Open Access
Add to BibSonomy
Abstract
We propose a structure of porous core photonic crystal fiber with a suspended and porous elliptical core that provides considerable properties for terahertz wave guidance. The propagation features of the structure are simulated and compared over the terahertz frequency region when TOPAS and silicon nitride are used as based materials. Our results reveal that the TOPAS based structure could enhance the birefringence up to 0.123 at the frequency of 1.4 THz, with a relatively comparable total loss of 0.2 cm−1. Moreover, it is confirmed that introducing silicon nitride as the base material to the proposed structure leads to a significant birefringence of 0.89 at the frequency of 0.6 THz, with a confinement loss of 10−4 cm−1 and effective material losses of 10−3 cm−1.
© 2022 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The increasing applications of terahertz waves in various fields of science and technology [1–5] have motivated the demands not only for more reliable and compact generation schemes [6–11] but also for efficient transmission media with much lower propagation loss comparing to that in quasi-optical free space transmission [12–16]. Among various methods proposed for terahertz wave guidance [17–23], fiber-based structures have attracted more attention and various types of such structures have been so far proposed [21–23]. Due to the structural flexibility and also the adaptability to a variety of technical requirements, the fibers can be used in different layouts for guiding the waves through a confined direction without the possibility of coupling with other devices [17]. In addition to decreasing the propagation loss, utilizing these terahertz waveguides would be accompanied by birefringence increase, which could be of interest for specific applications such as filtering [24], sensing [25], interferometry [26], and coherent communication [27]. So far, various polymer fibers, including plastic fibers [21], Bragg fibers [22], and photonic crystal fibers (PCF) [23], have been employed for terahertz guidance.
Photonic crystal fibers have been well studied and developed as suitable and flexible structures with relatively low propagation loss over the terahertz frequency range [17]. PCFs with subwavelength discontinuities stretching along the z-axis [28] provide two general light-guiding mechanisms based on total internal reflection and the photonic bandgap (PBG). When the refractive index of the core is large with respect to the cladding, the PC-PCF confines the optical signal in the core by utilizing a modified total internal reflection technique [29]. On the other hand, when the refractive index of the core is lower than cladding, PC-PCF confines the optical signal in the core by utilizing PBG effects [30]. Numerous designs have been proposed by using a solid dielectric material as the core and then inserting appropriately spaced air holes in the clad to impose a relative refractive index difference between the clad and the core and establish the initial condition for wave guidance [31]. Many works have focused on engineering the air filling ratio to minimize the effective material loss (EML) as well as the confinement loss to improve the transmission efficiency [32,33]. The polarization maintenance as another interesting feature of photonic crystal fibers has been also studied in structures with a broken symmetry obtained by applying a disruption in continuity of refractive index along one axis comparing to the other axis of the core or the clad [23]. This technique has been successfully employed in various materials and structures [34–36]. Ren et al. reported an air-core polarization-maintaining rectangular PCF with a square lattice of subwavelength air holes providing a birefringence in the order of $10^{-3}$ and a very high EML [37]. The earliest contribution in highly birefringent THz fibers was made by Atakaramians et al., who developed the idea of using a slotted core to break the symmetry of the design to obtain a birefringence of $2.6 \times 10^{-2}$ with a loss below $0.06\,cm^{-1}$ in the range 0.5$-$1 THz. Chen et al. [32] introduced a squeezed lattice with elliptical air holes to break the symmetry of the fiber and present a birefringence of $4.5 \times 10^{-2}$. A porous fiber with radially distributed elliptical air-holes was [38] proposed with a birefringence as high as $4.45 \times 10^{-2}$ at a wide frequency range from 0.73 to 1.22 THz. All of these groups used air claddings and only used core geometry to break the symmetry of the design. More recently, researchers have focused on breaking the symmetry of both the core and cladding geometry to achieve higher birefringence at lower transmission loss. Wang et al. [39] have used a suspended solid elliptical with a pair of struts to achieve a high birefringence of $6.23 \times 10^{-2}$ at 1THz. Utilizing suspended cores for reducing the loss has been confirmed in designing the polarization splitters [40–42]. A high birefringence of $7.5 \times 10^{-2}$ with an effective absorption loss of $0.07\,cm^{-1}$ has been demonstrated in a slotted core design inside a cladding formed with a circular array of air holes [34]. Moreover, because of the convenience in fabrication, circular designs are also highly regarded. Hasan et al. [43] have used a vertically aligned circular unit in the core inside a cladding formed with spiral arms of circular rings to realize a birefringence of $4.83 \times 10^{-2}$. By employing a kagome lattice as the cladding with a slotted core a birefringence of $8.22 \times 10^{-2}$ and a loss of $0.054\,cm^{-1}$ at 1 THz has been also realized [43]. As an advanced PC-PCF, a suspended core photonic crystal fiber has been introduced in [44]. More recently, Sadath et al. [45] have recently investigated the effect of the core’s shape on PCF performance in terms of the birefringence and the loss. By comparing various shapes, they have concluded that fibers with elliptical suspended porous core exhibit a higher birefringence of $1.043 \times 10^{-1}$. Their results are in agreement with those previously supported by many groups. This type of design has high porosity in cladding and confines more light to the core, which may result in low loss and very low dispersion.
