Image information encryption can hide the encrypted information among redundant information, which is very important in the field of information security and anti-counterfeiting^[1]. Optical metasurface consisting of periodically arranged artificial units (such as grating^[2], nano rod^[3], nano ring^[4], nano block^[5], etc.) on the surface can change the amplitude, phase, and polarization of incident electromagnetic waves, causing reflection, diffraction, and scattering of incident light ^[6], thus producing gorgeous structural colors and are widely used in image information encryption. Compared with dyes and pigments, metasurface imaging is widely used in different images of storage and encryption due to its long-term stability and high resolution^[7]. Most structural color metasurface uses a fixed shape and size sub-wavelength structure, and are designed in a single form, which can only produce static colors. The demand for dynamic color displays is increasing in order to achieve functional expansion. Structured color metasurfaces have been developed for anisotropic functional multiplexing, and the unique optical properties of a specific functional metasurface can be achieved by tuning the subwavelength structure of the surface^[8]. Due to their flexible and designable optical properties, they are widely used for anti-counterfeiting and information hiding. By modulating the spectral and polarization response, metasurface imaging has been successfully applied in operations where different information is written to different angular^[9] and polarization^[10-12] channels. The most classic application is to store, hide and encrypted image information by using interleaved anisotropic antennas (such as grating^[13], nano block^[14,15], cross-shaped protrusion^[16], etc.). Among them, due to its sensitivity to polarization in response, one-dimensional gratings can easily arrange their grating orientation to cause angular deflection of dispersion, providing a solution for multiplexed information encryption metasurface devices.

Although the development of advanced processing technologies such as photolithography^[4] and electron beam etching^[5] has provided more design space for on-demand structured color metasurfaces, this method that requires masks or individual printing greatly limits the processing efficiency of devices. Due to its ultra fast and ultra strong characteristics and flexible control methods, ultrafast lasers have unique advantages in precision manufacturing of structural color metasurfaces on demand. Laser Induced Periodic Surface Structures (LIPSS)^[17] is a unique method of obtaining grating structures on various materials such as metals, semiconductors, and ceramics through direct laser writing. Due to the ability to process large area sub wavelength structures in one step, this structure has been widely used in metasurface devices. By utilizing the sensitivity and dispersion effects of LIPSS on laser polarization, various metasurface devices based on modified or ablated gratings have been developed, achieving dynamic color display ^[18]. The LIPSS structure is formed at the interface between the material and air by excitation of Surface Plasmon Polaritons (SPP) resulting from incident laser irradiation^[19]. Periodic electromagnetic fields are generated on the surface of the material, resulting in the formation of periodic structures. Although the surface roughness of the material can promote the excitation of SPP^[20], the severe material ejection during the processing of the ablated grating seriously affects the coupling between SPP and light, leading to the problem of poor grating quality, thereby affecting the color rendering performance of this type of information encryption metasurface for reading pattern information and the anti-crosstalk performance between different information. Although modified gratings can avoid the problems of ablating gratings, they require subsequent chemical etching operations^[21], which greatly reduces the convenience of using LIPSS processing method. Therefore, there is an urgent need for a high-quality and multiplexed grating structure processing method.

As a widely studied phase-change material, Ge₂Sb₂Te₅ can undergo rapid phase switching under the action of laser, which has been extensively studied in the field of tunable devices^[22-23]. GST can undergo a phase-change from the amorphous (a-GST) to the crystalline (c-GST) state under ultrafast laser multi-pulse irradiation, during which its reflectivity increases^[24]. More importantly, the rearrangement of the lattice at the microscopic level leads to a change in the macroscopic volume^[25]. In addtion, as a narrow bandgap semiconductor, GST can excite free carriers into metallic states under ultrafast laser irradiation and form modified LIPSS under the action of surface SPP. For periodically alternating crystalline and amorphous stripes. Due to the absence of material spluttering and subsequent operations, this modified grating structure can achieve uniform and consistent processing effects while maintaining high processing efficiency. In addition, the combination of reflectivity changes and volume shrinkage effects within the visible wavelength of GST provides a new possibility for the design of multiplexed metasurfaces.

