Focused Ion Beam Direct Fabrication of Subwavelength Nanostructures on Silicon for Multicolor Generation

Color filtering via interaction of visible light with nanostructured surfaces offers high resolution printing of structural colors. A novel approach for color filtering in reflection mode via direct fabrication of subwavelength nanostructures on high-index, low-loss, and inexpensive silicon (Si) substrate is developed. Nanostructures having a unique geometry of tapered holes are fabricated exploiting the Gaussian nature of a gallium source focused ion beam (FIB). The fabrication process is rapid and single-step, i.e., without any pre- or postprocessing or mask preparation in contrast to previously reported nanostructures for color filtering. These nanostructures are tunable via FIB parameters and a wide color palette is created. Finite-difference time-domain (FDTD) calculations reveal that the unique tapered nanohole geometry facilitates enhanced color purity via selective absorption of a narrow band of incident light wavelengths and makes it possible to obtain a wide variety of colors suitable for realistic color printing applications. The proposed approach is demonstrated for color printing applications via fabrication of butterflies and letters on Si.