Photonic Crystal Fibers in the Terahertz Regime: Design Strategies, Performance Metrics, and Emerging Applications: A Systematic Review

This chapter deals with terahertz (THz) sensor-based photonic crystal fiber (PCF) and its various useful applications in multiple domain such as chemical, gas, biomedical and environmental. In the visible and infrared areas of optical fiber sensor, the probing length of evanescent fields is sub wavelength, limiting its use for identification of bigger targets. Because THz signal has a higher wavelength and THz sensors could be fit for a variety of sensing applications. The THz radiation (0.1–10 THz) has a huge potential in a wide application domain like sensing, biomedical industry, astronomy, medical imaging, security, agricultural industry, etc. Due to their adaptive design flexibility and wide range of applications in the THz spectrum, PCFs have emerged as an appealing alternative to electronics technology in this regard. Owing to the recent revolution in biomedical industry, nowadays the scientists are more focused in designing optical biosensors, where the PCF-based sensors are the most researched devices and almost controlling the modern photonic industry. PCFs-based sensors in THz regime have been extensively investigated owing to their high birefringence, ultra-high sensitivity, effective mode area, ultra-high sensitivity and minimum loss. With the advancement in technology, different hollow core and porous core PCFs have been successfully fabricated with extensive analysis of propagation characteristics and tested for various sensing applications like blood, gas, salinity, DNA, alcohol, liquid, food, hormones, enzymes, cells, urine, glucose, chemicals, etc. From the numerical analysis, the suitable background materials such as Teflon, Zeonex, PMMA and TOPAS are used for operation in THz region due to their high transparency in THz spectrum and feeble loss coefficient. Recently, a more effective THz sensors are envisaged through plasmonic principle by integrating novel metals and transition-metal dichalcogenide (TMDC) materials in the design of PCFs. In this chapter, different geometries of PCF and their light guiding mechanisms are thoroughly studied. Moreover, an extensive investigation is carried out vis-à-vis numerous sensing applications of PCF in THz spectrum, and their sensing performances are compared.KeywordsPhotonic crystal fiberTHz sensorChemical sensor and Biosensor

A novel kind of high birefringent terahertz (THz) photonic crystal fibers (PCFs) with material-filled structure is proposed in this paper. Based on the material-filled technology, which different materials are selectively filled into four air holes of the inner first circle near the central core in the designed THz PCFs, high birefringence are obtained from the structural and material-filled induced asymmetry in large frequency ranges near 1THz. Modal birefringence with different structural parameters and diverse refractive indices of the filled materials are investigated by plane wave expansion (PWE) method. The numerical results show that high birefringence up to 10-3can be obtained and its structure is simpler than that of the early proposed highly birefringent THz PCFs. It is helpful for PCFs design and real fabrication in the potential THz applications.

In this paper, we propose a novel high birefringent terahertz (THz) photonic crystal fiber (PCF) with subwavelength circular air hole pairs in the core which are arranged as a hybrid crystal lattice structure. And its high mode birefringence is realized by reducing structure symmetry in core. A professional software COMSOL Multiphysics 4.0 is used for modeling, and the simulation results show that this kind of THz PCF exhibits a high birefringence on a level of 10-2and low confinement loss over a wide THz frequency range. Moreover, the birefringence or confinement loss can be controlled flexibly by adjusting some fiber parameters. Compared with a similar structure PCF for the optical communication, the THz PCF is easier to practically fabricate, owing to its large wavelength size.

A novel terahertz (THz) photonic crystal fiber (PCF) that yields single-polarization single-mode (SPSM) propagation over an ultra-wide bandwidth is designed and analyzed. The PCF is based upon a triangle-based lattice of air holes in a high resistivity silicon substrate with three selectively-filled rectangular slots introduced into the core area. Four air holes surrounding the core region are chosen to be loaded with an epsilon-near-zero (ENZ) material. The configuration, and the large loss of the ENZ material establish a large loss difference (LD) between the two fundamental propagating polarization modes, and any higher order modes. When the central slot of the three in the core is filled with a gain material, and the adjacent two slots are air-filled, the LD values between the one desired propagating mode, and all other modes are significantly enhanced. Consequently, essentially only the desired mode will exist in the PCF after a short propagation distance resulting in the SPSM behavior. The optimized design provides large LD values, greater than 9.4 dB/cm, over a SPSM spectrum of 0.64 THz (from 1.10 to 1.74 THz), which, to the best of our knowledge, is the widest SPSM bandwidth achieved to date in the THz regime. The unwanted modes are 30 dB smaller than the wanted mode after a 3.2 cm length of the PCF. This outcome is highly desired for polarization sensitive THz communications, and sensor systems that rely on waveguiding structures.

