**1. Introduction**

Chemical sensing, especially the trace-gas detection, is of a significant importance for a wide range of applications in practical fields including the medical inspection, the environmental monitoring, the manufacturing control, as well as the nation security surveillance. For example, controlling respiratory gases in the medical sector can be a matter of life or death [1, 2]. Monitoring the toxic, explosive and air polluting gases such as NOx, CO2, CO, CH4, and O3 is vital to prevent harms to human communities [2]. Nowadays, there are many choices of gas sensors based on different methods such as semiconductor [3, 4], catalytic [5], field effect [6], electrochemical [7], and optical gas sensors [8, 9]. Among them, the optical-based gas sensors have been well-received as a fast, precise and reliable technique, which has a long life expectancy, immunes to all chemical poisoning,

and requires low maintenance for high-precision operation [9]. Furthermore, with the rise of the emerging technology, Internet of Things (IoTs), such high standard of performance requirements become more and more desired from the industrial world [10]. It is also necessary to point out that, besides of the aforementioned features, the importance of reducing the size, weight, power, and cost (SWaP-C) to enable the chip scale integration is revealed clearly in the development of IoT technologies [11]. Unfortunately, limited by the Beer-Lambert Law, to achieve a suitable sensitivity, a large optical interaction length is required in the conventional optical gas sensor systems. This long path of interaction makes the optical gas sensors relatively large and costly to be manufactured [12], which consequently sets a fundamental barrier to satisfy the SWaP-C requirements. Therefore, in such a high-speed technology innovating era, more and more researchers have paid significant attention to develop new technologies that can overcome these limitations.

Photonic crystals (PhCs), which can be regarded as one of the most advanced modern photonic technologies [13–16] was firstly proposed by Yablonvitch and John since 1980s [17, 18]. Owing to the unique non-linear optical dispersive properties [19, 20], PhCs become a powerful tool for control and manipulation of light-matter interactions on micrometer length scales [21, 22]. Considering the aforementioned optical sensing limitations, this technology is capable of addressing the size issue, and potentially suitable for on-chip gas detection applications. In recent year, the research on PhC-based gas sensing research has developed rapidly. Various sensing techniques have been proposed by using PhCs to detect chemicals in gas, vapor and even liquid environment [23–25]. Several review articles have been reported as well [26, 27]. For example, in 2013, Xu et al. [27] investigated different photonic crystal structures, such as morpho-butterfly wing, porous silicon PhCs, multilayer PhC films, colloidal PhCs, and Inverse opal colloidal crystals. In 2015, Zhang et al. [26] reviewed optical sensors based on photonic crystal cavity enhancing mechanisms. Overall speaking, the reported technologies can be considered in two categories. The first approach can be concluded as the "refractive index sensing", which could sensitively measure refractive index changes with gas involvement. Another one is known as "photonic crystal light absorption spectroscopy". This method is to detect the distinctive absorption patterns of gas molecules in the infrared spectrum. Because photonic crystals can slow light propagation and enhance light intensity in the space where gas fills [4], this new spectroscopy not only shares the advantages of the conventional spectroscopy but also eliminates the issues caused by large optical interaction length.

To the best of our knowledge, the research emphasis on the PhC-based gas sensing development has been mainly focused on the near-infrared spectral range. However, compared with the near-infrared region, molecular species in the mid-infrared range show intrinsic absorption bands with much larger absorption coefficients. **Figure 1** presents that the mid-infrared portion of the spectrum with several trace gas chemical species placed where their strong absorptions occur. As can be seen, taking the carbon dioxide (CO2) as an example, its absorption strength in the mid-infrared range (~4.2 μm) is about two orders of magnetite higher than the one in near-infrared range (~2 μm). Such significant difference also exists in almost all the gas molecules including xylene, methane, and so on [29]. Therefore, fundamentally speaking, the optical sensors functioning in the mid-infrared range offer much higher device sensitivity [30]. Consequently, much richer information can be found for those wishing to probe, detect, image, or quantify these and many other species including explosives, nerve agents, and toxins [30]. Nevertheless, there are some hurdles preventing the development of PhC based optical sensors in

**75**

*The Mid-Infrared Photonic Crystals for Gas Sensing Applications*

the mid-infrared spectrum, such as the limited availability of the low-cost, highefficient light sources or photodetectors. Also, it was pointed out in a report that the difficult process of alignment of the beam for coupling light in and out of the sample could also be very challenging, due to limitations of available equipment [31]. In order to have a systematic understanding of the current progress of PhC based gas sensing research in mid-infrared range, in this chapter, we are going to provide a comprehensive review on the existing mid-infrared PhC-based gas sensor technologies, evaluate their performance in a practical point of views, and also

It is known that most of the molecules have their distinctive absorption patterns in the mid-infrared spectrum, and as it was pointed out that their absorption coefficients are much higher than the ones in near-infrared range. Majorly due to these reasons, the most active research efforts of using PhC technology for gas sensing in mid-infrared range focuses on measuring the spectral intensity change caused by the gas resonant mode absorption mechanisms. But it is necessary to point out that, other than this typical methodology, some other approaches using PhC structures have also been reported, which rely on the detection of the peak position drifting of transmission or reflection spectrums caused by the gas-induced refractive index variation, or the electrical conductance behavior in the presence of gases. These methods certainly help to enrich the PhC sensing capabilities in the mid-infrared range. With all these being said, based on the different sensing mechanisms, in the following contents, we will elaborate on the recent development of the mid-infrared

Being as the most popular technology in the mid-infrared range, the gas absorption spectroscopy can be used to precisely measure both the gas composition and concentrations. In this method, the mid-infrared radiation is absorbed at some specific frequencies due to the presence of gas molecules. These frequencies correspond to vibrational modes of the molecular structures [32]. Typically, this type of gas sensor consists of three parts: the mid-infrared light source, the light/gas interaction

discuss the future of the PhC-based mid-infrared sensing technologies.

