## 1 Introduction  
More generally, the terahertz (THz) wave is part of the electromagnetic wave with a frequency range of 0.1-10 THz. Since it is positioned in the transition region of electronics and photonics, its unique characteristics have been found in different types of applications [1, 2]. Firstly, as compared with X-rays, the photon energies within this THz region are extremely low. It causes no damage to the living organism and has therefore promising application prospects in the field of biological biopsy [3, 4, 5]. Secondly, the wide spectral range of the THz waves can be used to characterize the composition of matter [6; 7]. Thirdly, the THz waves are highly penetrating for many substances, providing a different and more efficient solution for search and rescue [8; 9]. Based on these advantages of the THz waves, an increasing number of scientific publications can be found in the field of THz technology. Most of these research activities have focused on THz radiation sources, functional devices, and more practical applications [10; 11; 12]. Examples of THz functional devices are filters [13; 14; 15], sensors [16; 17], slow-light devices [18; 19], switches [20; 21], absorbers [22], and modulators [23; 24]. In recent years, THz sensors have been developed and used in a wide variety of fields. [PERSON] et al. reported on a broadband THz sensor, based on quantum-cascade laser arrays, which could realize the sensing of the refractive index of solids, or liquids, in a reflective geometry [25]. [PERSON] et al. used THz fiber Bragg gratings to implement the resonant THz sensor, and it was applied to monitor the quality of paper [26]. [PERSON] et al. proposed a different method for in situ measurement of coating thickness (on individual tablets) during film deposition, by employing a THz line sensor [27]. [PERSON] et al. presented an ultrasensitive THz sensor, which consisted of a sub-wavelength graphene disk and an annular gold ring within a unit cell. The numerical results showed that a high frequency sensitivity of 1.908 THz, per refractive index unit (RIU), was achieved [28]. [PERSON] et al. took advantage of a thin metal sheet, perforated to a periodic hole array, in constructing a THz surface plasmon sensor that could distinguish the chemical contents in gasoline [29]. In addition, [PERSON] et al. demonstrated a THz multichannel microfluidic sensor that was based on a parallel-plate waveguide with different resonant cavities (two in number). The experimental results showed that the sensitivities of these cavities were \(1.21\times 10^{6}\) nm/RIU and \(6.77\times 10^{5}\) nm/RIU, respectively [30]. Moreover, [PERSON] reported on a glucose sensor based on a metamaterial absorber, to be used in food engineering [31]. [PERSON] et al. developed a tunable localized surface plasmon graphene metasurface that should be used as a multiband superabsorber. It will then function as a refractive index sensor for measurements in the surrounding environment [32]. [PERSON] et al. reported on a THz metamaterial biosensor, to be used in the measurement of the thermally induced denaturation temperature of insulin [33]. There has also been some research in the field of thermal sensing. For example, [PERSON] et al. used the temperature dependencies of the dielectric constants of two materials, graphene and indium antimonide, in the design of a terahertz thermal sensor (with a sensitivity of 160 nm/K) [20]. In terms of patents, [PERSON] published a patent entitled \"Teramos-terahertz thermal sensor and focal plane array\" in 2014 [34]. Despite all these earlier investigations in the field of THz sensors, an efficient THz thermal sensor, of a waveguide type, has not yet been reported. This type of sensor is expected to be extremely useful in applications based on high-precision temperature measurements and control. A THz sensor has been demonstrated in the present study, which is based on a defect-containing periodically corrugated metal tube.