LCoS, short for Liquid Crystal on Silicon, is an optical component based on liquid crystal technology integrated with silicon. Essentially, LCoS is a type of reflective display device composed of liquid crystal material combined with silicon-based integrated circuit technology [1]. In essence, LCoS utilizes the birefringence properties of liquid crystal molecules to modulate the amplitude or phase of incident light by controlling the polarization state of light. Thus, LCoS can be categorized into amplitude modulation and phase modulation types^[1].

Amplitude Modulation LCoS - Twisted Nematic Configuration

The arrangement of amplitude modulation LCoS, also known as amplitude-only LCoS, includes the Twisted Nematic (TN) configuration and the Vertical Aligned Nematic (VAN) configuration. By applying different voltages to pixel units and using orthogonal polarizers, grayscale modulation of the incident light's amplitude is achieved, as illustrated in Figure (1)^[2].

 

Amplitude modulation LCoS modulates light in a manner similar to traditional LCD principles, where pixel voltages are applied, and the birefringence effect of liquid crystal molecules is utilized to change the polarization state of light ^[3]. For optimal performance, the polarization direction of the incident light should be parallel to the polarization direction of the LCoS's incident polarizer. When an external voltage is applied to the liquid crystal molecules in the pixel, they rotate, altering the polarization characteristics of the incident light. Depending on these polarization characteristics, the outgoing light can be classified into three types as depicted in Figure (2)^[3]:

When there is no external electric field, the liquid crystal molecules are twisted at 90° between the two polarizers. Since the light travels along the direction of the molecular alignment, the light passing through the liquid crystal is also twisted by 90°, allowing it to pass through the device and be observed by the human eye (see Figure (2)(A)^[4].

1. When the liquid crystal molecules start to be subjected to voltage, they rotate under the influence of the electric field, gradually transitioning from a twist aligned parallel to the polarizer to a vertical alignment. At this point, the degree of twist of the light passing through the liquid crystal ranges from 0° to 90°, allowing some light to pass through the device and be observed by the human eye (see Figure (2)(B))^[4].

2. When the liquid crystal molecules are subjected to sufficient voltage, their alignment becomes completely perpendicular to the polarizer. In this case, the light passing through the liquid crystal does not twist, resulting in this alignment blocking the propagation of light, preventing the light from passing through the device (see Figure (2)(C))^[4].

Currently, Amplitude-type LCoS has mature applications in home and industrial projection fields. Recently, Amplitude-type LCoS has also begun to be used in automotive Head-Up Displays (HUDs).

Phase-type LCoS - Zero Twisted Configuration

In contrast to Amplitude-type LCoS, Phase-only LCoS typically adopts a zero twisted liquid crystal arrangement, paired with parallel polarizers^[5]. The phase of light can be understood as the relative position relationship of light (as shown in Figure (3)). Altering the phase of light means changing the relative positions of light in space, a process known as phase modulation^[5].

Phase-only LCoS is commonly used in applications such as laser lithography, laser imaging, and holographic imaging^[6]. It can be seen that Phase-only LCoS is usually used in conjunction with laser light sources, as only high-coherence light sources can ensure the required enhancement or attenuation of the modulated light after phase modulation, achieving the purpose of light modulation^[5].

As shown in Figure (4), when voltage is applied to the liquid crystal layer of Phase-type LCoS, the liquid crystal molecules undergo deflection, yet the incident light of Phase-type LCoS still manages to fully exit.

However, due to the different refractive indices of the biaxial liquid crystal molecules along their long and short axes (with the refractive index of the long axis, ne, being greater than that of the short axis, no), when no voltage is applied, the light passes through the short axis no portion of the biaxial molecules (as shown in Figure 4(A))^[5]; when voltage is adequately applied, the liquid crystal molecules deflect, and the light passes through the long axis ne portion of the biaxial molecules (as depicted in Figure 4(B)).