This paper presents a method for nested processing of angle-multiplexed structural color metasurfaces. This method uses modified structures (i.e., modified gratings and crystallized stripes) obtained by direct writing of the GST surface of the phase-change material at different picosecond laser parameters to form encrypted different patterns and background patterns, respectively, and the resulting modified gratings have high orientation uniformity compared to conventional ablated gratings. The macroscopic pattern information used for encryption is divided into multiple parallel slices, and each slice is processed using a modified grating structure obtained by single-step direct scanning with a picosecond laser. The two encrypted patterns overlap macroscopically, while the slices forming the two patterns at the subwavelength scale are nested with each other. The background pattern after slicing is filled with pure crystalline stripes in the peripheral unprocessed area of the pattern information to achieve information hiding under natural light irradiation conditions. Based on the polarization multiplexing and grating dispersion capability of the metasurface device, the pattern information composed of two sets of gratings with 16° difference can be selectively read and dynamically displayed under the decryption conditions of different combinations of observation pitch and rotation angles under the condition of strong light incidence. The proposed processing method achieves the angular multiplexing effect with good color rendering effect while ensuring the processing efficiency, and has a wide application prospect in information hiding, multidimensional storage and flexible wearable display devices.

A picosecond laser (Atlantic 355-20) from EKSPLA is used as the processing light output source with a laser wavelength of 1064 nm, a pulse width of 13 ps, and a Repeat frequency setting of 1 kHz. The pulse energy is adjusted through a continuous neutral density attenuator, and a half-wave plate is used to adjust the polarization direction of the linearly polarized picosecond laser. The half-wave plate angle is fixed during the processing. The picosecond laser with a spot diameter of 5 mm is focused on the sample surface by a planoconvex lens with a focal length of 100 mm. 100-nm thick amorphous GST is deposited on a 1 mm thick flexible polydimethylsiloxane (PDMS) substrate by a magnetron sputtering device (JINSHENGWEINA MSP-620). The sample is fixed on a high-precision six-dimensional translation stage (PI H-811.I2), and the laser processing position needed to be adjusted by controlling the translation stage during the scanning process, with the translation speed of 0.5 mm/s.

The surface phase properties, structural features and reflectivity information of the prepared samples are characterized by SEM (SU9000), AFM (Bruker Corporation, Dimension Icon) and optical microscopy (BX53, Olympus). The optical properties are characterized by optical images taken by a smartphone with a digital camera fixed on a three-dimensional translation stage, illuminated by a halogen lamp (SCHOTT KL 1500 electronic) treated with near-parallel light, and this optical observation experiment is performed in the dark.

The overall schematic diagram of the designed angle multiplexed information encryption metasurface is shown in Figure 1 (color online), and the entire processing process is completed by picosecond laser direct writing. As shown in Figure 1(a), the picosecond laser can induce the modification of the surface of the GST material at the appropriate power density, leading to a phase transition from a-GST to c-GST. The crystal lattice changes from disordered to ordered, showing an increase in surface reflectivity at the macro level and a shrinkage in volume at the same time. By simply changing the direction of laser polarization, picosecond laser direct writing GST produces a modified structure with three features, as shown in Figure 1(b). Two of them are modified grating structures with opposite orientations, and one is a pure crystalline strip, each of which will be used for the corresponding single pattern processing on the right side. The three patterns are sliced separately and divided into parallel slices with equal spacing. At the macroscopic level, the processing takes place in the same area, while at the grating scale, it is nested with each other. To better understand this methods, the last step of processing of three patterns is used as a demonstration, with slice locations marked in yellow, red, and purple on the patterns. In the magnified surface structure diagram in Figure 1(c), the yellow, red, and purple dotted boxes represent the processing position relationships of the three pattern slices. Figure 1(c) shows the metasurface functional effect obtained in this way, which exhibits an overall increase in reflectivity under natural light and effectively hides the nested pattern information. Under the condition of strong light illumination different pattern information can be selectively decoded at a specific angle.