Among various materials used in fabrication of such structures, TOPAS with an absorption coefficient of 0.2 $cm^{-1}$ at frequency of 1 THz has attracted more attention [40]. By employing TOPAS in a recently proposed structure, a birefringence of 0.0973 and an effective material loss of 0.056 $cm^{-1}$ at frequency of 1 THz have been reported [22]. Faisal et al. have also reported an ultrahigh birefringence of 0.1057 obtained by a TOPAS based porous core photonic crystal structure for terahertz wave guidance at 1 THz with an ultralow material loss and confinement loss of 0.047 $cm^{-1}$ and $9.4\times 10^{-3}\,cm^{-1}$, respectively [23]. Yang et al. have shown that the use of high resistivity silicon as the based material significantly increases the birefringence to 0.82 with a very low total loss of 0.011 $cm^{-1}$ at 1 THz [17]. In this paper, we propose a precisely designed fiber structure with a suspended and porous elliptical core to explore the optimal transmission at the terahertz frequency range. By employing TOPAS in this structure we could relatively improve the birefringences to 0.123 at the frequency of 1.4 THz comparing to the other TOPAS base structures [3,4,13,14,16]. Moreover, silicon nitride $(Si_{3}N_{4})$ as an alternative material with remarkable properties is embedded in the proposed structure. Silicon nitride has unique properties including the constant refractive index of 2.7 in the range of 0.1 to 4 THz, very low absorption coefficient (< 0.01 $cm^{-1}$) in the range of 0.1 to 1.1 THz, low material dispersion, biocompatibility, low density, and considerable thermal, mechanical, and chemical features [10,46], which make it a suitable candidate for terahertz transmission purposes. The simulation results reveal that applying silicon nitride as the base material in the proposed structure could significantly enhance the features and lead to birefringence as high as 0.89 that is accompanied by a very low effective material loss of $10^{-3}\,cm^{-1}$ and a confinement loss of $10^{-4}\,cm^{-1}$ at the frequency of 0.6 THz.
2. Structure and design
The cross-section of the proposed design for the photonic crystal fiber is shown in Fig. 1. A suspended slotted elliptical core, with the major axis length and the minor axis length defined as $2a$ and $2b$, is formed by introducing five horizontally rectangular slotted air holes. The radius of the overall structure is considered to be 660 $\mu$m. The core is suspended by a pair of horizontal strut with thickness of $\Lambda _{1}=4d$ and four pairs tilted struts with a thickness of $\Lambda _{2}=2d$ of silicon nitride where $d=0.04p$, where $p$ is a constant parameter we use to normalize all the parameters of the designed structure. Moreover, twelve diagonal struts with thickness of $\Lambda _{2}$ are employed in four directions in order to break the structural symmetry, which is a prerequisite for high birefringence, and provide the clad design. Such structure could be implemented based on recently developed techniques such as extrusion method and 3D printing [22–24]. We have explored various possible structures with different dimensions aiming at two main goals i.e., high birefringence and polarization maintenance. Therefore, five pairs of struts that support the elliptical core inside, the tilted struts, and twelve diagonal struts with the thickness of $\Lambda _{2}$ in four directions are employed in the cladding area to assist the breaking of geometric symmetry of the design. Five slotted rectangular air holes with the identical thickness of $w$ are embedded inside the core at different distances of $L_{1}=0.48b$ and $L_{2}=0.85b$ from the central air slot, and consequently with different corresponding lengths. The purpose of these air slots is to break the structural symmetry and increase the birefringence. In the design that is presented based on silicon nitride, the optimum value of the $p$ parameter is obtained to be 73 $\mu$m. We have also utilized a perfectly matched layer (PML) with a thickness of 10$\%$ of the fiber radius as a non-absorbent boundary condition outside the cladding area.
