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文中采用超表面耦出结构的AR近眼显示光波导结构示意图如图1 所示,图1(a)为入射光在波导全反射角为50°时出射光准直耦出示意图,图1(b) 为20°视场角的出射光示意图,其中采用Micro-LED显示芯片作为近眼显示系统的Micro-projector像源,Micro-LED具有亮度高、高发光效率、低能耗、高反应速度、高对比度与色彩饱和度,被认为是新一代理想显示技术[24-28]。图1中大面积蓝色部分为光波导结构,波导内填充材料为二氧化硅(SiO2)(折射率nc =1. 46),其全反射临界角为43.23°,当入射光在波导内传播角度大于43.23°时可实现全反射(n0 = 1) ,其耦入部分采用切角为60°的波导,通过改变入射光角度,使入射光在波导内传播全反射角度在50°~75°。在光耦出区域设计了超表面结构,该超表面由衬底和周期性纳米柱组成,其衬底材料为SiO2,衬底和波导为同一材料,纳米柱材料为氮化硅(Si3N4),Si3N4折射率虚部为0,是无损介质,即在超表面材料中使用氮化硅可以实现更高的传输效率,且具有较高的热稳定性和化学稳定性,可以在高温环境下工作而不会发生严重的性能退化,工作的时间更长而损耗小。其共振特性会引起相位变化,使得结构表面形成相位分布的周期性梯度变化,并使入射光发生偏转。
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传统的光学元件根据光在传播过程中逐步累积相位差,实现了对光束进行偏转或聚焦,这一过程相位是渐变的,不存在相位突变。而超表面通过人为设计的纳米尺度结构单元,使该界面具有传统界面所不具备的特性,当光波经过该界面时,实现对光波或电磁波的任意调制。超表面单元结构可以在二维平面形成突变相位,可以用费马原理解释突变相位的产生,即当光波在介质中进行传播时,所经过的光路的光程是相等的[29]。对于由两种介质构成的光学界面,平面波从A点以入射角θi入射,并经过界面上B点,若平面波沿界面方向的相位不连续,则A、B两点之间存在两条无限接近的光传播路径,它们之间的相位差为零,即:
$$ \begin{gathered} \left[ {{k_0}{n_i}\sin {\theta _i}{\rm{d}}x + (\phi + {\rm{d}}\phi )} \right] - ({k_0}{n_t}\sin {\theta _t}{\rm{d}}x + \phi ) = 0 \\ \end{gathered} $$ (1) 式中:θt为折射角;ϕ和ϕ+dϕ为两条路径中的光分别穿过界面时的相位;
dx 为两条路径在界面处的距离;ni和nt分别为两个介质的折射率;k0=2π/λ0,λ0为波长。整理公式(1)可得: $$ {n}_{t}\mathrm{sin}{\theta }_{t}-{n}_{i}\mathrm{sin}{\theta }_{i}=\frac{{\lambda }_{0}}{2\pi }\cdot \frac{{\rm{d}}\phi }{{\rm{d}}x} $$ (2) 在入射角不变的情况下,改变界面的相位梯度dϕ/dx,则折射角θt随之改变,符合广义折射定律(图2)。
传输相位型超表面主要为基于介质材料的单层周期结构,可以用等效折射率理论解释,即通过改变亚波长纳米柱的占空比来改变等效折射率,从而实现相位调控光波经过微结构所产生的相位累积,可以用下式近似表示为:
$$ \Delta \phi = 2\pi {n_{eff}}\frac{{{\lambda _0}}}{h} $$ (3) 式中:neff、h、λ0分别为微结构的等效折射率、微结构的高度和工作波长。通常情况下,为了方便工艺的制备,微结构的高度是固定的,通过改变微结构的有效折射率来实现在特定波长和特定高度条件下的波前控制。利用时域有限差分算法可得到垂直入射时光束透过超表面结构所产生的相位延迟,该相位延迟使得界面的相位呈现梯度变化,从而改变光的透射方向,据此可设计透射式超表面结构以实现垂直入射时的光束偏转。
Design of waveguide decoupled metasurface for augmented reality display optical engine (invited)
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摘要: 增强现实(AR)近眼显示光学引擎是新型显示光学设计领域的研究热点之一,它将虚拟图像投射到现实物理环境中进行显示,在空间上增强、融合和补充了物理世界。AR 近眼显示光学引擎在光学系统集成化和微型化方面有较高要求,眼镜形态的AR近眼显示光学设备是未来必然发展趋势。光学超表面是一种由亚波长单元结构在二维平面上周期排布而成的人工结构阵列,通过单元结构和电磁波的相互作用实现对光场中振幅、相位和偏振的任意调控,同时具有体积小、效率高、结构紧凑等特点,在近眼显示应用中具有很大潜力。文中在AR光学引擎设计中引入一种传输相位型超表面光波导耦出结构,该超表面单元引入了突变相位,通过对超表面的等相位面调控改变光经过波导耦出的角度,使出射光效率最高达到77%,并实现20°视场角,为AR光波导结构设计提供一种可行方案,有望为下一代人机交互显示平台提供解决方案。Abstract:
Object The optical engine design of augmented reality (AR) near-eye display is one of the research hotspots in the field of display technology. It projects virtual images to the real physical environment for display, and simultaneously enhances, merges, and complements the physical world in space. AR near-eye display optical engine has high requirements for the integration and miniaturization of optical system, and the glass-like AR near-eye display optical device is an inevitable development trend in the future. Optical metasurface is an artificial structure array composed of subwavelength unit structure periodically arranged on a two-dimensional plane. It realizes arbitrary regulation of the amplitude, phase, and polarization of the light field through the interaction of the unit structure and electromagnetic wave. At the same time, it has the characteristics of small size, high efficiency, and compact structure, and has great potential in near-eye display applications. Methods In this paper, a metasurface structure is designed as the decoupled structure of the AR near-eye display optical waveguide (Fig.1). The decoupled part adopts a waveguide with a cutting angle of 60°. By changing the angle of incident light, the incident light propagates inside the waveguide at 50°-75°. The coupled part of the metasurface has a height of 900 nm and a radius of 50-120 nm (Fig.6). The AR near-eye display optical waveguide is simplified and simulated in FDTD. The light source is placed inside the waveguide to simulate the total reflection of the incident light, and the decoupling angle is simulated by changing the incident angle. Results and Discussions When the collimated light is incident into the metasurface structure, the outgoing light deviates from the z-axis by −35° (Fig.7). The field intensity distribution is observed by placing a monitor or