Fabrication and characterization of plasmonic gold Sierpinski nanocarpets

Deterministic fractals [1] are self-similar objects generated by geometrical rules having non-integer dimensionality and exhibiting non-discrete Fourier spectra [2]. The multi-scale property of fractals can be exploited to... [ view full abstract ]

Deterministic fractals [1] are self-similar objects generated by geometrical rules having non-integer dimensionality and exhibiting non-discrete Fourier spectra [2]. The multi-scale property of fractals can be exploited to realize broadband spectral response in engineered nanostructures [2,3]. Tunable plasmonic near-field enhancement at multiple frequencies inspires realization of novel structures with potential applications in multi-band antennas [4], bio/chemical sensors [5], solar cells [6], and photodetectors [7]. We present fabrication, optical characterization and finite-difference time-domain simulation of gold plasmonic nanostructures arranged in Sierpinski carpet (SC) fractal [1,4], and propose their application in surface enhanced Raman spectroscopy (SERS) and localized surface plasmon resonance sensing.

Gold SCs were realized starting from a unit cell of side L that is divided into a 3x3 array of sub-cells scaled by L/3, with a gold square replacing the central sub-cell. Recursively applying the same rule to each generated sub-cells, fractals of higher orders were obtained. Employing electron beam lithography, evaporation, and lift-off, we fabricated first five orders of gold SCs with element size ranging from 3 µm to 40 nm and thicknesses 25-45 nm on CaF₂ and Si substrates. In addition, we fabricated periodic arrays of squares with size and period corresponding to the different orders of the SC.

We observed by Fourier transform infrared spectroscopy, a broadband plasmonic response from visible to mid-infrared range, exhibiting a fractal spectrum with multiple resonances. The electric field enhancement was investigated by SERS measurements carried out on SCs coated by Brilliant Cresyl Blue dye. Our results are in good agreement with the electromagnetic simulations.

References: 

1) B.B. Mandelbrot, The fractal geometry of nature (W.H. Freeman, San Francisco, 1983). 

2) L. Dal Negro & S. Boriskina, Laser & Photonics Reviews 6 (2011). 

3) G. Volpe et al., Optics Express 19 (2011). 

4) T. L. Chen et al., New J. Phys. 16, 1-13 (2014). 

5) E. Aslan et al., ACS Photonics 3.11, 20102-20111 (2016). 

6) L. Zhu et al., Optics Express 21.103, A313-A323 (2013). 

7) J. Fang et al., Nano Letters 17, 57-62 (2017).