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Abstract
We present a volume holographic waveguide display by dispersing gold nanoparticles (Au-NPs) in acrylate-based photopolymer. The diffractive bandwidth and diffraction efficiency (DE) of the volume holographic grating (VHG) applied for waveguide displays are characterized and analyzed through both the simulations and experiments. The results show that the wavelength bandwidth of the VHG can be enlarged to 30 nm with a corresponding refractive index modulation (RIM) increased to around 0.08 by dispersing the Au-NPs with a concentration of 0.012 g/ml into the acrylate-based photopolymer. Finally, the green monochromatic waveguide display system with 30° horizontal field of view (FOV) is realized.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As one of the promising optical coupling elements for waveguide based augmented reality (AR) display, volume holographic gratings (VHGs) have received immense attentions in recent years because of high first order diffraction efficiency (DE), high see-through transmittance, small size and light weight [1–3]. A typical VHG-coupler is fabricated by recording the interference fringes of two coherent laser beams in the light sensitive materials, such as silver halide, dichromate gelatin, photo-resist and photopolymer [4]. Compared with other holographic recording materials, photopolymer has obvious advantages of low cost, long storage time, humidity-resistance and ease of fabrication, which are suitable for the industrial production [5,6]. The photochemical kinetics of photo-polymerization are shown in Fig. 1. Generally, the photopolymer mainly contains monomers, initiators, photo-sensitizers and other components [7]. Under illumination, the photo-sensitizer absorbs photons that respond to light waves of sensitive wavelengths and excited to a high energy level, and the energy released again in the relaxation process [8,9]. The released energy is transferred to the initiator to produce active molecules (free radicals) [10]. The radicals continuously transfer to a large amount of monomer molecules, which generate new radicals. The chains are continuously growing when two monomers with same radicals meet. On the other hand, a chain termination reaction occurs when two monomers containing different radical groups, then the polymerization process ends [11].
The VHG is formed in interference exposure based on the above described photo-induced polymerization. The bright and dark fringes are formed inside the VHG by the interference exposure. At the initial exposure, the monomers are firstly polymerized in the bright region, the monomer concentration continuously decreases in the process. Therefore, the monomer concentration gradient is formed between the bright and dark regions, which causes monomers in the dark region diffusing to the bright region. In the polymerization process, monomers are continuously consumed for polymerization, and diffuses from dark region to bright region. Finally, the refractive index modulation (RIM) is initially generated between dark and bright regions.
Field of view (FOV) is an important indicator parameter for evaluating the AR display system. For VHG-based waveguide near-to-eye display systems, FOV depends on the response bandwidth of the VHG, which is limited by the RIM of the photopolymer [12,13]. Sony Corporation realized monochromatic green holographic waveguide (AR glasses SED-E1) with only 20° diagonal FOV in 2009 and also proposed the full color AR glass prototype with 24° diagonal FOV in IDW'18 [14].
The RIM of the photopolymer represents the refractive index difference between the binder and monomer. Selecting high refractive index monomer is an effective method to increase the RIM of photopolymer. However, the refractive index of commercial acrylate-based monomer only reach 1.55-1.58 and the refractive index of binder is around 1.5, which limits the conventional RIM for photopolymer in consumer market to only around 0.035 (such as Bayfol HX200) [23,24]. In recent years, researchers have done excellent work to increase the RIM of the photopolymer. Some kinds of high refractive index inorganic nonmetallic nanoparticles were dispersed into the water solubility photopolymer for increasing the RIM [15–18]. However, the Inorganic non-metallic nanoparticles are difficult to disperse in the organic solvents or water, which cannot satisfy the requirements of clarity for waveguide display.
In 2014, Cao et al. proposed a holographic kinetic model of the mixed volume gratings in a bulk Au-NPs doped photopolymer and successfully fabricated gold nanoparticle-doped PQ/PMMA photopolymer for multiplexing holography [19,21]. In the Au-NPs dispersed photopolymer, Au-NPs will diffuse from the bright to the dark regions driven by the photopolymerization process of the monomers. Since Au-NPs are not assumed, the NPs concentration will increase in the bright regions as a result of the consumption of monomers. Consequently, the spatially periodic distributions of Au-NPs and photoproduct of monomers are formed under the interference fringes in the Au-NPs dispersed photopolymer. At the resonance wavelength of Au-NPs, a strong absorption grating can be activated in phase with the refractive index grating. The periodic spatial distribution of Au-NPs in the photopolymer can theoretically increase the RIM of the VHG. However, PQ/PMMA is not suitably applied to the waveguide display because of the toxicity, low RIM and exposure sensitivity, compared with the acrylate-based photopolymer [25–28]. In this work, the Au-NPs was applied to the acrylate-based photopolymer system, which confirmed that it can also improve the RIM of the acrylate-based photopolymer and enlarge the response bandwidth of VHG, thus benefiting the FOV of waveguide display.
