1. Introduction

Augmented reality (AR) display, which is capable of superimposing virtual images on real environment, has attracted significant attention in education, healthcare, military, entertainment, and other fields [1–3]. As an important optical element in AR display, optical combiner plays a critical role in the system, which has been realized through several methods including beam splitting prisms [4], free-form surface combination lenses [5], off-axis optics [6] and optical waveguides [7]. Liquid crystal (LC)-based Pancharatnam-Berry phase lens (PBL), as a planar diffractive optical element with desired phase distribution from spatially varied LC director orientation, has been implemented as optical combiner of in various AR display architecture due to intrinsic features of high diffraction efficiency, strong polarization dependency, high optical quality, and small form factor [8–15].

Cholesteric liquid crystals (CLC) reflective PBL, which features of wide spectral bandwidth and large angular response bandwidth due to existing of chiral photonic crystal structure with reflective band gap, have been explored as coupler or optical combiner in various AR display. Circularly polarized light with the same or opposite handedness in specific wavelength range of band gap will be reflected or transmitted when incidents on the reflective CLC PBL optical devices, respectively [16]. A flat CLC PBL with small f-number and large diffraction angle at the same time has been demonstrated, providing promise application in waveguide-based near-eye AR display [17]. Multi-plane optical see-through AR display with customized polarization-dependent CLC PBL has been proposed, which are capable of display different images at different depths [18]. Maxwellian displays with CLC PBL devices are also demonstrated for pupil steering [19] and foveated AR display [20] based on their polarization selectivity through two polarization-selective CLC lenses. However, currently reports on CLC PBL for AR display either limited by monochrome display that can only imaging in single color or suffers from complicated multi-stacking design for colorful display that brings to complicated fabrication process as well as increased costs, which hinders their applications in AR display. Therefore, colorful AR display based on single CLC PBL device features of simple, compact, light and low-cost is high desirable.

In this letter, we propose a colorful multi-plane AR display system with dynamically tunable reflective PBL. The reflective PBL is fabricated by polymer-stabilized cholesteric liquid crystal that provides dynamical and continuous tunability of color and focal length simultaneously by direct current voltage. A proof-of-concept colorful multi-plane AR device is demonstrated. The designed system shows application potential in compact, lightweight colorful AR display specially in head-up display (HUD) that requires color-imaging with separated imaging depths.

2. Experiments

The schematic fabrication process of tunable reflective PBL is shown in Fig. 1(a). An indium tin oxide (ITO) coated glass substrate was firstly cleaned with acetone, isopropyl alcohol and ethanol solutions in sequence, and then irradiated in an ozone cleaner for 20 minutes. The photoalignment material azobenzene sulfonic dye (SD1, 0.5 wt.%) dissolved in dimethylformamide solution (DMF, 99.5 wt.%) was spin-coated onto a clean ITO glass at a speed of 500 rpm for 5 s and then 3,000 rpm for 30 s to make a uniform photo-aligned film (step i). The SD1 coated ITO glass was dried on a hot plate at 100°C for 10 minutes to remove residual DMF solvent. Two ITO glass substrates with SD1 were assembled to form a cell, where gap of cell was controlled by sphere spacer with diameter of 10 µm (step ii). Then the cell was exposed with a Sagnac polarization interference method where a right-handed circularly polarization (RCP) light and left-handed circularly polarization (LCP) light were applied to generate a PB phase lens pattern (step iii). After exposure, a lens pattern with a diameter of 13 mm was obtained and the polarization distribution of interference was recorded by the SD1 molecules on both two glass substrates where the direction of SD1 molecules were preferred to align perpendicularly to the polarization field. A cholesteric liquid crystal (CLC)/monomer mixture, which consists of 78.6 wt.% negative liquid crystal HNG30400-200, 6 wt.% diacrylate monomer RM82, 7.2 wt.% chiral dopant S1011, 7.2 wt.% chiral dopant S811, and 1 wt. % photoinitiator IR369, was filled into the cell and then exposed under ultraviolet (UV) light with intensity of 250 mW/cm² for 4 minutes for curing (step iv).

 

Fig. 1. (a) The schematic fabrication process of reflective PBL. (b) Schematic illustration of the Sagnac polarization interference exposure system. (c) The distribution of LC moleclues (rod-like) directors of PBL on the glass substrate plane.