For the convenience of description, the coordinate axes in Fig. 1(a) are defined, and the processing direction is constant along the x direction. The angle between the laser polarization and the x-axis is defined as the polarization angle, and the angle between the modified structure orientation and the y-axis is defined as the grating orientation angle. Under the conditions set in this experiment, when the polarization angle is 0° and the laser power is between 45 μW and 60 μW, the LIPSS with alternating light and dark under the light microscope can be processed in direct writing, as shown in Figure 2(a) (color online). The light microscope diagram shows that this laser-induced GST-modified grating method is free of material sputtering, thus enabling the formation of a highly uniform grating structure with high consistency. Numerous studies on the crystallization properties of GST have demonstrated that surface reflectance enhancement is an important judgment factor for amorphous GST crystallization^[24]. As shown in Figures 2(b), 2(c) (color online), the ripple structure with periodic distributed stripes are also presented in the SEM results. The generation of this ripple structure is widely believed to be due to the interaction of incident light with the surface SPP^[26]. Kolobov et al. found that the interatomic potential in amorphous GST is anharmonic, thus leading to an increase in bond length in the disordered state. During crystallization, the disorder within the GST decreases, leading to volume contraction and density increase of the material^[27], and this anomaly has also been experimentally observed^[25]. As shown in Figure 2(d) (color online), the modified ripple structure was subjected to AFM tests, and the obtained three-dimensional plots show that they are accompanied by periodically distributed variations in height. The cross-sectional height profile is plotted in Fig. 2(e) (color online). It can be seen that more intuitive information about its period height distribution can be obtained with a period of about 1 μm and a height difference close to 8 nm due to the crystallization shrinkage effect, so that the modified grating structure can be processed by picosecond laser direct writing GST under this condition. This one-step printing-like method simplifies the steps and improves the efficiency compared to the conventional mask-based grating structures.

The laser power is kept constant at 60 μW and the effect of laser polarization on the processed structure is investigated. The laser polarization is varied in steps of 10° between −90° and 90° by rotating the half-wave plate. The grating linewidths obtained under positive and negative polarization conditions are the same but the orientation angles are symmetric to each other. The linewidths and grating orientation angles of the modified structures after direct-write processing under 0° to 90° polarization are shown in Fig.3(a) (color online), and two representative schematic diagrams of the modified structures are indicated in the figure. The effect of laser polarization on the modified grating can be divided into two stages. At laser polarization angles between 10° and 30°, the grating linewidth decreases slightly, but the grating orientation angle remains at 8°. When the polarization angle is greater than 30°, the processed structure is a pure crystalline strip, at which time there is no grating structure and the line width of the pure crystalline strip is basically unchanged. The relevant optical microscope images are shown in Fig.3(b)−3(e) (color online), with laser polarization of 0°, 10°, 30° and 40°, respectively. It is noteworthy that the orientation angle of the conventional ablative grating shows an approximately linear relationship with the laser polarization, while the GST surface-modified grating exhibits a peculiar phenomenon with a constant orientation angle of 8°. Although this phenomenon has not been reported, it can be tentatively explained by the fact that the SPP propagation direction is orthogonal to the laser polarization due to the constant laser direct writing direction along the positive direction of the x-axis^[28]. When the polarization direction is 0°, the SPP propagation direction is the same as the laser direct writing direction, and the joint action of SPP and subsequent pulses leads to the continuous induction of the ripple structure. When the polarization angle gradually increases, the angle between the SPP propagation direction and the direct writing direction also increases, and the SPP propagation direction has no subsequent pulse, thus the ripple structure cannot be formed.