Design and performance analysis of background material of zeonex based high core power fraction and extremely low effective material loss of photonic crystal fiber in the terahertz (THz) wave pulse for many types of communication areas

A type of high-birefringent terahertz (THz) photonic crystal fiber (PCF) with all circle air holes is proposed. The characteristics including birefringence, dispersion, and confinement loss are numerically analyzed in detail by using the finite element methods. Simulation results show that the proposed THz PCFs exhibit high birefringence on the level of 10−2 in the frequency range of 2 to 4 THz, which is realized by the minor position adjustment of air holes in the first ring of the cladding. We believe that the proposed THz PCFs can be fabricated without complications due to their simple structure. In addition, two porous-core THz PCFs are proposed and the birefringence property is investigated.

The technologies emerging from Terahertz (THz) research have attracted many previously unexplored areas, such as the characterization of negative index materials. Recently, the progress with THz waveguides results in potential of not only flexible THz guiding but also for THz sensing. The variety of THz waveguides made of metals [1] and dielectrics as well as THz photonic crystal fibers (PCFs) [2,3] reported low-loss, low-dispersion THz waveguiding characteristics. We present a new design of THz fiber with metal wires inclusion that enables low loss and low dispersion THz guidance in an essentially air-core region. The fibers are constructed from the PCF fabrication technology, eliminating altogether the mounting complexity of the bare metal wires [4].

A novel kind of terahertz (THz) photonic crystal fibers (PCFs) based on the material-filled structure is proposed in this paper. Different materials can be selectively filled into parts of air-holes in the designed THz PCFs, and then perfect features such as single-mode transmission and ultra-flattened dispersion are obtained easily in large frequency ranges near 1THz. Employing full-vectorial finite element method (FEM) and plane wave expansion method (PWE), confinement losses and modal dispersion with different structural parameters and diverse refractive indices of the filled materials are investigated respectively. Numerical results show that the proposed PCFs have acceptable confinement losses, low and flattened dispersions whose absolute values are lower than 1ps/nm*km. Moreover, its structure is simple and its feature is insensitive to variations of parameters. It is helpful for PCFs design and real fabrication in the potential THz applications.

A terahertz (THz) photonic crystal fiber (PCF) with an ultra-wide bandwidth and single-polarization-single-mode (SPSM) operation is designed and analyzed. Two air slots are introduced in the core region and epsilon-near-zero (ENZ) material is deposited in four specific air holes in the cladding of the PCF. The design achieves significantly different electric (E)-field distributions of the X-polarized (XP) and Y-polarized (YP) modes. The E-field components overlapping the ENZ material are attenuated because it is lossy. Gain material is then deposited in a rectangular slot in the core center to provide amplification of the E-field components overlapping this gain region. Changing the dimensions of the PCF modifies the amplification and attenuation rates to the wanted XP mode, the unwanted YP mode, and any unwanted higher order (HO) modes. The amplification of the wanted mode and the attenuation of the unwanted modes are maximized through optimization. The result is a PCF with an ultra-wide SPSM spectrum of 0.53 THz, from 1.00 to 1.53 THz. The minimum loss difference (MLD) across this bandwidth between the wanted mode and any unwanted modes is over 7.4 dB/cm. To the best of our knowledge, this is the widest SPSM bandwidth of a PCF fiber reported in THz regime.

Liquid crystal (LC) is an excellent tunable functional material which can be controlled by the external stimulus such as electric field, magnetic field and temperature. Terahertz (THz) radiation in a frequency range of 0.1−10.0 THz, has enormous advantages such as a low photon energy, sensitivity to crystal lattice vibration, magnetic spins, hydrogen bonds, intermolecular interaction, and water, and high transparency to non-conducting materials. The THz technology, therefore, has great potential in a diverse range of applications from spectroscopy, security screening to biomedical technology and high-speed wireless communication. But the development of high-performance LC based tunable THz functional devices is still in its infancy stage. The dispersion of LC refractive index induces a comparatively low birefringence in the THz regime. The lack of transparent electrodes makes the electric tuning of LCs difficult to achieve. To achieve certain modulations requires a very thick THz layer, leading to several disadvantages such as high operating voltage, slow response and poor pre-alignment. In this paper, we first present the research progress of large birefringence LCs in THz range. A room-temperature nematic LC NJU-LDn-4 with an average birefringence greater than 0.3 in a frequency range from 0.5 to 2.5 THz is shown in detail. This kind of LC can remarkably reduce the required cell gap, thus reducing the operating voltage and response time. Then we summarize varieties of conventional THz devices based on LC. Many electrodes are used for THz range. Graphene which can be used as a perfect transparent electrode material in THz band is proposed. Not only tunable transmissive but also reflective THz waveplates are introduced. The thickness of the LC layer of the reflective one can be reduced to ~10% of that needed for the same phase shift at a given frequency in a transmissive waveplate. The same tunability as that in the transmissive type just needs half the thickness. We also introduce that LC can generate THz vortex beam based on a photopatterned large birefringence LC. In the area of LC based versatile THz metamaterial devices, the adjacent units of a metasurface layer, such as a fishnet or grating, are usually connected to each other which may cause low-quality (<i>Q</i>) factor and polarization sensitivity, which is undesirable. We emphasize a graphene-assisted high-efficiency tunable THz metamaterial absorber. Few-layer porous graphene is integrated onto the surface of a metasurface layer to provide a uniform static electric field to efficiently control the LC, thereby enabling flexible metamaterial designs. The THz far-field and near-field with large modulation and fast response are realized. A magnetically and electrically polarization-tunable terahertz emitter that integrates a ferromagnetic heterostructure and the large-birefringence liquid crystals is also demonstrated to be able to generate broadband THz radiation and control the polarization of THz waves perfectly as well as LC based THz reflectarray. Last but not least, a temperature-supersensitive cholesteric LC used for THz detection is shown. It can not only measure the beam profiles but also detect the power values of THz waves generated from a nonlinear crystal pumped by a table-top laser. Quantitative visualization based on not only the thermochromic but also the thermal diffusion effect, can be used conveniently and effectively at room temperature. In this review, we summarize the latest progress of liquid crystal materials and components in THz and discuss the possible prospects of the combination of liquid crystal technology and THz technology. We envision that LCs will play a unique role in THz sources, THz functional devices and THz detectors.

