Open Access
Add to BibSonomy
Abstract
Angular bandwidth, which is critical to field-of-view, plays important role in diffractive optical waveguide augmented reality display. However, design and fabrication of large angular bandwidth is still a challenge. Herein, we demonstrate a liquid crystal reflective gradient polarization volume grating with three-dimensional gradient periodic structure for waveguide near-eye display. Two-beam polarization interference with special designed periodic gradient photomask are applied to chiral-dopant reactive mesogens doped with ultraviolet dye for generating gradient three-dimensional configuration of liquid crystals, resulting in gradient polarization volume grating with extended angle bandwidth of 61° while keeping 80% diffraction efficiency, with peak efficiency near 100%. The proposed gradient polarization volume grating provides an effective method to broaden the angular bandwidth in waveguide for wide field-of-view augmented reality display.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Augmented reality (AR) near-eye display, as a promising next-generation display, provides a novel display method for people to perceive the world [1–4]. As a critical component, optical combiner is usually applied to superimpose virtual images in the real environment under the premise of maintaining high visibility in AR display [5–7]. In order to obtain a better human-computer interaction experience, optical combiner with large field of view (FOV) that is more beneficial to human vision is highly required. The existing optical combiners mainly include the following types: polarizing beam splitting prisms, free-form surface coupling, off-axis optics, and optical waveguides [8,9]. Among them, the optical waveguide, which can realize the replication and expansion of the exit pupil that is difficult to achieve in traditional visual optical system, features thin and compact shape, wide FOV and high transmittance to external ambient light and draw people’s more and more attention in recent years [10–12].
Diffractive optical waveguide, as the major method of optical waveguide technology, can be generally separated to three types: volume holographic gratings, surface relief gratings and liquid crystal polarizer gratings [13–15]. For volume holographic grating (HVG), the angular bandwidth is usually limited by the small refractive index of photosensitive recording material, resulting in small FOV in display [16]. The surface relief grating (SRG) can obtain larger angular bandwidth than HVG, but also suffers from high fabrication cost in large title-angle structure for larger FOV [17]. In recent years, the reflective polarization volume grating (PVG) based on liquid crystal polymer with characteristic of high diffraction efficiency, large refractive index modulation, unique polarization property, and simple fabrication process, has attracted significant attention with application potential in optical combiners [18–24]. To increase the FOV, a reflection chirped PVG has been proposed with expanded angular bandwidth of 54° by introducing a gradient pitch structure along the vertical direction [25]. Therefore, angular bandwidth, which is critical to field-of-view, plays important role in diffractive optical waveguide augmented reality display. However, design and fabrication of large angular bandwidth is still a challenge, and the angular bandwidth needs further increase for waveguide-based AR display.
In this paper, we demonstrate a gradient LC PVG with three-dimensional (3D) gradient periodic structure for waveguide near-eye display. Two-beam polarization interference with special designed periodic gradient photomask are applied to chiral-dopant reactive mesogens doped with UV dye for generating gradient 3D configuration of LC, resulting in gradient PVG with extended angle bandwidth of 61° while keeping 80% diffraction efficiency, with peak efficiency near 100%. The proposed gradient PVG provides an effective method to broaden the angular bandwidth in waveguide for wide FOV AR display.
2. Experiments
The formed structure is actually a reflective polarization volume grating with slanted helical axis featured by gradient pitch in both horizontal plane (x-z plane) and vertical direction (y axis). The device can realize the function of reflecting circularly polarized light of the same chirality and transmitting circularly polarized light of opposite chirality. Within the absorption spectrum of the sulfonic acid dye SD-1, due to the isomerization properties of the azo group [26,27], the molecules of photoalignment material SD-1 rotate perpendicularly to light polarization direction. Then the Pancharatnam-Berry (PB) grating pattern is generated through Mach-Zehnder interferometer by two orthogonal circularly polarized light beams. PB grating is also called geometric phase grating, which can flip the handedness of circularly polarized light. The CLC molecules on the surface of the photo-alignment layer will follow the orientation arrangement of the bottom SD1 molecules. The CLC molecules then self-assemble into a helical structure. The modification from fixed to gradient pitches of PVGs in both horizontal plane and vertical direction will further extend the reflective band in spectral and thus lead to wider angular bandwidth.
