# Pursuing High Quality Phase-Only Liquid Crystal on Silicon (LCoS) Devices
## 1 Introduction  
Photo-activated liquid crystal on zinc-sulfide (ZnS) [1], cadmium-sulfide (CdS) [2], and active-matrix addressed liquid-crystal-on-silicon (LCoS) [3] were first introduced by Hughes Research Laboratories for display applications in the early 1970s. Later, this approach was extended to the reflective spatial light modulator (SLM) [4; 5]. The LCoS-based SLM panel resolution increased from 16 \(\times\) 16 to 176 \(\times\) 176 pixels, as was reported by the University of Edinburgh (Edinburgh, UK), in the period 1986-1989 [6]. In 1993, resolution was further increased to a 254 \(\times\) 254 pixel array by DisplayTech (now CITIZEN FINEDEVICE (CFD) Co., LTD, Yamanashi, Japan). Their panel adopted the fast-switching ferroelectric liquid crystal (FLC) and implemented the field-sequential color system to generate a full color display. The ferroelectric-LCoS remains their distinctive product until today [7]. The projection display technology using LCoS as the core optical engine is not able to compete with digital micro-mirror device (DMD) technology with a resolution of 720p due to the cost structure. However, a high-resolution LCoS panel with a small pixel size can be commercialized owing to the advancement in integrated circuit (IC) technology during the past 10 years. Since 2000, LCoS has evolved from 720p to not only 2K1K Full High Definition (FHD) but also 4K2K resolution. Photos shown in Figure 1 of high-resolution display products are provided by Jasper Display Corp.  
Holographic near-eye displays using LCoS as a display panel opened a new era for virtual and augmented realities. Oculus Rift and HTC VIVE announced and demonstrated compelling 3D virtual reality (VR) experiences using JD5552 LCoS-SLM (Jasper Display Corp.; Hsinchu, Taiwan R.O.C.) in 2016 [8]. Microsoft demonstrated the HoloLens, its first self-contained augmented reality (AR) device, using PLUTO LCoS-SLM (Holoeye Photonics; AG, Berlin, Germany) in the same year [9]. In addition to Jasper Display Corp. and Holoeye Photonics, there are other worldwide providers of LCoS micro-displays, such as Hamamatsu Photonics, Meadowlark Optics, Santec Corp., and Himax. Each company has its own focus within its products. Jasper Display Corp. has its own digital drive 2K1K and 4K2K panels with corresponding drivers. Holoeye Photonics has a distinctive solution for 60 Hz 4K-SLM and computer-generated hologram (CGH) information input software development kit (SDK) interface design [10]. Hamamatsu provides pure-phase SLM solutions with high linearity and precision. It also provides SLMs for high-power laser applications [11]. Meadowlark Optics features the analog drive LCoS-SLM solution, which has a fast liquid crystal (LC) response time, a near-millisecond response, and a near-kHz picture frame rate [12]. Santec's LCoS-SLM phase modulator can provide ultra-high flatness (i.e., high phase precision) and a 10-bit linear curve [13]. Table 1 summarizes their unique products. A phase-only spatial light modulator can be used as a key optical element for displays, adaptive optics for sensing, lithography, and telecommunication, as shown in Figure 2. The linearity of phase modulation, response time, phase precision, and phase stability are key characteristics for appraising or selecting phase-only LCoS-SLM panels for the applications designed. However, it is difficult to obtain a single LCoS possessing all the desired features for all applications. Their specification and performance optimization are application-driven. For example, response time is the current limitation in a holographic display. It may be more important than phase stability, because an excellent holographic image can still be achieved using a well linearly calibrated and high-phase precision LCoS-SLM panel. On the other hand, the phase stability of the LCOS-SLM is a vital property in the field of optical communication, because an instantaneous disturbance resulting from an unstable phase