Geometrical optimization study of diabolo nanoantenna

Journal Pre-proof Geometrical optimization study of diabolo nanoantenna Nyha M. Hameed, Mohammed AL Lethawe

PII:

S0030-4026(19)31432-9

DOI:

https://doi.org/10.1016/j.ijleo.2019.163534

Reference:

IJLEO 163534

To appear in:

Optik

Received Date:

3 July 2019

Accepted Date:

2 October 2019

Please cite this article as: Hameed NM, Lethawe MA, Geometrical optimization study of diabolo nanoantenna, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163534

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Geometrical optimization study of diabolo nanoantenna NYHA M. HAMEED1* AND MOHAMMED AL LETHAWE1,2 1Al

Muthanna University, College of Science, Department of Physics, Al Muthanna, Iraq MN2S, Institut FEMTO-ST, UMR 6174 CNRS, Université Bourgogne Franche– Comté, 15B Avenue des Montboucons, 25030, Besançon Cedex, France

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2Département

*[email protected]

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Key words:

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Abstract: We numerically simulate and investigate different dimensional geometries of the diabolo antenna (DA) in gold. The results show that the resonance wavelength is linearly related to the DA gap and length, but nonlinearly to the thickness. The electric and magnetic field enhancements of different DA geometries can be considered as a database for other researchers to review. This DA shows a higher magnetic field enhancement increasing with geometric changes, compared to that of the electric field. The results obtained provide a map of the factors of enhancement with nanoscale antenna dimensions.

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This research did not receive any specific grant from funding agencies in the public, commercial, ornot-for-profit sectors.

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1. Introduction Babinet’s principle is a common approach to light wave theory. The principle has been used for the simple analysis of some diffraction problems [1,2] and as an approximation for metamaterials with limited thickness [3,4]. The study of the diabolo antenna (DA) has attracted interest for electric and magnetic field enhancement. The DA structure contributes to increases in the intensity of optical forces generated on dielectric nanoparticles, which can be exploited for optical trapping. By using the designed plasmonic dipole nanoantennas, 10-nm gold nanoparticles were successfully trapped [5–7]. To further enhance the optical force, the confinement of both the electric and magnetic fields can be increased. Metallic nanorings [8,9] were recently proposed as magnetic field detectors because of their specific magnetic properties. The DA was recently demonstrated to generate a high-strength magnetic near field when illuminated by linearly polarized wave along its axis. Such metal nanostructures can generate greatly enhanced electromagnetic (EM) fields because of their forms and compositions [10,11]. Additionally, in a recent

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study, the confinement and enhancement of magnetic fields were obtained by applying such structures. Numerical 3D-finite-difference time-domain (FDTD) simulation results demonstrated high confinement of the EM field near the DA. Here, we propose exploitation of this enhanced magnetic field. Our results show that, by adjusting the DA geometry, a double-resonance EM confinement effect can be achieved near the DA gap zone. This doubly resonant structure permits the design of a new generation of efficient optical nano-tweezers. Moreover, the trapping process depends on the nanoparticle dimensions; for specific geometries, non-contact trapping can be achieved [12].

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2. Proposed geometry The DA is inspired by Babinet’s principle, which specifies that the electromagnetic field diffraction by any aperture in an infinitely thin layer of perfect metal is equal to that diffracted a complementary structure [13], provided that the electric field replaces the magnetic field and vice versa. To numerically check this principle, we consider a DA for which the complementary structure is a bowtie nano-aperture (BNA), as shown in Figure 1(a) and (b). BNA allows electric field enhancement because of capacitive effects (charge accumulation at the edges of the metallic arms, while the DA is an inductive component allowing charge to travel between the arms . Let us note that the two above effects (capacitive and inductive) are induced by the EM field component that is directed along the antenna-axis that is defined by the metallic arm axis.

Fig. 1. 3D views of (a) BNA and (b) its complementary DA nanoantenna.

Numerical simulations were performed for two different cases of a DA and BNA made using perfect conductors or real metal (gold, for instance). The

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near-field (5 nm above NA) enhancement spectra of the electric and magnetic fields are presented in Figure 2(a) and (b), respectively, for D = 135 nm, G = 15 nm, and T = 20 nm. As shown, the Babinet principle is almost verified in the near-field region only for the perfect conductor, while a discrepancy appears for the gold structure. This is attributed to the EM field penetration within the metallic NA.

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Fig. 2. Near-field electric and magnetic spectral responses of DA and its complementary BNA of (a) perfect conductor and (b) real metal.

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We intend to provide a database of electric and magnetic fields around a DA and their changes with varying media. Many recent papers in optical trapping have investigated liquids in order to compensate for particle weight by buoyancy. Consequently, we have performed a 3D simulation of the DA placed in water for varying DA dimensions. Figures 3(a) and (b) show the electric and magnetic spectral responses of DAs of different lengths D with the fixed G = 25 nm and T = 20 nm. The four values of D = 135, 145, 155, and 165 nm are considered to present the influence of this parameter on both the enhancement factor and the resonance wavelength. The results clearly demonstrate a high enhancement in magnetic field enhancement compared to that of the electric field.

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Fig. 3. (a) Magnetic and (b) electric spectral responses as a function of length D with fixed values of T = 20 nm and G = 25 nm of a gold DA immersed in water.

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Fig. 4. (a) Magnetic and (b) electric spectral responses as a function of gap G with fixed values of D = 135 nm and T = 20 nm of a gold DA immersed in water.

Fig. 5. (a) Magnetic and (b) electric spectral responses as a function of thickness T with fixed values of D = 135 nm and G = 25 nm of a gold DA immersed in water.

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We note from Figure 3 that the electric and magnetic resonances occur at the same wavelength values, regardless of D. This is very important because we are attempting to obtain simultaneous electric and magnetic confinement. In addition, the electric field enhancement factor seems to be independent of D, the magnetic field enhancement factor linearly increases with D. Similar studies are performed by varying the gap size G (see Figure 4) with D = 135 nm and T = 20 nm and the thickness T (see Figure 5) with D = 135 nm and G = 25 nm. For these cases, the resonance wavelength (RW) changes with variations in G and T. In addition, the growth of the RW accompanies increases in the electric and magnetic field enhancements. The results of Figures 3, 4, and 5 can be applied to determine the geometry of the DA to obtain the desired RW. To elucidate the relationship between the RW and enhancement factor and these geometrical parameters, we plot the RW and the maximum enhancement factors against the DA geometrical parameters of D, G, and T as shown in Figure 6, 7, and 8, respectively. The obtained results show linear behaviors with each DA parameter, except for the DA thickness T, which is nonlinearly related to the RW and enhancement factors.

Fig 6. Relationships between RW and EM enhancement factors with DA geometrical parameters: DA length D.

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Fig 7. Relationships between RW and EM enhancement factors with DA geometrical parameters: DA gap G.

Fig 8. Relationships between RW and EM enhancement factors with DA geometrical parameters: DA thickness T.

3. Conclusion

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In summary, we have theoretically and numerically studied the enhancement factors of electric and magnetic fields in DA structures of varying dimensions. Our results show that this configuration can confine electric and magnetic fields from incident illumination with large enhancement factors. Numerical simulations were performed to determine the interaction between the DA’s enhancement factors and its geometry. Linear relationships were found between the DA enhancement factors and RWs with its dimensions of length and gap width. The DA can thus be designed to fulfill experimental constraints to yield a specific RW.

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