*Metalens Antennas in Microwave, Terahertz and Optical Domain Applications DOI: http://dx.doi.org/10.5772/intechopen.99034*

#### **Figure 4.**

*(a) The skeptic view of metalens antenna with SIW feeding components from the bottom. (b) Jerusalem cross is used as the unit cell of metalens (c) tapered SIW with radiation patches are used for transmit/receive feeding array unit (d) Fabricated metalens with copper elements on the polymer substrate and (e) simulated radiation pattern with different incident angles in the focus plane.*

#### **Table 1.**

*Design parameters used in MIMO Metalens antenna.*

#### **Figure 5.**

*The silicon micro-brick unit: P = 150* μ*m, h = 195* μ*m, Lx = 120* μ*m and Ly = 60* μ*m.*

#### **Figure 6.**

*Electromagnetic field presence of the Si micro-brick unit cell with Lx and Ly variations at 1.0 THz. The phase changes* Φ*x (a) and transmission amplitude tx (b) of the Si unit as a function of Lx and Ly with the x-polarized incidence. The phase changes* Φ*y (c) and transmission amplitude ty (d) as a function of Lx and Ly with the y-polarized incidence.*

fine-tuned from 30 μm to 140 μm to match the desired electromagnetic profile. Si micro-bricks array is taken as homogeneous and of infinite extent in both in-plane dimensions. The transmitted phase shifts Φx, Φy and the transmittance tx and ty of the x, y-polarized light, are shown in **Figure 6(a**–**d)**. The simulated 2π phase

*Metalens Antennas in Microwave, Terahertz and Optical Domain Applications DOI: http://dx.doi.org/10.5772/intechopen.99034*

**Figure 7.**

*The phase distribution of the designed DFM along the x direction for XLP (a) and YLP (b). The transmitted intensity distributions of the designed DFM under the XLP (c), 30o LP (d), 45o LP (e) and YLP (f) incidences respectively.*

variations with transmission over 90% are shown in **Figure 6**. Nearly any combination of phase shifts from 0 to 2π with high transmittance for two polarizations can be obtained from single unit block by setting Lx and Ly properly. Conclusively, the double-phase modulation could be realized by the modulating two polarizations separately for this structure.

In **Figure 7(c)** and **(f )**, the XLP light will be focused to the left side position at x = −x0; y = f. Parallelly, the YLP light will be focused to the right side position at x = x0; y = f. In fact, any polarized light, such as the 30° LP and 45° LP, can be decomposed to x and y polarization components and be focused separately. If designing a dual-focus metalens, there will be two focus spots in the focal plane as shown in **Figure 7(d)** and **(e)**. 30° LP and 45° LP lights can be two focusing spots corresponding to the x and y-polarization. **Figure 7** shows that the focusing intensity of y-polarization component is increased with the LP angles, and the focus intensity of x-polarization component is decreased. The result shows that the splitting energy ratio could be adjusted by manipulating the two polarization states separately.

From this example, the lattice constants of the unit cell are set to 150 μm for both directions, which is chosen to be slightly less than the half of wavelength to avoid the diffraction effect. The height of Si unit is set to 195 μm, and this is obtained from the parametric sweeping by commercial FDTD simulator. The phase profile is obtained from the resonance contribution of each Si unit cell, and the height of the unit should be enough to cover 2π phase modulation and efficient transmission.

#### **3.3 Metalens with high efficiency and numerical aperture**

An imaging system normally requires a high numerical aperture with enough efficiency to meet the requirement on the resolution and image contrast.

#### **Figure 8.**

*(a) Schematic of the THz metalens. (b) Schematic of the silicon cross resonators with the incident THz field propagating along z direction and electric field along x direction. The arm width W and the height H are 12* μ*m and 20* μ*m, respectively, by varying the arm length L. (c) Transmission amplitudes and (d) corresponding phases as functions of the arm length of the cross resonator and the frequency.*

#### **Figure 9.**

*(a) Setup schematic of the THz metalens imaging experiment. (b) Measured field distribution plane along z direction parallel to the metalens plane. (c) FWHMs of the focus spots along z direction and* Δ*z represent the different distances from the focal plane. (d) Normalized experimental curve in red and theoretical curve in blue indicate the intensity profiles along the y direction at the focal plane.*


*Non-destructive Characterizations of Natural Yarns and Fabrics DOI: http://dx.doi.org/10.5772/intechopen.102587*

**Figure 7.**

*Yarn characteristics obtained from ASTM test standards and XRM-CT methods. (a) Yarn diameter, (b) yarn packing factor (PF), and (c) yarn crimp% [12, 13].*

#### **Figure 8.**

*Fabric characteristics obtained from ASTM test standards, theoretical values and XRM-CT method. (a) Fabric thickness, (b) fabric volumetric density calculated using ASTM thickness and CT thickness, and (c) fabric cover factor calculated using theoretical yarn diameters and CT yarn diameters [12, 13].*

#### **Figure 9.**

*X-ray micro-computed tomographic 2D images of the fabrics studied. (a) P40L, (b) P40H, (c) S40L, (d) S40H, (e) P60L, (f) P60H, (g) S60L, and (h) S60H [12, 13].*

However, despite the different twists in different count-yarns in the fabrics, twist multiplier were nearly the same suggesting the same internal structure (helix angle) of the yarns [12, 13].

