## 3 Materials and Methods  
### Fabrication of Grating Surfaces  
The parallel-superimposed bi-grating nanostructures were fabricated using laser interference lithography, which takes advantage of the macroscopic photo-induced mass transport of azobenzene molecules in a thin film through photoisomerization [30]. Disperse Red-1 azobenzene molecular glass (gDR1) powders were synthesized following a previously described method [31]. A 3 wt% solution of gDR1 in dichloromethane (CH\({}_{2}\)Cl\({}_{2}\)) was spin-coated onto clean 75 \(\times\) 38 mm glass microscope slides (Corning 0215, [PERSON], Redding, CA, USA) at 1400 rpm for 35 s, resulting in a thin film approximately 400 nm thick. The coated slides were then dried in an oven at 70 \({}^{\circ}\)C for 15 min to ensure complete solvent evaporation. Linear surface relief gratings were inscribed on the azobenzene-coated glass substrates using a 532 nm diode-pumped solid-state continuous wave laser (Verdi V6, Coherent, Santa Clara, CA, USA) in conjunction with a Lloyd mirror interferometer setup, as previously described [32]. This interferometric technique generates a sinusoidal interference pattern by splitting the laser beam, with one-half incident directly on the sample and the other half reflected off an orthogonal mirror. The grating pitch depends on the incidence angle of the laser beam onto the Lloyd interferometer. The parallel-superimposed bi-gratings were fabricated through two sequential exposures. The first grating was inscribed at a specific pitch by exposing the sample to 227 mW/cm\({}^{2}\) laser power for 95 s. The Lloyd interferometer and the sample were subsequently rotated with respect to the incident laser beam and a second exposure was performed using the same laser power, but for only 70 s. This resulted in the superposition in two gratings of similar modulation depths, each approximately 60 nm. Finally, a 50 nm gold layer was deposited onto the grating structures using a sputter coater (Q 150V ES Plus, Quorum, Laughton, East Sussex, UK) to achieve SPR. The overall fabrication process is illustrated in Figure 2.  
To achieve retroreflection at multiple angles of incidence, six separate parallel-superimposed bi-gratings were fabricated side by side on the azobenzene thin film sample, as shown in Figure 3 and summarized in Table 1. Each bi-grating was designed to retroreflect an arbitrarily chosen wavelength of 785 nm at a specific angle of incidence  
Figure 2: Side-view schematic (not to scale) illustrating the fabrication process of a gold-coated superimposed bi-grating. The schematic outlines the step-by-step process, including the sequential inscription of gratings and subsequent gold coating on an azobenzene thin film. (summarized in Table 1). When light was incident on the entire azobenzene sample, each bi-grating provided a retroreflection at a specific angle; thus, when the backscattered light from the entire sample was captured, a combined piecewise retroreflection occurred in which a broad angular coverage was achieved for monochromatic retroreflection. In addition to monochromatic retroreflection, a multi-wavelength angular-dependent retroreflector was fabricated. This design utilized three separate regions of parallel-superimposed bi-gratings fabricated side by side on the same azobenzene thin-film sample. Each bi-grating pair was independently designed to retroreflect a specific wavelength, red, orange, and green wavelengths, corresponding to different angles of incidence: 15\({}^{\circ}\), 10\({}^{\circ}\), and 5\({}^{\circ}\), respectively, as summarized in Table 2.  
Furthermore, a creative approach was employed to achieve azimuthally independent retroreflection using bi-gratings in a circular mosaic design. The details of this method, including its fabrication process and underlying design principles, are provided in the Supplementary Information (SI S1.0). The depth and profile of the bi-gratings were analyzed using an atomic force microscope (Dimension Edge AFM, Bruker, San Jose, CA, USA) and processed with Nanoscope