Figure 1(a) shows the schematic diagram of a single unit with 16 gradient regions in PVG sample fabricated by using a customized gradient photomask, where the photomask consists of 35500 repeated units (250 × 142). The unit of photomask consists of 4 × 4 regions with gradually varied transmittance from 37%−7% (from region R1 to R16), which is designed according to the relationship of transmittance spectral and total UV exposure dosage collected in our experiment When exposed under UV light through the photomask, two types of gradient pitch of CLC arrangement will be achieved. The first type of gradient pitch is obtained in the xz plane, where the pitch of CLC decreases gradually due to reduced UV exposure dosage from region of R1 to R16. The second type of gradient pitch occurs along the thickness direction (y axis). In single region along y direction, the pitch decreases gradually from top to bottom due to reducing UV absorption, where pitches in top and bottom regions are represented by pt, pb, respectively. When UV light is transmitted through the sample, UV dyes will lead to the formation of the gradient of light intensity. The gradient pitches of CLC are highly related to the gradient of UV light intensity, where the higher UV light intensity leads to faster polymerization rate of RM monomer as well as the diffusion of unpolymerized monomer spread to the region with higher light intensity due to the difference in depletion of RM monomer. After polymerization, the ratio of chiral dopant/polymer will decrease (increase) in region with high (low) UV light intensity, thus the pitch of obtained CLC is long (short) in region with high (low) UV light intensity.
Fig. 1. (a) Schematic diagram of a single unit with 16 gradient regions in PVG sample. (b) Schematic profile diagram of four different regions in PVGs and the orientation profile of SD-1 molecules on bottom. (c) Schematic cross-section diagram of LC molecules within a ${\Lambda _x}$ period length. (d) The schematic fabrication processes of reflective PVGs with gradient periods. (e) Schematic of optical setup for two-beam interference.
Figure 1(b) shows the schematic diagram of a single unit with 16 gradient regions (named by R1-R16) in x-z plane of PVG sample, where the CLCs twist in vertical direction along a slanted helix axis which are represented here by the above schematic diagram for convenience of description. α is the twist angle of molecular optical axis and z axis. Here, configuration of four regions of R1, R4, R13, R16 are selected for schematic expression. Within regions from R1 to R16 in x-z plane, the transmittance of corresponding photomask reduces gradually, resulting in gradually reduced exposed UV light intensity thus gradually reduced average pitch of CLC (longest in R1 and shortest in R16). In addition, as the UV light is exposed along -y direction from top to bottom, the pitch of CLC in all regions gradually reduces from top to bottom along -y direction due to gradient absorption of UV dye, resulting in the longest pitch in top part (${\Lambda _t}$) and the shortest pitch in bottom part (${\Lambda _b}$). The schematic cross sections (x-y plane) of LC configuration for selected regions of R1, R4, R13, R16 are demonstrated in Fig. 1(b). Figure 1(c) demonstrates the orientation of SD-1 molecules within ${\Lambda _x}$ (${\Lambda _x}$ is the horizontal period at the bottom surface in x-z plane), which is used to precisely align the LC molecules on the horizontal plane. φ is the tilted angle of grating vector K. In our experiment, the period length in vertical direction ${\Lambda _y}$ and Bragg period length ${\Lambda _B}$ in slanted direction are both gradient.
