Optical Properties of Plasmonic Mirror-Image Nanoepsilon
© The Author(s). 2016
Received: 15 May 2016
Accepted: 7 July 2016
Published: 12 July 2016
We propose a novel mirror-image nanoepsilon (MINE) structure to achieve highly localized and enhanced near field at its gap and systematically investigate its plasmonic behaviors. The MINE can be regarded as a combination of two fundamental plasmonic nanostructures: a nanorod dimer and nanoring. By adapting a nanoring surrounding a nanorod dimer structure, the nanorod is regarded as a bridge pulling the charges from the nanoring to the nanorod, which induces stronger plasmon coupling in the gap to boost local near-field enhancement. Two resonance peaks are identified as the symmetric and anti-symmetric modes according to the symmetries of the charge distributions on the ring and rod dimer in the MINE. The symmetric mode in the MINE structure is preferred because its charge distribution leads to stronger near-field enhancement with a concentrated distribution around the gap. In addition, we investigate the influence of geometry on the optical properties of MINE structures by performing experiments and simulations. These results indicate that the MINE possesses highly tunable optical properties and that significant near-field enhancement at the gap region and rod tips can be realized by the gap and lightning-rod effects. The results improve understanding of such complex systems, and it is expected to guide and facilitate the design of optimum MINE structures for various plasmonic applications.
Metallic nanostructures with localized surface plasmon resonance (LSPR) provide a method for manipulating light at the nanoscale beyond the optical diffraction limit [1, 2]. The collective oscillation of free electrons on the metallic nanostructure can be excited by incident light at specific frequencies with some unique optical phenomena, such as the subwavelength localization of electromagnetic energy, the directional scattering of light out of the nanostructure, and the formation of high-intensity hot spots at the structure surface . The unique optical phenomena of metallic nanostructures have been extensively applied to various fields, including vibrational spectroscopies (surface-enhanced Raman spectroscopy and surface-enhanced infrared absorption) [4–6], plasmonic solar cells [7, 8], nanomedicine [9–11], enhanced single-emitter fluorescence [12, 13], gas detections [14–16], and optical tweezers [17–19]. However, to promote their performance, these applications require a highly localized and enhanced field, which depends on the plasmonic modes that dominate the optical properties of the nanostructure, while the plasmonic modes are quite sensitive to the geometry of nanostructure [20–22]. Nanostructures have been explored with various shapes, such as nanodisks , nanostars , nanotriangles , and nanospheres . In addition, plasmonic resonances can be tuned and positioned within a large wavelength range from ultraviolet to mid-infrared wavelengths by adjusting the structural aspect ratio using hybrid nanostructures, such as nanoshells , nanorice , and nanorings . Among these hybrid nanostructures, nanorings are particularly notable as the nanoring possesses a highly tunable plasmon resonance, uniformly enhanced field distribution in the cavity, large surface-to-volume ratio, and an open-cavity structure where the analyte could be easily accessed. These characteristics make this ring-shaped structure particularly suitable for developing biosensors .
The field intensity can be dramatically enhanced in the gap region of the metallic dimer due to the near-field coupling effect when two nanostructures are sufficiently close to each other. Various plasmonic dimers have been investigated, including nanosphere dimers , nanodisk dimers , and nanorod dimers . In particular, nanorod dimers provide a strongly coupled mode to achieve a large field enhancement in the gap because the rod-shaped structure possesses a sharper shape that allows more charges to be accumulated at the rod tip, i.e., the lightning-rod effect . In other words, the optical field can be improved by several orders of magnitude by modifying the gap distance or the rod width of a nanorod dimer. Such highly localized and enhanced near fields in the dimer system have been utilized to miniaturize the trapping spot size and simultaneously promote the induced optical forces for optical tweezers. For instance, Zhang et al. have demonstrated that 10-nm metal nanoparticles can be trapped and sensed with the nanorod dimer .
Combining the features of a nanoring and a nanorod dimer, we propose a novel mirror-image nanoepsilon (MINE), which is equal to two mirrored nanoscale ϵ-shaped structures, to boost the local near-field enhancement for various applications that require highly localized and enhanced field. Taking advantage of the nanorod dimer which can gain a high local field at the gap of the structure via lightning-rod and gap effects, an auxiliary nanoring structure is adopted surrounding the nanorod dimer to form the MINE structure for further field enhancement. The nanoring can first induce and support excess charges, and then the nanorod functions as a bridge to pull these excess charges from the nanoring to the nanorod, inducing stronger plasmon coupling to provide a larger near field around the gap. The MINE structure is fabricated and simulated to explore its plasmonic behaviors. Two resonance peaks corresponding to symmetric and anti-symmetric modes are identified. The symmetric mode is preferred because its charge distribution can support a stronger local near field with a concentrated distribution around the gap. We then investigate the influence of geometry on the optical properties of MINE structure for obtaining a highly localized and enhanced near field.