As another determinative parameter, we compare the effective refractive indices on both orthogonal polarization modes in x and y directions as $n_{\mathrm {eff}}^{y}$ and $n_{\mathrm {eff}}^{x}$, respectively, to calculate the birefringence (B) as [47]
Besides the high birefringence, the proposed structure is examined in terms of the power absorption by controlling the effective material loss (EML), which is obtained as [48]
Fig. 2. The power flow distributions for different core porosities and major-axis lengths at frequency of 1 THz for the $Si_{3}N_{4}$-based PCFs. The arrows represent the electric field vector.
3. Results and discussion
To design the structure and simulate the associated propagation modes we have employed a simulation algorithms based on the finite element method. To solve Maxwell’s equations for the proposed design we have used the finite element method (FEM) based software COMSOL Multiphysics to simulate the associated propagation modes. Figure 2 depicts the power flow spatial distributions in the central regions of the PCF based on $Si_{3}N_{4}$ for the x and y polarization modes at the frequency of 1 THz. The results have been compared in this figure for various values of $a$ and $w$ parameters. Porosity, as the ratio of the air area to the entire core area, is controlled by changing the thickness of the air slot ($w$). It can be seen that the X-polarized fields are confined to the core more strongly compared to that with Y polarization.
By using Eq. (1), we explore among various dimensional parameters to approach the conditions with the highest birefringence. Among them, we have reached a minor axis with length of $b = 0.55p$ as the optimum value for the proposed elliptical-core structure. The variations of the birefringence versus the frequency obtained for three elliptical-core PCFs with minor axis fixed at optimal length but major axes with different lengths of $a =3.2p$, $2.2p$, and $1.5p$ are presented in Fig. 3. It can be seen that a very high birefringence of 0.88 is obtained by elliptical axes of $a=3.2p$ and $b=0.55p$ at a frequency of 0.65 THz. According to this figure, one could expect the present design to yield lower birefringence at frequencies above 0.8 THz, regardless of the dimensional parameters. To confirm that the length of the minor axis is optimal, we have further compared the variations of the birefringence versus the frequency for different minor axis lengths of $b=0.45p$, $0.5p$, $0.55p$, and $0.6p$, when the major axis length is kept fixed at $a=3.2$. As shown in Fig. 4, the birefringence could be spectrally tuned at almost comparable values by changing the minor axis length. Although a relatively higher birefringence of 0.89 is obtained by $b=0.6p$ at the frequency of 0.6 THz, when compared with that by $b=0.55p$, our further calculations reveal that it would be suppressed due to the higher propagation loss at this frequency. Thus, we present the results in the following for a minor axis length of $b=0.55p$. Figure 4 also reveals that the frequency with the maximum birefringence increases when the shorter minor axis are applied in the structure.
Fig. 3. Birefringence versus frequency for the core with a fixed minor axis length of $b=0.55p$ and different major axis lengths.
Fig. 4. Frequency birefringence for sub-diameters different from the core with a major diameter fixed at $a=3.2p$.
Moreover, by adjusting the core diameters at their optimum values, we examine how the thickness of the core’s air slots ($w$) can affect the birefringence of the proposed structure. As shown in Fig. 5, by increasing the thickness of the air slot, and consequently increasing the porosity in the fiber core, the birefringence tends to follow a slower spectral dependence with a maximum obtained at relatively higher frequencies. Moreover, the maximum birefringence of 0.88 is obtained at 0.65 THz when the thickness of the core’s air slot is adjusted at $0.8d$.
