**2.2 Mid-infrared "refractive index" sensing**

Besides of exploring the use of PhCs in the mid-infrared spectroscopy, there is another popular approach measuring the shift of PhC modulated Bragg resonant peak due to the refractive index change, which can lead to the detection of gas

#### **Figure 7.**

*Measured (solid curves) and theoretical (dash curves) absorption lines of methane for concentrations of a, 5%; b, 1%; c, 0.5%; and d, 0.1% [55].*

concentrations. For instance, in a porous silicon PhC, when pore areas are filled by a gas the effective refractive index of the PhC will be increased. Moreover, it is possible that the lattice constant of the PhC increase due to the swelling of the gas or vapor. Understanding this procedure can be easier by considering Bragg law expression [27].

$$m\lambda \, = \, 2nd \sin \theta \,\tag{1}$$

where m is the diffraction order, λ is the incident wavelength, n is the effective refractive index, d is the lattice constant, and θ is the glancing angle between the incident light and diffraction crystal planes [56, 57]. This method has been used in near and visible range [58–60], more frequently rather than mid-infrared range.

In 2015 Zou et al. [45] designed a holey, slotted, and a regular PCW for detection the chemical warfare simulant in the mid-infrared range. A section of this research has been assigned to the investigation of the relationship between the value of electrical field overlap with the analyte and the sensitivity of the refractive change based sensors. In order to study this correlation, they used 3D FDTD simulation. They selected C2Cl4 (refractive index = 1.5) as a top cladding. As shown in **Figure 8** the sensitivity of a PCW sensor is depended directly on the percent of electrical field overlap with the analyte. So that, the shifted transmission in slotted PCW is 2 times higher than holey one because its electrical field overlap with the analyte is almost 2 times higher as well. In 2017 Turduev et al. [61] presented an optical refractive index sensor (T-slotted PC sensor) design for mid-IR photonics. They used numerical methods based on finite-difference time-domain and plane-wave expansion method. an overall sensitivity is calculated to be around 500 nm/RIU for the case of higher refractive indices of analytes n = 1.10–1.30.

Detecting an unknown gas through this method can be challenging because the principles of this method are based on the refractive index changes. Thus, detecting two different gases with the same refractive index is simply impossible. On the other hand, for gases with refractive indexes close to air refractive indexes (n = 1), the sensitivity of this kind of sensors can be strongly reduced. However, the complexity of this method is lower than absorption-based sensing methods.

#### **2.3 Mid-infrared electrical conductance sensing**

In 2000 Boarino et al. [38] studied changes in electrical conductivity in the presence of NO2 through P+ porous silicon layers (PSL) at the room temperature and the atmospheric pressure. The recovery time and response to interfering gases were tested as well. PSL is well-known in humidity sensors area, while its application in the field of gas sensing has been considered only recently. This structure has obtained high importance in the field of gas sensors due to its high surface to

#### **Figure 8.**

*Simulated transmission with air-clad and C2Cl4-clad conditions for (a) conventional PCW, (b) holey PCW, and (c) slotted PCW [45].*

**83**

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

Accordingly, due to the inherent characteristic of P+

(∆G/G) for different porosity (38, 43, 53, 62 and 75%).

is studied in 2.1.1 section in details.

**3. Conclusion**

**Table 2.**

volume ratio, and its reactivity to the environment. Changes in work function, refractive index, photoluminescence, and conductivity variation can be used as indices in the gas sensing. Boarino et al. used the last feature to detect NO2.

*Relative response of PS samples of different porosity to the listed NO2 concentrations [38].*

**Porosity (%) ΔG/G (3 ppm) ΔG/G (5 ppm) ΔG/G (10 ppm)** 6 87,837 0.7 1 1.9 3.5 0.3 1.7 4.6 29.3 84.3 197 3.5 45 164

changes can be observed in resistivity in the presence of polar liquids, vapors, and gases. They measured the PS change of conductance in presence of different gases in humid conditions at constant bias V = 5 v. The PS response to NO2 was tested for different value of porosity. **Table 2** shows the relative conductance variation

Since the high available surface for gas adsorption plays a key role in obtaining an efficient chemical sensor, the 55% porosity for PS surface gives the best result, due to the maximum value of surface to volume ratio. The lowest concentration that could be examined, was 1 ppm and the relative response was 1.6 for the 60% porosity sample. In comparison with NO2, under the same concentration, the conductivity change in the presence of NO was significantly lower. Likewise, the relative response of PS to interfering species (CO (up to 1000 ppm) and CH4 (up to 5000 ppm)) and alcohol, such as methanol, at concentrations up to 800 ppm was negligible. They also used FTIR spectroscopy for detecting NO2 through PSL which

