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

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 PhC based gas sensing technologies.

#### **2.1 Mid-infrared light absorption sensing**

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 chamber, and the radiation detector [33]. In recent years, PhC has been proven to be an effective technology to improve the performance of the light sources, especially for the laser devices [34]. But more significantly, PhC has also played a critical role to downsize the sensor module footprint by minimizing the interaction chamber volume under the chip scale level [35–38].

#### *2.1.1 Advanced porous gas sampling structure for gas sensing*

As proposed by Soref [29], silicon is a proper candidate for the mid-IR wavelength range due to its transparency window between 1.1 and 8.5 μm. While silicon dioxide is also transparent up to around 3.6 [29], silicon-on-insulator (SOI) is a suitable structural form for mid-IR integrated photonics [39]. In 2007, Lambrecht et al. [12] for the first time, suggested the implementation of a two dimensional (2D) macroporous silicon PhC in the interaction volume to slow the light and enhance the gas-light interaction. The experimental results with CO2 showed more than two times enhancement in absorption line. In fact, these experimental results became a base for the realization of high sensitivity and miniature gas sensors. The device configuration is shown in **Figure 2**. Gas flows through the sampling cell from the top hole to the bottom, as highlighted in blue color. The sampling cell containing a PhC membrane is positioned between a thermal emitter and a pyrodetector with an IR bandpass filter centered at the absorption peak of CO2 (λ = 4.24 μm). The PhC membrane is placed in between two BaF2 light guiding rods which help to couple the IR radiation among the thermal emitter, PhC membrane and the pyrodetector. Here, one thing is worth noting that, the PhC membrane could be easily removed from the plastic holder without changing positions of the BaF2 rods. This offers the possibility of measuring the empty cell with the same optical path length. In detail, the operation voltage is 10 Hz and the pyrodetector is measured by digital lock-in amplifier with a time constant of 2 s. With such setup, this method does offer the capability to detect the presence of CO2 gas, which proves the possibility of using PhC technology for developing gas sensors with compact footprint. However, there is a crucial drawback with this simple PhC gas sensor. Typically, a low group velocity corresponds to a high effective refractive index, which leads to difficulties in and out couplings of radiation.

**77**

**Table 1.**

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

to allow for a reasonable transmission in the 1 mm device.

presence of NO2 is intense while this value, at 3748 cm<sup>−</sup><sup>1</sup>

the enhanced gas absorption by a factor of 5.8 at 5400 nm [41].

*Experimental and theoretical value of the gas absorption enhancement [35].*

the data are presented in the following **Table 1**.

In the course of the experiment, several sample lengths, varying from 0.25 to 1 mm, were investigated. Results showed that their absorption enhancements changed at the range of 2.6–3.5. By taking all the mismatch factors into consideration, the numbers are in good agreement with the numerical simulation results. All

In 2000, Boarino et al. [40] applied this porous silicon method to the environmental analysis of NO2 gas because of its high sensitivity to surface molecular structures. **Figure 3** shows the enhanced absorption value due to the presence of NO2. In this figure, spectrum-1 is related to the sample outgassed under dynamic vacuum, spectrum-2 is given after the dosage of 1 Torr of the pure NO2 sample, and spectrum-3 is recorded after outgassing. As can be seen, in the range of 1000–1250 cm<sup>−</sup><sup>1</sup> (assigned to the stretching vibrations of Si–O species) and range of 2150–2300 cm<sup>−</sup><sup>1</sup> (Si–H stretching modes of SiO–Hx species) the absorption enhancement in the

tion), is negligible. They claimed that this broad absorption can be attributed to electrons populating of the conduction band. Moreover, from spectrum-3 is an evidence that removing NO2 from the gas phase leads to a complete restoration of initial conditions. Moreover, the most recent research based on this method showed

It can be concluded that porous silicon is an effective gas sampling structure that can help in minimizing the overall size of the sensor device. However, the main

**Length (mm) Gas Filter[μm/(a/λ)] ζ***exp* **ζ***theo,TM* **ζ***theo,TE* 1 CO2 4.24/0.472 3.5 3.7 2.9 0.5 CO2 4.24/0.472 2.6 3.7 2.9 0.5 CO2 4.24/0.472 3.0 3.7 2.9