By using Eq. (2), integrating over the area filled by the material in the numerator and over the entire PCF cross-section in the denominator, the variation of effective material losses versus the frequency is obtained for different core minor-major axis lengths. It is clear from Fig. 6 that the effective material loss depends on the core major axis, especially at higher frequencies and regardless of the polarization mode. This could be simply attributed to the increase of the material interaction area and implies how the appropriate choice of the material could be essential for minimizing the propagation loss. This figure indicates that effective material losses in the case with silicon nitride as the base material would be too low in the order of $10^{-4}\,cm^{-1}$ to $10^{-2}\,cm^{-1}$ in the range of 0.5 to 1.1 THz for both polarization modes.
Fig. 6. Effective material loss versus the frequency for different core major axes at both polarization modes.
In addition, calculations on confinement loss (CL) of the structure by using Eq. (3) reveal that the confinement loss is negligible in order of $10^{-2}$ to $10^{-11}\,cm^{-1}$ over the frequency range of 0.5 to 1.1 THz for both polarization modes (Fig. 7). Finally, we have quantitatively compared the mode area covered by the main fiber mode in transverse dimensions at different frequencies. For such a purpose, we have calculated the product of beam waist across transverse directions, $w_{opt}^{x} w_{opt}^{y}$, that is proportional to the transverse mode area. The values of $w_{opt}^{x}$ and $w_{opt}^{y}$ are obtained according to those presented in Fig. 2. As illustrated in Fig. 8, it can be seen that the mode area decreases with increasing frequency because more light is limited in the core area.
To investigate to what extent the material of silicon nitride contributes to the obtained results, we examine the features of the presented structure by replacing it with TOPAS that is commonly used in previous studies. In these calculations, the core dimensions are considered as $a=2.2p$ and $b = 0.55p$ in order to achieve optimal birefringence and losses. According to the diagrams presented in Fig. 9, a birefringence of 0.123 could be obtained at frequency of 1.4 THz, which is higher than those previously obtained in the structures based on Topas. Comparing to the results obtained in the case of silicon nitride as the base material, one may conclude that the improvement in birefringence could be mainly attributed to the properties of silicon nitride. This result also shows that the proposed structure could also reduce the total propagation loss. The total loss obtained for x and y polarization modes are 0.029$cm^{-1}$ and 0.105$cm^{-1}$, respectively, which is lower than those obtained by Topas in other PCF structures. We further discuss the coupling efficiency versus terahertz frequency for both x and y polarization. For this purpose, we employ a parameter recently proposed [44,50] based on overlapping integral between the input field and the core-guided mode field. The results are illustrated in Fig. 10. It can be seen that the coupling efficiency is relatively smaller for y-polarization compared to those of x-polarization, which is due to the more confined field distribution at y-polarization. Finally, we summarize the best values obtained for the proposed structure in Table 1.
Fig. 9. Birefringence and total loss versus terahertz frequency when TOPAS is embedded as base material.
