Efficiency-enhanced reflective nanosieve holograms

Photon sieves composed of etched holes on an opaque film have been proposed firstly, to the best of our knowledge, to reduce the focal spot size and alleviate high diffraction orders in soft X-ray and the optical spectrum. With the rapid development of nano-fabrication technology, photon sieves have been demonstrated at nanoscale and worked as binary-amplitude metasurfaces for optical focusing. The photon nanosieves have the advantages of polarization independence and more degrees of freedom in design than the concentric rings in zone plates, which therefore enable more complex manipulation of light, such as hologram, by arranging the locations of holes in a customized way. Due to the subwavelength feature of nanosieves, their related holograms usually support broadband operation. In addition, the non-resonating mechanism of amplitude modulation makes the nanosieve hologram have a wider spectrum than other metasurface devices with resonating nano-structures. The nanosieve hologram also enables a large field of view for holographic display when combined with tunable phase realized by a spatial light modulator. Beyond the circular shape, rectangle nanosieves have also been proposed to control the geometric phase of a circularly polarized light by rotating the orientations of the rectangular nanosieves, thus enabling full-color holography and the generation of optical vortices in various electromagnetic spectra such as X-ray and vacuum ultraviolet wavelengths.

Nanosieves operate in a transmission mode, where optical transmission of light through the nanosieves is low because of the weak coupling between the propagating light and the waveguide modes supported within the nanoholes. Persistent efforts have been made to solve this efficiency issue. For example, a 15-layer MoS₂ film with a thickness of 10 nm can enhance the coupling for higher transmission, meanwhile blocking the undesired background with strong exciton absorption. The resulting hologram has a record-high efficiency of ∼22%, where the contribution from the background light is also involved. Dielectric nanobricks located at only specially chosen positions, as an extension of classic photon nanosieves, are used to sieve the circularly polarized light with an additional geometric phase for realizing a wavelength- and polarization-sensitive hologram. Although their demonstrated efficiency for one single channel is only 2% at the wavelength of 670 nm due to small bandgap of the employed silicon, it indeed generalizes the concept of photon sieves, hereby suggesting new approaches for improving the efficiency of photon nanosieves.

The research group led by Prof. Kun Huang from the University of Science and Technology of China proposed well-designed and location-optimized meta-mirrors that could sieve the reflected photons for holography in Chinese Optics Letters, Volume 20, Issue 5, 2022 (S. O. H. Mohammed, et al., Efficiency-enhanced reflective nanosieve holograms).

These aluminum meta-mirrors are located on a transparent quartz substrate, working as an analogue of transmissive photon nanosieves, as shown in Fig. 1(a). Compared with other metals such as silver and gold, aluminum is employed here due to the lower cost and less absorption of visible and ultraviolet light. For these aluminum meta-mirrors, their periods along the x and y directions are chosen to be 250 nm, which is almost below half of the entire visible wavelength. In addition, the heights of these aluminum meta-mirrors are optimized to be 100 nm, which is a good balance between optical efficiency and fabrication issues. Fig. 1(b) sketches the experimental setup for hologram characterization. Fig. 1(c) shows the measured intensity profiles at the exemplified wavelengths from 450 nm to 650 nm.

These holographic images cover blue, green, yellow, and red colors, which makes this technique suitable for optical static display. The hologram showed a broadband operation over 200 nm covering the visible range due to the subwavelength features of the meta-mirrors, with performance matching well with the simulation. Meanwhile, the imaging plane shifts at the different wavelength with a dispersion relationship of zλ = z₀λ₀, where z is the distance between the imaging plane at the wavelength λ and the meta-mirror hologram. Due to the Fresnel imaging location of the demonstrated hologram, these reconstructed images have the same size, which is similar to the case of classic photon nanosieves. In addition, no aberration or distortion is observed at the recorded images, implying good quality of the holographic reconstruction. More importantly, they demonstration is focused on the metamirror hologram that has an experimental efficiency of ∼7% over a broad visible spectrum, exhibiting an enhancement factor of ∼3.5 compared with the previous transmissive nanosieves.

