## 5 Discussion  
The experimental results demonstrate the capability of parallel-superimposed bi-gratings to achieve monochromatic retroreflection at a broad range of incident angles. The retroreflective behavior of the bi-gratings can be attributed to the efficient in-coupling and out-coupling of SPR modes facilitated by the unique configuration of the bi-gratings. The mechanism relies on the selective generation of SPRs through the first grating and subsequent out-coupling through the second, as illustrated by the dispersion relationships. This configuration allows for a highly tunable and predictable retroreflective response that can be adapted for multiple wavelengths and varied incidence angles. The alignment of bi-gratings -1 to -6 facilitated effective monochromatic retroreflection over a range of angles, specifically from 5\({}^{\circ}\) to 10\({}^{\circ}\), with a consistent output of 785 nm light. This result, illustrated in Figure 6a,b underscores the precision with which the system can reflect light across different incident angles. The variation in peak intensity observed among the bi-gratings arises from differences in coupling efficiency, which are  
Figure 7: Multi-wavelength retroreflection observed in samples Bi-grating-7, Bi-grating-8, and Bi-grating-9 simulating a stoplight effect. (**a**) Normalized spectral response of each sample, indicating distinct retroreflected wavelengths: 550 nm (green) at 5\({}^{\circ}\), 590 nm (orange) at 10\({}^{\circ}\), and 650 nm (red) at 15\({}^{\circ}\). (**b**) A 2D graph presenting the observed spectra at 5\({}^{\circ}\), 10\({}^{\circ}\), and 15\({}^{\circ}\) angles of incidence. (**c**) SPRi results showing distinct wavelengths meeting SPR conditions at their respective angles of incidence. influenced by fabrication inaccuracies. The bi-gratings are highly sensitive to the periodicity and amplitude of the base gratings; minor deviations during fabrication, such as slight misalignments, exposure times, or inconsistencies in the interference pattern, can lead to differences in reflection intensity. Fabrication finetuning is needed to obtain equal retroreflection for the various gratings. The significant intensity of the retroreflected wavelength across these angles suggests that the bi-gratings are functioning optimally, allowing for practical implementations in optical devices where narrowband reflection is critical. The multi-wavelength retroreflection observed in samples Bi-grating-7, Bi-grating-8, and Bi-grating-9 provides further evidence of the system's versatility. As depicted in Figure 7, each bi-grating successfully retroreflects distinct wavelengths, 550 nm (green), 590 nm (orange), and 650 nm (red), at their designated angles of incidence. This outcome illustrates the precise control achieved in the design of the bi-gratings, enabling tailored optical responses for specific applications. Regarding SPRi observed in Figure 7c, the gratings are designed to retroreflect light only when the SPR conditions are met, achieved through the in-coupling and out-coupling of light. The camera images reveal glowing bi-grating regions surrounded by dark areas of flat gold, a result of SPR excitation and subsequent light emission retroreflected into the camera. Thus, the imaging captures areas where SPR occurs. The SPRi results support these findings, confirming that the gratings effectively satisfy SPR conditions at specified angles. The SPRi results corroborate these findings, confirming that the gratings are effectively meeting SPR conditions at the specified angles. The ability to co-relate the angle of incidence with retroreflection at specific wavelengths opens avenues for advanced signaling and communication technologies. However, as depicted in Figure 7b, there is a noticeable variation in the observed reflection intensity and linewidth. Similar to the spectra in Figure 6, the bi-gratings may experience slight deviations due to fabrication imperfections.