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Figure 1(b) shows the schematic illustration of the Sagnac polarization interference exposure system for fabrication of reflective PBL [21]. The polarized laser light (488 nm) from Argon ion laser after collimation and beam expansion was incident on the polarizing beam splitter (PBS) and then divided into p and s waves. The half wave plate (HWP) was used to obtain equal intensity of p and s waves. The two beams after PBS passed through the mirrors M₁, M₂ and a convex lens respectively, and then output from the PBS. The convex lens was placed between the two mirrors to generate the lens phase. Finally, the s and p waves passed through a quarter-wave plate (QWP) and were converted into circularly polarized light with opposite chirality, and the two beams were interfered and exposed on the cell assembled by two glass substrates with photo-alignment layer to generate a PBL sample. In our experiment, the exposure power density was 30 mw/cm². The distribution of LC moleclues (rod-like) directors of PBL on glass substrate plane is plotted in Fig. 1(c), which is in a parabolic pattern aligned with SD1 molecules. The filled liquid crystal/reactive mesogens molecules will be arranged gradually in the plane of the glass substrate, changing continuously from 0 to π according to the orientation of SD1 molecules. For the on-axis lens patterns recorded in this experiment, the sample substrate was placed in a plane perpendicular to the direction of beam propagation, where the phase profile of $\varphi $ represents the phase difference between the center and edge of a lens [22]. The phase profile can be expressed by $\varphi = \frac{{2\pi }}{\lambda }\left( {\sqrt {{r^2} + {f^2}} - f} \right)$, where $\lambda $ is the wavelength for the imaging system, r and f are the sample lens radius and focal length, respectively.

3. Results and discussion

In our experiment, the UV curing of CLC/monomer mixture leads to a PSCLC with tunable reflection bandgap under external applied direct current (DC) voltage. The exist of photoinitiator in PSCLC system increases the density of ion impurities under UV light irradiation, leading to a higher sensitivity to the applied DC voltage due to the strong cation trapping effect of the polymer network. When DC voltage applies, the polymer network will be displaced and concentrated at the negative electrode of the cell, which leads to higher and lower concentration of chiral dopant near to the positive and negative electrode, respectively. Thus, the helical pitch of CLC polymer is increased near the positive electrode and compressed near the negative electrode. The pitch increases evenly in the center of the liquid crystal cell, resulting in extension of whole helical pitch of PSCLC thus the red-shifting of reflection band gap. The main reason for choosing negative liquid crystal here is that under the control of DC voltage, the long axis direction of negative liquid crystal molecules tends to orient perpendicularly to the direction of electric field. For CLC PBL based on photo-alignment technology, the directors of negative liquid crystal molecules are hardly affected by the electric field induced from DC voltages [23].

The application of PSCLC materials in PBL enables its dynamic tuning on reflection wavelength as well as circularly polarization selectivity, which is quite useful in AR displays [24–26]. Figure 2(a) shows the cross-sectional schematic diagram of our proposed reflective DC voltage tunable PBL. The pitch of PSCLC increases with the applied DC voltage. When the CLC/monomer was filled and cured in the cell, the liquid crystal molecules on the x-y plane would orient following the PBL pattern on the photo-alignment SD1 layer and exhibit periodic self-assembled helical arrangement in the z-axis direction. Here, the pitch length is defined the length corresponding to 360° rotating of the liquid crystal molecular director along z-axis. The central wavelength ($\lambda $) of the reflection band gap is determined by the optical refractive index ($n$) and the pitch length ($p$) of the liquid crystal molecules: $\lambda = np$.

 

Fig. 2. (a) The cross-sectional schematic diagram of proposed reflective DC voltage tunable PBL. (b) Schematic illustration of optical property of PBL with left-handed PSCLC. (c) Transmittance spectrum of proposed PSCLC PBL at different applied DC voltage. (d) The relationship of applied DC voltage versus central wavelength of band gap in transmittance spectrum. (e) The curve of transmittance versus time. (f)-(h) POM images of PBL at DC voltages of 0 V, 16 V and 30 V. The scale bar is 200 µm.

