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Photonic-crystal-fiber (PCF) based sensors in the terahertz spectrum have been immensely studied and implemented due to their unique advantages and high sensitivity. At an early stage, conventional and hybrid structured porous core PCF-based sensors were proposed, but the sensitivity was not so high. With the advancement of PCF fabrication technology, hybrid structured hollow-core PCFs have been reported and offer superior sensing characteristics than the previous types. In this chapter, both porous core and hollow-core PCF-based THz sensors are analyzed and the propagation characteristics are explained using terahertz spectrum. Finally, some promising terahertz sensors are studied and compared at the end of this chapter.
Department of Electrical and Electronic Engineering, Rajshahi University of Engineering and Technology, Bangladesh
Md. Shamim Anower
Department of Electrical and Electronic Engineering, Rajshahi University of Engineering and Technology, Bangladesh
Md. Nazmul Islam
Department of Electrical and Electronic Engineering, Bangabandhu Sheikh Mujibur Rahman Science and Technology University, Bangladesh
*Address all correspondence to: habib.eee.116.ah@gmail.com
1. Introduction
The electromagnetic signal whose frequency ranges between 0.1 and 10 THz is termed the terahertz radiation band [1, 2, 3, 4]. This radiation band has been effectively used in numerous applications, such as sensing, medical imaging, biotechnology, and genetic engineering, security, chemical spectroscopy, diagnosis of different cancerous cells, radar, and astronomy [5, 6, 7, 8, 9, 10]. Due to the massive advancement in the optoelectronics sector, numerous types of terahertz sources and terahertz detectors (photoconductive antennas, bolometers, etc.) are already available on the market [11, 12, 13]. On the contrary, efficient and favorable terahertz waveguides are still under research. The major barrier for the expansion of terahertz waveguides in different applications is the selection of waveguide base material. Metallic waveguides are suitable for microwave signal propagation, but they offer high ohmic loss for higher frequency signals. A circular-shaped metallic waveguide [14] was proposed in 1999, which suffered only ohmic loss for terahertz signals, whereas coplanar and microstrip transmission lines exhibited other types of losses [15]. One of the major drawbacks of this circular waveguide was the strong dispersion near the cut-off frequency. One year later, a rectangular metallic waveguide [16] was demonstrated theoretically and experimentally, which experienced the same type of loss and dispersion problem as the circular metallic waveguide [15]. In 2001, a copper-based parallel plate terahertz waveguide was proposed, which exhibited only ohmic loss but the dispersion was absent for this type of waveguide [17]. After that, Wang and Mittleman first introduced bare metal or Sommerfeld wire as a terahertz waveguide, and they proved that this waveguide offered less loss than the metallic waveguides [18]. However, in metallic waveguides, the ohmic loss was low, but this waveguide experienced fewer light confinement problems for terahertz waves. This problem can be solved by replacing metallic slit waveguides instead of the bare metal waveguide, and it was first introduced in 2007 [19]. Along with the various types of metallic waveguides, some remarkable dielectric waveguides were also proposed by the researchers, which also include those in the terahertz frequency range. The dielectric waveguides were used to transmit higher frequency signals (such as infrared and optical frequencies) as metallic wires experienced higher losses at these frequencies. On the contrary, the dielectric waveguides were subjected to dielectric absorption loss dependent on waveguide frequencies. Dielectric waveguides can be classified into two categories, which are hollow- and solid-core-fiber waveguides. The dielectric waveguides offer better light confinement capability than metallic waveguides, so the dielectric waveguides are more popular for terahertz wave propagation.
In hollow-core dielectric waveguide, the light is confined by an omnidirectional mirror and the dielectric material is transparent to a high frequency signal. So that, the hollow-core dielectric waveguide eliminates the limitations existing in silica fiber and hollow metallic waveguide. The light-guiding mechanism in PCFs are exactly similar to the hollow-core dielectric waveguide so that the PCFs can be considered as a dielectric waveguide. The invention of photonic crystal fibers (PCFs) has opened a new door for optoelectronics researchers. However, the selection of the base material for PCF is a tough task as most of the materials experience wavelength-dependent attenuation loss for wide-band signals. In the recent past, a huge number of fused silica-based solid core optical waveguides have been theoretically and experimentally investigated due to some excellent characteristics of this material compared to other glasses or plastics [20]. Fused silica has higher tensile strength, higher transparency to light waves, lower absorption loss, higher availability in nature, high melting point, lower dispersion characteristics, and so on, which increase its popularity as an optical waveguide [21]. However, the silica-based fibers are not suitable for the terahertz spectrum as they offer high absorption loss and modal dispersion to terahertz signals. The most commonly used materials for terahertz dielectric waveguides are TOPAS, Zeonex, Teflon, PMMA, etc. due to their lower loss coefficients [15]. Among them, TOPAS and Zeonex offer the lowest loss coefficient and highest transparency towards the terahertz spectrum, so that various application dependent fiber waveguides based on those materials have been proposed [22].