A modified single-polarization THz fiber with epsilon-near-zero (ENZ) material

A hollow-core hexagonal lattice terahertz (THz) photonic crystal fiber (PCF) is designed. The proposed THz PCF supports multi-pole modes spanning monopole-like, dipole-like, quadrupole-like, and hexapole-like modes. A low confinement loss of 0.087dB/cm with a high Q-factor on the order of 106 has been obtained for a hexapole-like (LP31) mode at 0.210THz. Moreover, a dipole-like (LP11) mode at 0.126THz yields a maximum relative sensitivity of 88.35% for an effective electromagnetic (EM) field-analyte interaction. By introducing a rotating dielectric septum inside the PCF, the dipole-mode conservation for various C6v symmetry rotational angles is demonstrated. Using the optomechanical mode rotation of the proposed PCF, a method to sense two different gases (methane and carbon dioxide) is presented.

A low loss and broadband photonic bandgap (PBG) terahertz photonic crystal fiber (PCF) with high birefingence is proposed. Terahertz wave is confined within the core surrounded by the cladding with triangular lattice arrangement of subwavelength air holes. The birefringence and the loss of the fiber are investigated by using a full-vectorial finite element method. The numercal simulation shows that within a broadband area of about 0.3THz, the loss of the near-rectangle core THz PBG photonic crystal fiber is less than 0.009cm-1, the phase birefringence is on the order of 10-3, and the group birefringence even can reach 10-2.

Stimulant abuse enhances dopamine release, thereby causing increased excitation. Any extent of stimulant abuse can considerably harm the user. Thus, methods of detecting stimulants must be precise, accurate, and reliable. A novel terahertz (THz) photonic crystal fiber with a Topas substrate is designed and rigorously investigated for detecting liquid amphetamine, cocaine, and ketamine. The fiber structure has a pentagonal shape and comprises circular air holes in the core and cladding spatial extents. As shown in finite element simulation, the proposed fiber yields a high relative sensitivity of approximately 80 % when any of the liquid stimulants is infiltrated in the core air holes. At 1 THz operating frequency, the proposed fiber produces a large effective mode area, negligible confinement loss, and extremely low bending and effective material losses. Other THz waveguiding properties, such as core power fraction and total loss, are also studied. Lastly, a positive and negative 2 % fabrication tolerance is set to ensure seamless potential practical realization of the fiber.

A novel hollow core Kagome lattice photonic crystal fiber(PCF) has been designed and analyzed for terahertz (THz) wave guidance by pumping Xenon (Xe) gas inside the hollow core of the PCF. Using finite element method (FEM), optical properties have been simulated and optimized with different strut width of Kagome lattice PCF. In addition, those properties have been optimized by changing Xe-gas pressure inside the hollow core in the THz regime from 1 to 10 THz. Simulation results show tightly mode field confinement of light through the hollow core for single mode operation within the band of frequency and the lowest EML of 5.18E-5 dB/cm has been found in operating frequency of 10 THz for strut width of 5 μm and the core diameter of 800 μm with constant pressure at 1000 bar. These losses have been further reduced up to 20% i.e. 4.16E-5 dB/cm, by increasing the pressure of Xe-gas to 3000 bar. Findings of this paper can help to make extremely low loss THz waveguide for potential beneficial biomedical applications.