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The schematic illustration of optical property of PBL with left-handed PSCLC (L-PSCLC) is depicted in Fig. 2(b). When unpolarized light incidents from the positive power side of the PBL, the left-handed circularly polarized light (LCP) will be reflected and converged by the left-handed PSCLC, while the right-handed circularly polarized light (RCP) directly passes through the PBL. In contrast, when unpolarized light incidents from the negative power side, the RCP light will be reflected and diverged while the LCP light will be directly transmitted [16]. In addition, due to the inherent diffraction optical negative dispersion characteristics of the PBL, light with different wavelength will be reflected at different focal length depth when dynamically tuning the reflection band gap by DC voltage.

Figure 2(c) demonstrates the transmittance spectrum of proposed PSCLC PBL at different applied DC voltage. When no voltage is applied, the reflection band gap of the PBL is in blue light (centered at 461.5 nm) with band gap range from 440 nm to 483 nm. When the voltage is increased to 16 V, the reflection band gap moves to green light (centered at 525.5 nm), with band gap range from 502 nm to 549 nm. When the applied voltage increases to 30 V, the reflection band gap shifts to red light (the center wavelength is 616.5 nm), and the band gap range is from 591 nm to 642 nm. When the voltage increases to 46 V, the reflection bandgap moves to the near-infrared band (the central wavelength is 737 nm), and the band gap range is from 710 nm to 764 nm. The band gap continuously red-shifts as the applied voltage further increases. Those obtained spectra curve with nearly 50% transmittance in range of band gap indicates excellent optical quality as well as good tunability of proposed devices, and the see-through visibility of the device is also high.

The relationship of applied DC voltage versus central wavelength of band gap in transmittance spectrum has also been investigated, as shown in Fig. 2(d). When the applied voltage increases from 0 V to 62 V, the central wavelength red-shifts from 461.5 nm to 776 nm, corresponding to shifting range of 314.5 nm. Furthermore, in order to quantitatively study the reflection characteristics of the device, three circularly polarized lasers in red, green, and blue color (wavelength of 457 nm, 532 nm and 633 nm) were used in our experiment, respectively. The corresponding diffraction efficiency of the tunable reflective PBL based on PSCLC were measured to be 98.1%, 92.3%, and 93.6%, respectively.

The response time is critical to the electro-optic response characteristics of tunable PBL, which was measured through the curve of transmittance versus time (Fig. 2(e)) by switching DC voltage between 0 V and 30 V. A red laser (633 nm) was used as the light source and a photodetector was used as the receiver of transmitting signal. The measured decay and rising time are 462.4 ms (90% to 10% transmittance) and 215.2 ms (10% to 90% transmittance), respectively.

In order to clearly describe the reflection characteristics of the PBL under dynamic modulation of DC voltage, a polarization optical microscope (POM) was used to observe the pattern of the sample in reflection mode, which was consistent with the designed lens pattern. Figure 2 (f)- (h) demonstrate the polarization optical microscopy of PBL at DC voltage of 0 V, 16 V and 30 V, respectively. The scale bar of all micrographs is 200 µm, and the sample was kept at room temperature (25 °C). The color change of the different reflective textures is caused by the change of the pitch length during the voltage application.

For CLC PBL, the chromatic aberration mainly comes from the dependence of optical power and wavelength (${\lambda _R}/{K_R} = {\lambda _G}/{K_G} = {\lambda _B}/{K_B}$), where K is the optical power and $\lambda $ is the wavelength for red (R), green (G) and blue (B), respectively. The focal length $f = 1/K$ [23]. Therefore, the focal length is the largest for blue light and smallest for red light, which is also referred to the optical negative dispersion. Herein, the optical negative dispersion characteristics of CLC PB diffractive lens were utilized for fabrication of optical element capable of dynamic tuning of different colors with different focal length in AR display system.

