## 7 Conclusions and Perspectives  
Here, we introduce the basic optical geometry of the ATR system and summed up the factors that influence the system. High-performance, broadband, active, and multi-function modulators based on ATR have been proposed and discussed, showing attractive potentiality and unrivalled advantages in amplitude and polarization modulation. Surface plasmon sensing methods based on ATR configuration, Otto and Kretschmann, have been proposed and theoretically validated the feasibility, which will open a new window for high performance plasmonic sensor for gas, liquid, and so forth. The advantage of the ATR method makes it convenient for charactering liquid, powder, and thin-film materials, especially for the surface layer characteristics, which will be further developed in the rapid and noninvasive method for both spectroscopy and imaging. Due to the effective control of intensive absorption of water, the research of solute and water molecules around, including molecular dynamics, hydrogen bonds, and hydration in aqueous solution, can be realized through ATR geometry. In particular, the biomaterial-related picosecond dynamics of hydration lies in the THz band, making it suitable to obtain the characteristics of nucleic acid, protein, cell, and tissue for further application in biomedicine. To conclude, ATR in the THz region shows unique strengths as following:  
1. Ultra-sensitivity to the conductive interface makes ATR an ideal platform for modulation; active modulation would also be realized by materials with tunable conductive properties. 2. The evanescent field supported by ATR offers great opportunity for realizing SPR with specific materials and structures which support the SPP mode. 3. In situ research for biomaterials and biomedicine can be convenient and effective through ATR geometry. Despite ATR being applied as an interesting platform in THz modulation, sensing, spectroscopy and imaging, the limitations also make it challenging for further application. The performance of modulation and SP sensing with ATR geometry is based on the properties of materials, such as graphene and PVDF and configuration of the device. Thus, the fabrication of this modulator and sensor remains the biggest challenge for achieving high performance. For spectroscopy and imaging, the ambiguous response mechanism limits the applications, especially when it comes to both complex external and internal situations for samples. In addition, the influence factors, such as difference in  
Figure 7: THz-ATR imaging of (**a**) distilled water, (**b**) a piece of blood agar, and (**c**) porcine tissue. Reproduced with permission from [33], Copyright © 2017, IOP Publishing. environment and preparation for the samples, can affect the results substantially. Based on these, further research could be developed from these aspects:  
1. For modulation and sensing, the performance is determined by the fabrication, thus attention on the uniformity, repeatability, and stability of graphene, PVDF, and other tunable conductive layers should be paid. Especially for SPR sensing, a tiny difference from thickness and properties in each layer even would make it invalid. 2. Present SPR sensing based on ATR could be a challenge for their application due to the optical path needed to be adjusted accordingly. Fixing the incident angle while using continuous-wave could be a better way to solve this problem. 3. Spectra characteristics of the material should be further mined, which could be realized by some advanced algorithms such as deep learning. Besides, theoretical explanations for the spectroscopy response in the THz region also need to be further supplemented and completed. A simulation study, such as molecular dynamic and vibration, could offer guidance. 4. Reducing environmental disturbance and standardizing the test process are in great need for reliable spectroscopy and imaging results. The ambient temperature should be stabilized in a certain range and humidity should be controlled as low as possible.