For reflective polarization volume grating (PVG) device, it exhibits uniform molecular rotation and a slanted helical axis. The horizontal period (${\Lambda _x}$) and vertical period (${\Lambda _y}$) represent the distance that LC molecules rotate 180° in the x and y directions, respectively. The relationship between the three parameters is [28],
Under the condition of photoalignment material and chiral liquid crystal self-spiral assembly function, LC molecules rotate periodically in three-dimensional space, and α is expressed as:
where the lengths of ${\varLambda _x}$ and ${\varLambda _y}$ can be determined by [15],For PVGs, when the thickness of liquid crystal polymer is larger than 10 times of pitch (p), the Bragg diffraction condition can be satisfied after the vertically incident light enters the volume grating, which can be expressed by [29]:
The Bragg period length (${\Lambda _B}$) is equal to half the pitch length (p) of the chiral liquid crystal, and the chiral liquid crystal pitch length can be expressed as [30]:
where HTP and c are the helical twist force constant and concentration of the chiral dopant, respectively.The schematic fabrication processes of reflective PVGs with gradient periods mentioned above are shown in Fig. 1(d). The glass substrate was cleaned with acetone, isopropyl alcohol and ethanol in sequence. The photo-alignment material azobenzene sulfonic dye (SD-1) (dissolves in dimethylformamide with 1 wt.%) was spin-coated onto cleaned glass substrates at 500 rpm for 5 s and then 3,000 rpm for 30 s to fabricate uniform thin films (step i). The film was dried on a hotplate at 100 °C for 10 minutes. The photo-alignment substrate coated with SD-1 was obtained by two-beam interferometer of left-handed circularly polarized light (LCP) an right-handed circularly polarized light (RCP) to achieve PB grating pattern (step ii). Next, the PVGs precursor was spin-coated onto the exposed sample, where the precursor was prepared by a mixture of RM257, chiral dopant R5011, photo-initiator Irgacure 651, UV dye (Avobenzone) and surfactant (Zonyl 8857A). The RM mixture was firstly spin-coated on the glass with photo-orientation pattern at the speed of 500 rpm for 30 s, and then it was placed it on a hot stage at 80°C for two minutes. This step was to volatilize the remaining toluene solvent and make the chiral liquid crystal better self-spirally aligned (step iii). Finally, the PVG sample was cured by UV light on a hot plate at 60°C through the designed photomask with gradient transmittance (step iv).
Figure 1(e) demonstrates the schematic of optical setup for two-beam interference. The beam from a linearly polarized Ar+ laser (${\lambda _e}$= 488 nm) was expanded, collimated by lens, and then separated into to two beams (s and p waves) by a polarizing beam splitter (PBS) cube, while a half wave plate (HWP) was applied before the PBS to obtain equal intensity for two waves. Those two beams then passed through a quarter wave plate (QWP) separately to generate two opposite-handed circularly polarized beams, where the glass substrate coated with SD-1 was placed in the interference path to receive the PB grating pattern. In this experiment, the exposure dosage of UV light was 5 J/cm2, the angle between two exposure beams is 2$\theta $=59°, and the horizontal period of the grating pattern was estimated to be ${\varLambda _x} = {\lambda _e}/2sin\theta $=495 nm.
In our experiment, several samples of precursor with different material compositions have been tested, as shown in Table 1. The purpose of designing such a control group is to find the material parameters of the precursor sample with the largest central wavelength shift range. From samples A1 to A4, the concentration of RM257 (chiral dopant R5011) decreases (increases) from 93.15 wt.% (2.15 wt.%) to 93.09 wt.% (2.21 wt. %), while the concentration of IR651, Avobenzone, and Zonyl 8857A are fixed at 3.00 wt.%, 1.50 wt.%, and 0.20 wt.%, respectively. From sample B1 to B3, the concentration of RM257 (UV dye Avobenzone) decreases (increases) from 93.11 wt.% (1.50 wt.%) to 92.71 wt.% (1.90 wt.%), while the concentration of IR651, Zonyl 8857A, and R5011 are fixed at 3.00 wt.%, 0.20 wt.%, and 2.19 wt.%, respectively. In our experiment, all the samples of precursor were diluted in toluene with mass ratio of liquid crystal mixture/toluene solution to be 1:3. The different UV exposure dosages are demonstrated in Table 2, where the conditions of C1∼C6 represents the different UV exposure dosage that is determined by times of UV intensity and exposure time for all 7 samples in Table 1. It can be seen that the applied UV dosage is varied from 4.661 J/cm2 to 0.458 J/cm2.
Table 1. The material compositions of samples.
Table 2. The exposure dosage of samples.