Table 1. Parameters obtained by the proposed structure for terahertz wave guidance. (P: polarization, CL: Confinement loss, EML: effective material loss, TL: Total loss, BR:Birefringence)
4. Conclusion
We have presented a structure of porous core photonic crystal fiber with a suspended and porous elliptical core for the efficient transmission of terahertz waves. The simulation results have shown that the proposed structure could improve the birefringence and reduce the propagation loss. When Topas is utilized as the base material, the proposed PCF could provide a high birefringence of 0.123 at the frequency of 1.4 THz, with a total loss of 0.2$cm^{-1}$. Moreover, a significant birefringence of 0.89 at the frequency of 0.6 THz, with a confinement loss of $10^{-4}\,cm^{-1}$ and effective material losses of $10^{-3}\,cm^{-1}$ have been provided when the alternative material of silicon nitride is introduced to the structure.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. H. Hafez, X. Chai, A. Ibrahim, S. Mondal, D. Férachou, X. Ropagnol, and T. Ozaki, “Intense terahertz radiation and their applications,” J. Opt. 18(9), 093004 (2016). [CrossRef]
2. R. Gill, S. Punia, and H. K. Malik, “Terahertz radiation for medical application,” EPL (Europhysics Letters) 123(6), 65003 (2018). [CrossRef]
3. T. Nagatsuma, G. Ducournau, and C. C. Renaud, “Advances in terahertz communications accelerated by photonics,” Nat. Photonics 10(6), 371–379 (2016). [CrossRef]
4. J. Zhang and D. Grischkowsky, “Waveguide terahertz time-domain spectroscopy of nanometer water layers,” Opt. Lett. 29(14), 1617–1619 (2004). [CrossRef]
5. E. Pickwell and V. Wallace, “Biomedical applications of terahertz technology,” J. Phys. D: Appl. Phys. 39(17), R301–R310 (2006). [CrossRef]
6. R. A. Lewis, “A review of terahertz sources,” J. Phys. D: Appl. Phys. 47(37), 374001 (2014). [CrossRef]
7. J. A. Fülöp, S. Tzortzakis, and T. Kampfrath, “Laser-driven strong-field terahertz sources,” Adv. Opt. Mater. 8(3), 1900681 (2020). [CrossRef]
8. T. Amini and F. Jahangiri, “Regenerative thz parametric amplifier based on four-wave mixing in asynchronously pumped graphene oxide integrated topas,” Optics Express pp. 10.1364/OE.430297–(in press) (2021).
9. K. Zhong, W. Shi, D. Xu, P. Liu, Y. Wang, J. Mei, C. Yan, S. Fu, and J. Yao, “Optically pumped terahertz sources,” Sci. China Technol. Sci. 60(12), 1801–1818 (2017). [CrossRef]
10. T. Amini and F. Jahangiri, “Optimized design for a terahertz parametric oscillator based on degenerate four-wave mixing in silicon nitride,” J. Opt. Soc. Am. B 37(9), 2725–2731 (2020). [CrossRef]
11. F. Jahangiri, M. Hashida, S. Tokita, T. Nagashima, K. Ohtani, M. Hangyo, and S. Sakabe, “Directional terahertz emission from air plasma generated by linearly polarized intense femtosecond laser pulses,” Appl. Phys. Express 5(2), 026201 (2012). [CrossRef]
12. M. Tamosiunaite, S. Tamosiunas, M. Zilinskas, and G. Valusis, “Atmospheric attenuation of the terahertz wireless networks,” Broadband Communications Networks-Recent Advances and Lessons from Practice pp. 143–156 (2017).
13. W. Yuan, L. Khan, D. J. Webb, K. Kalli, H. K. Rasmussen, A. Stefani, and O. Bang, “Humidity insensitive topas polymer fiber bragg grating sensor,” Opt. Express 19(20), 19731–19739 (2011). [CrossRef]
14. J. Sultana, M. S. Islam, M. Islam, and D. Abbott, “High numerical aperture, highly birefringent novel photonic crystal fibre for medical imaging applications,” Electron. Lett. 54(2), 61–62 (2018). [CrossRef]