In this paper, we fabricate the high RIM acrylate-based photopolymer by dispersing the Au-NPs. The monochromatic green holographic waveguide using the Au-NPs dispersed photopolymer is realized to achieve large FOV. A prototype with complete near-to-eye display system is demonstrated, and actual display results are also shown. This work confirms the possibility of Au-NPs dispersed acrylate-based photopolymer as the recording material for the VHG-coupler, and provides a new approach for enlarging the FOV for the volume holographic waveguide display.
2. Experiment
2.1 Preparation of the Au-NPs dispersed acrylate-based photopolymer
The acrylate-based photopolymer was fabricated by mixing four components [11], including polycellulose acetate (55.4 wt. %), N-vinyl carbazole (23.5 wt. %), tetrahydrofurfuryl acrylate (9.8 wt. %), 2-phenoxyethy acrylate (6.1 wt. %) in the mixed organic solvents which contains benzene 60% and dimethylformamide (DMF) 40%. These four binders and monomers are the key components to polymerize in the photo-initiation process. (2, 2’-bis (2-dichlorophenyl) −4, 4’, 5, 5’tetraphenyl- 1, 2’-biimidazole (5.14 wt. %) was served as the free radical photo-initiator to promote the polymerization of the monomers. The light-sensitive dye erythrosine B (0.06 wt. %) molecules firstly absorbs the photon’s energy and transforms the initiator intra molecularly into a reactive species, then initiating radical attacks monomers, which finally converts into the polymer [9].
Then Au-NPs were utilized to increase the RIM of acrylate-based photopolymer. To avoid the agglomerate, we employed the Au-NPs modified with the lipophilic groups (Macklin Corporation). The average diameter of the Au-NPs is 6-8nm characterized by the transmission electron microscope (TEM), as shown in Fig. 2. The concentration of Au-NPs in pure photopolymer solution was 0.012 g/ml.
The fabrication process is illustrated in Fig. 3, all the mentioned reagents were added into the mixed organic solvents and stirred at 1000 rpm for 1 h until completely dissolved. Then the dissolved solution was scraped onto the transparent A4 size 50um thick Polyethylene terephthalate (PET) film using the spatula. The film coated with the photopolymer solution was dried for 30 minutes in the dark clean room (25 °C, 40% RH), then a 15um thick photopolymer dry film was obtained. After that, the film was laminated on the clean 1mm thick transparent silica glass to get the dry holographic plate. The transmission spectra of the prepared photopolymer film is also measured. It can be seen that there are two absorption peaks at 452nm and 550nm, which represents the photopolymer is sensitive to ultraviolet, blue and green light. The spectra was measured by the UV/VIS spectrophotometer (TU-1901 from PERSEE Corporation).
Fig. 3. Flowchart for fabricating the holographic dry plate. Transmission spectra of the Au-NPs dispersed photopolymer is also shown in the figure.
2.2 Design of the holographic waveguide
Here, two reflective type VHGs are respectively laminated at the both ends of waveguide as the in-coupler and out-coupler. The glass substrate here has a 25mm×75mm rectangular shape, the substrate thickness is 1mm, and the refractive index is 1.52, which is same as the photopolymer. The size of in-coupling VHG is 20mm×15mm, the out-coupling VHG is 20mm×45mm. Figure 4(a) shows the image transmission diagram of the designed holographic waveguide. The image firstly enters the waveguide, then diffracted with large angle by the in-coupling VHG, and propagates in the waveguide with total-reflection, finally the image is diffracted out of the opposite side of the waveguide by the out-coupling VHG.
Fig. 4. (a) Image transmission diagram of the holographic waveguide; (b) Vector circle analysis of VHG-couplers; (c) Schematic diagram of the grating cross section.
The grating vector ${\textrm{K}_\textrm{g}}$ of in-coupling and out-coupling VHGs are completely consistent, as shown in Fig. 4(b). The grating vector ${\textrm{K}_\textrm{g}}$ determines the central operation wavelength of the VHG-couplers, as well as the diffractive angles. In the fabrication process, we can control the direction and size of the grating vector ${\textrm{K}_\textrm{g}}$ by tuning the angle between the object light vector ${\textrm{K}_\textrm{o}}$ and reference light vector ${\textrm{K}_\textrm{r}}$.
The grating parameters of in-coupling and out-coupling VHGs can be calculated by the Bragg equation we reported in Ref. [30]:
where ${\lambda }$ is incident wavelength, n is the refractive index of the grating, $\Lambda $ is the grating period, and ${{\theta }_B}$ is the diffraction angle when Bragg diffraction happens.In this work, the construction wavelength ${\lambda }$ at normal incidence is designed as 532 nm. The grating period $\Lambda $ of 30$^\circ $slanted VHG is 202.07 nm, which is corresponded to diffractive angle of 60$^\circ $ for normal incidence of green light.