The dispersion effect of the grating on the incident light is related to the grating orientation, irradiation angle and viewing angle, so a device is designed to characterize the color rendering performance of the grating. The experimental configuration consists of a smartphone with digital camera capability, a halogen light source and a sample holder. As shown in Fig. 3(f) (color online). When a white light source is irradiated to the surface of a periodic grating structure at an angle of incidence a, the central wavelength of diffracted light λ at angle b is given by the diffraction equation: mλ = Λ(sina±sinb), where Λ is the grating period and m is the diffraction level^[21]. In this experiment, the modified grating orientation angle is oriented along the x-direction and the sample is fixed; a white light source is used for illumination, and the incident light emitted through a planoconvex lens converges to an approximately parallel light, which is incident at an angle of −27° to the z-axis in the yoz plane, i.e., a = 27°. The incident light passes through the dispersion of the grating structure on the surface of the sample to produce colorful colors. The color information of the dispersion region throughout the visible range is captured by moving the phone to different positions in a plane 10 cm high from the sample. As the viewing angle changes within the plane of capture, the pattern can show rich structural colors throughout the visible range. The cell phone lens is panned in a plane 10 cm directly above the sample and photographed sequentially at 0.4 cm intervals. Figures 3(g)−3(i) (color online) show the RGB colors of the processed samples, and the captured picture information is pure and vivid in color, which reflects the high orientation consistency of the modified grating from the side. It is worth noting that since the spot size does not cover the whole processed area, the color information at the same location on the sample (see Fig. 3(i) at the position of the white circle) is extracted to fill the squares and displayed as shown in Fig. 3(j) (color online). The color information collected at different tilt angles b in the range of −2.5° to 19° is displayed horizontally, showing the whole visible color from red to violet, respectively. More colors will be obtained if the interval when collecting colors is reduced. In addition, the brightness information is also shown vertically at different pitch angles. It can be seen that the brightestness of color collected directly above the sample is the strongest. As the pitch angle becomes larger, the color brightness becomes progressively weaker, and the information is visible for pitch angles ranging from −2° to 2°. This large viewing range at the tilt angle with very small information leakage effect at the pitch angle has unique advantages in terms of crosstalk-free dynamic display of structural colors, and in addition, the color uniformity indicates a high degree of consistency in the orientation angle of the processed modified grating.

Inspired by the color rendering properties of the above-mentioned modified gratings observed at different angles, their potential for information hiding is further explored, and the preparation of information encryption metasurfaces is achieved by smart design. As shown in Fig. 4(a) (color online), pattern 1 "city wall", pattern 2 "palace" and pattern 3 (background pattern) are designed for image information hiding, and each pattern is divided into parallel slices spaced 25 μm apart, and the slices of different patterns are nested with each other so that their information is recorded in the same macroscopic region on the sample without crosstalk. The laser power is fixed at 50 μW, and the line width of the modified grating obtained under this condition is 9 μm. This processing accuracy is higher than the resolution of the human eye, thus ensuring the continuity of the patterns, increasing the information density, and matching the spacing of the processing lines to facilitate the nesting of the two patterns. Each slice of the two patterns to be encrypted is processed by modified gratings with orientation angles of 8° and −8° obtained by the picosecond laser at ±30° polarization, respectively, while the slice of pattern 3 used as a background is processed by a pure crystalline stripe obtained by the picosecond laser at 90° polarization, corresponding to the three modified structures shown in the lower right corner of Figure 1(a). The reason for filling pattern 3 outside the encrypted image information is that the modified area has the effect of enhanced reflectivity when observed under natural light and the partial contours of the pattern can still be identified under macroscopic conditions. Figure 4(b) (color online) shows the macroscopic results of the information encryption metasurface processed on the GST surface of the flexible substrate under natural light conditions. Since the reflectivity of the crystalline GST is higher than that of the amorphous GST, square regions with hidden pattern information can be seen. Under natural light conditions, the pattern information is hidden in the background and cannot be read due to the small depth of the grating and the weak dispersion effect on natural light. Figure 4(c) (color online) shows a modified grating with 16° difference in orientation angle for encrypted patterns, and Figure 4(d) (color online) shows a pure crystalline strip for background patterns.

The above shows the information hiding function of the prepared metasurface, however, under the condition of strong light incidence, the grating structures with different orientations perform a dispersion effect on the incident light in different directions without crosstalk to each other, which leads to the selective reading of different patterns under strong light incidence in fixed directions, as shown in Figure 4(e) (color online). The corresponding information was decrypted and read at different pitch and tilt angles, respectively. For example, when the rotation angle a’ = ±8°, pattern 1 and pattern 2 can be observed separately without crosstalk with each other, while when the rotation angle is fixed and the tilt angle b’ is from −2.5° to 19°, the dynamic display of individual pattern information can be achieved again. The results show that the metasurface can achieve dual image information encryption, crosstalk-free and dynamic information display.