To verify the proposed experimental feasibility, a compact DC voltage dynamically tuned multi-plane augmented reality display system was demonstrated, in which the reflective PSCLC PBL was used as a tunable optical device for color tuning. The object distance ($u$) in the imaging system is fixed at 18.5 cm. According to the relationship $1/u + 1/v = 1/f$ [23], when lasers with wavelengths of 457 nm, 532 nm and 633 nm incident on the reflective PSCLC PBL sample, focal lengths ($f$) of 37.2 cm, 30.9 cm and 26.1 cm are obtained at 0 V, 16 V, and 30 V, respectively, which leads to image distances ($v$) of -36.81 cm (plane 1), -46.1 cm (plane 2) and -63.53 cm (plane 3), respectively. The schematic diagram of proposed augmented reality display system is shown in Fig. 3(a). The input image source used was a commercial laser projector. The beam was collimated by a lens and passed through a linear polarizer (LP) and wide-band quarter-wave plate (QWP) to be modulated into left-handed circularly polarized light. After the beam passes through the beam splitter BS1, it was directly vertically incident on the surface of the reflective PBL, and then reflected and converged. By changing the DC voltage on sample, the reflection wavelength band gap of the lens can be shifted from blue to green, and red, leading to different focal lengths. After passing through BS1 again, the image was captured by a camera through beam splitter BS2 as the optical combiner. The proposed multi-plane augmented reality images could be observed at plane 1-3 for in blue, green, and red color. It worth mention that, other colors (not show here) different from red, green, and blue can also been generated at different planes. The real optical setup of proposed multi-plane augmented reality display system is demonstrated in Fig. 3(b).

 

Fig. 3. (a) Schematic diagram of proposed augmented reality display system. (b) The real optical setup of proposed multi-plane augmented reality display system.

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To demonstrate the multi-plane AR system with tunable reflective PBL in potential application for vehicle HUD, three car models were placed at different depths positions corresponding to blue image at plane 1, green image at plane 2, and red image at plane 3, respectively, as shown in Fig. 4(a). For the initial voltage-free state, blue images of number “72” and sign “right turn arrow” were captured at plane 1 (blue car) when the camera was focused at 24 cm, as shown in Fig. 4(b)-4(c). When the DC voltage was increased to 16 V, green images of number “90” and sign “left turn arrow” were captured at plane 2 (green car) when the camera was focused at 33 cm, as shown in Fig. 4(d)-4(e). When the DC voltage was increased to 30 V, red images of number “70 in ring” and “forward arrow” were captured at plane 3 (red car) when the camera was focused at 51 cm, as shown in Fig. 4(f)-4(g). The proposed tunable AR display system with ability of exhibiting full colors at different depth of field in time sequence, exhibiting great application potential in head-up display, where virtual images with different color and separated depth are high desirable for drivers to receive information at specific color-depth pair. It is worth noticing that, long-term use of DC voltage for liquid crystals may cause material degradation, increasing the number of impurity ions in the device. This is unavoidable due to the inherent properties of liquid crystal materials. However, in our experiments, a certain concentration of liquid crystal monomers that can form a polymer network can be used to capture some ionic impurities, which reduces the number of ions directly adhered to the glass substrate as well as long-term instability. Another possible method is to dope a small amount of inorganic nanoparticles in the liquid crystal system to reduce some impurity ions and thereby improve the performance of the device. When polymers are used in liquid crystal devices, haze may occur under the influence of voltage. For our proposed PSCLC PBL, a high transmittance of greater than 80% can be maintained after applying voltage. Since the application point of this device in the AR display system is an intermediate optical device in the optical combiner, it is not directly used as a window for the human eye to receive imaging but only the reflection. Therefore, we believe that a little haze in the device will have little impact on the entire AR display system.

 

Fig. 4. (a) Schematic diagram of multi-plane AR system with tunable reflective PBL. (b)-(c) Blue, (d)-(e) green, and (f)-(g) red images at different image positions under DC voltage of 0 V, 16 V, and 30 V, respectively. The corresponding cars are highlighted by dash rectangles to mark positions in each figure.

