Self-Similarity of Plasmon Edge Modes on Koch Fractal Antennas

Abstract

We investigate the plasmonic behavior of Koch snowflake fractal geometries and their possible application as broadband optical antennas. Lithographically defined planar silver Koch fractal antennas were fabricated and characterized with high spatial and spectral resolution using electron energy loss spectroscopy. The experimental data are supported by numerical calculations carried out with a surface integral equation method. Multiple surface plasmon edge modes supported by the fractal structures have been imaged and analyzed. Furthermore, by isolating and reproducing self-similar features in long silver strip antennas, the edge modes present in the Koch snowflake fractals are identified. We demonstrate that the fractal response can be obtained by the sum of basic self-similar segments called characteristic edge units. Interestingly, the plasmon edge modes follow a fractal-scaling rule that depends on these self-similar segments formed in the structure after a fractal iteration. As the size of a fractal structure is reduced, coupling of the modes in the characteristic edge units becomes relevant, and the symmetry of the fractal affects the formation of hybrid modes. This analysis can be utilized not only to understand the edge modes in other planar structures but also in the design and fabrication of fractal structures for nanophotonic applications.

Plasmon response evaluation based on image-derived arbitrary nanostructures

“…All SERS-active substrate films were prepared by a PVD method with subsequent annealing at 290°C for 60 s, following a protocol described in detail previously. 8 The pre-cleaned silicon wafer on which the particles were formed was tilted by ∼15°to mimic the angle of silicon AFM tips (used for the TERS-active tips) to facilitate the comparison between the two types of plasmonic samples. The TERS active tips were produced using a similar procedure, by evaporating a silver particle film on commercial AFM tips (Tap150Al-G, BudgetSensors).…”

“…A proper understanding of the parameters affecting the amplitude and the frequency of the plasmon resonance, as well as the specific modes involved at a particular wavelength is a key prerequisite for reaching well-defined and desired plasmonic characteristics.In a perfect scenario, time consuming and costly experimental trial-and-error experiments should thus be avoided and replaced by a detailed theoretical description of the electric field enhancement and its spatial distribution for any plasmonic system under investigation (unless complex techniques such as electron energy loss scattering or scattering near field optical microscopy are used to map the spatial distribution of the field intensity). [6][7][8] The plasmon resonance simulated is then linked to the expected Raman response, although several theories and other factors also come into play. [9][10][11] Unfortunately, tremendous computational capacity and calculation time are generally unavoidable when it comes to predicting the specific behavior of plasmonic substrates.…”

Surface Plasmon Resonances in Sierpinski-Like Photonic Crystal Fibers: Polarization Filters and Sensing Applications

“…The use of fractal geometries, on the other hand, has been successfully exploited during the last decades for the realization of multiband (or broadband) compact and high-performance antennas [ 34 , 35 , 36 ]. More recently, these self-similar geometries have also enabled multispectral compatibility and multiple applications when used for patterning one- and two-dimensional plasmonic superlattices [ 37 , 38 , 39 , 40 , 41 ]. In contrast, self-similar plasmonic properties in SPR-PCFs remain unexplored.…”

Bio-inspired plasmonic leaf for enhanced light-matter interactions

“…Fractal is an abstract object that describes the geometry with a high degree of self-similarity [1]. Stimulated by this mathematical concept, different fractal geometries such as Hilbert curves, Sierpinski carpets, and Cayley trees are utilised in a variety of two-dimensional (2D) planar photonic systems, ranging from terahertz resonators [2][3][4][5], optical antennas [6,7] frequency-selective photonic quasi-crystals [8][9][10][11], to sub-wavelength focusing [12,13], photovoltaics [14,15], surface-enhanced spectroscopy [16,17], and photodetectors [18]. Meanwhile, fractal-like structures widely exist in nature at different scales, including spiral galaxies, coastlines and lightning bolts, seashells, and parts of living organisms such as the human lungs.…”

“…A prominent temperature increment under one sun is achieved with an ultra-thin (10 nm) Au film, owing to the broadband absorption enhancement from visible to infrared. We attribute such advantages to the 3D fractal geometry of the plasmonic leaf that enlarges the region of interaction between the metallic film and incident light, while previously used planar configurations [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] align fractal structures in the plane perpendicular to the direction of light propagation.…”