To select the optimized sample for gradient PVG fabrication, the relationship of central wavelength position in transmittance spectrum as a function of exposure dosage has investigated in different groups of samples. It worth mentioning that there is no photomask applied in exposure process for sample selection. The first group of PVG samples (A1∼A4) with different concentration of chiral dopant ranging from 2.15 to 2.21 wt.% were prepared and cured under the same range of UV exposure dosage conditions (C1-C6) without photomask, where the relationship of center wavelength in transmittance spectrum as a function of UV exposure dosage are plotted in Fig. 2(a). It can be seen that, for chiral dopant’s concentration of 2.15 wt.% (A1), 2.17 wt.% (A2), 2.19 wt.% (A3) and 2.21 wt.% (A4), the corresponding central wavelength varies from 539 nm to 540.5 nm, 534 nm to 539.5 nm, 523 nm to 532.5 nm, and 515 nm to 521 nm when the UV exposure dosage increases from 0.458 J/cm2 to 4.661 J/cm2, leading to a shift of central wavelength of 1.5 nm, 5.5 nm, 9.5 nm, and 6 nm, respectively. With the increase of concentration of chiral dopant from 2.15 wt.% to 2.21 wt.%, the central wavelength decreases as blue shift as a whole. In contrast, for single sample of A1 to A4, the central wavelength shows gradually increase trend (red shift) with increase of UV exposure dosage, where the largest red shift of 9.5 nm is achieved in sample A3 with 2.19 wt.% of chiral dopant. Samples of group B is selected based on sample of chosen A3, where the B1 is exactly the same to A3, to investigate the effect of concentration of UV dye of on central wavelength.
Fig. 2. (a) The relationship of center wavelength in transmittance spectrum as a function of UV exposure dosage for samples of A1-A4. (b) The center wavelength position as a function of exposure dosage at different UV dye.
Figure 2(b) shows the relationship of center wavelength in transmittance spectrum as a function of UV exposure dosage are plotted for the second group of samples B1-B3 under the same range of UV exposure dosage conditions (C1-C6) without photomask. It can be seen that, for UV dye’s concentration of 1.50 wt.% (B1), 1.70 wt.% (B2), and 1.90 wt.% (B3), the corresponding central wavelength varies from 523 nm to 532.5 nm, 512 nm to 523.5 mm, and 509 nm to 514 nn, when the UV exposure dosage increases from 0.458 J/cm2 to 4.661 J/cm2, leading to a shift of central wavelength of 9.5 nm, 11.5 nm, and 5 nm, respectively. With the increase of concentration of UV dye from 1.50 wt.% to 1.90 wt.%, the central wavelength decreases as blue shift as a whole. In contrast, with the increasing exposure dosage from 0.458 J/cm2 to 4.661 J/cm2, red shift occurs for all three samples B1-B3, where the largest red shift of 11.5 nm is obtained in sample B2. In the following gradient PVG fabrication process, sample B2 is selected due to the largest shift of central wavelength within certain exposure range thus the highest sensitivity of ratio of CLC pitch/UV intensity, which is critical in fabrication of gradually change pitch in CLC by varying exposure UV intensity. For B2 used in our experiment in fixed exposure dosage of 4.661 J/cm2 (100%), the most obvious blue shift of central wavelength is obtained in range of 7%−31% (0.326-1.450 J/cm2). Beyond 31%, the change (gradient) saturates up to 100%. Therefore, we chose the 37%−7% (the circle in Fig. 2(b) as following) to cover the region mentioned above to achieve the largest blue shift in different regions (R1-R16).
3. Results and discussion
Figure 3(a) shows the photograph of designed photomask, with size of 3 cm × 1.7 cm, including 250 × 142 (35500) repeated units. Each unit (with size of 120 µm × 120 µm), as shown in polarization optical microscopy (POM) image in Fig. 3(b), consists of 16 regions (4 × 4 matrix in yellow rectangle, named R1-R16, POM image is shown Fig. 3(c)), where the size of each region is 30 µm × 30 µm. The gradient photomask is designed according to optimized data of sample B2 (the detail is shown in Fig. 2) and the UV exposure dosage is set to be 4.661 J/cm2. Herein, the transmittance of R1-R16 is set to be 37%−7% (Fig. 3(d)), with −2% difference in between, which results in exposure dosage of 1.725 J/cm2−0.326 J/cm2 to obtained the largest gradient or shift in central wavelength.
