## 2 Optical Data Information Encoding  
### Data Encoding Strategy  
Direct laser writing lithography (DLWL) [27; 28] is a widely used technique to fabricate a 3D structure. It is a maskless lithography technique that can customize arbitrary 3D structure on a photoresist with sub-micron accuracy. A real 3D device can be formed with selected materials and structures, following etching, deposition, and other processes. Compared to the disadvantages of the digital micro-mirror device (DMD) with surface roughness over 1 \(\upmu\)m [29] and the femtosecond-laser-induced two-photon polymerization (TPP) with low efficiency for large area manufacturing [30], DLWL device used in our lab can achieve 300 nm resolution with the surface roughness below 6 nm, which meansguaranteed optical properties. In addition, the DLWL technology can reach 4096 exposure grayscale, i.e., a high resolution of 12-bit grayscale level, which can make it achieve high writing resolution in the vertical direction. Compared with other methods, large-area microstructures can be transferred to other materials without the need for complex molds [31]. Only one mold needs to be made for mass production. In this condition, we propose a 3D optical storage nanostructure scheme based on a DLWL device. In the experiments, four samples with different slope angles were produced using a laser direct writing device (PICOMASTER 100, 4 PICO, Eindhoven, The Netherlands). The surface smoothness measurements using a 3D profilometer are shown in Figures S1 and S2 of the Supplementary Materials. The scheme uses different reflection spectral responses from different patterns to encode information. The structure and materials, used as inputs to the numerical model are shown in Figure 1a. As a thin film, the Si\({}_{3}\)N\({}_{4}\) layer, which has a height of 200 nm, was deposited on a silicon-on-insulator (SOI) substrate, which has a 900 nm silicon overlay. The SOI substrate has excellent stability, oxidation, and aging resistance [32]. This size was chosen according to the etching ability of grayscale lithography. The Z-axis height was precisely controlled in the 3D laser lithography process, and the selection of SOI substrate at this height allowed for better fine tuning of the slope angle. Depositing a Si\({}_{3}\)N\({}_{4}\) film of 200 nm thickness can largely increase the reflectivity of the structure due to the high reflectivity of the Si\({}_{3}\)N\({}_{4}\) film, thus improving the accuracy of the readout and enabling a high density of optical storage. In order to simulate the reflective spectra, a rigorous coupled-wave analysis method was applied. In this study, we used the DiffractMOD module of the simulation software R-soft, setting the harmonics in the module to 15. The simulated results of different structures with different slope angles (Figure 1b) revealed different wavelength-based reflectance spectra, with spectra processed by a 3 \(\times\) 3 high-pass filter (Figure 1c). In this study, light in the wavelength range of 200 nm to 1000 nm was chosen in order to simulate the real experimental setting of the readout system. In the spectra, the blue and red lines represent Transverse Electric (_TE_)/Transverse Magnetic (_TM_)  
Figure 1: The flow chart of the manufacturing process and encoding strategy. (**a**) The nanostructure manufacturing process, (**b**) Variation in structure according to slope angle, and (**c**) Wavelength-based reflectivity spectra of different slope angles. polarization, respectively. The spectral responses of _TE_ and _TM_ were significantly different, suggesting that more information can be collected with both _TE_ and _TM_ polarizations. The spectral responses can be easily distinguished for different structures, suggesting the potential applications of special structures with different slope angles in optical storage encoding. We designed two different storage strategies, i.e., slope angle-based and azimuth angle-based, to use the capability and flexibility of 3D laser direct writing.