# Non-Mechanical Beam Steering with Polarization Gratings: A Review
## 1 Introduction
This combination has produced discrete beam steerers that can steer over \(\pm\)45\({}^{\circ}\) with good efficiency. This technique, however, requires the use of two large-aperture LC OPAs for discrete steering and four for quasi-continuous steering, by placing two more large-aperture OPAs at the end of the stack to fill in between the discrete angle deflections from the Bragg gratings. As with all LC OPAs, the steering efficiency rolled off to 60% in a few degrees [1,3,5]. This efficiency roll-off results from light coupling into sidelobes and zero-order leakage due primarily to phase wrapping (i.e, 0\(\pi\) to 2\(\pi\) transitions) required by blazed-grating phase profiles. These phase wraps must occur over a small spatial duration compared to the grating period and must provide a precise 2\(\pi\) phase shift or the resulting phase discontinuities produce sidelobes. In the pursuit of a comparatively less expensive and more producible approach, there were also researchers working with patterned polymer plates that formed blazed grating structures that could be switched on or off through index matching with a LC layer to null out the grating [6]. These LC-based devices were composed of thin films and could be scaled to large apertures but relied on 0 to 2\(\pi\) phase wrapping. Since this was also a stacked element approach similar to the solid crystal approach discussed above, it did not take many stages to cause the steering efficiency to fall below 50% (i.e., \(-\)3 dB) if the diffraction efficiency of each deflector plate was not near 100%. Pancharatnam-Berry phase modulation eliminates the need for phase wrapping in the grating profile, since the phase modulation continuously progresses with the rotation of the birefringent material's optical axis [7,8]. Removal of phase discontinuities by eliminating the reset paved the way for PGs with near 100% diffraction efficiencies. Unlike optical path delay (OPD) modulators and Bragg gratings, this type of phase shift also made the deflector plates less sensitive to the steered light's wavelength, because the phase shift depends only on the relative optic-axis orientation of the birefringent material. These theoretical advantages eliminated some difficult fabrication issues and provided a means for achieving a practical non-mechanical scanner with a large angle-aperture product. Early on, PG films had less steering efficiency than blazed grating films until the patterning and fabrication processes were improved to the point where practice started matching up with theoretical expectations [9]. At that point, the thought of cascading devices to form a digital beam scanner became a possibility, if the other losses such as Fresnel, LC scatter and electrode absorption could be reduced to workable levels. Among non-mechanical beam steering approaches, polymer polarization gratings (PGs) offer a number of unique advantages [10, 11, 12, 13]. In the following article, we will review the design and construction of non-mechanical steering devices using PGs, also known as Pancharatnam-Berry deflectors (PBD) or cycloidal diffractive waveplates (CDW). We aim to cover the relative strengths and weaknesses of the approach and highlight several demonstrations of the technology in different applications. Specifically, we present demon strations of PG beam steering in three compelling use cases: a time-of-flight camera, a coherent Doppler lidar, and a midwave infrared (MWIR) broadband passive imager. While the scope of this review is aimed specifically at PG-based approaches to non-mechanical beam steering, the two reviews by [PERSON] et al. [1; 2], provide a broader overview of non-mechanical optical beam steering, while the review by [PERSON] et al. [5] provides comparison with other liquid crystal (LC)-based approaches. By viewing the larger beam steering topic with a wider lens, these reviews can serve to place the PG-based approaches in a broader context than provided here.