Although the achieved efficiency in the reflective photon nanosieve is still lower than those of dielectric metasurfaces, it provides potential technology for optical display and anti-counterfeiting if the lower-cost nano-printing technique is employed for mass production.

反射式纳米筛实现高效全息成像

光子筛最初被用来减少软x射线、减小光谱中的焦斑尺寸和抑制高衍射级次的产生。随着纳米加工技术的迅速发展，纳米光子筛也得到了广泛的应用。其作为二元振幅型超表面能够实现光学聚焦等功能，并具有与偏振响应无关的优点。在设计上纳米光子筛比菲涅尔波带片的自由度更高，可通过调整小孔的位置来实现更复杂的波前调控，如全息成像。

纳米光子筛的亚波长特性使其能够在宽谱下实现全息成像。当纳米光子筛与空间光调制器相结合时，还可实现大视场的全息显示。此外，矩形纳米光子筛还可通过旋转矩形小孔来调控几何相位，从而在x射线和真空紫外波长下，实现彩色全息和光学涡旋等功能。

目前报道的纳米光子筛都是在透射模式下工作，由于入射光和纳米孔所支持的波导模式之间的耦合作用很弱，入射光经过纳米光子筛的透射效率很低。为提高纳米光子筛效率，研究者使用厚度为10 nm的多层MoS₂薄膜来增强耦合，提高透射率，同时利用强激子吸收过滤不需要的背景光。作为经典纳米光子筛的延伸，其可用于特定位置的电介质纳米砖来过滤不同偏振的圆偏振光，以实现具有波长和偏振响应的全息成像。但硅在670 nm波长下的带隙较小，其单通道效率仅为2%，使得该器件效率较低。因此，亟需提出一种高效率的纳米光子筛。

中国科学技术大学的黄坤教授课题组和新加坡材料与工程研究所（IMRE）合作在Chinese Optics Letters第20卷第5期上介绍了一种提高光子筛效率的新方法，提出了一种精心设计的反射式纳米光子筛，可过滤反射光子生成全息图像，并具有较高的全息效率，被选为当期封面（S. O. H. Mohammed, et al., Efficiency-enhanced reflective nanosieve holograms）。

该研究团队利用铝圆片阵列排布在透明的石英基底上设计了作用类似于透射式光子纳米光子筛，如图1（a）所示。其成本相对较低，且对可见光和紫外光的吸收更少。铝圆片阵列x和y方向上的周期均为250 nm，在可见波长范围内符合“周期等于半波长”的经验规律。该团队还通过时域有限差分法优化，确定了铝圆片阵列的高度为100 nm，很好地平衡了光学效率和加工难度。图1 (b)为表征全息成像的实验装置示意图。图1 (c)展示了其在450 nm~650 nm可见光波长下的实测图像。

全息图像具有蓝、绿、黄和红等不同颜色，适用于光学彩色显示。同时，由于铝片结构单元的亚波长特性，全息图可在超过200 nm的可见光宽谱范围内工作，该实验结果与仿真结果相吻合。尽管成像平面在不同波长处会发生偏移，但其色散关系符合zλ = z₀λ₀（z为波长λ处成像平面与纳米筛之间的距离） 。

此外，该反射式纳米光子筛生成的全息图像位于菲涅耳成像区域，与经典纳米光子筛类似，其重建图像具有相同的尺寸。并且其采集的图像没有明显像差或失真，全息成像质量较好。更重要的是，该反射式纳米光子筛在可见光宽谱内的实测效率为~ 7%，这与之前报道的透射纳米光子筛相比，效率提高了~ 3.5倍。

目前，虽然反射式纳米光子筛的效率仍低于电介质型超表面，但如果能与成本较低的纳米印刷技术结合并用于大规模生产，反射式纳米光子筛将大大促进光学显示和光学防伪的发展。基于该技术，团队已申请一项美国专利。