*The absorption strength of some typical trace gas molecules in the mid-infrared range [28].*

**2. PhC-based mid-infrared gas sensing methods**

PhC based gas sensing technologies.

**2.1 Mid-infrared light absorption sensing**

*DOI: http://dx.doi.org/10.5772/intechopen.80042*

**Figure 1.**

*The Mid-Infrared Photonic Crystals for Gas Sensing Applications DOI: http://dx.doi.org/10.5772/intechopen.80042*

*Photonic Crystals - A Glimpse of the Current Research Trends*

overcome these limitations.

cal interaction length.

and requires low maintenance for high-precision operation [9]. Furthermore, with the rise of the emerging technology, Internet of Things (IoTs), such high standard of performance requirements become more and more desired from the industrial world [10]. It is also necessary to point out that, besides of the aforementioned features, the importance of reducing the size, weight, power, and cost (SWaP-C) to enable the chip scale integration is revealed clearly in the development of IoT technologies [11]. Unfortunately, limited by the Beer-Lambert Law, to achieve a suitable sensitivity, a large optical interaction length is required in the conventional optical gas sensor systems. This long path of interaction makes the optical gas sensors relatively large and costly to be manufactured [12], which consequently sets a fundamental barrier to satisfy the SWaP-C requirements. Therefore, in such a high-speed technology innovating era, more and more researchers have paid significant attention to develop new technologies that can

Photonic crystals (PhCs), which can be regarded as one of the most advanced modern photonic technologies [13–16] was firstly proposed by Yablonvitch and John since 1980s [17, 18]. Owing to the unique non-linear optical dispersive properties [19, 20], PhCs become a powerful tool for control and manipulation of light-matter interactions on micrometer length scales [21, 22]. Considering the aforementioned optical sensing limitations, this technology is capable of addressing the size issue, and potentially suitable for on-chip gas detection applications. In recent year, the research on PhC-based gas sensing research has developed rapidly. Various sensing techniques have been proposed by using PhCs to detect chemicals in gas, vapor and even liquid environment [23–25]. Several review articles have been reported as well [26, 27]. For example, in 2013, Xu et al. [27] investigated different photonic crystal structures, such as morpho-butterfly wing, porous silicon PhCs, multilayer PhC films, colloidal PhCs, and Inverse opal colloidal crystals. In 2015, Zhang et al. [26] reviewed optical sensors based on photonic crystal cavity enhancing mechanisms. Overall speaking, the reported technologies can be considered in two categories. The first approach can be concluded as the "refractive index sensing", which could sensitively measure refractive index changes with gas involvement. Another one is known as "photonic crystal light absorption spectroscopy". This method is to detect the distinctive absorption patterns of gas molecules in the infrared spectrum. Because photonic crystals can slow light propagation and enhance light intensity in the space where gas fills [4], this new spectroscopy not only shares the advantages of the conventional spectroscopy but also eliminates the issues caused by large opti-

To the best of our knowledge, the research emphasis on the PhC-based gas sensing development has been mainly focused on the near-infrared spectral range. However, compared with the near-infrared region, molecular species in the mid-infrared range show intrinsic absorption bands with much larger absorption coefficients. **Figure 1** presents that the mid-infrared portion of the spectrum with several trace gas chemical species placed where their strong absorptions occur. As can be seen, taking the carbon dioxide (CO2) as an example, its absorption strength in the mid-infrared range (~4.2 μm) is about two orders of magnetite higher than the one in near-infrared range (~2 μm). Such significant difference also exists in almost all the gas molecules including xylene, methane, and so on [29]. Therefore, fundamentally speaking, the optical sensors functioning in the mid-infrared range offer much higher device sensitivity [30]. Consequently, much richer information can be found for those wishing to probe, detect, image, or quantify these and many other species including explosives, nerve agents, and toxins [30]. Nevertheless, there are some hurdles preventing the development of PhC based optical sensors in

**74**

**Figure 1.** *The absorption strength of some typical trace gas molecules in the mid-infrared range [28].*

the mid-infrared spectrum, such as the limited availability of the low-cost, highefficient light sources or photodetectors. Also, it was pointed out in a report that the difficult process of alignment of the beam for coupling light in and out of the sample could also be very challenging, due to limitations of available equipment [31]. In order to have a systematic understanding of the current progress of PhC based gas sensing research in mid-infrared range, in this chapter, we are going to provide a comprehensive review on the existing mid-infrared PhC-based gas sensor technologies, evaluate their performance in a practical point of views, and also discuss the future of the PhC-based mid-infrared sensing technologies.