Fig. 3. (a) Photography of designed photomask. (b) POM image of 5 × 5 units. (c) POM diagram of a single unit with 16 regions. (d) Schematic diagram of a single unit with transmittance of R1-R16 regions. (e) Schematic polarization responses of proposed gradient PVG with right-handedness on glass substrate. (f) Experiment setup for measurement of reflection or diffraction parameters of proposed gradient PVG. (g) The photograph of gradient PVG sample.
Figure 3(e) demonstrates the schematic polarization response of proposed gradient PVG with right-handedness on glass substrate. For linearly polarized (LP) light incidence, the LCP light will directly pass through as transmitted light while the RCP light (with wavelength in reflection band of gradient PVG) will be reflected with diffraction angle of β. The photo of experiment setup for measurement of reflection or diffraction parameters of proposed gradient PVG is demonstrated in Fig. 3(f), where the PVG sample was firstly fixed on a rotatable platform by a clamp and then immersed in a refractive index matching liquid (n = 1.56). The diffraction efficiency can be calculated by testing the optical power of incident light and reflected or diffracted beam. The photograph of gradient PVG sample is shown in Fig. 3(g).
Figure 4(a) plots the transmittance spectrum in range of 400-700 nm of proposed gradient PVG fabricated with photomask (red curve, expose dosage of 4.661 J/cm2) and uniform PVG fabricated without photomask (black curve, expose dosage of 0.663 J/cm2) for comparison, respectively. For the uniform PVG (based on sample of RM257:R5011:IR651:Zonyl 8857A = 94.59:2.21:3:0.2), the structure is uniformed due to non-existence of UV dye. It can be seen that, with similar position of central wavelength (515 nm and 516 nm, respectively), the bandwidth of gradient PVG (84 nm) is much wider than that of uniform PVG (53 nm), which is consistent with expectation.
Fig. 4. (a) Transmittance spectrum in range of 400-700 nm of proposed gradient PVG fabricated with photomask (red curve) and PVG fabricated without photomask (black curve), respectively. (b) The results of diffraction efficiency as a function of incident angle, black curve denote the uniform PVG (measured result), red curve denote the gradient PVG (measured result), and green curve denote the gradient PVG (simulated result). (c) The transmittance spectra of gradient PVG as whole region and four separated regions (R1, R6, R11, R16). (d) Simulated transmittance spectra of gradient PVG as whole region and four separated regions (R1, R6, R11, R16).
The diffraction efficiency as a function of incident angle for proposed gradient PVG and uniform PVG are plotted in Fig. 4(b). Herein, the wavelength of incident light is 532 nm. For PVG (without photomask), the angular bandwidth with diffraction efficiency greater than 80% is only 38°(from 0° to 38°), while for the gradient PVG (with photomask) the angular bandwidth with diffraction efficiency greater than 80% is largely broaden to 61° (from −8° to 53°), with increase of 60.5%. It can be seen that, with the application of proposed photomask with gradient grey, the achieved angular bandwidth with high diffraction efficiency (>80%) is significantly improved (with peak efficiency of 98%), which is larger than previously reported result of 54° by chirp PVG with gradient pitch along vertical direction only [25]. The simulation of average angular response of gradient PVG is also plotted in Fig. 4(b) as GP simulation. The angular bandwidth of gradient PVG is 58° (from −7° to 51°), with difference of 4.9% from the experimental data. Since the gradient model of 16 regions is very complex, the boundary condition setting in the simulation model will have certain limitations, so the average angular response of the 16 regions has a certain difference from the experiment.
