1. Introduction

Gas sensing for pollutant monitoring and leaky molecules detection is important when the environmental issues on breath health are revealed. Various gas sensors based on different principles are presented, such as the gas chromatography-mass spectroscopy [1–3], electrochemical [4], and optical sensors. For the electrochemical sensors [5, 6], the high sensitivity is requested from the high operation temperature, which is risky for explosive gas detection due to the high-power consumption at electrodes. The optical sensing scheme can solve the unsafe problem because of the room-temperature operation without electric contact [7–9]. THz radiation, which lies between the infrared ray (IR) and microwave regions, can strongly perturb polar gas molecules with the energy level transitions of rotation or vibration. The absorption strength of gas molecules in the THz frequency range is typically on the same order of magnitude as the IR region, and is <sup>10</sup><sup>3</sup> –10<sup>6</sup> stronger than that in the microwave region [10]. The low photon energy of the THz wave is

relatively safer than that of IR wave and has the stronger interaction response than that in the microwave region [11–13].

The cylindrical layer acts as a FP etalon, and THz waves satisfying the FP resonance condition enables field resonance inside the cylindrical layer, which becomes leaky to form multiple transmission dips. Based on the FP criteria, the resonant wavelength of THz waves in the cylindrical layer is defined as,

order resonant wavelengths, thicknesses of a cylindrical layer, orders of the resonant modes, and effective waveguide refractive indices of cylindrical layer and the pipe core [21]. Figure 1 shows the transmission spectrum of a blank glass pipe, where the spectral dips with low power represent the FP resonant waves along the glass layer. The measured resonant frequencies are approximate to the calculated results via finite-difference time-domain (FDTD) method. The cross section of the glass-pipe sensor is shown in the inset of Figure 1, where the inner core diameter Din and the thickness of pipe-wall are 5.57 and 1.17 mm, respectively. The refractive indices of the air-core and glass used in simulation are 1 and 2.6 [17], respectively. There are five spectral dips of the glass-pipe waveguide in the transmission spectrum, 0.2–0.5 THz (Figure 1), which are 0.201, 0.266, 0.326, 0.392 and 0.452 THz frequencies. According to resonance condition of FP etalon, the resonant dip frequency changes with ncor, and thus various vapors can be identified in the hollow

THz resonant fields inside the cylindrical layer must be sufficiently evanescent toward the air core to make the pipe-waveguide resonator sensitive to the vapor molecules. Interaction between the THz resonant field and the vapor molecules is therefore enhanced, consequently resulting in the sensitive detection. Based on the investigation of THz dielectric pipe waveguides, the high order resonance waves have strong evanescent field inside the pipe core [22]. That is, the resonance dips at the high frequency range have stronger core field than those at the low frequency range. The resonant dip at 0.452 THz as shown in Figure 1 is thus suitable to probe vapors within the glass pipe core because of the powerful resonant field to interact

To approve the sensing principle of a pipe waveguide sensor, the vapor molecules of the water, hydrochloric acid (HCl), acetone and ammonia liquids are used

Transmission spectra of a glass-pipe waveguide: (inset) the cross section of a glass-pipe-waveguide sensor

(reprinted from Opt. Express 20, 5858-5866 (2012). © 2012 OSA.

core by detecting the spectral shifts of the resonant dips.

Optical Gas Sensors Using Terahertz Waves in the Layered Media

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

=m, where λm, d, m, ncld, and ncor are, respectively, the mth-

λ<sup>m</sup> ¼ 2d

vapor molecules.

Figure 1.

39

2.2 Vapor sensing principle and results

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n2 cld � n<sup>2</sup> cor <sup>q</sup>

THz gas sensing methods have been demonstrated based on two main styles. The first style is normally illuminating THz radiation directly on the gaseous analytes and acquiring their spectral response for the sensing purposes. For example, the strong absorption lines at specific frequencies (i.e., fingerprint spectra) or the pulse power decay within one certain THz spectral width have been applied to analyze the gaseous analytes [14]. Such the sensing performance are presented from the photo-mixing [15], heterodyne detection [3, 16], and chirped-pulse THz spectroscopy [12]. Such the spectroscopic systems successfully analyze the gas mixtures of more than 30 chemicals [3] and distinguish gases that possess similar compositions. This recognition scheme provides high selectivity based on the rotational/ vibrational transition of gas molecules. Nevertheless, the THz spectroscopic system should be equipped with a long gas cell [3, 12, 14], a cryo/sorbent pre-concentration system, and a heating apparatus [3, 16] to improve the sensing limit from the ppm concentration to the ppb level. The overall configuration is complicated, bulky, expensive, and consumes high power. Although the quantum cascade laser is presented as a compact THz laser source for gas sensing applications to simplify THz wave generation [17]. However, the THz laser source should be operated in the low-temperature condition and is limited for practical applications.

The other method of THz gas sensing is to use the THz resonance field in the photonic or periodic structures [18, 19]. For example, one- and two-dimensional photonic structures respectively based on the silicon slabs [18] and pillar arrays [19] have been validated for the non-specific gas sensing in the THz frequency range. The proposed photonic structures have high-quality factors on THz field resonance and are sensitive to slight changes of refractive index. The demonstrated detection limit for hydrogen gas is about 6% concentration change [18]. The approved minimum detectable amount of oxygen or argon is 1 μmol [19]. Although the resonator-type THz gas sensor is relatively compact, portable, and consumes low power, its short interaction length inside the chip essentially leads to the limited sensitivity and poor selectivity.

In this chapter, THz gas sensors with sufficiently long interaction lengths are presented based on THz wave propagation along the dielectric layers. The layered media specifically perform the enhancement of THz resonance and absorption in the dielectric waveguides and porous structures, respectively. The interaction between THz wave and gas analytes thus becomes efficient. The presented waveguide sensor contributes the detection of THz refractive index variation based on the resonator principle of Fabry-Pérot (FP) etalon, and the porous structures detect vapor molecules based on the response of molecular dipole moment for THz wave absorption.
