Abstract
Abstract The mathematical concept of fractals is widely applied to photonics as planar structures ranging from terahertz resonators, optical antennas, to photodetectors. Here, instead of a direct mathematical abstract, we design a plasmonic leaf with fractal geometry from the outline of a leaf from Wargrave Pink. The enhanced light-matter interactions are observed numerically from the improvement in both absorption and near-field intensification. To demonstrate the effect experimentally, a three-dimensional fractal structure is realised through direct laser writing, which significantly improves the photothermal conversion. By virtue of the self-similarity in geometry, the artificial leaf improves the absorption of a 10-nm-thick gold film with 14 × temperature increment compared to flat Au film. Not limited to the proof-of-concept photothermal experiment demonstrated here, the fractal structure with improved light-matter interactions can be utilised in a variety of applications ranging from non-linear harmonic generation, plasmonic-enhanced fluorescence, to hot electron generation for photocatalysis.
Highlights
The mathematical concept of fractals is widely applied to photonics as planar structures ranging from terahertz resonators, optical antennas, to photodetectors
We discover the Koch curve, one of the earliest fractal geometry described by Helge von Koch in 1904 [1], a good abstraction to mimic the shape of the leaf
The plasmonic leaf is achieved through deposition of Au film on a fractal template made of photoresist through direct laser writing
Summary
For the structures with high order (K3, K4), dramatic absorption boost is achieved in the infrared regime, where only tiny absorption occurs inside the flat thin film (K0, solid red line). Such strong localisation of photons with different is always desired for optical systems for energy harvesting and harmonic generations.
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