3.5 Geometry-dependent sensitivity: micropore number effect

The pore number effect on MPS sensitivity is studied by changing the stacking layer numbers of Periodic-45-133 and Uniform-45 MPSs. As shown in Table 1, three different layer numbers of MPSs with identical pore size are prepared for sensitivity comparison in both the Periodic-45-133 and Uniform-45 MPSs. Figure 10 shows αeff and Δneff for Periodic-45-133 and Uniform-45 MPSs, which are the small-pore MPSs, respectively, with periodic and uniform configurations. Each type of MPS possesses three kinds of thicknesses formed by stacking different layer numbers of PET mesh. The αeff and Δneff of Periodic-45-133 MPS for 6-layered thickness are evidently larger than those of 34- and 46-layered thicknesses under all vapor densities (Figure 10a and b). For the response of Uniform-45 MPS, both αeff and Δneff are increased with decreasing device thickness from 34 layers to 12 and 6 layers (Figure 10c and d). The highest sensitivities for the periodic and uniform MPSs occur in their thinnest conditions (i.e., 6-layered structure). The experiment in this case indicates that the small pore numbers also increase THz wave responses in αeff and Δneff of the micropores, thereby enhancing the vapor detection sensitivity.

Under a constant amount of vapor exposure, increasing pore quantity or size is equivalent to expanding the pore volume of the microporous structure. The vapor density, congregated within the micropore and adsorbed on the hydrophilic surface, is thus diluted and eventually decreases the measured values in αeff and Δneff. Figure 10 also shows that 6-layered Uniform-45 MPS requires less acetone vapor amounts to saturate the responsivities of αeff and Δneff because of its less micropore volume/number, comparing to those of 12- or 34-layered Uniform-45 MPS. The vapor saturation density of 6-layered Uniform-45 MPS occurs at around 4 nmol/mm<sup>3</sup> (i.e., 200 ppm), indicating the dynamic range of linear responsivity is sufficiently wide for minute vapor detection.

## 3.6 Sensing applications

(i.e., ρ < 200 ppm) and the measurement inaccuracy of the THz absorption coefficient, the minimum detectable concentration changes of acetone vapor using Uni-

The detection sensitivity of Uniform-45 MPS is apparently higher than that of Uniform-90 MPS, consistent to the comparison result between Periodic-90-249 and Periodic-45-133 MPSs. The performance emphasizes again that half of the pore width, whether periodic or uniform configuration, facilitates the infiltration and adsorption of acetone vapor in the micropores and on the pore surface, leading to an enhanced vapor-field interaction to increase the sensitivity. In addition, the two uniform MPSs in Figure 9 have obviously higher αeff and Δneff than those of the periodic MPSs in Figure 8 under the same vapor density exposure. For example, the largest THz absorption coefficient of Uniform-45 MPS is around 80 cm�<sup>1</sup> and evidently larger than 50 cm�<sup>1</sup> of Periodic-45-133 MPS based on the same 6-layered MPS thickness. The responsivity of linear fitting slopes in Figures 8 and 9 presents that the two sensing parameters, αeff and Δneff, of uniform MPSs are increased more rapidly within a narrower sensitive region compared with those of periodic MPSs. It means only fewer amounts of vapor molecules that infiltrate the uniform MPSs can drastically increase the THz absorption coefficient and refractive index variation until the desired chamber saturation is achieved. The average pore width of Uniform-90/-45 MPS is smaller than that of Periodic-90-249/-45-133 MPS under the similar MPS thickness (i.e., the same stacked layer number of the PET microporous mesh). Therefore, the simple uniform MPS is particularly advantageous for minute vapor sensing with a detection limit of even lower ppm level compared with the

Detecting (a, c) effective absorption coefficients and (b, d) refractive index variations within unit pore volume by Periodic-45-133 and Uniform-45 MPSs with different thicknesses (reprinted from Opt. Express 25,

, corresponding

form-90 and Uniform-45 MPS gas sensors are <46 and 31 pmol/mm<sup>3</sup>

to 2.68 and 1.83 ppm, respectively.