This paper addresses the urgent need for efficient processing of multiplexed metasurfaces and addresses the challenges of low efficiency and poor color rendering in conventional LIPSS processing. We propose a method for encryption metasurfaces by using different modified structures of ultra-fast laser induced phase-change material GST. By combining nested design with grating dispersion effect, a pure crystalline strip, and a modified grating with an orientation angle difference of 16° are formed on the surface of GST using picosecond laser, and a double encrypted pattern with angle multiplexing is prepared. Firstly, the dispersion properties of the prepared GST-modified gratings are first characterized, and the angle-multiplexed information encryption metasurfaces are designed by combining the polarization dependence of the modified grating, and the metasurface prepared by the proposed method is further demonstrated. In addition, the performance of encryption under natural light conditions and selective decryption reading and dynamic display under strong light conditions has been achieved. By cleverly utilizing the characteristics of this one-step GST modified structure in terms of phase and structure, the shortcomings of conventional pattern encryption metasurface efficiency and poor color rendering effect are solved. In addition, as a reconfigurable phase-change material, GST will play an increasingly important role in various fields such as anti-counterfeiting, multi-dimensional information storage, flexible wearable display devices, and reconfigurable devices.

图像信息加密能够在冗余信息中隐藏被加密信息，在信息安全和防伪领域非常重要^[1]。由表面周期性排布的人造单元(如光栅^[2]、纳米棒^[3]、纳米环^[4]和纳米方块^[5]等)组成的光学超表面，其可以改变入射电磁波的振幅、相位和偏振，使入射光产生反射、衍射及散射^[6]，从而产生绚丽的结构色，被广泛应用于图像信息加密。与染料和颜料相比，超表面成像因其长期的稳定性和高分辨率而被广泛用于存储和加密不同的图像^[7]。大多数的结构色超表面使用固定形状和尺寸的亚波长结构，且设计单一，仅能够产生静态颜色。为了扩展功能，对动态彩色显示的需求正在增加。基于各向异性功能复用的结构色超表面得到发展，通过对表面亚波长结构的调节，可以实现特定功能超表面的独特光学特性^[8]。由于其具有灵活和可设计的光学特性，被广泛应用于防伪及信息隐藏。通过调制光谱和偏振响应，超表面成像已成功应用于将不同信息写入不同角度^[9]和偏振^[10-12]通道。最经典的应用是通过使用交错排列的各向异性天线(如光栅^[13]、纳米块^[14-15]、十字形突起^[16]等)来存储、隐藏和加密图像信息。其中，一维光栅由于其响应对偏振具有敏感性，通过简单地排布光栅取向，就能够使色散发生角度偏转，为多路复用的信息加密超表面器件提供了解决途径。

尽管光刻^[4] 、电子束刻蚀^[5]等先进加工技术的发展使按需设计的结构色超表面有了更多的设计空间，然而这类需要掩模或逐个打印的方法极大地限制了器件的加工效率。超快激光由于具有超快超强的特性及灵活的操控方式，在按需精密制造结构色超表面方面具有独特的优势。其中激光诱导表面周期结构^[17]（Laser-Induced Periodic Surface Structures, LIPSS）是一种通过激光直写就可在金属、半导体、陶瓷等多种材料上获得光栅结构的独特方法。由于其可一步实现加工大面积亚波长结构，故在超表面器件中应用广泛。利用LIPSS对激光偏振的敏感性及色散效应，已经开发了基于改性或烧蚀光栅的多种超表面器件以实现颜色动态展示^[18]。LIPSS结构形成原理为：材料与空气交界处由于入射激光辐照而激发表面等离子体激元(Surface Plasmon Polaritons, SPP)^[19]，在材料表面产生了周期分布的电磁场，进而产生周期性结构。尽管材料表面粗糙度会促进SPP的激发^[20]，然而在上述烧蚀光栅加工时，剧烈的材料喷发会严重影响SPP与光的耦合，导致光栅质量差，从而影响此类信息加密超表面读取图案信息的显色性能及不同信息间的抗串扰性能。改性光栅尽管避免了烧蚀光栅存在的问题，然而其需要化学刻蚀的后续操作^[21]，严重牺牲了LIPSS加工方法的便捷性。由此可知，目前亟需一种高质量的可多路复用的光栅结构加工方法。

Ge₂Sb₂Te₅(GST)作为一种被广泛研究的相变材料，可在激光的作用下实现相态的快速切换，在可调谐器件领域有诸多研究^[22-23]。GST可在超快激光多脉冲辐照下发生由非晶态(a-GST)到晶态(c-GST)的相变，在此过程中其反射率有所提升^[24]，更为重要的是，微观上晶格的重新排列会导致宏观体积的改变^[25]。同时，GST作为一种窄禁带半导体，可在超快激光辐照下激发自由载流子成为金属态，在表面SPP的作用下形成改性LIPSS，即周期交替排布的晶态与非晶态条纹，由于没有材料喷溅及后续操作，这种改性光栅结构在保持高加工效率的同时能够获得均匀一致的加工效果。GST在可见光波长内的反射率变化及体积收缩效应的组合为多路复用超表面的设计提供了一种新的可能。