2.3 Simulation model for the waveguide applied VHG
The setup of the reflective type VHG in a FEM simulation model established with the commercial software COMSOL is depicted in Fig. 5(a). The 532nm light beam is incident from bottom to top along y direction, the perfect matched layer is set to absorb the unwanted boundary reflections and stray light on both the top and bottom ends of the model. The incidence port are used to set the light field and the incident light energy. In the simulation model, the grating parameters of VHG is set consistent with the above design. The refractive index n distribution inner the grating cross-section is described by the equation [22].
The material of VHG is set to have a value of RIM ranging from 0.035 to 0.1, and the glass has a refractive index of 1.52, which is consistent with the material we used in experiments.
In addition, at the resonance wavelength of Au-NPs, a strong absorption modulation will be activated in phase with the refractive index modulation grating, the conductivity ${\sigma }$ represents the absorption factor, which can be described by the equation in the Ref. [21].
As depicted in Fig. 5(b), the VHG is capable of diffracting the incident beams at wavelength of 532 nm with high efficiency and large angles of 60$^\circ $ (satisfying the total inner reflection condition in glass).
2.4 Fabrication of the holographic waveguide
In order to evaluate the improvement on the RIM, two holographic waveguides are both fabricated using photopolymer without Au-NPs and Au-NPs dispersed photopolymer, and the interference exposure optical setup is shown in Fig. 6(a). The green laser light beam which comes from the single longitudinal solid state laser (200mW, CNI) is firstly separated into two beams through the beam splitter (GCC-4010, Daheng Optics), and expanded to the large circular spot by the optical filter (Newport) separately, then these two expanded spots are collimated with the Fourier lens (GCO-02, Daheng Optics). The size of spots are adjusted by the iris and the recording angle between object beam and reference beam is controlled by the mirror. To ensure the diffracted light of the VHG can be totally reflected off the air-glass interfaces and propagate in the waveguide, a K9 right-angle prism is used here to fabricate the VHG-couplers. The reference light is normally incident to the holographic dry plate, the object light is incident towards the hypotenuse of prism at a certain angle ${{\theta }_1}$ . According to Fresnel's law, the refraction will occur when the object light from the air into the prism, the actual incident angle ${{\theta }_2} = \textrm{arcsin}({\textrm{n}_1}\textrm{sin}{{\theta }_1}/{\textrm{n}_2})$ towards the holographic dry plate. Considering that the photopolymer syrup has a similar refractive index to the glass substrate and prism, the angle of the two recording beams in the photopolymer is about 60$^\circ $. In addition, the prism is also used to avoid the trapping of the recording beam due to the TIR on the glass substrate, and the refractive index matching oil is used to fill up the air gap between the holographic dry plate and prism. When the first grating was done, the second grating was recorded in the other end of the sample by mirror operation.
Fig. 6. (a) Holographic recording optical setup for volume holographic waveguide; (b) Post-treatment flow diagram.
For achieving high diffraction efficiency, the intensity ratio between object and reference light beams entering onto the holographic dry plate should be controlled as 1:1 by the adjustable attenuator (GCO-07, Daheng Optics). The light beam intensity is 5 mW/cm2 measured by the laser power meter (OPHIR Vega) and the exposure dosage is 30 mJ/cm2.
The post-treatment process after exposure shown in Fig. 6(b) should be conducted to fabricate the final sample. To ensure the monomers diffusing completely, the plate should be placed for 2 minutes in dark room after exposure. After the UV curing process for 5 minutes, a fixed polymer network can be generated. Finally, the holographic waveguide sample is heated in an oven at 110 $\circ{\textrm{C}}$ for 5 minutes to allow the high-folding monomers curing fully.
3. Simulation and experiment results
3.1 Simulated and experimental DE curve of VHG
Generally, the RIM of the Au-NPs dispersed photopolymer can be characterized by the diffractive wavelength bandwidth of the VHG [20]. As shown in Fig. 7, the relationship between diffractive wavelength bandwidth of VHG and RIM is quantitatively analyzed in FEM model. We can see from the figure that when RIM increases from 0.01 to 0.1, the diffractive wavelength bandwidth of VHG also increases from 6nm to 39nm.
