Tunable Optical Performances on a Periodic Array of Plasmonic Bowtie Nanoantennas with Hollow Cavities
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We propose a design method to tune the near-field intensities and absorption spectra of a periodic array of plasmonic bowtie nanoantennas (PBNAs) by introducing the hollow cavities inside the metal nanostructures. The numerical method is performed by finite element method that demonstrates the engineered hollow PBNAs can tune the optical spectrum in the range of 400–3000 nm. Simulation results show the hollow number is a key factor for enhancing the cavity plasmon resonance with respect to the hotspot region in PBNAs. The design efforts primarily concentrate on shifting the operation wavelength and enhancing the local fields by manipulating the filling dielectric medium, outline film thickness, and hollow number in PBNAs. Such characteristics indicate that the proposed hollow PBNAs can be a potential candidate for plasmonic enhancers and absorbers in multifunctional opto-electronic biosensors.
KeywordsPlasmonic bowtie nanoantennas Finite element method Hollow number Cavity plasmon resonance
Broadband nanoantennas play a potential role in the nanophotonic field. Recently, plasmonic optical nanoantennas [1, 2, 3, 4] made by novel metal nanoparticles (MNPs) have generated great research interest due to their capability or dramatically localizing and enhancing electromagnetic (EM) fields on the surface of the MNPs [5, 6] and have attracted much attention for near field applications in biosensing , spectroscopy , nanolithography [9, 10], etc. Among these traditional optical nanoantennas, plasmonic bowtie nanoantennas (PBNAs) [11, 12], which can efficiently convert light from free space into subwavelength scale with the local field enhancement at optical frequency in the small air gap between their two triangle MNPs. PBNAs are usually designed to induce high local EM fields in between the gap to be used in sensing applications.
To deeply exploit the surface plasmon resonance (SPR) effects on a periodic array of PBNAs, a fully understanding of the interaction between the incident EM wave and PBNAs is urgently needed. Optical transmittance measurements revealed that the PBNAs supported both the bonding and anti-bonding SPR modes, and finite element method (FEM) calculations show very high field intensities within the bowtie gap for the bonding SPR modes [13, 14]. Recently, absorber MNPs are also employed in the quantum dot solar cells to trap solar radiation in the form of localized SPRs, which lead to a broad spectral photocurrent enhancement . In a gold nanofilm, strongly enhanced local field due to the excitation of SPRs will give rise to two-photon absorption and further lead to two-photon excite photoluminescence (TPPL). TPPL can be served as an efficient way to probe and image SPR modes in MNPs and resonant nanoantennas . In the recent literatures, it was demonstrated that hollow PBNAs are capable of absorption radiation [1, 2, 17] and the operation wavelength is confined in the range of visible light. In fact, it should be considered that broadband resonances are commonly correlated to efficient absorption over the wider spectrum. Tuning the properties of PBNAs for the vast variety of applications relies heavily on a careful design approach which involves three major parameters: dimension, aspect ratio, and the inner/outer material in MNPs [18, 19]. The expanding application spectrum of PBNAs demand versatile design approaches to tailor the antenna properties for specific requirements.
The fabrication of the periodic array of hollow PBNAs based on secondary electron lithography was previously demonstrated [1, 2]. In fact, being hollow cavities, the PBNAs can work as an optical nanocavity that generates both outer and inner SPR modes along with the high hotspots exiting in hollow and gap regions far below the diffraction limit. This value is significantly high if we consider that the hollow works in nanoscale regime. It could open promising applications where high near-field intensity is needed in a free volume (air, gas, dielectric medium, or liquid) of subdiffraction dimension [1, 2]. In this paper, we numerically analyze and quantitatively compare the near-field intensities and absorption spectra in a periodic array of PBNAs by introducing the hollow cavities into the MNPs. Although several outline plasmonic nanoantenna structures have been proposed for tuning SPRs, for manipulating the damping of the plasmonic resonances, for achieving deep subwavelength light modulation, and for improving dielectric constant sensing capabilities [3, 20, 21], our study will demonstrate the significant freedom gained by a simple outline design to broaden and tune the SPR effects without changing the outer dimension of MNPs. We focused our attention on a period array of hollow PBNAs which show the accumulation of the optical energy in the hollow regions. Numerical simulations are performed by using three-dimensional (3-D) FEM . We examine the influence of its structural parameters on the antenna resonance conditions (i.e., the effective resonant wavelength (λ res)). In addition, the characteristic of near-field intensities and absorption spectra of a periodic array of hollow PBNAs corresponding to their bonding mode has also been investigated.