Since ne > no, the transmission speed of light through the liquid crystal layer is ve < vo. Therefore, after applying voltage, the phase position of light passing through the liquid crystal layer lags behind that of the situation without voltage^[5]. LCoS can apply different voltages at each pixel point, and the refractive index of liquid crystal molecules will also vary between no and ne, thus achieving pixel-level phase modulation.

 

Difference between LCoS and TFT-LCD

Similar to Amplitude-type LCoS, TFT-LCD (Thin-Film Transistor Liquid Crystal Display) also controls the deflection of liquid crystals through electric fields to modulate the amplitude of light^[7]. The main difference between LCoS and TFT-LCD lies in their methods of controlling light: LCoS imaging primarily utilizes silicon substrates for reflection (see Figure (5))^[1], while TFT-LCD adopts a transmission approach using double-sided glass substrates^[7].

 

TFT-LCD is a transmission-type display, with glass used as the substrate at the bottom of the panel, and the light source positioned behind this layer of substrate (Figure (5)(A))^[7]. Incident light passes through the glass substrate, and each pixel of the TFT-LCD screen contains a Thin-Film Transistor (TFT), which controls the rotation of liquid crystal molecules by regulating the voltage of the TFT. The light is modulated after passing through the liquid crystal layer, then continues to pass through the upper glass panel of the display, propagating towards the human eye^[7].

In comparison to TFT-LCD, the mainstream LCoS solution employs reflective imaging, with only the upper layer panel using a glass substrate, while the bottom utilizes a backplane made of reflective silicon (Figure (5)(B))^[8]. The control circuit chips on the backplane primarily use semiconductor material silicon, with CMOS active display driver matrices on the bottom providing MOSFET switches, storage capacitors, light-shielding layers, and pixel electrodes for each pixel, used for integrated circuitry and electronic control operations. Between the silicon substrate and the liquid crystal, there is a layer of aluminum or other highly reflective material used as a reflective surface for the light. This means that after light enters the liquid crystal layer and is modulated by the liquid crystal, it is reflected to the human eye by the mirror plate^[8].

Advantages of LCoS Imaging Principle

High Optical Efficiency

Firstly, LCoS boasts a higher optical efficiency^[9]. As mentioned earlier, the bottom backplane of LCoS is composed of single-crystal silicon, which exhibits excellent electron mobility and facilitates the formation of finer circuits^[10]. This presents two implications: on a microscopic level, electrons within single-crystal silicon move faster under the influence of an electric field^[10]; on a macroscopic level, both transistors and circuits of LCoS can be fabricated within the CMOS chip, situated beneath the reflective surface, occupying only pixel gaps in surface area^[10].

Therefore, under identical driving conditions, the overall dimensions of LCoS circuits shrink, allowing pixels to utilize a larger area. Consequently, LCoS achieves a higher aperture ratio and significantly improves optical efficiency, with amplitude-type LCoS reaching around 40%, four times that of transmissive LCDs [11]. Hence, LCoS can achieve greater light output and higher luminance per unit projection area, thus possessing higher brightness^[11].

High Resolution

Secondly, LCoS offers superior resolution^[12], primarily due to its higher pixel density (Pixels Per Inch, PPI). Compared to TFT-LCD, which fabricates thin-film transistors on glass substrates, LCoS produces complementary metal-oxide-semiconductor transistors (CMOS Transistor) on silicon substrates with smaller dimensions^[12]. Consequently, within the same space, the PPI of LCoS based on silicon substrates is significantly greater than that of TFT-LCD based on glass substrates. This means that for TFT-LCD, to achieve the same pixel count, either a larger glass substrate must be designed or the size of individual pixels must be reduced. However, the design size of glass substrates is limited, highlighting the advantage of LCoS silicon substrates in achieving greater pixel density and ensuring better resolution^[12].

