# Pursuing High Quality Phase-Only Liquid Crystal on Silicon (LCoS) Devices
## 3 Switching Time for Phase Modulation  
There are four major issues for holographic display applications: (1) the slow liquid crystal response time; (2) the small field of view (FoV); (3) the laser speckle effect degrading the image quality when using lasers as a backlight unit; and (4) a reduction in the frame rate for achieving field-sequential color displays [25]. There are many studies where the aforementioned issues have been addressed, such as: (1) 3-4 ms phase-only panel response time of thin cell gap at \(\lambda\) = 633 nm to reduce the dynamic reconstruction holographic image blur [26]; (2) the time-division multiplexing method to increase the FoV [27]; and (3) reduction of the speckle effect in image projection using temporal averaging techniques [28]. The ultimate turn-key solution to these challenges is the SLM panel with a sub-millisecond response time and a sub-kHz input frame rate. Ferroelectric liquid crystal (FLC) modulators [29; 30] and digital micro-mirror devices (DMD) [31] are two commercial spatial light modulators capable of achieving sub-millisecond response times. Significant quantization noise lowers the diffraction efficiency due to binary phase modulation. Several  
Figure 4: Pixel density and pixel pitch specifications for current LCoS-SLM integrated circuit (IC) backplanes. approaches have been proposed to achieve a fast response time, such as dual frequency LC (DFLC) [32], polymer network LC [33; 34], and polymer-stabilized blue phase LC [35]. High driving voltage (greater than 10 V) and light scattering in the visible region are the two major drawbacks of these LC material systems. In addition, limited driving voltage in a high-resolution backplane often results in a slower LC response time. Lately, [PERSON]'s group reported that an average phase-to-phase response time of \(\sim\)2 ms can be achieved using high \(\Delta\)n LCs in a thin cell gap of 1.7 \(\upmu\)m at an operating voltage of 5 V at 40 \({}^{\circ}\)C [36; 37]. The reported LCs can be used with 240 Hz frame rate without complicated overdrive or undershoot circuitries. Additionally, the effective birefringence of liquid crystals slightly decreases when the cell gap is below 1.8 \(\upmu\)m due to the strong surface boundary effect of the alignment layers [38]. Thus, a higher birefringence LC material may be needed in order to achieve the calculated effective birefringence in an ultra-thin cell for a fast-response LCoS phase modulator. [PERSON] et al. utilized the overdrive approach to boost the LC response time to 1-2 ms for a low-resolution panel (256 \(\times\) 256) with 500 Hz input frame rate and \(\sim\)1.6 kHz data frame rate [39]. However, the input frame rate dropped to 30 Hz when using a High Definition Multimedia Interface (HDMI) controller in their high resolution (1920 \(\times\) 1152) LCoS-SLM. Holoeye Photonics presented a HDMI 2.0 Field Programmable Gate Array (FPGA) driver to achieve 1920 \(\times\) 1080 resolution (4000 ppi) for 240 Hz and 720 Hz data frame rates for high performance 3D sensing systems [40]. However, the driver scheme cannot support their LETO phase modulator (also 4000 ppi) due to the slow LC response time, which is not comparable with the sub-kHz input frame rates. Meadowark Optics launched a new 1.8 ms response time phase-only 1920 \(\times\) 1152 pixels (\(\sim\)2764 ppi) LCoS-SLM without overdrive. In 2018, the maximum frame rate reached has been 714 Hz by using a PCIe controller [41]. In the same year, [PERSON]'s team announced their PCU-3-01 LCoS-SLM, capable of reaching 1.6 ms at 45 \({}^{\circ}\)C. The panel can be driven at 240 Hz input frame rate with the OCM-ASIC SDK driver (Jasper Display Corp.) and at 720 Hz data frame rate by using an HDMI controller [17]. Higher resolution of 4K2K or 8K4K LCoS panels (greater than 5000 ppi) is limited by low input frame rate (\(\lesssim\)60 Hz) and low driving voltage (\(<\)3.3 V). Future electronic hardware updates are required to achieve full phase modulation.