本文提出了一种嵌套加工角度复用的结构色超表面方法。利用在不同皮秒激光参数下直写相变材料GST表面得到的改性结构，即改性光栅与纯晶化条，分别用来构成加密的不同图案和背景图案，所得改性光栅与传统烧蚀光栅相比具有高度取向一致性。用于加密的宏观图案信息被划分为多个平行切片，每个切片均由皮秒激光单步直写扫描得到的改性光栅结构加工而成。两个加密图案在宏观上重合，而在亚波长尺度上构成两个图案的切片相互嵌套。在图案信息的外围未加工区域用纯晶化条填充切片处理后的背景图案，以实现自然光照射条件下的信息隐藏功能。基于超表面器件的偏振复用及光栅色散能力，由相差16°的两组光栅组成的图案信息在强光入射条件下，可在不同观察俯仰角与旋转角组合的解密条件下实现选择性信息读取及信息动态展示。所提加工方法在保证加工效率的同时实现了具有良好显色效果的角度复用。在信息隐藏、多维存储及柔性可穿戴显示器件方面有广泛的应用前景。

实验采用EKSPLA公司的皮秒激光器（Atlantic 355-20）作为加工光输出光源，激光波长为1064 nm，脉宽为13 ps，重频设置为1k Hz。脉冲能量通过中性密度连续衰减片进行调节，半波片用于调节线偏振皮秒激光的偏振方向，在加工过程中半波片的角度固定。光斑直径为5 mm的皮秒激光通过焦距为100 mm的平凸透镜聚焦于样品表面。100 nm厚的非晶态GST通过磁控溅射设备(金盛微纳 MSP-620)沉积在1 mm厚的柔性聚二甲基硅氧烷(PDMS)基底上。样品被固定于高精度六维平移台(PI H-811.I2)上。在扫描过程中需通过控制平移台调整激光加工位置，平移速度设置为0.5 mm/s。

所制备样品的表面相态性质、结构特征与反射率信息分别由SEM(SU9000)、AFM(Bruker Corporation，Dimension Icon)和光学显微镜(BX53，Olympus)进行表征。其光学特性由固定在三维平移台上带有数码相机的智能手机拍摄的光学图像所表征，照明光源为经过近似平行光处理的卤素灯(SCHOTT KL 1500 electronic),光学观察实验在黑暗中进行。

本文设计的角度复用信息加密超表面整体示意图如图1(彩图见期刊电子版)所示，整个加工过程由皮秒激光直写完成。如图1(a)所示，皮秒激光在合适的功率密度下能够诱导GST材料表面发生改性，发生由a-GST到c-GST的相变，其晶格发生了从无序到有序的变化，宏观上表现为表面反射率提升，同时其体积发生收缩。仅改变激光偏振方向，皮秒激光直写GST会产生3种特征的改性结构，如图1(b)所示。两种为取向相反的改性光栅结构，一种为纯晶化条，每一种改性结构都将用于右侧对应的单个图案加工。将3个图案分别进行切片处理，划分为具有相等间隔的平行切片，宏观尺度上在同一区域加工，而在光栅尺度相互嵌套。为了更好地理解这种处理方式，以3个图案的最后一步加工为例进行演示。分别在图案上以黄色、红色及紫色标注出切片位置，在图1(c)放大的表面结构示意图中，黄色、红色及紫色的虚线框分别代表3个图案切片的加工位置关系。图1(c)展示了利用这种方式得到的超表面的功能效果。可以看出，在自然光下反射率整体提升而其中嵌套的图案信息得以隐藏，而在强光照射的条件下以特定的角度可选择性解码不同的图案信息。