Moreover, TFT-LCD typically adopts a traditional RGB pixel layout, where each pixel consists of red, green, and blue sub-pixels, each with corresponding color filters, to display images through the brightness and color of the backlight ^[13]. In contrast, LCoS can separate the red, green, and blue information in color images using temporal color driving, sending images of different channels to the display screen at different time intervals. As long as the alternating frequency of RGB three primary colors is high enough, the image perceived by the human eye is the color effect formed by the combination of RGB three primary colors^[11]. Therefore, LCoS pixels can be smaller, achieving higher resolution on the same-sized display screen. Image (7) illustrates a brief principle of the internal pixel structures of these two technologies.

 

In summary, in terms of optical efficiency and resolution, LCoS offers a better contrast, higher definition, and finer display effect compared to TFT-LCD (refer to Image (8)).

Advantages of LCoS

Technological Advantages

Additionally, the production process of LCoS offers certain efficiency advantages^[11]. As the silicon substrate used as the optical reflective surface can be manufactured through modern integrated circuit fabrication processes, the production of LCoS can be integrated with the manufacturing process of integrated circuits. This integration reduces manufacturing complexity, enabling the mass production of microelectronic structural modules with high reliability and precision, thereby achieving miniaturization and lightweighting of LCoS modules and potentially reducing costs^[11].

Moreover, as an open-source technology, unlike the proprietary DLP technology from Texas Instruments, the design and manufacturing of LCoS face fewer restrictions, allowing more companies and individuals to participate, thereby providing greater opportunities for the further development of LCoS.

Disadvantages of LCoS

Temperature Challenges

Despite the maturity of LCoS technology and its significant applications in the projection and optical fields, its adoption in the automotive industry faces challenges in meeting regulatory standards due to time-consuming compliance processes. The main reason is the thermal management challenges faced by LCoS. While both TFT-LCD and LCoS use liquid crystal materials^[14], there are thousands of types of liquid crystal materials available, and the types used by TFT-LCD and LCoS differ. Additionally, TN-LCoS and VAN-LCoS use different liquid crystal materials. Liquid crystal devices exhibit performance degradation to some extent at high or low temperatures. To meet automotive regulatory requirements (-40°C to +85°C), suitable liquid crystals must be selected for automotive LCoS applications. However, the primary challenge for automotive LCoS currently lies in high-temperature failure issues. Therefore, there are few production-grade LCoS optical module assemblies that meet automotive regulations.

Response Delay

Compared to the DLP optical modules commonly used in current HUDs, LCoS has a slower response speed, with differences of more than two to three orders of magnitude (approximately 100 to 1000 times)^[15]. Additionally, the color switching of LCoS screens is achieved by controlling the rotation of liquid crystal molecules. The speed of rotation of the liquid crystal layer affects the gray response time of LCoS, which is temperature-dependent. At low temperatures, LCoS screens may exhibit noticeable ghosting, affecting visual performance^[16].

In theory, ferroelectric liquid crystals can rotate faster, thus improving the switching speed of LCoS^[17]. However, the preparation and integration of ferroelectric liquid crystals are relatively complex, and there may be issues of consumption and degradation with prolonged use or frequent switching. Therefore, current LCoS does not use ferroelectric liquid crystals. Moreover, for phase-type LCoS, in holographic construction, there are three degrees of black, white, and gray, and ferroelectric liquid crystals may lead to the loss of gray stripes, resulting in reduced information content and image quality.

Recent Developments in LCoS

Based on publicly available specifications, the pixel size of current LCoS technology ranges from 3.74 to 25 μm^[18], and the response speed is in the millisecond (ms) range, approximately within the range of 1 to 200 ms^[18]. In terms of resolution, current LCoS technology can achieve a maximum original resolution of 8K (4096×2160)^[11]. In December 2020, LCoS achieved mass production in China for the first time, with the pixel density increasing from 4300 PPI (Pixels Per Inch) to 6000 PPI.