Fig. 7. The relationship between diffractive wavelength bandwidth and RIM of Au-NPs dispersed photopolymer simulated by COMSOL software
The simulated and measured DE curves of VHGs fabricated with photopolymer without Au-NPs are shown respectively in Fig. 8(a). When the reconstructing light wavelength changes from 450 nm to 600 nm at normal incidence, the simulated diffractive wavelength bandwidth is around 13 nm and the maximum DE reaches 100% in FEM model, as well as the corresponding RIM is 0.03. It is certain that the measured DE curve match well with the simulation results. We can see that the maximum DE of the VHG using the acrylate-based photopolymer without Au-NPs nearly reaches 95%, and the diffractive wavelength bandwidth is same as that of simulated results. Compared with the RIM of acrylate-based photopolymer without Au-NPs, the simulated and measured results of Au-NPs dispersed photopolymer are shown in Fig. 8(b). It can be seen that the diffractive wavelength bandwidth can reach about 30 nm, which is much larger than that of the photopolymer without Au-NPs. Moreover, the maximum DE is also increased from 95% to 100%. Depending on the correspondence between the diffractive bandwidth and RIM analyzed in Fig. 7, the RIM of Au-NPs dispersed photopolymer reaches around 0.08, and the simulated and measured DE curves fit well, as shown in Fig. 8(b).
Fig. 8. (a) Simulated and measured DE curves of waveguide applied VHG using pure photopolymer, the RIM of simulated VHG is set as 0.03; (b) Simulated and measured DE curves of waveguide applied VHG using Au-NPs dispersed photopolymer, the RIM of simulated VHG is 0.08. All the above DE curves are measured by the UV/VIS spectrophotometer TU-1901 made by PERSEE Corporation.
The results show that dispersing Au-NPs into the acrylate-based photopolymer can help the RIM increase from 0.03 to 0.08. Thus, the diffractive wavelength bandwidth is enlarged from 13 nm to 30 nm. Furthermore, the DE is also increased from 95% to 100%.
3.2 Display system construction and display results
In order to verify the display function of the holographic waveguide based on the Au-NPs dispersed photopolymer, we developed the near-to-eye waveguide display system, which is illustrated in Fig. 9. Here, images exited from the LCoS based imaging source are coupled into human eyes with two reflective VHGs on the both ends of the waveguide. The in-coupling VHG is laminated on the bottom side of the glass substrate, and the out-coupling VHG is placed on the other side. The collimator is used to collimate the light coming from the pixels of micro-display [29].
Fig. 9. Schematic diagram of the volume holographic waveguide display. VHGs are used as the optical couplers for coupling the green spectral lights propagating in the waveguide layers. The photograph shows the appearance of the prototype we designed.
The appearance of the display system is shown on the top right corner of Fig. 9. A white box is used to assemble the micro-projection optical system including LCoS (HX7816, Himax) optical module and collimation lens. The imaging system can provide a diagonal FOV of 30° with 4mm entrance pupil.
The display result is shown in Fig. 10. A green image which consists of three circles is used as the example input, and the green circles shown in the Fig. 10(a) represent the FOV of 10°, 20° and 30°, respectively. The camera is placed in front of the horizontally placed waveguide out-coupler at the position 16mm which was the designed eye-relief. As shown in Fig. 10(b), the horizontal FOV of display frame is around 15°. By contrast, the horizontal FOV of the volume holographic waveguide using Au-NPs dispersed photopolymer can be increased to 30°, shown in Fig. 10(c). The experimental results show that the monochromatic green holographic waveguide can realize the function of image transmission. And compared to the holographic waveguide using photopolymer without Au-NPs, the technical solution can increase the FOV effectively.
Fig. 10. Photograph of the output image of the prototype using two different volume holographic waveguide samples. (a) Original input image; (b) Sample fabricated with photopolymer without Au-NPs; (c) Sample fabricated with Au-NPs dispersed photopolymer.
4. Summary
In summary, we have proposed volume holographic waveguide based on Au-NPs dispersed acrylate-based photopolymer. Due to the local surface plasma resonance properties of Au-NPs, the Au-NPs dispersed photopolymer has higher RIM than the conventional photopolymer, which benefit the FOV of the volume holographic waveguide. Therefore, we employed the high RIM photopolymer to realize a monochromatic green volume holographic waveguide display with the large FOV. The interference exposure with photo-initiated polymerization method was utilized to fabricate the VHG-couplers based on Au-NPs dispersed acrylate photopolymer. The holographic diffraction characteristics and processing parameters of reflective VHG-couplers were studied through both of the rigorous FEM model and experimental results. The experimental DE of volume grating increases from 95% to 100%, and the wavelength diffractive bandwidth can be enlarged from 13nm to 30nm. We also demonstrated a prototype to validate the display function of our designs. The results showed that a clear monochromatic green display with a diagonal FOV of around 30° was realized. This work proves that volume holographic waveguide based on Au-NPs dispersed photopolymer is an effective and feasible solution for wide-field AR display.
Funding
National Key R&D Program of China (2016YFB0401201); Nanjing Industry Academia Research Funding Project (201722003).
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