Results and discussions
Note that the λ res of near-field intensity and absorption spectra shows a red shifting with the increasing hollow number (the number of triangular wings) in PBNAs due to the fact that more hollow regions result in enhancing the cavity plasmon resonance among them. These results in the splitting of the plasmon mode into two resonance modes, i.e., “bonding” mode (ranging in longer wavelengths) and “anti-bonding” mode (ranging in shorter wavelengths) . The λ res are found in the wavelength range of 1.2–2.5 μm for bonding mode and 0.6–1.3 μm for anti-bonding mode, respectively. The bonding mode occurs at longer wavelengths owing to the attractive near-field interactions across the gap and hollow regions lower than the resonant frequency . Additionally, the more hollow number in PBNAs can contribute a change of λ res toward longer wavelength with respect to the retarding effects among the MNPs. It is worthy to note that the advantage of case 4 is the polarization independence compared to the other cases with less hollow number. The λ res of both bonding and anti-bonding modes can be easily tuned in the broad spectra range by adjusting their structure parameters, e.g., a, b, c, d, t, ε, and P, useful for various plasmonic optical sensing, solar cells, and optical device applications, owing to the coupling effects among neighboring gold-shell outline structures [13, 14]. This ability relies on the hollow cavity vertically aligned (with respect to the dielectric substrate plane) PBNAs. The multiband behavior exhibited by these hollow PBNAs may be a key factor for highly sensitive Raman measurements on the various components of the whole cell using an extended range of excitation light sources [1, 2].
The schematic charge densities of cases 1–4 are also depicted in Fig. 3b, respectively. In case 1, the charge pairs distribute locally on the edge surface of PBNAs, showing dipole-like distributions and resulting in bonding mode resonance. The charge pairs of cases 2–4 distribute overall on the inner/outer outline surface of PBNAs and exhibit stronger dipole-like distributions than that of case 1 due to the combination of SPRs and the cavity-like effects among the MNPs, gaps, and hollows. It is worthy to note that the segment of charge pair distribution is dependent on the hollow number being used in PBNAs. The mechanism can be explained by the symmetries and asymmetric modes of the charge pair distribution and the dipole and quadrupolar (or higher order) resonances of hollow PBNAs with/without central hollows or cavities [13, 14, 25, 26, 27, 28, 29].
We numerically analyze and quantitatively compare the near-field intensities and absorption spectra of SPR modes on a periodic array of hollow PBNAs by means of 3-D FEM. We have found that the resonance width can be reduced as the increasing hollow number in PBNA due to the fact that more hollow regions result in enhancing the cavity plasmon resonance among them. It is worthy to note that the advantage of the proposed case 4 is the polarization independence compared to the other cases with less hollow number. The region of hotspot can be increased as the increasing of the hollow number in PBNAs, which demonstrates that the cavity plasmon resonance contributes the SPR effects on PBNAs with hollow cavities, showing the accumulation of the optical energy in the hollow regions. These promising results warrant a more detailed understanding of the performance of a periodic array of hollow PBNAs. The results shown in this work open new opportunities for design the broadband, flexible, and compact plasmonic devices for general purpose while the outer size of hollow PBNAs is kept constant.
This work was supported by the University Research Grant of Universiti Brunei Darussalam (Grant No. UBD-ORI-URC-RG331-U01) and the Ministry of Science and Technology of Taiwan (Grant No. MOST 103-2112-M-019-MY3).
YFCC drafted the manuscript. CTCC and JYR carried out the whole simulation works. HPC designed the simulation works, revised the manuscript, and approved the final version. CML, RCL, and NYV participated in the data analysis and discussions. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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