(SiO–H stretching vibra-

In 2011, Pergande et al. [35] solved this problem via designing antireflection layers (ARL) at two interfaces, including the air-ARL interface and the ARL-PhC interface. Because of that, it enhanced the light absorption of CO2 up to 3.6 times. To explain the mechanism, introducing this ARL leads to the generation of surface modes which are used for coupling light into slow light PhC modes. These modes are confined in air-ARL interference due to the forbidden propagation of light in photonic band-gap. The thickness of ARL affects the spectral position of surface modes. When ARL thickness is equal to 0.57a (a is the lattice constant of PhC), the similarity of field distributions of surface modes and the slow light PhC modes would be able to couple together much easily. In addition, this ARL could also help to effectively reduce the interface optical loss. Theoretically, absorption of the device can be enhanced up to 60 by using this ARL enhanced PhC configuration. The difference between theoretical and experimental results comes from several factors, such as the non-optimal lattice constant and pore diameter fluctuation. They all could lead to the off-resonance operation mode to the CO2 absorption line. In this work, 2D PhCs were fabricated by photoelectrochemical etching of n-type silicon. ARL is designed by photolithography and was transferred by photoelectrochemical etching into the silicon wafer. The lattice constant was 2 μm an (r/a), varying from 0.360 to 0.385. lengths of PhC changed from 100 μm to 1 mm while the height and width were approximately 330 μm and 1 mm, respectively. The porosity of the sample was about 64%. Pergande et al., according to numerical estimates, suggested that the positional variations and pore diameter fluctuations should be below 0.5% in order

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

**Figure 2.** *Schematic design used for gas absorption measurements [35].*

#### *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*

*2.1.1 Advanced porous gas sampling structure for gas sensing*

volume under the chip scale level [35–38].

out couplings of radiation.

chamber, and the radiation detector [33]. In recent years, PhC has been proven to be an effective technology to improve the performance of the light sources, especially for the laser devices [34]. But more significantly, PhC has also played a critical role to downsize the sensor module footprint by minimizing the interaction chamber

As proposed by Soref [29], silicon is a proper candidate for the mid-IR wavelength range due to its transparency window between 1.1 and 8.5 μm. While silicon dioxide is also transparent up to around 3.6 [29], silicon-on-insulator (SOI) is a suitable structural form for mid-IR integrated photonics [39]. In 2007, Lambrecht et al. [12] for the first time, suggested the implementation of a two dimensional (2D) macroporous silicon PhC in the interaction volume to slow the light and enhance the gas-light interaction. The experimental results with CO2 showed more than two times enhancement in absorption line. In fact, these experimental results became a base for the realization of high sensitivity and miniature gas sensors. The device configuration is shown in **Figure 2**. Gas flows through the sampling cell from the top hole to the bottom, as highlighted in blue color. The sampling cell containing a PhC membrane is positioned between a thermal emitter and a pyrodetector with an IR bandpass filter centered at the absorption peak of CO2 (λ = 4.24 μm). The PhC membrane is placed in between two BaF2 light guiding rods which help to couple the IR radiation among the thermal emitter, PhC membrane and the pyrodetector. Here, one thing is worth noting that, the PhC membrane could be easily removed from the plastic holder without changing positions of the BaF2 rods. This offers the possibility of measuring the empty cell with the same optical path length. In detail, the operation voltage is 10 Hz and the pyrodetector is measured by digital lock-in amplifier with a time constant of 2 s. With such setup, this method does offer the capability to detect the presence of CO2 gas, which proves the possibility of using PhC technology for developing gas sensors with compact footprint. However, there is a crucial drawback with this simple PhC gas sensor. Typically, a low group velocity corresponds to a high effective refractive index, which leads to difficulties in and

