Fabrication and characterization of well-aligned plasmonic nanopillars with ultrasmall separations
© Si et al.; licensee Springer. 2014
Received: 2 May 2014
Accepted: 7 June 2014
Published: 13 June 2014
We show the fabrication of well-aligned gold and silver nanopillars with various array parameters via interference lithography followed by ion beam milling and compare the etching rates of these two metallic materials. Silver is suitable for fabricating ultrafine arrays with ultrasmall separations due to high milling rates. The optical properties of the fabricated nanopillars are specifically characterized from both normal incidence and oblique incident angles. Tunable surface plasmon resonances are achieved with varying structural parameters. Strong coupling effects are enabled when the separation between adjacent nanopillars is dramatically reduced, leading to useful applications in sensing and waveguiding.
KeywordsPlasmonic Nanopillars Dense arrays
Known as the electromagnetic waves propagating along metal-dielectric interfaces, surface plasmons (SPs) have drawn increasing attention in recent years[1–5]. Many plasmon-enabled applications have been developed due to their unique optical properties and particular ability of manipulating light at the nanometer scale. Additionally, SP-based waveguides are useful for developing devices with ultrahigh sensitivity and figure of merit because the near-field of electromagnetic waves can be significantly enhanced using different plasmonic nanostructures. Various plasmonic nanostructures, including nanopillars for waveguiding[6–8], and bio-sensing[9–11], or photonic crystals for efficient cavity coupling, have been demonstrated recently. Moreover, extensive useful applications have been triggered by plasmonics in super-resolution imaging[13–15], cloaking[16–18], energy harvesting[19–21], and color filtering[22–25]. Various applications (plasmonic absorbers, for instance) have been reported by using nanodisks[26–28] or nanopillars to modify the surface profile, allowing for tight confinement of more energy inside the functional layer of a solar cell. Such nanodisks/nanopillars that act as plasmonic absorbers (also known as plasmonic blackbodies) are extremely useful for energy harvesting. Metal nanopillars or wires excited by electromagnetic waves show resonance characteristics which are highly dependent on geometric parameters. In the optical regime, metals are dispersive materials with finite conductivity. Either surface plasmon polaritons (SPPs) or localized surface plasmon resonances (LSPRs) reveal salient resonance features, and the optical properties of metal nanopillars are mainly determined by their shape, size, and even the dielectric environment. Recently, the fascinating optical properties of small nanopillars/particles[30–34] and other different geometries[35–40] have been extensively investigated both experimentally and theoretically, providing a new pathway for manipulating light at the subwavelength scale.
Due to important advances in nanofabrication techniques, plasmonic nanostructures and related devices are presently gaining tremendous technological significance in nanophotonics and optics. Nanostructures could provide intriguing possibilities for resolving those challenges and improving device performance. Well-aligned nanopillars with perpendicular orientations to the substrate are becoming the main building blocks for new optical devices with promising potential applications. Here we explore, experimentally and theoretically, the optical properties of periodic nanopillars perpendicularly aligned on the supporting substrate. Combination of interference lithography (IL) and ion beam milling (IBM) techniques enables scalable fabrication of such nanopillars with excellent dimensional control and high uniformity. Detailed experimental results show that silver (Ag) has a much higher etching rate than gold (Au) under the same milling conditions, making Ag a perfect candidate for the construction of plasmonic ultrasmall features. In addition, nanopillar arrays with ultrasmall inter-pillar separations are fabricated and optically characterized.
Quartz substrates were first cleaned with acetone in an ultrasonic bath followed by isopropyl alcohol (IPA) and deionized water washing and finally blow-dried with a nitrogen gun. Subsequently, Au or Ag films with different thicknesses were deposited on quartz substrates with 4-nm titanium as the adhesion layer by electron beam evaporation (Auto 306, Edwards, Crawley, UK) at a base pressure of about 3 × 10-7 mbar. In order to minimize the deposition-introduced roughness, low evaporation rates were applied (less than 0.03 nm/s). Afterwards, positive resist (S1805, Dow, Midland, MI, USA) was used to define nanopillar arrays on the metal (Au or Ag) layer supported by a quartz substrate (refractive index = 1.46) with a laser holography system using a 325-nm helium-cadmium laser, serving as the IBM mask after development.
Parameters summary for the IBM process in this work
The optical properties of the fabricated nanopillars under normal incidence were measured using a commercial system (UV-VIS-NIR microspectrophotometer QDI 2010™, CRAIC Technologies, Inc., San Dimas, CA, USA). A × 36 objective lens with the numerical aperture of 0.5 was employed with a 75-W xenon lamp which provided a broadband spectrum. Using a beam splitter, the partial power of the incident light beam was focused onto the sample surface through the objective lens. The spectrum acquisition for all measurements was performed with a sampling aperture size of 7.1 × 7.1 μm2. Transmission and reflection were measured with respect to the light through a bare quartz substrate and an aluminum mirror, respectively. To characterize the optical properties from oblique angles, an ellipsometry setup (Uvisel, Horiba Jobin Yvon, Kyoto, Japan) was employed with a broadband light source.
Results and discussion
To conclude, we have demonstrated the fabrication of well-aligned plasmonic nanopillars by combining IL and IBM techniques. Using arrays with different geometric parameters, tunable plasmon resonances are simply achieved. It is found that Ag has a much higher milling rate than Au under the same experimental conditions, which makes Ag suitable for constructing fine nanostructures with ultrasmall features and high aspect ratios. The optical properties of the fabricated nanopillars are characterized both experimentally and theoretically. The approach developed in this work may trigger new applications in plasmon-assisted sensing and detecting.
atomic force microscopy
ion beam milling
localized surface plasmon resonances
scanning electron microscopy
surface plasmon polaritons.
This work was supported by the NEU internal funding (Grant Nos. XNB201302 and XNK201406), Natural Science Foundation of Hebei Province (Grant Nos. A2013501049 and F2014501127), Science and Technology Research Funds for Higher Education of Hebei Province (Grant No. ZD20132011), Fundamental Research Funds for the Central Universities (Grant No. N120323002), Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20130042120048), Science and Technology Foundation of Liaoning Province (Grant No. 20131031), and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Grant No. 47-4).
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