15. F. Cunin, T. A. Schmedake, J. R. Link, Y. Y. Li, J. Koh, S. N. Bhatia, and M. J. Sailor, “Biomolecular screening with encoded porous-silicon photonic crystals,” Nat. Mater. 1(1), 39–41 (2002). [CrossRef]
16. G. Emiliyanov, J. B. Jensen, O. Bang, P. E. Hoiby, L. H. Pedersen, E. M. Kjaer, and L. Lindvold, “Localized biosensing with topas microstructured polymer optical fiber,” Opt. Lett. 32(5), 460–462 (2007). [CrossRef]
17. T. Yang, C. Ding, R. W. Ziolkowski, and Y. J. Guo, “A scalable thz photonic crystal fiber with partially-slotted core that exhibits improved birefringence and reduced loss,” J. Lightwave Technol. 36(16), 3408–3417 (2018). [CrossRef]
18. B. Bowden, J. A. Harrington, and O. Mitrofanov, “Silver/polystyrene-coated hollow glass waveguides for the transmission of terahertz radiation,” Opt. Lett. 32(20), 2945–2947 (2007). [CrossRef]
19. H. Han, H. Park, M. Cho, and J. Kim, “Terahertz pulse propagation in a plastic photonic crystal fiber,” Appl. Phys. Lett. 80(15), 2634–2636 (2002). [CrossRef]
20. J. Li, K. Nallappan, H. Guerboukha, and M. Skorobogatiy, “3d printed hollow core terahertz bragg waveguides with defect layers for surface sensing applications,” Opt. Express 25(4), 4126–4144 (2017). [CrossRef]
21. T. Yang, E. Wang, H. Jiang, Z. Hu, and K. Xie, “High birefringence photonic crystal fiber with high nonlinearity and low confinement loss,” Opt. Express 23(7), 8329–8337 (2015). [CrossRef]
22. M. S. Islam, M. Faisal, and S. A. Razzak, “Dispersion flattened extremely high-birefringent kagome lattice elliptic core photonic crystal fiber in thz regime,” Opt. Quantum Electron. 51(1), 35 (2019). [CrossRef]
23. M. Faisal and M. S. Islam, “Extremely high birefringent terahertz fiber using a suspended elliptic core with slotted airholes,” Appl. Opt. 57(13), 3340–3347 (2018). [CrossRef]
24. Y. Song, L. Zhan, S. Hu, Q. Ye, and Y. Xia, “Tunable multiwavelength brillouin-erbium fiber laser with a polarization-maintaining fiber sagnac loop filter,” IEEE Photonics Technol. Lett. 16(9), 2015–2017 (2004). [CrossRef]
25. H. Fu, H. Y. Tam, L.-Y. Shao, X. Dong, P. K. A. Wai, C. Lu, and S. K. Khijwania, “Pressure sensor realized with polarization-maintaining photonic crystal fiber-based sagnac interferometer,” Appl. Opt. 47(15), 2835–2839 (2008). [CrossRef]
26. D.-H. Kim and J. U. Kang, “Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity,” Opt. Express 12(19), 4490–4495 (2004). [CrossRef]
27. K. Wang and D. M. Mittleman, “Metal wires for terahertz wave guiding,” Nature 432(7015), 376–379 (2004). [CrossRef]
28. I. Yakasai, P. E. Abas, S. Kaijage, and F. Begum, “Novel hrs-based porous core photonic crystal fibre for terahertz wave guidance,” International Journal of Electrical and Electronic Engineering & Telecommunications 9(1), 1 (2020). [CrossRef]
29. S. Olyaee and A. Naraghi, “Design and optimization of index-guiding photonic crystal fiber gas sensor,” Photonic Sens. 3(2), 131–136 (2013). [CrossRef]
30. F. Benabid, J. C. Knight, G. Antonopoulos, and P. S. J. Russell, “Stimulated raman scattering in hydrogen-filled hollow-core photonic crystal fiber,” Science 298(5592), 399–402 (2002). [CrossRef]
31. P. S. J. Russell, “Photonic-crystal fibers,” J. Lightwave Technol. 24(12), 4729–4749 (2006). [CrossRef]
32. S. Atakaramians, S. Afshar, H. Ebendorff-Heidepriem, M. Nagel, B. M. Fischer, D. Abbott, and T. M. Monro, “Thz porous fibers: design, fabrication and experimental characterization,” Opt. Express 17(16), 14053–14062 (2009). [CrossRef]