**76**

**Figure 2.**

*Schematic design used for gas absorption measurements [35].*

In 2011, Pergande et al. [35] solved this problem via designing antireflection layers (ARL) at two interfaces, including the air-ARL interface and the ARL-PhC interface. Because of that, it enhanced the light absorption of CO2 up to 3.6 times. To explain the mechanism, introducing this ARL leads to the generation of surface modes which are used for coupling light into slow light PhC modes. These modes are confined in air-ARL interference due to the forbidden propagation of light in photonic band-gap. The thickness of ARL affects the spectral position of surface modes. When ARL thickness is equal to 0.57a (a is the lattice constant of PhC), the similarity of field distributions of surface modes and the slow light PhC modes would be able to couple together much easily. In addition, this ARL could also help to effectively reduce the interface optical loss. Theoretically, absorption of the device can be enhanced up to 60 by using this ARL enhanced PhC configuration. The difference between theoretical and experimental results comes from several factors, such as the non-optimal lattice constant and pore diameter fluctuation. They all could lead to the off-resonance operation mode to the CO2 absorption line. In this work, 2D PhCs were fabricated by photoelectrochemical etching of n-type silicon. ARL is designed by photolithography and was transferred by photoelectrochemical etching into the silicon wafer. The lattice constant was 2 μm an (r/a), varying from 0.360 to 0.385. lengths of PhC changed from 100 μm to 1 mm while the height and width were approximately 330 μm and 1 mm, respectively. The porosity of the sample was about 64%. Pergande et al., according to numerical estimates, suggested that the positional variations and pore diameter fluctuations should be below 0.5% in order to allow for a reasonable transmission in the 1 mm device.

In the course of the experiment, several sample lengths, varying from 0.25 to 1 mm, were investigated. Results showed that their absorption enhancements changed at the range of 2.6–3.5. By taking all the mismatch factors into consideration, the numbers are in good agreement with the numerical simulation results. All the data are presented in the following **Table 1**.

In 2000, Boarino et al. [40] applied this porous silicon method to the environmental analysis of NO2 gas because of its high sensitivity to surface molecular structures. **Figure 3** shows the enhanced absorption value due to the presence of NO2. In this figure, spectrum-1 is related to the sample outgassed under dynamic vacuum, spectrum-2 is given after the dosage of 1 Torr of the pure NO2 sample, and spectrum-3 is recorded after outgassing. As can be seen, in the range of 1000–1250 cm<sup>−</sup><sup>1</sup> (assigned to the stretching vibrations of Si–O species) and range of 2150–2300 cm<sup>−</sup><sup>1</sup> (Si–H stretching modes of SiO–Hx species) the absorption enhancement in the presence of NO2 is intense while this value, at 3748 cm<sup>−</sup><sup>1</sup> (SiO–H stretching vibration), is negligible. They claimed that this broad absorption can be attributed to electrons populating of the conduction band. Moreover, from spectrum-3 is an evidence that removing NO2 from the gas phase leads to a complete restoration of initial conditions. Moreover, the most recent research based on this method showed the enhanced gas absorption by a factor of 5.8 at 5400 nm [41].

It can be concluded that porous silicon is an effective gas sampling structure that can help in minimizing the overall size of the sensor device. However, the main


**Table 1.** *Experimental and theoretical value of the gas absorption enhancement [35].*

#### **Figure 3.**

*FTIR spectra of free-standing p + PSL: Spectrum-1 is related to sample outgassed under dynamic vacuum. Spectrum-2 has been obtained after dosage of 1 Torr of pure NO2, spectrum-3 has been recorded after outgassing the sample, under dynamic vacuum [40].*

restrictive factor for using porous silicon PhCs in gas sensing is its high sensitivity to the small fluctuation of the pore diameter and the lattice constant. For instance, more than 0.5% pore diameter fluctuation in the 1 mm device eliminates the advantage of using the porous silicon. Thus, a high technology is needed for fabricating this porous silicon, which limits the implementation of this method. However, for cases where high-sensitive and small sensors are needed, using porous silicon PhCs to decrease the interaction path and increase the sensitivity is a suitable option.

