## 1 Introduction  
Terahertz (THz) radiation generally refers to a band of electromagnetic waves (from 0.1 to 10 THz) which spans the region between the mid-infrared (MIR) and the microwave [1; 2; 3]. In this range, the electromagnetic waves are characterized in properties as follow: The energy levels of photon are quite low (around 1-10 meV), and THz waves are transparent to most dielectric materials while intensely absorbed by water. Besides, most motions like the hydrogen-bonding stretches and torsion, molecular rotations, crystalline phonon vibrations, and low-frequency bond lie in the THz range [4; 5], which means THz waves are suitable to explore and analyze these motion modes. In the past decades, along with the development of THz science and technology, both THz spectroscopy and imaging technique have been widely applied in various fields including material [6; 7], communication [8; 9], imaging [10; 11; 12], biomedicine [13; 14], pharmaceutics [15; 16], food [17; 18], agriculture [19; 20; 21; 22], and others, with heated discussions around modulation, sensing, spectroscopy and imaging analysis. THz time-domain spectroscopy (THz-TDS) is very popular in the THz region, which uses pump and probe pulses to directly measure the electric field. Both amplitude and phase information can be extracted by using fast Fourier transform (FFT), further, the optical or dielectric parameters could be calculated [2; 4]. The interactions between the THz wave and material have been discussed from various perspectives and the features of the material were obtained by THz technique, thus making THz spectroscopy and imaging technique become an emerging characterization method. According to the sample properties and geometrical constraint, three geometries are usually configured in the THz systems: transmission, reflection, and attenuated total reflection (ATR). Transmission geometry is the most common method for samples with moderate absorption. Reflection and ATR geometries are suitable for the strongly absorbing or scattering samples. Restrictions are inevitable for the surface of the sample when using reflection geometry. ATR geometry is advantageous for liquid, powder, or thin film samples without further preparation compared to the others [4]. Research on THz wave modulation, especially amplitude and polarization, is essential for promoting the rapid applications in THz spectroscopy and imaging [23]. Modulation of visible and near-infrared light has been easily achieved for decades, while there are still plenty problems to be solved for modulating THz waves. Modulation devices with metasurfaces, photodoped semiconductors, conductive thin film, many other artificial or natural materials and structures, show drawbacks such as limited bandwidth, poor attenuation ability, and slow switching speed. Many THz polarization controlling components or structures, such as polarization rotators and wave plates, using stacked birefringent slides, dielectric grating, metamaterials, or prism reflections, still exhibit shortcomings like the unexpected thickness of the structures, narrow operational bandwidth, and restricted conditions for large phase differences [24]. Thus, the utilization of ATR geometry in realizing THz wave modulation have been proposed, which has been proved to be highly efficient and easy-to-fabricate and can reach high or multiple modulation performance over a broadband THz region [23; 24]. Surface plasmon resonance (SPR) sensing in the visible and infrared region has been proven to be highly sensitive to the changes in the optical properties of the neighboring media. This sensing method has been widely applied for compounds analysis, especially for biological or chemical molecules [25]. However, in the THz region, large negative permittivity strongly prohibits electromagnetic fields from penetrating inside a metal [26], thus surface plasmon polariton (SPP) excitation becomes an attractive challenge.