## 1 Introduction  
Retroreflectors are optical devices that reflect incident light back to its source. They are utilized in various applications, including sensing [1], optical communication [2, 3], and signal enhancement [4, 5]. Traditional retroreflectors, such as corner-cubes [6] and spherical retroreflectors [7, 8], have been extensively studied and widely implemented. However, the evolving demands of modern optical systems, including miniaturization, tunability, and enhanced functionality, have spurred the development of advanced retroreflector designs [9]. A range of differently shaped nanostructures, including nanoshells and fiber optics, have been explored for retroreflection purposes [10, 11]. Among these, grating-based retroreflectors have emerged as a promising alternative, offering a versatile platform for tailoring optical characteristics [12]. Grating-based retroreflectors can be engineered to have specific optical characteristics, and they offer several distinct advantages over traditional designs, including compact size, tunability, and the ability to support resonant light interactions [13]. Among the various approaches to enhance the performance of grating-based retroreflectors, the integration of surface plasmon resonance (SPR) has emerged as a particularly promising avenue. SPR occurs when incident light couples into the plasmon mode to produce collective oscillations of surface electrons at a metal-dielectric interface, leading to significant confinement of the electromagnetic field at the boundary [14]. While SPR is not unique to gratings, it can also be generated by various other structures, such as C-shape, bowtie apertures, circular, and metasurface structures [15; 16; 17; 18]. In the case of gratings, only incident light that is polarized along the grating vector will couple into the SPR mode. The capability of surface relief gratings to couple light into an SPR mode has garnered considerable attention, as these nanostructures can be finely engineered to achieve desired optical responses [19; 20; 21], including the SPR wavelength band. Recent advancements have demonstrated the potential of grating-based retroreflectors for applications such as resonant beam communication [22], omnidirectional retroreflection [23], and enhanced optical performance [24; 25]. Within this context, parallel-superimposed bi-linear surface relief diffraction gratings have demonstrated particular effectiveness in generating counter-propagating surface plasmon waves. This configuration facilitates the in-coupling and out-coupling of light waves, allowing for controlled retroreflection with tailored optical properties [26; 27]. Despite the semi-analytical demonstrations of grating-based retroreflectors, prior studies have not fully explored their potential for practical retroreflection-based applications. In this work, we fabricated a novel monochromatic retroreflector that uses surface plasmon in-coupling/out-coupling within a surface relief bi-grating structure. Two superimposed gratings are inscribed on the surface of an azobenzene molecular glass thin film, which is then covered by a thin film of gold. The parameters of the gratings are selected to enhance retroreflection at a chosen fixed wavelength and at various angles. Additionally, we developed a multi-wavelength retroreflector capable of selectively reflecting specific wavelengths, including in the near-infrared (NIR), at selected specific angles. To support this design, we provide a detailed theoretical analysis of the bi-grating structure, explaining the mechanisms underlying SPR in-coupling and out-coupling. Furthermore, we explore the potential for multi-wavelength retroreflectors, highlighting their ability to retroreflect multiple distinct wavelengths at different angular positions. Retroreflectors with these designed characteristics can have significant implications in critical areas such as search and rescue operations, where effective light feedback is essential [28; 29].