#### *2.1.2 Photonic crystal waveguide (PCW) for gas sensing enhancement*

Mid-IR PhC waveguides (PCW) with high-quality factor are powerful tools for nonlinear optical applications because they can achieve slow-light enhancement and low linear propagation loss simultaneously [31]. Therefore, this component is of major interests in the gas sensing research area. The enhanced detection sensitivity achieved by using PCWs (strip and slot waveguides) in the near-infrared range have been previously reported [42, 43]. However, based on our knowledge, very few works about the PCW enhanced gas sensor in the mid-infrared range has been reported so far [44]. In 2015, Zou et al. [45] provided the first experimental demonstration of transmission characteristics of holey and slotted PCWs in silicon-on-sapphire at the wavelength of 3.43 μm with a fixed-wavelength inter-band cascade laser (ICL). They used an 800 μm long holey PCW to detect gas-phase Triethyl phosphate (TEP) with the concentration of 10 ppm (parts per million).

They investigated a holey PCW with a row of smaller holes (rs = 0.65r), which was located in the center of PCW (hexagonal lattice of air holes in silicon) where air hole radius was r = 0.25*a* (*a* is lattice constant). As shown in **Figure 4** the optimization of (rs/r) value has been conducted through considering four conditions: having a large guiding bandwidth for the propagating PCW modes, broad mode bandwidth, large electrical field overlap with the analyte, and high peak enhancement factor which yield to more efficient light-matter interaction. The other value that should be optimized is the lattice constant. As shown in **Figure 5** for a less than 830 nm, due to stopgap, there is no transmission. Moreover, when *a* is located between 840 and 845 nm the propagation loss is approximately 15 dB/cm, while

**79**

is 3 times higher than holey PCW.

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

this value increases dramatically for *a* > 845 nm. Therefore, it can be concluded that in order to have reasonable transmission and propagation loss, the lattice constant

*Optimization of rs/r according to bandwidth, stop gap electrical field overlap, peak enhancement factor [45].*

On the other hand, the slotted type PCW was also investigated. Likewise, the same optimization process was conducted for slotted PCW to determine slot width (130 nm) and lattice constant (between 830 and 840 nm). However, the optimized value showed propagation loss of 55 dB/cm, which is almost 3 times greater than the loss propagation in holey PCW. Afterward, they compared the peak of electric field enhancement factor in the holey PCW, and the conventional rectangular slotted PCW, to a regular PCW. This comparison showed 3.5 times and 13 times enhancement for holey and slotted PCW (respectively) relative to regular PCW. Furthermore, the electrical field overlap with the analyte in regular PCW was 5%, while this value for holey and slotted one was 8 and 15% respectively. However, propagation loss for holey PCW was 15 dB/cm while this value was 55 dB/cm for slotted one. Thus, we can conclude that the holey PCW can be a better candidate for absorption spectroscopic gas sensing because while its electrical field overlap with the analyte is 2 times lower than slotted PCW, the propagation loss of slotted PCW

The transmitted light through an 800 μm long holey PCW with *a* = 845 nm is measured in the presence and absence of TEP. They investigated changes in

should have a value between dashed lines in **Figure 5**.

*Lattice constant dependent transmission and propagating loss [45].*

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

**Figure 4.**

**Figure 5.**

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

**Figure 4.** *Optimization of rs/r according to bandwidth, stop gap electrical field overlap, peak enhancement factor [45].*

#### **Figure 5.**

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

restrictive factor for using porous silicon PhCs in gas sensing is its high sensitivity to the small fluctuation of the pore diameter and the lattice constant. For instance, more than 0.5% pore diameter fluctuation in the 1 mm device eliminates the advantage of using the porous silicon. Thus, a high technology is needed for fabricating this porous silicon, which limits the implementation of this method. However, for cases where high-sensitive and small sensors are needed, using porous silicon PhCs to decrease the interaction path and increase the sensitivity is a suitable option.

*FTIR spectra of free-standing p + PSL: Spectrum-1 is related to sample outgassed under dynamic vacuum. Spectrum-2 has been obtained after dosage of 1 Torr of pure NO2, spectrum-3 has been recorded after* 