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Several LC-based tunable HUD methods have been demonstrated. Wang et al. proposed a single green holographic multi-plane AR HUD system based on polymer dispersed liquid crystal (PDLC) and lens holographic optical elements [27]. The system can provide two switchable display modes of dual virtual image mode and virtual real image mode. But this method mainly focuses on monochrome display, which suffers from complicated multi-stacking design for colorful display. Liu et al. used the polarization multiplexing method to realize a single green dual focal plane AR HUD without losing image resolution [28]. In this solution, liquid crystals were used to prepare twisted-nematic cell to achieve polarization multiplexing. Freeform mirror is used in this scheme, which leads to complicated manufacturing process and high cost. Richter et al. proposed an AR HUD designed using holographic waveguide technology [29]. For this method, a specially developed reactive monomer liquid crystal mixture is used as the holographic material, and a high-efficiency holographic grating is produced through laser interference method. Although this solution can be applied to full-color display, it cannot achieve multi-depth plane display, which can easily cause visual fatigue for the driver. Hou et al. proposed an optical switch using PDLC devices as projection display systems to control the shape of the projected image. RGB super-power LED lamp sets were used as backlight sources to achieve color projection display [30]. This propose realizes projection images of different colors, but the three-color projection images are in the same focal plane, where the depth information cannot be achieved, and the backlight power consumption is large. Wang et al. changed the optical paths of the liquid crystal plates through voltage modulation, allowing imaging at different depth positions [31]. However, it has problems such as large driving voltage (about 100 V) and long response time (10 s).

Therefore, traditional LC based tunable AR-HUD cannot achieve colorful display with dynamically adjustable depth image, which will produce a depth mismatch between the real environment the driver sees for a long time and the virtual image in the HUD, resulting in problems such as dizziness [28]. Our proposed method can produce full-color, multi-depth image manipulation through continuous tuning of voltage, which is achieved through the negative dispersion characteristics of PSCLC Lens. This solution can significantly reduce the driver's visual fatigue problem.

In this experiment, the total volume of AR optical architecture is about 2.1 L including the projector, optical elements and camera, wherein the size of the sample device is a 2.5 cm × 2.5 cm liquid crystal cell, and the actual area of the lens is a circle with a diameter of 1.2 cm, as shown in the upper left corner of Fig. 2(d). The weight of sample is about 10 g. The measured power consumption of the device is 8.8 × 10-4 W. In actual vehicle head-up display applications, the second BS component in Fig. 3(a) can be replaced by the windshield. Since the designed device is just a single liquid crystal box, it is thinner and lighter than the lens group in a traditional HUD. In terms of energy efficiency, our devices have an optical efficiency of over 90%, which can meet practical applications when the brightness of the image source is sufficient.

The tolerance of focal length was measured at wavelength of 532 nm and 633 nm with applied voltage, respectively. By turning on and off applied voltage, 3 data of focal length were obtained to be 30.9 cm, 30.8 cm, and 30.9 cm for 532 nm at 16 V and 26.1 cm, 26.1 cm, and 26.0 cm for 633 nm at 30 V, where the original focal length for 457 nm at 0 V is 37.2 cm. It can be seen that the error range of the focal length is about 0.3%, which is reliable. In addition, the left-handed circular polarization purity is 0.972, 0.979 and 0.974 for wavelengths of 457 nm, 532 nm and 633 nm, respectively.

Furthermore, colorful multi-plane AR display capable of displaying multi-images at different colors can be realized by combing N pieces of single PBL together, where each PBL piece can be tuned independently. Herein, multi-plane AR imaging with 2 pieces of reflective PSCLC PBL is taken as an example for description, as shown in Fig. 5. In this case, the ITO part on cell of PBL can be divided to two regions during fabrication process with independent electrodes for DC voltage control, resulting in 3² display modes (RR, RG, RB, GR, GG, GB, BR, BG, and BB) can be realized by changing the value of the DC voltage independently. The display region can be further divided into N parts to obtain a n² display modes.

 

Fig. 5. Schematic diagram of 3² multi-plane display modes under different voltage tuning. V₁ and V₂ represent voltages for obtaining red, and green colors.

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4. Conclusion

In summary, a colorful multi-plane optical see-through AR display with a reflective PSCLC PBL dynamically tunable by DC voltage is designed. In the AR display prototype demonstrated, the modulation of different reflection wavelength band gaps is achieved by changing the DC voltage. At the same time, the color and focal length are dynamically controlled according to the inherent diffractive optical negative dispersion characteristics of the PBL device. The fabricated reflective PSCLC PBL exhibits over 90% diffraction efficiency at the desired wavelength. The proposed simple, compact, lightweight, and low-cost colorful AR display system capable of color-imaging with multi-depth shows great application potential in the vehicle-mounted head-up display.