为了方便描述，定义了图1(a)中的坐标轴，加工方向恒定沿x方向，将激光偏振与x轴的夹角定义为偏振角，将改性结构取向与y轴的夹角定义为光栅取向角。在本实验设置的条件下，当偏振角为0°时，激光功率为45~60 μW，能够直写加工出在光镜下明暗交替的LIPSS，如图2(a)(彩图见期刊电子版)所示。从光镜图中发现，这种激光诱导GST改性光栅的方法没有材料喷溅，从而形成了具有高度一致性的均匀光栅结构。大量关于GST结晶性质的研究已经证明，表面反射率提升是非晶态GST结晶的重要判断因素^[24]。如图2(b)~2(c)(彩图见期刊电子版)所示，SEM结果中呈现了明暗交替的波纹结构。这种波纹结构的产生被广泛认为是入射光与表面SPP相互作用的结果^[26]。Kolobov等人发现，非晶态GST中的原子间电位具有非谐性，导致无序状态下的键长增加。在结晶过程中，GST内部的无序性降低，导致材料的体积收缩、密度提高^[27]，通过实验观测到了这一反常现象^[25]。对改性波纹结构进行了AFM测试，如图2(d)(彩图见期刊电子版)所示，得到的三维图显示其伴随有周期分布的高度上的变化，将其截面高度曲线绘制在图2(e)(彩图见期刊电子版)中，可以更直观地得到其周期高度分布信息。可知，周期大约为1 μm，晶化收缩效应导致的高度差接近8 nm。由此可知，在该条件下皮秒激光直写GST可加工出改性光栅结构。与传统基于掩模版制造的光栅结构相比，这种类似一步打印光栅结构的方法大幅简化了加工步骤并提高了效率。

保持激光功率恒定为60 μW，研究激光偏振对加工结构的影响，通过旋转半波片的方式使激光偏振在−90°到90°之间以10°为步长进行变化，正负偏振条件下得到的光栅线宽一致但取向角互相对称。在图3(a)(彩图见期刊电子版)展示了0°到90°偏振条件下直写加工后的改性结构的线宽和光栅取向角信息，并在插图中给出了两种代表性的改性结构示意图。激光偏振对改性光栅的影响可分为两个阶段。激光偏振角在10°~30°之间时，光栅线宽略有减小，但光栅取向角保持在8°；当偏振角大于30°时，加工结构为纯晶化条，无光栅结构的存在，且纯晶化条的线宽基本不变。相关光学显微镜图像如图3(b)~3(e)(彩图见期刊电子版)所示，激光偏振分别为0°、10°、30°和40°。值得注意的是，传统烧蚀光栅的取向角与激光偏振呈近似线性的关系，而GST表面改性光栅却呈现出奇特的现象，其取向角恒定为8°，尽管这种现象还未见报道，但可以初步解释如下：由于激光直写方向恒定沿x轴正方向，而SPP的传播方向与激光偏振正交^[28]；当偏振方向为0°时，SPP传播方向与激光直写方向相同，SPP与后续脉冲的共同作用导致波纹结构的持续诱导；当偏振角逐渐增大时，SPP传播方向与直写方向的夹角也随之增大，SPP传播方向无后续脉冲，从而无法形成波纹结构。

光栅对入射光的色散效应与光栅取向、照射角度和观察角度有关，因此设计了一套装置来表示光栅的显色性能。实验配置由具备数码相机功能的智能手机、卤素灯光源和样品架组成。如图3(f)(彩图见期刊电子版)所示。白光光源以入射角a照射到周期性光栅结构表面时，衍射光波长 λ 在某个角度 b 的中心波长由衍射方程给出：mλ = Λ(sina±sinb)，其中，Λ为光栅周期，m为衍射级^[21]。在本实验中，将改性光栅取向角沿x方向定向并固定样本。白光光源用于照明，发散的入射光经过平凸透镜后，会聚为近似平行光，在yoz平面中与z轴呈−27°角入射，即a = 27°。入射光通过样品表面光栅结构的色散，产生五颜六色的颜色。手机在距离样品10 cm高平面内的不同位置，采集整个可见光范围内色散区的颜色信息。当视角在拍摄平面内变化时，图案可以在整个可见光范围内呈现出丰富的结构色。距样品正上方10 cm的平面内平移手机镜头，间隔0.4 cm依次拍照。图3(g)~3(i)(彩图见期刊电子版)展示了加工样品的RGB色彩。可见，采集到的图片信息色彩单纯且鲜艳，侧面反映了改性光栅的高取向一致性。值得注意的是，由于光斑尺寸并未覆盖全部加工区域，因此提取了样品上同一位置(见图3(i)白圈位置)的颜色信息填充为方块并加以展示，如图3(j)(彩图见期刊电子版)所示。横向展示了不同倾斜角b在−2.5°到19°范围内采集到的颜色信息，分别显示了由红到紫的整个可见光颜色。若减小采集颜色时的间隔，将得到更多颜色。纵向展示了在不同俯仰角下的亮度信息，其中在样品正上方采集的颜色亮度最强，随着俯仰角变大，颜色亮度逐渐变弱，信息可见的俯仰角范围为−2°到2°。这种在倾斜角上较大的观察范围与俯仰角上极小的信息泄露效果，在结构色无串扰动态显示方面具有独特的优势，另外，颜色的均匀性也表明所加工的改性光栅取向角的高度一致性。