Figure 4(c) demonstrates the transmittance spectra of gradient PVG as whole region and four separated regions (R1, R6, R11, and R16) along the diagonal of 4 × 4 matrix measured by UV-Vis-NIR microspectrophotometer (20/30 PVTM, CRAIC Technologies, USA), where the measured central wavelengths of R1, R6, R11, and R16 are 520 nm, 516 nm, 513 nm, and 508.5 nm (averaged central wavelength of 514.4 nm with shift of 11.5 nm), respectively, and the bandwidth of R1, R6, R11, and R16 are 72 nm, 74 nm, 72 nm, 71 nm (averaged bandwidth of 72.25 nm), respectively. It is expected that the central wavelength and bandwidth of whole region are the averaged and superimposed (considering the shift of central wavelength and average bandwidth) results of all 16 region, respectively. In our experiment, the measured central wavelength and bandwidth of whole region are 515 nm and 84 nm, respectively, which is closed to the estimated value (based on the selected 4 regions mentioned above) of 514.4 nm, and 83.75 nm (11.5 nm + 72.25 nm).
For a cholesteric liquid crystal, the Bragg reflection bandwidth can be widened by changing the structure of pitch, which can be realized when the pitch length becomes gradually. In our PVGs, a proper amount of UV dye was doped into the liquid crystal polymer molecules, it will form a gradient pitch length in the thickness direction of y axis. In the simulation model, the gradual pitch length was assumed to be changed from 385.6 nm to 411.4 nm, the pitch length of chiral liquid crystal corresponds to the 2π rotation angle of molecules, where the lengths of ${\Lambda _{y,bottom}}$ and ${\Lambda _{y,top}}$ are half of the pitch, respectively. The horizontal periodicity ${\Lambda _x} $ along x direction is fixed. The twist angle of the optical axis of liquid crystal molecules with gradient pitch length is defined by [31]:
Figure 4(d) demonstrates the simulated transmittance spectra of gradient PVG as whole region and four separated regions (R1, R6, R11, and R16) using finite element method (FEM) by commercial COMSOL software. In the simulation, for those four regions (R1, R6, R11, and R16), the horizontal period is set to be equal to each other, where ${\varLambda _{x1}} = {\varLambda _{x6}} = {\varLambda _{x11}} = {\varLambda _{x16}}$=495 nm. The thickness of grating is set to be 4 µm. For each gray-scale region, due to the existence of UV dye, when liquid crystal monomers are polymerized, the pitch of CLC changes gradually and decreases from top to bottom in the thickness direction (-y direction). For region R1, R11, and R16, the length of ${\varLambda _y}$ is set to be ${\varLambda _{y1,top}}$=191 nm and ${\varLambda _{y1,bottom}}$=188 nm, ${\varLambda _{y6,top}}$=188 nm and ${\varLambda _{y6,bottom}}$=185 nm, ${\varLambda _{y11,top}}$=185 nm and ${\varLambda _{y11,bottom}}$=182 nm, and ${\varLambda _{y16,top}}$=182 nm and ${\varLambda _{y16,bottom}}$=179 nm, respectively, resulting in bandwidth of 71 nm (485-556 nm), 75 nm (479-554 nm), 73 nm (476-549 nm), and 69 nm (474-543 nm) and central wavelength of 520.5 nm, 516.5 nm, 512.5 nm, and 508.5 nm. In addition, the simulated bandwidth and central wavelength of whole region are 84 nm (473-557 nm) and 515 nm. It can be seen that, the calculated results are consistent with experimental results (Table 3), indicating the effectivity of the simulation method for proposed gradient PVG.
Table 3. Simulated and experimental bandwidth and central wavelength.
Figure 5(a) shows the schematic illustration of augmented reality (AR) display system based on optical waveguide with proposed gradient PVG as a prototype. The linearly polarized light from image source of liquid crystal on silicon (LCoS) is firstly passed through a broadband quarter wave-plate (QWP) to convert to a circularly polarized light (right-handed here) and then collimated by a collimating lens. Next, the chromatic light is incident on the input coupler of proposed gradient PVG and diffracted (or reflected) into the waveguide (n = 1.90) in diffraction angle that larger than the total internal reflection (TIR) angle. After reflected by output PVG, the light is coupled out of the waveguide with the same angle as incidence and received by human eye. The photography of waveguide with input gradient and output PVG and image source of LCoS are shown in Fig. 5(b) and 5(c), respectively. The AR system with two different images of “fireworks” and “butterfly” are demonstrated in Fig. 5(d) and 5(e), respectively, where a camera is placed 15 mm away from the pupil grating to capture the image instead of eye.
