# Non-Mechanical Beam Steering with Polarization Gratings: A Review
## 2 Non-Mechanical Beam Steering with PGs
### Optical Properties of PG-Based Non-Mechanical Steering Systems
#### 2.2.4 Switching Dynamics  
Non-mechanical steering with layered PGs and polarization switches provides random-access beam repointing. As the PGs are passive elements, the transition time between angles is entirely dependent on the means of polarization switching. As mentioned earlier, the PGs provide binary steering in response to the two handedness states of circularly polarized light. Therefore, all switching solutions must provide a means to either maintain or flip handedness depending on their driven state. Solutions must also be thin, to permit the layering of multiple stages with manageable beam walk-off, and should be operable over a wide angular range. Owing to these last two constraints, liquid crystal devices offer several attractive solutions. As previously mentioned, ECB cells using nematic liquid crystal provide a straightforward means of achieving switching in PG beam steering systems [13]. Such cells are well understood, simple to construct, and widely manufactured. In these cells, the response time is asymmetric, responding faster to an increasing drive voltage amplitude and relaxing more slowly. For true random-access scanning, the LC relaxation time becomes the limiting factor in beam repointing. This relaxation time is dependent largely upon the cell thickness and LC rotational viscosity [43; 44]. Through the retardance relationship cited earlier, \(\Delta nd/\lambda\), we observe that nematic LCs with high \(\Delta\)m permit thinner cell gaps and thus faster switching for a fixed wavelength. Similarly, longer wavelengths necessitate thicker cell gaps and, thus, slower switching for a given LC. In reality, this dependence on wavelength is exacerbated because most nematic liquid crystals exhibit decreasing \(\Delta\)n with increasing wavelength. Through viscosity, temperature also impacts the switching speed of ECB-based PG steering systems. While this dependence dictates that high-speed LC devices must be maintained within a desired operating temperature range, it also enables the application of heat to improve device speed [44]. For example, half-wave retardance ECB cells built for 915 nm illumination, and using LC similar to that described in [PERSON] et al., can achieve <700 us relaxation (Figure 6) when operating at a temperature of 65 \({}^{\circ}\)C and using the transient nematic effect (TNE) [45]. More generally speaking, high birefringence, low viscosity, nematic LC can provide relaxation times around 0.5-5 ms through the VIS-SWIR spectrum when both heating and TNE are utilized. Figure 5: Foreshortening of beam waist during beam steering from a fixed planar aperture. As with Risley prisms and phased arrays, non-mechanical beam steering with PGs leads to a progressive foreshortening of the beam waist along the direction of steering. Many other architectures exist for LC-based polarization handedness switches. A few have benefits for particular applications and will be mentioned here. For example, Pi cells present a modified ECB alignment that reduces back flow within the cell, resulting in faster relaxation times particularly for thin (VIS spectrum) cells [46]. Polymer network nematic LC approaches have been demonstrated to significantly improve relaxation times though often at a penalty of increased scattering and/or drive voltages [47; 48; 49]. Likewise, ferroelectric LC (FLC) cells can provide fast switching in response to both rising and falling drive signals, however they provide in-plane switching and behave naturally as polarization rotators, rather than variable retarders. As such, more sophisticated switch assemblies are required to produce the desired handedness switch [50; 51]. Both the polymer network nematic and ferroelectric LC approaches have applications in infrared switching, where the large cell gaps required for simple ECB switching otherwise result in 10's to 100's of millisecond relaxation times.