Lithography-Free and High-Generation Fractal Plasmonic Nanoantenna for Multispectral Infrared Sensing

Fractal plasmonic nanoantennas are widely used in plasmon-enhanced infrared spectroscopy and multiband sensing applications due to their inherent broadband and multispectral characteristics. However, conventional fractal antennas are typically fabricated by high-cost, unscalable, and complicated lithography processes. The inevitable diffraction limit restricts the fabrication of higher-order fractals and the improvement of multiband infrared response. In this study, a lithography-free Au fractal nanoantenna platform is developed via film dewetting. The antennas can support multiple resonances over a broad spectral range spanning from near-infrared to mid-infrared. The fractal orders can be controlled by adjusting dewetting times. Moreover, due to the spontaneous fractal growth mechanism, the iteration number is theoretically unlimited. Through electromagnetic field simulation and infrared spectroscopy, the fractal order-dependent multiband resonance mode and dense “hot spots” enabled electric-field enhancement are revealed. Based on the infrared-enhanced antenna, a minimum detection limit of 6 nm for the thickness of poly(methyl methacrylate) nanolayers is achieved. Additionally, a noninvasive sensor concept for glucose molecules in aqueous solution is demonstrated. This study presents a lithography-free approach for constructing high-generation and large-area fractal nanoantennas with multiband resonance capabilities, which holds great promise for trace detection and high-sensitivity biochemical sensing of various analytes in the near- to mid-infrared spectral region.