Fig. 5. (a) Schematic illustration of augmented reality (AR) display system based on optical waveguide with proposed gradient PVG. The photography of (b) waveguide with input gradient and output PVG and (c) image source of LCoS. The AR system with two different images of (d) “fireworks” and (e) “butterfly”.
4. Conclusion
In summary, a gradient PVG with three-dimensional (3D) gradient periodic structure is proposed for waveguide near-eye display. Two-beam polarization interference with special designed periodic gradient photomask are applied to chiral-dopant reactive mesogens doped with UV dye for generating gradient 3D configuration of LC, resulting in gradient PVG with extended angle bandwidth of 61° while keeping 80% diffraction efficiency, with peak efficiency near 100%. The pitch gradients both from UV absorption grade in vertical direction and UV exposure grade in horizontal plane result in the broaden of angular bandwidth, thus the FOV. The underlying mechanism can be attributed to the superimpose of periodic sub-regions with gradient central wavelength and similar bandwidth, where the simulation is consistent with experiment. The proposed gradient PVG provides an effective method to broaden the angular bandwidth in waveguide for wide FOV AR display.
Funding
National Natural Science Foundation of China (62175098, U22A20163); National Key Research and Q1 Development Program of China (2022YFA1203700); Guangdong Basic and Applied Basic Research Foundation (2021B1515020097).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
References
1. B. C. Kress and M. Pace, “Holographic optics in planar optical systems for next generation small form factor mixed reality headsets,” Light Adv. Manuf. 3(4), 1–801 (2022). [CrossRef]
2. H. J. Jang, J. Y. Lee, G. W. Baek, et al., “Progress in the development of the display performance of AR, VR, QLED and OLED devices in recent years,” J. Inf. Disp. 23(1), 1–17 (2022). [CrossRef]
3. O. Cakmakci, Y. Qin, P. Bosel, et al., “Holographic pancake optics for thin and lightweight optical see-through augmented reality,” Opt. Express 29(22), 35206–35215 (2021). [CrossRef]
4. Z. Lv, J. Liu, and L. Xu, “A multi-plane augmented reality head-up display system based on volume holographic optical elements with large area,” IEEE Photonics J. 13(5), 1–8 (2021). [CrossRef]
5. J. Xiong, E. L. Hsiang, Z. He, et al., “Augmented reality and virtual reality displays: emerging technologies and future perspectives,” Light: Sci. Appl. 10(1), 216 (2021). [CrossRef]
6. D. Cheng, Q. Wang, Y. Liu, et al., “Design and manufacture AR head-mounted displays: a review and outlook,” Light Adv. Manuf. 2(3), 336–369 (2021). [CrossRef]
7. K. Yin, E. L. Hsiang, J. Zou, et al., “Advanced liquid crystal devices for augmented reality and virtual reality displays: principles and applications,” Light: Sci. Appl. 11(1), 161 (2022). [CrossRef]
8. T. Zhan, Y. H. Lee, G. Tan, et al., “Pancharatnam-Berry optical elements for head-up and near-eye displays [Invited],” J. Opt. Soc. Am. B 36(5), D52–D65 (2019). [CrossRef]
9. J. H. Park and B. Lee, “Holographic techniques for augmented reality and virtual reality near-eye displays,” Light Adv. Manuf. 3(1), 1–150 (2022). [CrossRef]
10. K. S. Shin, M. H. Choi, J. Jang, et al., “Waveguide-type see-through dual focus near-eye display with a polarization grating,” Opt. Express 29(24), 40294–40309 (2021). [CrossRef]
11. M. H. Choi, K. S. Shin, J. Jang, et al., “Waveguide-type Maxwellian near-eye display using a pin-mirror holographic optical element array,” Opt. Lett. 47(2), 405–408 (2022). [CrossRef]
