Wet Chemical Engineering of Nanostructured GRIN Lenses

Gradient-index (GRIN) lenses have long been recognized for their importance in optics as a result of their ability to manipulate light. However, traditional GRIN lenses are limited on a scale of tens of microns, impeding their integration into nanoscale optical devices. This study presents a groundbreaking self-assembled method that overcomes this limitation, allowing for constructing GRIN lenses at an extremely small dimension. The self-assembly process offers several advantages, including creating highly precise, scalable, cost-effective, and complex structures that eliminate the need for intricate and time-consuming manual assembly. By engineering densely packed arrays of metallic nanoparticles, exceptional control over the local refractive index has been achieved. This is accomplished by layer-by-layer assembly of gold nanoparticles of different sizes over silica beads. A GRIN lens light-sink is built where light is preferentially directed toward the center, which is corroborated by measuring the fluorescence of Rhodamine B (RhB) in the inside. Unlike traditional bulky macroscopic GRIN lenses, light-sinks boast a size under 2.5 µm. Notably, the self-focusing effects of this design allowed us to track the growth of single-nanoparticle layers using SERS (Surface-Enhanced Raman Spectroscopy). These results pave the way for designing and developing lens-like devices at the nanoscale, allowing unprecedented light manipulation.