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Broadband cross-slots fractal nano-antenna and its extraordinary optical transmission characteristics
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Abstract
To overcome the disadvantages of narrow frequency band and low transmittance for traditional nano-antenna, a nano-antenna structure based on cross-slots fractal was designed. The extraordinary optical transmission characteristics of the cross-slots fractal nano-antenna and the differences between the cross-slots fractal nano-antenna and the uniform cross-slots nano-antenna were analyzed by the finite difference time domain method. Meanwhile, the influence of physical parameters on the extraordinary optical transmission characteristics of the cross-slots fractal nano-antenna and the relationship of transmission spectrum of the nano-antenna between the fractal size and the non-fractal size were discussed. The results show that the fractal cross-slots structure is more miniaturized, and realizes extraordinary optical transmission and full 2π phase control of transmission beam, and the transmittance is higher than the uniform cross-slots structure, the full width at half maximum (FWHM) is wider, and the highest transmittance is up to 99.51%. By adjusting the physical parameters, the transmission spectrum exhibits red-shift or blue-shift characteristics, achieving controllability of the transmission spectrum. When h=50 nm, the full width at half maximum is about 356 nm, and the transmittance is still as high as 95.66%, which is generally higher than traditional structures, and the peak transmittance is still greater than 74% at large incident angles (70 degrees). In short, the cross-slots fractal nano-antenna has the characteristics of wide frequency, controllable and adjustable, and more miniaturized structure compared with other nano-antenna structures, and realizes extraordinary optical transmission. -
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References
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Overview
Overview: The nano-antenna structure can break through the optical diffraction limit and achieve efficient transmission of light. For nano-antennas with specific wavelengths of radiation, transmission characteristics are an important characteristic of effective light transmission. Ebbesen et al. found optical extraordinary transmission phenomena when analyzing the transmission characteristics of metal film sub-wavelength aperture arrays. When light is incident on a sub-wavelength aperture array, the light transmission is higher than the ratio of the aperture area to the total area of the film at a particular wavelength, and the transmission is 1 to 2 orders of magnitude higher than the classical aperture transmission theory. Study has shown that the generation of extraordinary optical transmission is generally attributed to the mutual coupling of light waves with free electron oscillations at the surface of metal holes or slots structures, and localized surface plasmons at the edges of apertures or slots also have a non-negligible effect on extraordinary transmission. The coupling efficiency of the plasmon polarization of the upper and lower surfaces of the structure can be improved, thereby enhancing the transmission of light. In order to achieve wide-band extraordinary transmission and the purposes of controllable and adjustable, we introduce fractal theory, and utilize the properties of self-similarity and fractal dimension to propose an extraordinary transmitted cross-slots fractal nano-antenna. Furthermore, the finite-time-difference method is used to calculate the extraordinary transmission characteristics and surface electric field distribution of the cross-slots fractal nano-antenna structure, and the transmission characteristics mechanism is systematically analyzed and compared in detail. The results show that the cross-slots fractal structure is smaller in size, wider in the full width at half maximum (FWHM), and higher in transmittance, up to 99.51%. At 851.536 nm, the light transmittance is much higher than that of the uniform cross-slots structure. The ratio of the hole area to the Ag material area realizes the extraordinary optical transmission. By adjusting the physical parameters, the transmission spectrum exhibits a red-shift or blue-shift characteristic, and achieves the controllability of the transmission spectrum. Meanwhile, when h=50 nm, the FWHM is about 356 nm, the transmittance is still as high as 95.66%, which is generally higher than the traditional structures; At a large incident angle (70 degrees), the peak transmittance is still greater than 74%. In short, the cross-slots fractal nano-antenna has the characteristics of wide frequency, controllable and adjustable, and more miniaturized structure compared with other nano-antenna structures, and realizes the extraordinary transmission of light and full 2π phase transmission control. In addition, the nano-antenna produces a significant resonance in the short-band, which further enhances the transmission of light.
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Figure 1.
Cross-slots fractal nano-antenna structure model.(a) Cross-slot fractal nano-antenna structure; (b) 0-fractal; (c) 1-fractal; (d) 2-fractal
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Figure 2.
Transmission spectrum of cross-slots fractal nano-antenna structure
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Figure 3.
The field phase distribution of the transmitted beam. (a) Under dx size; (b) Under dy size
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Figure 4.
The electric field distribution |E| on the surface of the cross-slots fractal nano-antenna. (a) 0-fractal: 848.495 nm; (b) 1-fractal: 854.598 nm; (c) 2-fractal: 851.536 nm; (d) 1-fractal: 449.995 nm; (e) 1-fractal: 484.89 nm; (f) 1-fractal: 537.542 nm; (g) 2-fractal: 451.706 nm; (h) 2-fractal: 478.061 nm; (i) 2-fractal: 555.123 nm
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Figure 5.
Comparison of transmission spectra between uniform cross slots antenna structure and cross-slots fractal.(a) 0-fractal & 0-uniform; (b) 0-uniform; (c) 1-fractal & 1-uniform; (d) 2-fractal & 2-uniform
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Figure 6.
Transmission spectrum and FWHM of nano-antenna structure under different structural parameters.(a) Transmission spectrum with different lengths P; (b) Transmission spectrum with different thickness h; (c) FWHM with different lengths P; (d) FWHM with different thickness h
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Figure 7.
Transmission spectra of different materials and incident angles. (a) With different materials; (b) With different angles
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Figure 8.
Transmission spectrum under the cross slots parameter. (a) With different L1; (b) With different W1; (c) With different L2; (d) With different W2; (e) With different L3; (f) With different W3
