + 0.67ρ. The sensing result of Figure 11 reveals that the colorless and high THz-absorbed alcoholic aqueous solutions with different concentrations of toxic methanol adulteration can be easily distinguished using the

(a) THz-wave transmission spectrum for sensing toxic methanol adulterated in alcoholic solutions: (inset) detecting THz absorption coefficient spectra by the 6-layered Unifrom-45 MPS. (b) Detecting absorption coefficient and refractive index variation versus different concentrations of adulterated alcoholic aqueous

solutions at 0.4 THz (reprinted from Opt. Express 25, 5651-5661 (2017). © 2017 OSA).

Optical gas sensors are experimentally demonstrated using the THz refractive indices and THz absorption coefficients when THz waves propagating through the dielectric-layer media are monitored in a spectroscopic system (THz-TDS). The cylindrical layer is applied from a glass dielectric pipe to be the waveguide resona-

tor. Based on the FP criteria and FDTD simulation, the THz frequency of pipe-waveguide resonance field is approximately proportional to the refractive index of the pipe core. The experimental results present that only the high-order

MPS gas sensor composed of 6-layered Uniform-45 MPS.

4. Conclusions

52

Figure 11.

Gas Sensors

This work was supported by grants-in-aid for scientific research from the Ministry of Science and Technology of Taiwan (MOST 107-2221-E-006-183-MY3) and Japan Society for the Promotion of Science (JSPS, KAKENHI, JP16K17525).
