Surface Plasmon Mediated Angular and Wavelength Tunable Retroreflectors Using Parallel-Superimposed Surface Relief Bi-Gratings

This study presents the design and fabrication of light retroreflectors utilizing surface plasmon resonance (SPR) in parallel-superimposed bi-grating structures. The bi-gratings were inscribed onto a thin azobenzene molecular glass film via photolithography and subsequently coated with a thin gold layer to support SPR. The two superimposed gratings operate in tandem, with one grating coupling incident light into the SPR mode and the other coupling it back out toward the light source, thereby achieving retroreflection. Monochromatic retroreflection is demonstrated for a target wavelength (785 nm) at angles from 5° to 10°, while multi-wavelength retroreflection is achieved for red, orange, and green wavelengths at corresponding angles. The findings highlight the potential of these bi-gratings for applications in optical sensing, communication, and advanced photonic systems, where compact, tunable, and angularly responsive designs are essential.