# Liquid Crystal-Tuned Planar Optics in Terahertz Range
## 5 Prospect and Conclusions  
Liquid crystals exhibit broadband birefringence and can be tuned by an external field, occupying a place in the modulation of THz waves. From THz filters and phase shifters to integrated devices, we can see the ubiquitous shadows of LCs. Their characteristic of external-field responsiveness enables the free manipulation of functions. Pure-LC-based THz modulators mostly rely on thick LC layers to accumulate certain phase retardation. For LC geometric phase devices, half-wave conditions need to be satisfied to maximize the efficiency. These requirements cause LC THz devices to suffer from slow responses. As a mainstream THz modulation technique, metasurfaces can modulate the amplitude, phase, polarization and many other dimensions of THz waves. Because the design does not rely on the traditional Snell's law and propagation phase accumulation, arbitrary functions can be realized with reduced thickness and compact size. By combining LCs with metasurfaces, a THz modulator combining LC modulation and metasurface modulation can be obtained. The compact, efficient and fast-response modulations of THz waves meet the development tendency toward multifunction, integration and miniaturization.  
There are several aspects to be particularly pursued in future research. The first is the techniques of combining LCs and metasurfaces. For metal and dielectric metasurfaces, LCs are soaked on nanostructure arrays, as a refractive-index-tunable environment medium. For dielectric metasurfaces, LCs must be separated from the metasurface to provide PB phase modulation to avoid influences on the LC orientation of the nanostructures. Thus, new metadevice designs are needed to keep LCs separated from metasurfaces. The second is further improving the response time of LCs. The modulation speed of the THz modulator is a key requirement for most applications. The introduction of dual-frequency liquid crystals (DFLCs), blue-phase liquid crystals (BPLCs) and other fast response LCs may improve the modulation speed. The last is to introduce more kinds of external field. Most works in this review are electrically tuned. Their performance can be improved by further introducing other external fields, such as mechanical force, heat, light and magnetic fields. The responsiveness of LCs to multiple external fields enables the metasurface with free controllability. In addition to the innovations on LC materials and external-field excitations, the introduction of an artificial intelligence algorithm may facilitate many powerful application scenarios. The structure of a metasurface can be simplified by reverse design. The phase combination of both the LC and metasurface will inspire diverse THz photonic components. THz technology plays a key role in next-generation high-speed communication. Correspondingly, LC-integrated THz devices will also be extensively applied in communications.  
[PERSON] wrote the draft of sessions 1 & 3, [PERSON] wrote the draft of session 2 and part of session 5, and [PERSON] wrote the draft of session 4 and part of session 5. [PERSON], [PERSON] directed the preparation of the draft, and all authors participate in the preparation of the final manuscript. All authors have read and agreed to the published version of the manuscript.  
The authors gratefully acknowledge the support of the National Key Research and Development Program of China (SQ2022 YFA1200117), the National Natural Science Foundation of China (NSFC) (62035008, 62105143 and 61922038), Natural Science Foundation of Jiangsu Province (BK20210179) and Fundamental Research Funds for the Central Universities (021314380189).  
The authors declare no conflict of interest.