In this chapter, we presented a review work on the recent progress of PhC-based gas sensing research in the mid-infrared range. Various material structures including using porous silicon structure, photonic crystal waveguides, and hollow-core photonic crystal fibers, as well as both optical and electrical detection methods, have been thoroughly discussed. As mentioned, porous silicon structure enhanced sensing device achieved the highest sensitivity to detect NO2 at 1 ppm concentration level through measuring the conductance changes. But this method is restricted to a limited range of gases, and is unable to detect nonpolar gases such as CO, CH4, and alcohols. The other issue can be related to electrical components which are necessary for this method. These electrical components increase the risk of electrical discharge and augment the risk of explosion. Moreover, the electrical noises can strongly affect this kind of sensors. For the holey PCW, the sensor unit can deliver the measurement of Triethyl phosphate (TEP) with the concentration of 10 ppm. The small size (800 μm) of this PCW offers a great advantage which can potentially lead to the realization of SWaP sensors. The main drawback of this kind of sensors is that they are so sensitive to small fluctuation in the hole diameter. Thus, the fabrication process for this kind of sensors might be difficult and timeconsuming. However, the high energy overlap with gases within the holey PCWs, and its high power in slowing light and its small size make this sensor one of the

mesoporous silicon, strong

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


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

**Table 2.**

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

refractive indices of analytes n = 1.10–1.30.

**2.3 Mid-infrared electrical conductance sensing**

presence of NO2 through P+

concentrations. For instance, in a porous silicon PhC, when pore areas are filled by a gas the effective refractive index of the PhC will be increased. Moreover, it is possible that the lattice constant of the PhC increase due to the swelling of the gas or vapor. Understanding this procedure can be easier by considering Bragg law expression [27].

*m* = 2*nd sin* (1)

where m is the diffraction order, λ is the incident wavelength, n is the effective refractive index, d is the lattice constant, and θ is the glancing angle between the incident light and diffraction crystal planes [56, 57]. This method has been used in near and visible range [58–60], more frequently rather than mid-infrared range. In 2015 Zou et al. [45] designed a holey, slotted, and a regular PCW for detection the chemical warfare simulant in the mid-infrared range. A section of this research has been assigned to the investigation of the relationship between the value of electrical field overlap with the analyte and the sensitivity of the refractive change based sensors. In order to study this correlation, they used 3D FDTD simulation. They selected C2Cl4 (refractive index = 1.5) as a top cladding. As shown in **Figure 8** the sensitivity of a PCW sensor is depended directly on the percent of electrical field overlap with the analyte. So that, the shifted transmission in slotted PCW is 2 times higher than holey one because its electrical field overlap with the analyte is almost 2 times higher as well. In 2017 Turduev et al. [61] presented an optical refractive index sensor (T-slotted PC sensor) design for mid-IR photonics. They used numerical methods based on finite-difference time-domain and plane-wave expansion method. an overall sensitivity is calculated to be around 500 nm/RIU for the case of higher

Detecting an unknown gas through this method can be challenging because the principles of this method are based on the refractive index changes. Thus, detecting two different gases with the same refractive index is simply impossible. On the other hand, for gases with refractive indexes close to air refractive indexes (n = 1), the sensitivity of this kind of sensors can be strongly reduced. However, the com-

In 2000 Boarino et al. [38] studied changes in electrical conductivity in the

and the atmospheric pressure. The recovery time and response to interfering gases were tested as well. PSL is well-known in humidity sensors area, while its application in the field of gas sensing has been considered only recently. This structure has obtained high importance in the field of gas sensors due to its high surface to

*Simulated transmission with air-clad and C2Cl4-clad conditions for (a) conventional PCW, (b) holey PCW,* 

porous silicon layers (PSL) at the room temperature

plexity of this method is lower than absorption-based sensing methods.

**82**

**Figure 8.**

*and (c) slotted PCW [45].*

*Relative response of PS samples of different porosity to the listed NO2 concentrations [38].*

volume ratio, and its reactivity to the environment. Changes in work function, refractive index, photoluminescence, and conductivity variation can be used as indices in the gas sensing. Boarino et al. used the last feature to detect NO2. Accordingly, due to the inherent characteristic of P+ mesoporous silicon, strong changes can be observed in resistivity in the presence of polar liquids, vapors, and gases. They measured the PS change of conductance in presence of different gases in humid conditions at constant bias V = 5 v. The PS response to NO2 was tested for different value of porosity. **Table 2** shows the relative conductance variation (∆G/G) for different porosity (38, 43, 53, 62 and 75%).

Since the high available surface for gas adsorption plays a key role in obtaining an efficient chemical sensor, the 55% porosity for PS surface gives the best result, due to the maximum value of surface to volume ratio. The lowest concentration that could be examined, was 1 ppm and the relative response was 1.6 for the 60% porosity sample. In comparison with NO2, under the same concentration, the conductivity change in the presence of NO was significantly lower. Likewise, the relative response of PS to interfering species (CO (up to 1000 ppm) and CH4 (up to 5000 ppm)) and alcohol, such as methanol, at concentrations up to 800 ppm was negligible. They also used FTIR spectroscopy for detecting NO2 through PSL which is studied in 2.1.1 section in details.