PCFs can be classified into three categories on the basis of the core structure, which are solid core PCF [22], porous core PCF [23, 24], and hollow-core PCF [25]. In addition to wave communication functions, PCFs can also be used for optical sensing. To apply a PCF in sensing applications, the sample under test is filled in the core and after that, the electromagnetic wave is injected through the core [26]. The light interacts with the sample, and after analyzing the received signal, the unknown sample can be identified [26]. However, solid core PCF cannot be used in sensing applications as it is not possible to inject analyte inside of the solid core. So, numerous structures based on hollow-core and porous core dielectric fibers have been proposed as liquid or gas sensors [27, 28, 29, 30, 31] using the electromagnetic signal at 1.33 μm or 1.55 μm wavelengths. These proposed PCF-based sensors offer relatively high sensitivity and superior guiding characteristics, which are compared with commonly used mechanical sensors and transducers.
In order to claim a PCF as a good sensor, it must satisfy some criteria, including high relative sensitivity, low confinement loss, and high numerical aperture. The mathematical formulas or expressions are shown and explained, which are generally used to calculate the sensing and guiding characteristics of an optical sensor.
Relative sensitivity is the key parameter of any PCF-based sensor, which is an indication of how much change of an analyte can be sensed or detected by the sensor. Higher relative sensitivity is desirable from any sensor, as the higher the relative sensitivity, the smaller the change that can be identified by the sensor. The mathematical expression to calculate the relative sensitivity (r) is [27, 28, 29, 30, 31, 32, 33, 34],
r=nrneff×P%E1
where nr and neff are the effective refractive index of the actual propagating EM wave through the core and the analytes, respectively. In addition, P indicates how much light signal power interacts with the analytes filled in the fiber core. In the case of a PCF-based sensor, the parameter P is known as power fraction, and that can be easily calculated from the following mathematical expression [27],
where Ex, Ey, Hx, and Hy stand for x and y polarized electric field and magnetic field, respectively, when the propagation direction of light is in the z direction. The numerator of Eq. (2) integrates the energy that interacts with the sample under test and the denominator does the same for the complete sensor.
Another important propagation parameter for any kind of optical fiber-based waveguide or sensor is confinement loss. This loss parameter determines the actual fiber length as higher confinement loss restricts the actual fiber length. This parameter indicates the amount of power wastage due to the cladding air holes and it is usually calculated by employing the following expressions [27, 28, 29],
αCL=8.686×2πfcImneffE3
where f is the operating frequency of the propagating EM signal, c is the speed of light in free space, and Im (neff) stands for the imaginary part of the effective refractive index. In order to be considered a good sensor, the loss must be low. Till date, no optical sensor with lower confinement loss has been developed.
Along with the confinement loss, another loss is present in all optical sensors, which is called effective material loss (EML), which reflects the information power consumed by the background material of the sensor. However, many authors did not provide this parameter in their articles, but it is very important for the practical implementation or setup of a terahertz sensor. The mathematical equation to quantify the EML of a sensor is [1],
αeff=ε0μ01/2∫AmatnαmatE2dA2∫AllSZdAE4
where ε0 and μ0 indicate the relative permittivity and permeability of the vacuum, n is the refractive index of the guided light, and αmat is the bulk material loss coefficient of the base material of the sensor. Usually, Topas and Zeonex are the most commonly used materials for the terahertz sensor as they provide the lowest loss coefficient in the terahertz regime. However, different terahertz sensors with PMMA, Teflon, etc. were also proposed by the researchers in the recent past which offered slightly higher EML than the recently proposed PCF-based sensors.
Another important propagation parameter is birefringence, which is important in the case of sensing applications. Birefringence is the absolute difference between the effective refractive indexes of x and y polarization mode which is expressed by using the following expression [32, 33],
B=nx−nyE5
where nx and ny are the effective refractive indexes in the x and y directions, respectively.
In addition, some other propagation characteristics such as bending loss, numerical aperture, V parameter, spot size, beam divergence, nonlinearity, etc. are discussed in [32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45].
A large number of PCF-based terahertz sensors have been proposed in the recent past for the identification of different types of chemicals [32, 33, 34], bane chemicals [41], blood components [48, 49, 50], alcohols [36], and so on. The prime designing objective of any kind of PCF-based sensor is to propagate the maximum amount of light through the core so that maximum light-analyte interaction takes place. The cross-sectional view of some excellent PCF-based sensors in the terahertz regime is represented in Figure 1.
Figure 1.