Mid-IR PhC waveguides (PCW) with high-quality factor are powerful tools for nonlinear optical applications because they can achieve slow-light enhancement and low linear propagation loss simultaneously [31]. Therefore, this component is of major interests in the gas sensing research area. The enhanced detection sensitivity achieved by using PCWs (strip and slot waveguides) in the near-infrared range have been previously reported [42, 43]. However, based on our knowledge, very few works about the PCW enhanced gas sensor in the mid-infrared range has been reported so far [44]. In 2015, Zou et al. [45] provided the first experimental demonstration of transmission characteristics of holey and slotted PCWs in silicon-on-sapphire at the wavelength of 3.43 μm with a fixed-wavelength inter-band cascade laser (ICL). They used an 800 μm long holey PCW to detect gas-phase Triethyl phosphate (TEP) with

They investigated a holey PCW with a row of smaller holes (rs = 0.65r), which was located in the center of PCW (hexagonal lattice of air holes in silicon) where air hole radius was r = 0.25*a* (*a* is lattice constant). As shown in **Figure 4** the optimization of (rs/r) value has been conducted through considering four conditions: having a large guiding bandwidth for the propagating PCW modes, broad mode bandwidth, large electrical field overlap with the analyte, and high peak enhancement factor which yield to more efficient light-matter interaction. The other value that should be optimized is the lattice constant. As shown in **Figure 5** for a less than 830 nm, due to stopgap, there is no transmission. Moreover, when *a* is located between 840 and 845 nm the propagation loss is approximately 15 dB/cm, while

*2.1.2 Photonic crystal waveguide (PCW) for gas sensing enhancement*

the concentration of 10 ppm (parts per million).

*outgassing the sample, under dynamic vacuum [40].*

**78**

**Figure 3.**

*Lattice constant dependent transmission and propagating loss [45].*

this value increases dramatically for *a* > 845 nm. Therefore, it can be concluded that in order to have reasonable transmission and propagation loss, the lattice constant should have a value between dashed lines in **Figure 5**.

On the other hand, the slotted type PCW was also investigated. Likewise, the same optimization process was conducted for slotted PCW to determine slot width (130 nm) and lattice constant (between 830 and 840 nm). However, the optimized value showed propagation loss of 55 dB/cm, which is almost 3 times greater than the loss propagation in holey PCW. Afterward, they compared the peak of electric field enhancement factor in the holey PCW, and the conventional rectangular slotted PCW, to a regular PCW. This comparison showed 3.5 times and 13 times enhancement for holey and slotted PCW (respectively) relative to regular PCW. Furthermore, the electrical field overlap with the analyte in regular PCW was 5%, while this value for holey and slotted one was 8 and 15% respectively. However, propagation loss for holey PCW was 15 dB/cm while this value was 55 dB/cm for slotted one. Thus, we can conclude that the holey PCW can be a better candidate for absorption spectroscopic gas sensing because while its electrical field overlap with the analyte is 2 times lower than slotted PCW, the propagation loss of slotted PCW is 3 times higher than holey PCW.

The transmitted light through an 800 μm long holey PCW with *a* = 845 nm is measured in the presence and absence of TEP. They investigated changes in

transmitted light intensity at λ = 3.43 μm for switching and steady state TEP flow through the holey PCW. For the switching flow, transmitted signal intensity dropped to 80% of its original intensity due to the presence of TEP. However, for the steady-state condition, this value dropped to 60% of its original value. Also, the measurements are independent of the flow rate of nitrogen at 10 and 50 ppm respectively. The noise in measurement comes from the electrical noise of detector and the vibration of the optical fiber. Furthermore, by replacing the slot and strip waveguide for the steady-state gas flow, instead of the holey PCW, just a small change in intensity of transmitted light at 3.43 μm is observed at 28 ppm TEP concentration. Thus, as **Figure 6** shows, it can be concluded that the holey PCW shows more sensitivity relative to the slot and strip waveguide because the holey PCW enhances both f (fill factor denoting relative fraction of optical field residing in the analyte medium) and group index, which is inversely related to group velocity. However, the slot waveguide only enhances the f factor, and has no impact on group index. Moreover, the high detection capability of PCWs in gas sensing is revealed in [46] where the detection of carbon monoxide with the concentration of parts-per-billion is possible.