受上述改性光栅在不同角度观察的显色性的启发，进一步探索其在信息隐藏方面的潜力，通过巧妙的设计实现了信息加密超表面的制备。如图4(a)(彩图见期刊电子版)所示，图案1“城墙”、图案2“宫殿”和图案3背景图案是为图像信息隐藏而设计的，每个图案被划分为间隔25 μm的平行切片，不同图案的切片相互嵌套，使其信息互不串扰地记录在样品上的同一宏观区域。激光功率固定为50 μW，在这种条件下得到的改性光栅线宽为9 μm。这种加工精度高于人眼的分辨率，保证了图案的连续性，提高了信息密度，同时与加工线的间距相匹配，便于两个模式信息的嵌套处理。欲加密的两个图案的每个切片都分别由皮秒激光在±30°偏振下得到的取向角为8°和−8°的改性光栅加工所得，而用于充当背景的图案3的切片则由皮秒激光在90°偏振下得到的纯晶化条加工所得，对应于图1(a)右下角所示的3种改性结构。之所以在加密图像信息外填充图案三，是由于改性区域在自然光下观察会有反射率提升的效果，在宏观条件下仍能识别出图案的部分轮廓。图4(b)(彩图见期刊电子版)展示了在自然光条件下柔性基底的GST表面上所加工的信息加密超表面的宏观结果。由于晶态GST的反射率高于非晶 GST，因此可以看到带有隐藏图案信息的方形区域。在自然光条件下，由于光栅深度较小，对自然光的色散作用较弱，图案信息被隐藏在背景中无法读取。图4(c)(彩图见期刊电子版)展示了用于加密图案的取向角相差16°的改性光栅，图4(d)(彩图见期刊电子版)展示了用于背景图案的纯晶化条。

上述给出了所制备超表面的信息隐藏功能，而在强光入射条件下，不同取向的光栅结构对入射光进行不同方向互不串扰的色散效果，这导致在固定方向的强光入射下选择性读取不同的图案，如图4(e)(彩图见期刊电子版)所示，分别在不同的俯仰角和倾斜角下解密读取相应的信息。例如，当旋转角a’ = ± 8°，可互不串扰地分别观察到图案1与图案2，而当旋转角固定，倾斜角b’在−2.5°至19°时，又可实现单个图案信息的动态展示。结果表明，该超表面可以实现双图像信息加密、无串扰的信息显示及信息的动态展示。

本文面向多路复用超表面高效率加工的迫切需求，解决传统LIPSS加工效率低、显色效果差的难题，提出了利用超快激光诱导相变材料GST的不同改性结构设计信息加密超表面的方法。通过嵌套设计结合光栅色散效应，利用皮秒激光在GST表面形成纯晶化条及取向角相差 16°的改性光栅，制备了角度复用的双重加密图案。首先表征所制备的GST改性光栅的色散性能，结合改性光栅的偏振依赖性，设计了角度复用的信息加密超表面，进一步展示了通过所提方法制备的超表面，可实现在自然光条件下加密，在强光条件下选择性解密读取并动态展示的性能。通过巧妙利用这种一步成型的GST改性结构在物相及结构方面的特点，解决了传统图案加密超表面效率低、显色效果差等缺点。并且GST作为一种可重构的相变材料，将在防伪、信息多维存储、柔性可穿戴显示设备及可重构器件等多领域发挥越来越重要的作用。