Cross-sectional view of (a) kagome structured slotted core chemical sensor [33], (b) hybrid structured hollow-core chemical sensor [37], (c) hybrid structured rectangular hollow-core chemical sensor [40], (d) hexagonal structured rectangular porous core alcohol sensor [36], (e) hybrid structured polygonal hollow-core cholesterol sensor [45] and (f) hybrid structured rectangular hollow-core blood component sensor [48] in the terahertz regime.
A kagome lattice slotted core fiber is shown in Figure 1(a), which was proposed by Islam et al. to detect benzene (n = 1.366), ethanol (n = 1.354), and water (n = 1.33) by using the terahertz spectrum [33]. The kagome structure in Figure 1(a) offers high confinement of light through the core due to the compact structure in the cladding. That’s the reason why the proposed sensor in Figure 1(a) has high relative sensitivity in Figure 2(a) and (b) for the x and y polarization mode. As the core is not symmetrical, the power distribution in both polarization is unequal.
Figure 2.
Relative sensitivity of (a)-(b) kagome structured slotted core chemical sensor [33], (c)-(d) hybrid structured rectangular porous core chemical sensor [37], (e)-(f) hybrid structured rectangular hollow-core chemical sensor [40], (g)-(h) hexagonal structured rectangular porous core alcohol sensor [36], (i) hybrid structured polygonal hollow-core cholesterol sensor [45] for different operating frequencies in the terahertz regime.
Another hybrid structured PCF with all rectangular holes is proposed by Islam et al. in order to detect three different liquids [37]. Its cross-section is shown in Figure 1(b) with an enlarged view of the core section. Figure 1(b) shows that the structure is not symmetric, performing birefringence and unequal power distribution in two polarization directions. The relative sensitivity is shown in Figure 2(c) and (d) for two different core dimensions. As the W value is larger than the H value, more light travels with the x-polarization mode. The sensitivity of a x-polarization mode is thus high. Similarly, Figure 2(d) shows the relative sensitivity increases as enlarging the fiber core.
Habib et al. proposed a simple hollow-core PCF-based chemical detector [40] using terahertz spectrum and the two-dimensional view is represented in Figure 1(c). One rectangular air hole is introduced in the core region to be injected by analytes. Due to asymmetric structure, this sensor also has birefringence and the relative sensitivity for both polarization modes are presented in Figure 2(e) and (f). This sensor has extremely high sensitivity around 90% at 1.9 THz for the y-polarization mode.
A very simple hollow-core chemical sensor [36] was proposed in 2018 in Figure 1(d). Due to the asymmetric core, the relative sensitivity of that sensor for both polarization modes is reported in Figure 2(g) and (h). Figure 1(d) indicates that there are considerable amounts of high indexed solid material in the core region. This proposed sensor, therefore, extracts a small fractional THz signal for sensing and results in lower sensitivity. The maximum relative sensitivity is around 73% at 1 THz.
Figure 1(e) represents a hybrid structured hollow-core PCF [45] for the identification of cholesterol present in human blood and fruits. The proposed sensor offered a very high relative sensitivity of 98% at optimum conditions for two main reasons. Firstly, the hollow-core permits maximum light to travel through the core, which increases the power fraction, and finally, the higher refractive index cholesterol (n = 1.525) attracts more light to interact with itself. So that, according to the data of Table 1, the relative sensitivity of this cholesterol sensor is maximum and the graphical representation of the variation of relative sensitivity is shown in Figure 2(g). This figure also shows the relationship between the core dimension and sensitivity. For example, the maximum core dimension has the highest relative sensitivity than at 1.2 THz when the enlarged core has strong light-analyte interaction.
Comparison of different guiding and sensing parameters for numerous types of remarkable terahertz sensors.
Few PCF-based sensors can identify different components of human blood [48, 49, 50]. In 2019, Ahmed et al. proposed a hollow-core Zeonex-based blood component sensor in the terahertz spectrum [48], whose cross-sectional view is represented in Figure 1(f). The relative sensitivity of red blood cells (RBCs), hemoglobin, white blood cells (WBCs), plasma, and water for different operating frequencies is shown in Figure 3(a) and (b). From the above discussion, it is clear that the relative sensitivity of the terahertz sensors is higher than the silica-based sensors using IR spectrum [27, 28, 29, 30, 31]. In addition, hollow-core terahertz sensors offer higher sensitivity (Figure 2(c)–(f),(i)) than that of porous core sensors (Figure 2(a),(b),(g),(h)).
Figure 3.
Relative sensitivity as a function of frequency for different blood components in terahertz regime from (a)-(b) in ref. [48].