33. S. F. Kaijage, Z. Ouyang, and X. Jin, “Porous-core photonic crystal fiber for low loss terahertz wave guiding,” IEEE Photonics Technol. Lett. 25(15), 1454–1457 (2013). [CrossRef]
34. R. Islam, M. S. Habib, G. Hasanuzzaman, R. Ahmad, S. Rana, and S. F. Kaijage, “Extremely high-birefringent asymmetric slotted-core photonic crystal fiber in thz regime,” IEEE Photonics Technol. Lett. 27(21), 2222–2225 (2015). [CrossRef]
35. H. Chen, D. Chen, and Z. Hong, “Squeezed lattice elliptical-hole terahertz fiber with high birefringence,” Appl. Opt. 48(20), 3943–3947 (2009). [CrossRef]
36. M. S. Islam, J. Sultana, A. Dinovitser, M. Faisal, M. R. Islam, B. W.-H. Ng, and D. Abbott, “Zeonex-based asymmetrical terahertz photonic crystal fiber for multichannel communication and polarization maintaining applications,” Appl. Opt. 57(4), 666–672 (2018). [CrossRef]
37. G. Ren, Y. Gong, P. Shum, X. Yu, J. Hu, G. Wang, M. O. L. Chuen, and V. Paulose, “Low-loss air-core polarization maintaining terahertz fiber,” Opt. Express 16(18), 13593–13598 (2008). [CrossRef]
38. N.-n. Chen, J. Liang, and L.-y. Ren, “High-birefringence, low-loss porous fiber for single-mode terahertz-wave guidance,” Appl. Opt. 52(21), 5297–5302 (2013). [CrossRef]
39. J. Wang, H. Chen, and W. Shi, “A high birefringent polymer terahertz waveguide: suspended elliptical core fiber,” J. Opt. Soc. Korea 18(5), 453–458 (2014). [CrossRef]
40. Y.-F. Zhu, M.-Y. Chen, H. Wang, H.-B. Yao, Y.-K. Zhang, and J.-C. Yang, “Design and analysis of a low-loss suspended core terahertz fiber and its application to polarization splitter,” IEEE Photonics J. 5(6), 7101410 (2013). [CrossRef]
41. X. Jiang, D. Chen, and G. Hu, “Suspended hollow core fiber for terahertz wave guiding,” Appl. Opt. 52(4), 770–774 (2013). [CrossRef]
42. M. Roze, B. Ung, A. Mazhorova, M. Walther, and M. Skorobogatiy, “Suspended core subwavelength fibers: towards practical designs for low-loss terahertz guidance,” Opt. Express 19(10), 9127–9138 (2011). [CrossRef]
43. M. R. Hasan, M. S. Anower, M. A. Islam, and S. Razzak, “Polarization-maintaining low-loss porous-core spiral photonic crystal fiber for terahertz wave guidance,” Appl. Opt. 55(15), 4145–4152 (2016). [CrossRef]
44. J. Luo, S. Chen, H. Qu, Z. Su, L. Li, and F. Tian, “Highly birefringent single-mode suspended-core fiber in terahertz regime,” J. Lightwave Technol. 36(16), 3242–3248 (2018). [CrossRef]
45. M. A. Sadath, M. M. Rahman, M. S. Islam, M. S. Hossain, and M. Faisal, “Design optimization of suspended core photonic crystal fiber for polarization maintaining applications,” Opt. Fiber Technol. 65, 102613 (2021). [CrossRef]
46. E. D. Palik, Handbook of optical constants of solids, vol. 3 (Academic press, 1998).
47. K. Ahmed, S. Chowdhury, B. K. Paul, M. S. Islam, S. Sen, M. I. Islam, and S. Asaduzzaman, “Ultrahigh birefringence, ultralow material loss porous core single-mode fiber for terahertz wave guidance,” Appl. Opt. 56(12), 3477–3483 (2017). [CrossRef]
48. J. Luo, F. Tian, H. Qu, L. Li, J. Zhang, X. Yang, and L. Yuan, “Design and numerical analysis of a thz square porous-core photonic crystal fiber for low flattened dispersion, ultrahigh birefringence,” Appl. Opt. 56(24), 6993–7001 (2017). [CrossRef]
49. M. S. Islam, J. Sultana, J. Atai, M. R. Islam, and D. Abbott, “Design and characterization of a low-loss, dispersion-flattened photonic crystal fiber for terahertz wave propagation,” Optik 145, 398–406 (2017). [CrossRef]
50. L.-J. Chen, H.-W. Chen, T.-F. Kao, J.-Y. Lu, and C.-K. Sun, “Low-loss subwavelength plastic fiber for terahertz waveguiding,” Opt. Lett. 31(3), 308–310 (2006). [CrossRef]