12. A. Liu, Y. Zhang, Y. Weng, et al., “Diffraction efficiency distribution of output grating in holographic waveguide display system,” IEEE Photonics J. 10(4), 1–10 (2018). [CrossRef]
13. M. Shishova, A. Zherdev, S. Odinokov, et al., “Selective couplers based on multiplexed volume holographic gratings for waveguide displays,” Photonics 8(7), 232–244 (2021). [CrossRef]
14. B. Audia, P. Pagliusi, A. Mazzulla, et al., “Multi-wavelength optical patterning for multiscale materials design,” Photonics 8(11), 481–490 (2021). [CrossRef]
15. Y. Weng, Y. Zhang, J. Cui, et al., “Liquid-crystal-based polarization volume grating applied for full-color waveguide displays,” Opt. Lett. 43(23), 5773–5776 (2018). [CrossRef]
16. Z. Shen, Y. Zhang, A. Liu, et al., “Volume holographic waveguide display with large field of view using a Au-NPs dispersed acrylate-based photopolymer,” Opt. Mater. Express 10(2), 312–322 (2020). [CrossRef]
17. I. Stoica, I. Sava, E. L. Epure, et al., “Advanced morphological, statistical and molecular simulations analysis of laser-induced micro/nano multiscale surface relief gratings,” Surf. Interfaces 29(1), 101743 (2022). [CrossRef]
18. Y. H. Lee, K. Yin, and S. T. Wu, “Reflective polarization volume gratings for high efficiency waveguide-coupling augmented reality displays,” Opt. Express 25(22), 27008–27014 (2017). [CrossRef]
19. J. Kobashi, Y. Mohri, H. Yoshida, et al., “Circularly-polarized, large-angle reflective deflectors based on periodically patterned cholesteric liquid crystals,” Opt. Data Process. Storage 3(1), 61–66 (2017). [CrossRef]
20. J. Kobashi, H. Yoshida, and M. Ozaki, “Planar optics with patterned chiral liquid crystals,” Nat. Photonics 10(6), 389–392 (2016). [CrossRef]
21. Y. Weng, D. Xu, Y. Zhang, et al., “Polarization volume grating with high efficiency and large diffraction angle,” Opt. Express 24(16), 17746–17759 (2016). [CrossRef]
22. J. Xiong, G. Tan, T. Zhan, et al., “Breaking the field-of-view limit in augmented reality with a scanning waveguide display,” OSA Continuum 3(10), 2730–2740 (2020). [CrossRef]
23. C. Yoo, K. Bang, M. Chae, et al., “Extended-viewing-angle waveguide near-eye display with a polarization-dependent steering combiner,” Opt. Lett. 45(10), 2870–2873 (2020). [CrossRef]
24. Y. Gu, Y. Weng, R. Wei, et al., “Holographic waveguide display with large field of view and high light efficiency based on polarized volume holographic grating,” IEEE Photonics J. 14(1), 1–7 (2022). [CrossRef]
25. K. Yin, H. Y. Lin, and S. T. Wu, “Chirped polarization volume grating with ultra-wide angular bandwidth and high efficiency for see-through near-eye displays,” Opt. Express 27(24), 35895–35902 (2019). [CrossRef]
26. B. Y. Wei, W. Hu, Y. Ming, et al., “Generating switchable and reconfigurable optical vortices via photopatterning of liquid crystals,” Adv. Mater. 26(10), 1590–1595 (2014). [CrossRef]
27. L. L. Ma, S. S. Li, W. S. Li, et al., “Rationally designed dynamic superstructures enabled by photoaligning cholesteric liquid crystals,” Adv. Opt. Mater. 3(12), 1691–1696 (2015). [CrossRef]
28. Y. H. Lee, Z. He, and S. T. Wu, “Optical properties of reflective liquid crystal polarization volume gratings,” J. Opt. Soc. Am. B 36(5), D9–D12 (2019). [CrossRef]
29. M. Mitov, “Cholesteric liquid crystals with a broad light reflection band,” Adv. Mater. 24(47), 6260–6276 (2012). [CrossRef]
30. Q. Hong, T. X. Wu, and S. T. Wu, “Optical wave propagation in a cholesteric liquid crystal using the finite element method,” Liq. Cryst. 30(3), 367–375 (2003). [CrossRef]
31. C. L. Ting, T. H. Lin, C. C. Liao, et al., “Optical simulation of cholesteric liquid crystal displays using the finite-difference time-domain method,” Opt. Express 14(12), 5594–5606 (2006). [CrossRef]