Now, the loss characteristics of different types of terahertz PCF sensors are discussed below. To reduce the confinement loss and the effective material loss, numerous types of core and cladding structured PCFs were presented [26, 32, 33, 34, 35]. However, the light confinement inside the core is largely dependent on the geometric structure of PCF. Now, the variation of confinement loss of PCF-based sensors (Figure 1) for different THz frequencies is shown in Figure 4(a)–(f). The first three figures (Figure 4(a)–(c)) represent the confinement loss characteristics at certain PCF structures for sensing benzene, ethanol, and water [33, 37, 40]. At high THz frequency, the confinement loss is lower for all analytes because a few fractional THz wave at high frequency travels through the cladding air holes. The confinement loss of benzene is lowest than the other two samples due to the highest refractive index. The sensing results of Figure 1(d) and (e) are represented in Figure 4(d) and (e) respectively. The two figures inform that the loss is inversely proportional to the core dimension of the PCFs. Finally, Figure 4(f) shows the confinement loss for sensing blood samples [48]. The confinement loss is low at the high THz frequency and high refractive index conditions. THz waves with high frequencies have the tendency to travel through the high indexed zone, so less fractional power propagates through the cladding. The confinement loss is low at high THz frequencies for all PCF-based sensors. The confinement loss is minimum for the cholesterol sensor (Figure 1(e)).
Figure 4.
Confinement loss as a function of frequency of (a) Figure 1(a) ref. [33], (b) Figure 1(b) ref. [37], (c) Figure 1(c) ref. [40], (d) Figure 1(d) ref. [36], (e) Figure 1(e) ref. [45] and (f) Figure 1(f) ref. [48] for different PCF based sensors.
Now, the THz wave loss characteristics of different PCF-based sensors in the terahertz regime are investigated. The THz loss spectra are represented in Figure 5 for different proposed terahertz sensors shown in Figure 1(a)–(f). This loss comes from the solid materials around the core. The THz wave loss characteristics of benzene, ethanol, and water sensors in Figure 1(b) and (c) are shown in Figure 5(a) and (b) for different operating frequencies. However, according to Figure 5(a) and (b), it is clear that the loss shows an upward trend with the increase of operating frequency. Because THz waves at higher frequency can travel through high indexed solid material, the material more absorbs THz wave energy. Figure 5(c) and (d) represent THz wave loss characteristics of PCF-based sensors in Figure 1(d) and (e) for different structural conditions. Figure 5(c) shows that THz wave loss is lower for increasing the width of elliptical air holes (Figure 1(e)). Figure 5(d) shows that the THz wave loss is lowest for the largest core dimension. This structure in Figure 5(d) performs the lowest loss among Figure 5(a)–(d).
Figure 5.
Effective material loss variation of numerous structured terahertz sensorsas a function of frequency of (a) Figure 1(b) ref. [37], (b) Figure 1(c) ref. [40], (c) Figure 1(d) ref. [36], (d) Figure 1(e) ref. [45].
The following comparison shows the guiding characteristics for hybrid structured sensors [34, 35, 40, 42, 46, 48] and conventional structured sensors [33, 36, 37, 38, 39, 41, 43, 44, 45, 47, 49, 50, 51] in terahertz regime.
The most interesting characteristic of PCF-based sensors is that the guiding or sensing properties are highly dependent on the geometry or structure of the PCFs. However, from Table 1, it is quite clear that the relative sensitivity and loss profile is lower for nonconventional structured PCF than regular structured one. For example, the conventional structured liquid sensor in Ref. [44] offered the highest sensitivity of 82.26% among regular structured PCFs but the core loss is high (>10 dB−8 dB/m). On the other hand, a maximum of 93.5% relative sensitivity with lower confinement loss (< 10 dB−9 dB/m) can be achieved from PCF-based sensor in Ref. [46]. Moreover, the relative sensitivity and loss are also dependent on the test sample also. In Table 1 the highest relative sensitivity (98.75%) and lowest loss (core loss = 1.34 × 10 dB−17 dB/m and THz wave loss = 0.008 cm−1) was reported in Ref. [45] as the refractive index of cholesterol (refractive index = 1.525) is maximum than any other samples reported in Table 1.
In this chapter, different types of PCF-based terahertz sensors were discussed and compared for numerous chemicals, toxic agents, and blood components identification. In addition, a brief discussion about terahertz technology and different terahertz waveguides are included in this chapter. The sensing and guiding parameters of numerous terahertz sensors are numerically discussed and graphically represented for a better understanding of the readers. However, efficient application-specific terahertz sensors are still under research and we hope this chapter will help the optoelectronics researchers to propose new sensors.
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Written By
Md. Ahasan Habib, Md. Shamim Anower and Md. Nazmul Islam
Submitted: 07 March 2021Reviewed: 23 November 2021Published: 19 January 2022
Open access peer-reviewed chapter
