Transmission properties of dual-band cross-dipole fractal slit arrays for near- and mid-infrared wavelengths

The periodic arrays of two-stage cross-dipole fractal slits etched on a $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick gold film exhibit two resonance pass bands in the near- and mid-infrared region. The sample was fabricated by electron beam lithography and reactive ion etching system, and the width of the fractal slits is about $120\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The transmission coefficient exceeds 1 at the peak wavelength if normalized to the area of the fractal slits. The measured transmission spectrum in the frequency domain around the short wavelength pass band agrees well with the simulation results obtained via the transfer-matrix method. The existence of two resonance bands and their relative positions can be well explained by the geometric self-similarity of the fractal structure. Both transmission spectrum and electric field distribution analysis show that the two resonance bands result from the interaction of the incident light with the individual primary and secondary elements of the two-stage fractal structure.