With increasing fabric density (thread density), thickness decreased in 40Ne yarnmade fabrics, and increased in 60Ne yarn-made fabrics (**Figure 9**). With increasing fabric density, less twisted and larger diameter 40Ne yarns compressed within the structure, and filled up the voids. This resulted in a closer fiber and yarn packing, and reduced thickness in the 40Ne yarn-made fabrics (**Figures 7b** and **9**). In contrast, finer, tighter (with more twists) and smaller diameter 60Ne yarns had less voids (as discussed above in yarn packing factor). Hence, in the presence of low voids in the 60Ne yarns, the yarns expanded in the thickness direction with increasing fabric density (**Figures 7b** and **9**) [12, 13].

**Fabric Volumetric Density**: Both the ASTM thickness and CT thickness were used to calculate fabric volumetric density (g/cm3 ) for further comparison (**Figure 8b**), and were obtained by multiplying fabric basis weight (gsm) with thickness. The methods did not exhibit any statistical differences in the measurements (P-value 0.51) [12, 13].

**Fabric Cover Factor**: Fabric cover factor obtained from the theoretical values and the CT method (using CT yarn diameter) were compared and depicted in **Figure 8c**. A further statistical analysis showed that there was no significant difference between the measurements obtained from the two methods (*P*-value 0.62), and satin fabrics

### *Non-destructive Characterizations of Natural Yarns and Fabrics DOI: http://dx.doi.org/10.5772/intechopen.102587*

exhibited higher fabric cover factor. Irrespective of weave design, no significant difference was observed between 40 and 60Ne yarn-made fabrics [12, 13].

Therefore, unlike other classical test methods, the CT method are nondestructive and capable of providing realistic measurements from both the 2D and 3D images. Apart from the structural properties discussed in this Chapter, the CT method can be used for a more advanced analysis of the fabrics such as fiber area distribution [12, 13], porosity analysis [22], surface profile analysis [1] etc. Such in-depth analysis and understanding of the cotton-made yarn and fabric structures will help engineers and scientists optimize the structure and improve the performance of the textiles for diverse applications and end-uses.


**4.2 Extrinsic esophageal compression**

The presence of anterior cervical osteophyte, diffuse idiopathic skeletal hyperostosis can cause dysphagia secondary to mechanical impingement of esophagus, as well as inflammation causing adhesion and fibrosis.

In ACDF with plating, the presence of a plate can contributes to the same pathophysiology cause of dysphagia. Although the concept is not yet fully established, it has been proven that a thicker cervical plate is associated with a higher incidence of dysphagia [22].

Several studies confirmed that standalone cages are associated with less incidence of dysphagia when compared with ACDF with plating. However, this conclusion is not universally accepted [29].

#### **4.3 Esophageal retraction**

Retraction of esophagus during surgery to expose the anterior cervical spine is one of the possible cause of postoperative dysphagia while some studies concluded that esophageal retraction may cause ischemia of the esophageal wall which in turn compromise the motility [12], one study failed to confirm the association between the intraoperative pressure of esophageal retraction and postoperative dysphagia [30].

#### **4.4 Neural traction**

Intraoperative nerve traction or injury is another possible cause of postoperative dysphagia. Different nerves traction will cause different esophageal segment dysphagia. For example damage or traction to Hypoglossal nerve will impact the oral phase of swallowing, while injury or traction to the connection between the pharyngeal plexus and pharyngeal muscle will impact the pharyngeal phase of swallowing. Injury of the recurrent laryngeal nerve (RLN) and superior laryngeal nerve (SLN) are both operative in the development of postoperative dysphagia, reason why a sound knowledge of their anatomy and meticulous surgical technique are essential to decrease the postoperative dysphagia [11].

Anderson et al. [11] summarized the causes of oropharyngeal dysphagia as seen in **Table 1**.

Regarding the surgical techniques, the causes of dysphagia are described in **Table 2**.

### **5. Risk factors**

#### **5.1 Patient-related**

#### *5.1.1 Age*

Smith-Hammond et al. [20] found that older patients have an higher risk of dysphagia following ACDF, while Lee et al. [19] and Bazaz et al. [19] found the no correlation between the age and the risk of dysphagia.