In comparison to the porous silicon PhC Sensing method which enhances the sensor performance by only reducing group velocity, the PCW, especially the holey PCW, shows higher sensitivity because it enhances both f (filling factor denoting relative fraction of optical field residing in the analyte medium) and group velocity reduction simultaneously. In fact, using a PCW, instead of a simple 2D PhC, enables us to confine light and gas in at the same place, and increase the possibility of the interaction. Moreover, the propagation loss in PCWs is lower than the porous silicon PhC. However, the fabrication and optimization of a PCW are more complicated than the porous silicon PhC which potentially can lead to higher costs for PCW gas sensor development.

#### *2.1.3 Mid-infrared PhC fiber enhanced sensing*

Optical fibers offer significant advantages for gas sensing majorly due to its ability to confine optical radiation across long distances. It eliminates the need for beam collimation thus reduces device complexity significantly. However, their performance can be limited due to low mechanical flexibility, a weak overlap between light and gas or the requirement for conventional extrinsic gas cells [47]. Also, most of the fibers operate below 2 μm demonstrated due to the limited silica transmission window. In contrary to other microstructured fiber, Photonic bandgap fibers (PBFs) guide light in a gas (air) core rather than solid [48, 49]. This leads to the integration of gas sampling cell into the fiber, and the confinement of over 99% of the light in the hollow area rather than the silica, which makes PBFs an ideal

#### **Figure 6.**

*(a). Change in transmitted light intensity through an 800 μm long holey PCW with a = 845 nm with 10 ppm TEP (b). Transmitted light intensity through a silicon slot waveguide in SoS in the presence and absence of 28 ppm TEP. (c). Transmitted light intensity through a silicon strip waveguide in SoS in the presence and absence of 28 ppm TEP [45].*

**81**

**Figure 7.**

*b, 1%; c, 0.5%; and d, 0.1% [55].*

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

candidate for miniaturized gas sensing applications [50, 51]. Furthermore, compared with the fused silica, these fibers can transmit light in the mid-infrared with a

) [52, 53]. Shephard et al. reported a bandgap guidance at 3.14 μm in a silica-based aircore photonic crystal fiber in 2005 [54]. The year after, they then investigate the gas sensing functionality using this PBF technology [53, 55]. The optical guiding range of this PBF was specially developed for methane sensing at ∼3.2 μm [55]. The hollow-core PBF sensing unit was able to measure the methane vapor with the concentration of 1000 ppm. In the experiment, several fibers with a core diameter of ∼45 μm and a pitch (distance between two neighboring cladding holes) between 7 and 8 μm were examined thoroughly. First, an 80 cm PBF was filled with the mixture of methane and nitrogen with the ratio of 5 (methane) to 95 (nitrogen) at the pressure of 2 bar. Then, the fiber was filled with nitrogen to normalize the measurements. The results are shown in **Figure 7** for 5, 1, 0.5, and 0.1% methane concentration. (solid curve) shows experimental results and the calculated absorption demonstrated by the dashed curve. Even in this low concentration (**Figure 7d**), the main absorption line of methane close to 3.32 μm is still visible, and the transmission dropped to significantly at this wavelength. Moreover, authors claimed that the small difference between the theoretical and experimental results is due to

although processing errors during the spectral concatenation procedure.

than PCWs but is still higher compared to the porous silicon PhC devices.

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

In comparison to the porous silicon PhC as well as the PCW sensing mechanisms, the PBF gas sensor shows a higher energy overlap with gases and a lower optical loss. However, fibers are long in nature and this can be an obstacle to realization of SWaP sensors. Moreover, the degree of complexity for fabrication of PBFs can be lower

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

*Measured (solid curves) and theoretical (dash curves) absorption lines of methane for concentrations of a, 5%;* 

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

much lower optical loss (<1 dBm<sup>−</sup><sup>1</sup>

#### *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*

parts-per-billion is possible.

sensor development.

