## 1 Introduction  
The terahertz (THz) frequency region lies midway between the microwave and the far-infrared wave in the electromagnetic frequency spectrum [1, 2], ranging from 0.1 to 10 THz. In recent years, the focus on THz has increased because of its numerous potential applications, including broadband short-distance data transmission, imaging, sensing, defense and security screening, biomedical technology, and spectroscopy [3, 4, 5, 6, 7, 8, 9, 10]. However, THz is still in the development phase because the THz waveguides are bulky and only propagate in the vacuum, and the THz sources and detectors remain primitive compared with their optics counterparts in the other frequency region. In order to address these issues, various THz waveguides have been proposed. Various THz waveguides have been recently reported, including polymer fibers [11, 12, 13, 14, 15, 16, 17], metal tube waveguides [18], metal wire waveguides [19], and so on. The polymer fibers can achieve low loss and good flexibility. The types of THz fibers include solid-core fibers [16], hollow-core fibers [11, 12, 13, 14, 15], and porous-core fibers [17]. Among them, the hollow-core fibers have attracted a lot of research interest due to the low loss and easy inner modification with functional materials. One of its operating principles is based on photonic band gap guidance [20], and they are used in air propagation with low loss owing to the strong confinement of the mode in the core area [21, 22]. Another is an antiresonant effect. The hollow-core Bragg fibers, as one of the photonic band gap fibers, provide many advantages including flexible design, easy integration of microfluidic channels directly into the fiber cladding, and a wide operation range from far-infrared to THz compared to the other devices. The fibers, filled with functional materials, supply the chance to enhance the properties of the fibers, attain tunable fibers and related devices, which have been successfully developed in recent years. An argon gas-filled hollow-core fiber was designed as a pulse shaper to generate broadband and tunable time-domain spectroscopy in the THz by adjusting the argon gas pressure [11]. A fiber filled with methanol (CH\({}_{3}\)OH) gas in the gas laser was used as the reaction cell to shrink the volume of THz sources [12]. A THz fiber selectively filled with Potassium Chloride (KCL) was reported as Epsilon-Near-Zero (ENZ) material to ensure near-zero flattened dispersion [13]. The \(\alpha\)-lactose monohydrate powder was filled in the THz hollow-core Bragg fiber as resonant surface bio-sensor [14]. A THz hollow-core Bragg fiber with microfluidic channel was demonstrated for non-contact monitoring refractive index (RI) changes in flowing liquids (oil) [15]. Liquid water is one of the most common and easily available materials in nature compared to the aforementioned materials, and it can perform as a very good candidate for THz devices. Adjacent water molecules form a tetrahedral arrangement of hydrogen bonds. The typical time scales related to the relaxation dynamics of the hydrogen-bonded network of water molecules rotated from or displaced their equilibrium position falls in the picosecond to sub-picosecond [23]. Thereby, liquid water has a strong absorption characteristic in the THz frequency region. Further, THz waves, generated from liquid water, have recently been studied [24; 25; 26]. In addition, the optical characteristics of liquid water can be affected by the thermal relaxation and resonant process. Both the dipolar orientational and the translational stretch of the hydrogen band could result in absorbance [27]. Therefore, the complex dielectric constant of liquid water could be controlled by temperature. Thus, THz fibers filled with liquid water turn to be a choice for THz generation and tunable devices. In this paper, a hollow-core THz Bragg fiber with a defect layer in the cladding region is proposed. Liquid water of different temperatures is filled in the defect layer.