Gas Sensors

periodic MPS.

Figure 10.

50

5651-5661 (2017). © 2017 OSA).

The micropore size dependent sensitivity of the four types of MPSs is summarized in Table 2, where the sensitivity corresponds to the slope of linear fit. The blank chamber represents the vapor sensing performance of the microfluidic chamber without the MPS. It is THz vapor sensing in the free space measured by traditional THz-TDS. The Uniform-45 MPS with a 6-layered thickness has the highest sensitivity, down to 1 ppm-level acetone vapor molecule. Such the sensitivity is more excellent than that of the 23-layered Periodic-90-249 MPS and much higher than that of blank chamber.


#### Table 2.

Sensing performance of MPS for acetone vapor detection.

resonant modes are sensitive to the refractive-index variation due to the high evanescent power toward the pipe core. Different analytes with different vapor pressures, such as water, HCl, acetone and ammonia, are thus identified by a pipewaveguide resonator. To further improve the detection sensitivity and selectivity, the MPS structures are applied as 1 THz artificial material to adsorb vapor molecules. THz absorption coefficients of the unit volume are defined based on the effective medium concept and demonstrated to identify various vapor molecules in the investigation. The molecular dipole moment dominates THz absorption in the unit volume of micropore when several analytes, such as the acetone, methanol, ethanol and ammonia, are test in one MPS sensor. The sensing performance based on the MPS geometry is studied for the sensitivity and the possible detection limit. For the acetone molecule, the 6-layered Uniform-45 MPS sensor has the high sensitivity and the detection limit is potentially down to 1.8 ppm. The 6-layered Uniform-45 MPS sensor is eventually applied for one sensing application to distinct methanol and ethanol vapor molecules from various liquid mixtures. The MPS sensing

Optical Gas Sensors Using Terahertz Waves in the Layered Media

DOI: http://dx.doi.org/10.5772/intechopen.87146

scheme is therefore applicable to realize one optical gas sensor.

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

1 Department of Applied Physics, Faculty of Pure and Applied Sciences, University

2 Department of Photonics, National Cheng Kung University, Tainan, Taiwan

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Japan Society for the Promotion of Science (JSPS, KAKENHI, JP16K17525).

Acknowledgements

Author details

53

Borwen You<sup>1</sup> and Ja-Yu Lu2

of Tsukuba, Tsukuba, Ibaraki, Japan

provided the original work is properly cited.

\*

\*Address all correspondence to: jayu@mail.ncku.edu.tw

Figure 11.

(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).

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

### 4. Conclusions

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 resonator. 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

Optical Gas Sensors Using Terahertz Waves in the Layered Media DOI: http://dx.doi.org/10.5772/intechopen.87146

resonant modes are sensitive to the refractive-index variation due to the high evanescent power toward the pipe core. Different analytes with different vapor pressures, such as water, HCl, acetone and ammonia, are thus identified by a pipewaveguide resonator. To further improve the detection sensitivity and selectivity, the MPS structures are applied as 1 THz artificial material to adsorb vapor molecules. THz absorption coefficients of the unit volume are defined based on the effective medium concept and demonstrated to identify various vapor molecules in the investigation. The molecular dipole moment dominates THz absorption in the unit volume of micropore when several analytes, such as the acetone, methanol, ethanol and ammonia, are test in one MPS sensor. The sensing performance based on the MPS geometry is studied for the sensitivity and the possible detection limit. For the acetone molecule, the 6-layered Uniform-45 MPS sensor has the high sensitivity and the detection limit is potentially down to 1.8 ppm. The 6-layered Uniform-45 MPS sensor is eventually applied for one sensing application to distinct methanol and ethanol vapor molecules from various liquid mixtures. The MPS sensing scheme is therefore applicable to realize one optical gas sensor.