*2.1.3 Mid-infrared PhC fiber enhanced sensing*

transmitted light intensity at λ = 3.43 μm for switching and steady state TEP flow through the holey PCW. For the switching flow, transmitted signal intensity dropped to 80% of its original intensity due to the presence of TEP. However, for the steady-state condition, this value dropped to 60% of its original value. Also, the measurements are independent of the flow rate of nitrogen at 10 and 50 ppm respectively. The noise in measurement comes from the electrical noise of detector and the vibration of the optical fiber. Furthermore, by replacing the slot and strip waveguide for the steady-state gas flow, instead of the holey PCW, just a small change in intensity of transmitted light at 3.43 μm is observed at 28 ppm TEP concentration. Thus, as **Figure 6** shows, it can be concluded that the holey PCW shows more sensitivity relative to the slot and strip waveguide because the holey PCW enhances both f (fill factor denoting relative fraction of optical field residing in the analyte medium) and group index, which is inversely related to group velocity. However, the slot waveguide only enhances the f factor, and has no impact on group index. Moreover, the high detection capability of PCWs in gas sensing is revealed in [46] where the detection of carbon monoxide with the concentration of

In comparison to the porous silicon PhC Sensing method which enhances the sensor performance by only reducing group velocity, the PCW, especially the holey PCW, shows higher sensitivity because it enhances both f (filling factor denoting relative fraction of optical field residing in the analyte medium) and group velocity reduction simultaneously. In fact, using a PCW, instead of a simple 2D PhC, enables us to confine light and gas in at the same place, and increase the possibility of the interaction. Moreover, the propagation loss in PCWs is lower than the porous silicon PhC. However, the fabrication and optimization of a PCW are more complicated than the porous silicon PhC which potentially can lead to higher costs for PCW gas

Optical fibers offer significant advantages for gas sensing majorly due to its ability to confine optical radiation across long distances. It eliminates the need for beam collimation thus reduces device complexity significantly. However, their performance can be limited due to low mechanical flexibility, a weak overlap between light and gas or the requirement for conventional extrinsic gas cells [47]. Also, most of the fibers operate below 2 μm demonstrated due to the limited silica transmission window. In contrary to other microstructured fiber, Photonic bandgap fibers (PBFs) guide light in a gas (air) core rather than solid [48, 49]. This leads to the integration of gas sampling cell into the fiber, and the confinement of over 99% of the light in the hollow area rather than the silica, which makes PBFs an ideal

*(a). Change in transmitted light intensity through an 800 μm long holey PCW with a = 845 nm with 10 ppm TEP (b). Transmitted light intensity through a silicon slot waveguide in SoS in the presence and absence of 28 ppm TEP. (c). Transmitted light intensity through a silicon strip waveguide in SoS in the presence and* 

**80**

**Figure 6.**

*absence of 28 ppm TEP [45].*

candidate for miniaturized gas sensing applications [50, 51]. Furthermore, compared with the fused silica, these fibers can transmit light in the mid-infrared with a much lower optical loss (<1 dBm<sup>−</sup><sup>1</sup> ) [52, 53].

Shephard et al. reported a bandgap guidance at 3.14 μm in a silica-based aircore photonic crystal fiber in 2005 [54]. The year after, they then investigate the gas sensing functionality using this PBF technology [53, 55]. The optical guiding range of this PBF was specially developed for methane sensing at ∼3.2 μm [55]. The hollow-core PBF sensing unit was able to measure the methane vapor with the concentration of 1000 ppm. In the experiment, several fibers with a core diameter of ∼45 μm and a pitch (distance between two neighboring cladding holes) between 7 and 8 μm were examined thoroughly. First, an 80 cm PBF was filled with the mixture of methane and nitrogen with the ratio of 5 (methane) to 95 (nitrogen) at the pressure of 2 bar. Then, the fiber was filled with nitrogen to normalize the measurements. The results are shown in **Figure 7** for 5, 1, 0.5, and 0.1% methane concentration. (solid curve) shows experimental results and the calculated absorption demonstrated by the dashed curve. Even in this low concentration (**Figure 7d**), the main absorption line of methane close to 3.32 μm is still visible, and the transmission dropped to significantly at this wavelength. Moreover, authors claimed that the small difference between the theoretical and experimental results is due to although processing errors during the spectral concatenation procedure.

In comparison to the porous silicon PhC as well as the PCW sensing mechanisms, the PBF gas sensor shows a higher energy overlap with gases and a lower optical loss. However, fibers are long in nature and this can be an obstacle to realization of SWaP sensors. Moreover, the degree of complexity for fabrication of PBFs can be lower than PCWs but is still higher compared to the porous silicon PhC devices.