far-field calculation, and the deflection efficiency is calculated to reach 77%. In addition, the angle distribution of the outgoing light on the metasurface within the designed wavelength of ±30 nm is simulated, and it can be seen that the deflection angle of the device fluctuates within the designed angle of 5° (Fig.8). Since the same metasurface structure has a specific phase response to incident light at different angles, different wavefront adjustment of incident light at different angles can be realized. Waveguide with a cutting angle of 60° is adopted in the coupled part. By changing the incident light angle, the incident light can propagate in the waveguide at 50°-75°, and the variation range of the outgoing optical coupling angle is 0°-20°. There is a one-to-one correspondence between the angle change of the incident light and the angle change of the outgoing light (Fig.9). Conclusions A metasurface coupling structure for AR near-eye display optical waveguide is designed. The metasurface structure can be deflected by changing the radius and height of the structure, and the wavefront of the incident light at different angles is controlled. The results show that the deflection efficiency of light at a small incident angle can be as high as 77%. By changing the total reflection angle of the incident light in the waveguide, the coupling angle changes with the change of the incident angle, and finally the field of view angle of 20° can be achieved. The introduction of metasurface provides an effective scheme for the design of AR near-eye display optical engine, which is of great significance for the realization of light-weight and compact eyeglass for a AR module, and is expected to become a potential development direction of AR near-eye display optical engine. -
Key words:
- near-eye display /
- optical waveguide /
- metasurface /
- transmission phase /
- augmented reality /
- beam deflection
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图 4 相位和透射率扫描结果。(a)透射率随半径变化图;(b)相位随半径变化图;(c) 透射率随半径和高度变化图; (d)相位随半径和高度变化图
Figure 4. Phase and transmittance scanning results. (a) Diagram of transmittance variation with radius; (b) Diagram of phase variation with radius; (c) Diagram of transmittance variation with radius and height; (d) Diagram of phase variation with radius and height
图 6 (a) 超表面角度偏转示意图;(b)~(c) 具有高度H、半径为R的超表面单元结构的侧视图和俯视图,周期为P;(d)构建的角度偏转超表面的俯视图
Figure 6. (a) Metasurface angle deflection diagram; (b)-(c) Side view and top view of the metasurface cell structure with height H and radius R, the unit cell size is P; (d) Top view of the constructed angular deflection metasurface
图 8 超表面在设计波长±30 nm 范围内的出射光角度分布。(a)绿色Micro-LED的电致发光特性图[31];(b) 出射光在不同角度的透射率分布;(c) 偏转角度与入射波长的关系图
Figure 8. Dispersion characteristics of the metasurface in the range of ±30 nm. (a) Electroluminescence characteristics of green Micro-LED[31]; (b) Transmission distribution of the outgoing light at different angles; (c) The relation between deflection angle and incident wavelength
图 9 出射光角度偏转光强和角度分布图。(a)、(b)为耦出角0°的场强分布和角度分布图;(c)、(d)为耦出角+8°的场强分布和角度分布图;(e)、(f)为耦出角+20°的场强分布和角度分布图
Figure 9. Distribution of light intensity and angle of deflection. (a) and (b) are the field intensity distribution and angle distribution diagrams decoupled at angle 0°; (c) and (d) are the field intensity distribution and angle distribution diagram of decoupled angle +8°; (e) and (f) are the field intensity distribution and angle distribution diagram of decoupled angle +20°
图 10 (a)~(c)分别为入射角50°时光场在波导、超表面结构中和耦出超表面的分布图;(d)~(f)分别为入射角60°时光场在波导、超表面结构中和耦出超表面的分布图;(g)~(i)分别为入射角75°时光场在波导、超表面结构中和耦出超表面的分布图
Figure 10. (a)-(c) are the distribution of the time field in the waveguide, the metasurface structure and the coupled metasurface at the incidence angle of 50°, respectively; (d)-(f) are the distribution of the time field in the waveguide, the metasurface structure and the coupled metasurface at the incidence angle of 60°, respectively; (g)-(i) respectively represent the distribution of the time field in the waveguide and the metasurface structure and the coupled metasurface at the incident angle of 75°
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