Future Development of LCoS

The advantages of LCoS lie in its precise optical control. Therefore, LCoS has two main applications in the field of optical communication^[2]: one is for controlling electro-optic modulators, helping electro-optic modulators convert electrical signals into optical signals for high-speed and precise optical signal modulation^[2]. The other is for optical switches, which are used in optical communication systems to switch, route, and schedule optical signals^[19]. The precise optical control of LCoS contributes to improving the performance and transmission quality of optical switches^[2].

In the display field, with its excellent resolution and contrast, LCoS is suitable for various scenarios, including 3D projection, holographic projection, and laser projection. With its smaller module size, LCoS has recently been widely used in the field of AR technology, with LCoS being a common optical module in AR head-mounted displays^[2].

In the HUD field, amplitude-type LCoS has begun to be used in mass production projects, and Ruiwei Vision has also successfully developed a large field-of-view AR-HUD based on amplitude-type LCoS. Additionally, phase-only LCoS, due to its characteristics of pure phase modulation, also supports the implementation of digital holography (CGH), achieving real-time zooming 3D AR-HUD display.

 

#Reference：

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[3]Michalkiewicz, A., Kujawinska, M., Kozacki, T., Wang, X., & Bos, P. J. (2004). "Holographic three-dimensional displays with liquid crystal on silicon spatial light modulator". In Interferometry XII: Techniques and Analysis (Vol. 5531, pp. 85-94). SPIE.

[4]Frumker, E. and Silberberg, Y. (2007). "Phase and amplitude pulse shaping with two-dimensional phase-only spatial light modulators". J.Opt.Soc.Am.B,24(12).

[5]Zhang, Z., You, Z., & Chu, D. (2014). "Fundamentals of phase-only liquid crystal on silicon (LCOS) devices". Light: Science & Applications, 3(10), e213-e213.

[6]Dai, H., Liu, K. X. Y., Wang, X., & Liu, J. (2004). "Characteristics of LCoS phase-only spatial light modulator and its applications". Optics Communications, 238(4-6), 269-276.

[7]Ma, J., Ye, X., & Jin, B. (2011). "Structure and application of polarizer film for thin-film-transistor liquid crystal displays". Displays, 32(2), 49-57.

[8]Yin, Kun, et al. (2022). "Advanced liquid crystal devices for augmented reality and virtual reality displays: principles and applications." Light: Science & Applications 11.1: 161.

[9]Liu, H., Sun, C., Wang, W., & Zheng, J. (2015). "Design of a LCOS laser projector". Optik, 126(15-16), 1483-1486.

[10]Stauth, Sean A., and Babak A. Parviz. (2006). "Self-assembled single-crystal silicon circuits on plastic." Proceedings of the National Academy of Sciences 103.38: 13922-13927.

[11]Bleha Jr, William P., and Lijuan Alice Lei. (2013). "Advances in liquid crystal on silicon (LCOS) spatial light modulator technology." Display Technologies and Applications for Defense, Security, and Avionics VII 8736: 47-54.

[12]Chen, Huang-Ming Philip, et al. (2018). "Pursuing high quality phase-only liquid crystal on silicon (LCoS) devices." Applied Sciences 8.11: 2323.

[13]Lee, Baek‐woon, et al. (2003). "40.5 L: late‐news paper: TFT‐LCD with RGBW color system." SID symposium digest of technical papers. Vol. 34. No. 1. Oxford, UK: Blackwell Publishing Ltd,.

[14]Kacperski, Jacek, and Malgorzata Kujawinska. (2006). "Active, LCoS based laser interferometer for microelements studies." Optics express 14.21: 9664-9678.

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[17]Meyer, Robert B., et al. (1975). "Ferroelectric liquid crystals." Journal de Physique Lettres 36.3: 69-71.

[18]Holoeye、Sintec Optronics, etc.

[19]Mayer, Günter, and Alexander Heckel. (2006). "Biologically active molecules with a “light switch". Angewandte Chemie International Edition 45.30: 4900-4921.