The extinction spectra of the gold MINE arrays were measured using a homemade horizontal transmission spectroscopy with a halogen lamp as a light source. The optical setup for the transmission spectrum measurement can be separated into three main components, including the front focusing system, the back focusing system, and an optical spectrum analyzer. The front and back focusing systems ensured that the focal planes of the 20× objective lens were situated at the MINE array, and the visible charge-coupled device (CCD) and television screen were utilized to display the focused image of the MINE structures. The halogen lamp served as the incident light source that passed through an aperture to form collimated light. The polarization of the incident light was parallel to the major axis of the rod dimer in the MINE structure. Then, the light was focused on the gold MINE arrays through a 20× objective lens, and the spot size was approximately 60 μm in diameter. Finally, the transmitted light was collected by another 20× objective lens and fed into a multimode glass fiber connected to the optical spectrum analyzer. The extinction spectrum was calculated by − log [I out(λ)/I ref(λ)], where I out(λ) and I ref(λ) are the intensities of transmitted light with and without the MINE arrays, respectively.
Results and Discussion
LSPR Characteristic of MINE Structure
Figure 1b shows SEM and AFM images of the fabricated gold MINE array with average t = 31 nm, g = 51 nm, w = 77 nm, r i = 174 nm, r o = 304 nm, and p = 1000 nm. The inset in the SEM image shows a single MINE structure at high magnification. No apparent electron-beam-resist PMMA residue remains on the surrounding surfaces or in the cavities of the nanostructures. The three-dimensional finite element method (3D FEM), processed via COMSOL Multiphysics software, is implemented to investigate the optical properties of the MINE structure. In our simulation, the dielectric function of the gold MINE structure is described by the Drude-Lorenz model . The environmental refractive index is set as 1.0 for air, and dispersion of the ITO substrate is considered . The scattering boundary condition is applied for all external boundaries of the computational domain to investigate the optical behavior without considering any optical couplings. All simulation results come from a single MINE structure. The dimensions of the simulated structure with t = 30 nm, g = 50 nm, w = 75 nm, r i = 175 nm, and r o = 305 nm are chosen to match the fabricated dimensions. The incident optical field (from the top) with amplitude of 1 V/m is polarized in x-direction to excite LSPR in the central gap. In addition, considering fabrication feasibility, the right-angle apexes of the nanorod were curved with a radius of curvature of 10 nm. The measured and simulated extinction spectra are provided in Fig. 1c. Two main resonance peaks (green and purple arrows) occur in the near-infrared (NIR) region.
Influences of Gap, Outer Radius, and Rod Width on Plasmonic Properties
When the gap distance reduces from 120 to 30 nm, a greatly enhanced and strong near field is induced in the gap region, and the plasmonic mode is gradually localized to an extremely small range as well. Such significantly enhanced and localized near-field property dominates the optical interaction with absorbed biomolecules or with fluorophores passing in close proximity, which is helpful for the improvement of spectroscopic signal and fluorescent efficiency. Therefore, this novel MINE structure has great potential for use in practical applications.
A novel MINE structure, which has the combined features of the nanorod dimer and nanoring, is proposed and fabricated, and its plasmonic behaviors are investigated numerically and experimentally. Two resonance peaks are identified as the symmetric and anti-symmetric modes according to the symmetries of the charge distributions on the nanoring and nanorod dimer in MINE. The symmetric mode is preferred because its charge distribution leads to a stronger near field with a concentrated distribution around the gap. In addition, the optical properties of the MINE structure highly depend on the structural geometry. The near-field intensity can be greatly enhanced by adjusting the rod width and gap distance, promoting the lightning-rod effect and plasmon coupling. The extinction ability can be raised by enlarging the outer radius because more charges are supported by the larger structural size. Based on the strong and localized near field at the center of nanorod dimer, our studies show that adopting an auxiliary nanoring surrounding the nanorod dimer can further enhance its near field. Compared to individual nanoring and nanorod dimer, our proposed MINE structure can dramatically boost the local near-field intensity because excess charges can be induced by the ring-shaped nanostructure and the rod-shaped nanostructure can function as a bridge to pull these excess charges from the ring to the rods, thereby inducing stronger plasmon coupling between the rods and resulting considerably local near-field enhancement. Hence, this MINE structure, which has a significantly enhanced and localized near field, has great potential for applications such as improving single-molecule detection and exploring biochemistry, including molecular bonding and chemical reactions. Our studies provide illuminating insight into complex systems and guide the design of an optimal MINE structure. Moreover, we expect that this nanostructure can serve as a building block for various applications such as nanomedicine, biochemistry, single-emitter fluorescence, vibrational spectroscopy, and optical tweezers.
This work is supported by Taiwan’s Ministry of Science and Technology (MOST) under contract number MOST 103-2221-E-009-096-MY3. The authors would like to thank the equipment support from the Center for Nano Science and Technology (CNST) and the Nano Facility Center (NFC) at National Chiao Tung University (NCTU), Taiwan.
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