## 3 Challenges of LC-RIS and Future Outlook  
All three technologies (LCOS, LCRA, and LC-RIS) rely on the unique properties of liquid crystal materials, which can be continuously manipulated by applying a low-frequency electric field. They are also all capable of reconfiguring their properties in real time, making them useful for a variety of applications in communication, sensing, and imaging. However, there are several technical hurdles (illustrated in Figure 3) that must be overcome to realize their full potential. These drawbacks are attributed mainly to the use of LC, more specifically to the intrinsic properties of LC, as discussed below. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Technology** & **Phase-Shift** & **Power** & **Insertion** & **Response** \\  & **Resolution** & **Consumption** & **Loss (in GHz)** & **Speed** \\ \hline LC phase shifters & Analog tuning (stepless) & Low for transmission line. Moderate for waveguide. & Low for waveguide. Low/moderate for transmission line. & Slow (milliseconds to seconds) \\ \hline Semiconductors and MEMS phase shifters & Digital tuning (binary) & Relatively moderate & High/moderate & Fast (nanoseconds to microseconds) \\ \hline Ferrite and ferroelectric phase shifters & Digital tuning (binary) & Relatively high & High & Moderate (micoseconds to milliseconds) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Selective performance indicators of diverse phase-shift mechanisms for constructing RIS. Firstly, LC materials in nematic phase typically have relatively slow response times (in seconds or milliseconds) compared to other materials, e.g., semiconductors (in microseconds). This can limit the speed at which an RIS can operate, particularly for mission-critical and agile applications. To address this challenge, researchers are exploring various techniques to improve the response speed of LC-based reconfigurable components. One approach is to use materials with faster response times, e.g., ferroelectric LC [26]. Another approach is to optimize the design of RIS to reduce the thickness of the LC layer without compromising the insertion loss significantly. Additionally, recent advances in integrated circuits and digital signal processing [30] have enabled the use of sophisticated algorithms to control RIS and compensate for the slower response times of LC materials. By carefully designing the control algorithms, it is possible to achieve high performance even with slower response times. Secondly, the response of LC materials to electric (or magnetic) field is largely nonlinear [31], which makes it difficult to predict the exact behavior of an RIS in response to a given input signal. This nonlinearity can lead to distortion and signal degradation, which can affect the overall performance of the RIS. Promisingly, a LC-based 66 GHz enclosed coplanar waveguide phase shifter reported by [18] achieves a linear phase shift-voltage response between the threshold bias voltage (4 V) and the saturated bias voltage (10 V) in the experiments. Although the response is not fully linear between 0 V and 20 V, the effective tuning range achieves linearity for the first time. The performance is only slightly impacted by the nonlinearities that occur below the Freedericksz transition voltage (4 V) and above the saturation voltage (10 V). Upgrading further to the RIS panel level, an LC-RIS must be carefully designed and optimized to achieve the desired performance. This requires precise control over the material properties, the geometry of the RIS elements, and the operating conditions. Additionally, an RIS must be able to adapt to changing environmental conditions, such as variations in temperature and humidity; this is a quintessential multi-physics problem requiring structural, thermal, and electrical performance analysis of LC via numerical simulation. Powering LC units for an RIS can also be a significant challenge, particularly for large-scale applications.