Quasiperiodic Nanohole Arrays on Optical Fibers as Plasmonic

Aug 10, 2016 - Department of Mechanical & Materials Engineering, Western University, London N6A 3K7, Canada. ACS Sens. , 2016, 1 (8), pp 1078–1083...
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Quasiperiodic nanohole arrays on optical fibres as plasmonic sensors: fabrication and sensitivity determination Peipei Jia, Zhaoliang Yang, Jun Yang, and Heike Ebendorff-Heidepriem ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00436 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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Quasiperiodic nanohole arrays on optical fibres as plasmonic sensors: fabrication and sensitivity determination Peipei Jia, ,‡ Zhaoliang Yang,‡ Jun Yang,*,‡ and Heike Ebendorff-Heidepriem †

†

†

ARC Centre of Excellence for Nanoscale BioPhotonics and Institute for Photonics and Advanced Sensing, School of Physical Sciences, The University of Adelaide, Adelaide 5005 Australia

‡

Department of Mechanical & Materials Engineering, Western University, London N6A 3K7 Canada

KEYWORDS : Quasiperiodic nanohole array, surface plasmon resonance (SPR), optical fibre, sensitivity, plasmonic sensing

ABSTRACT: Surface plasmon resonance enhanced optical transmission has been observed in periodic nanohole arrays and found plenty of plasmonic applications from label-free biosensing to surface-enhanced spectroscopies on various platforms. Recently this effect has been also demonstrated for nanohole arrays with quasiperiodic patterns such as the Penrose tiling. Here we pattern and transfer quasiperiodic nanohole arrays onto optical fibres and investigate their optical performance in refractive index sensing. These quasiperiodic arrays show multiple resonances closely related to their geometric features. The resonances are narrow and sensitive to the dielectric changes on the probe surface due to our high quality fabrication. We find the measured sensitivity of our quasiperiodic nanohole arrays is as high as that of periodic nanohole arrays and reaches the theoretical sensitivity limit as predicted by our universal sensitivity analysis. This result in turn verifies our sensitivity theory on propagating surface plasmon resonance in a wider range beyond periodic nanostructure arrays. Our study demonstrates the quasiperiodic nanohole array based optical fibre is a high-performance plasmonic sensor.

Surface plasmon resonance (SPR) is the collective oscillation of electrons excited by light at the metal/dielectric interfaces.1 The resulting field enhancement is highly sensitive to refractive index changes at the interface. Thus SPR is adopted in many optical tools for measuring material adsorption onto the surface. Specially, SPR enhanced transmission2 through nanohole arrays perforated in metallic films provides a well-established route for label-free biological sensing from various molecular detection3,4 to protein dynamics.5,6 The SPR field distribution and spectral properties can be easily modulated by controlling the geometry of nanohole arrays.2,7 Besides the shape and size of nanoholes, periodicity plays a critical role in forming the transmission resonances of the periodic arrays. However, the enhanced transmission was also observed in quasiperiodic nanohole arrays,8-10 which contain the longrange order but lack translational Bravais symmetry. Bragg condition was used to predict the peak wavelengths of transmission spectra through these aperiodic nanostructures. It was revealed that enhanced transmission resonances comply with frequencies that closely match the discrete Fourier transform vectors in the array structure factor.11 The lack of periodicity results in distinctive scattering dynamics of resonances and therefore increases the localized characters of the resonances.12 These resonance modes can lead to efficient photon trapping

and surface interaction, resulting in high refractive index sensitivity. In addition, quasiperiodic arrays exhibit an increased number of SPR modes compared to periodic structures.12 Such multiple modes promise the implementation of multiplexing for sensing in one single array. Despite all these potentials, quasiperiodic nanohole arrays have not been applied to SPR sensing and their refractive index sensitivity has not been determined. To achieve the full potential of nanohole arrays for sensing applications, they need to be integrated with a portable and cost-effective optical platform in an efficient way to ease light delivery and signal collection.13 However, most of current nanohole arrays are implemented on planar substrates and excited using a benchtop microscope,3 which restricts the utilization of these sensors for in-situ applications due to the use of bulk optical component. In contrast, the optical fibre inherently provides a lightcoupled microscopic platform for plasmonic sensors. Moreover, multiplexing is possible by covering the tip of a fibre bundle with a single quasiperiodic nanohole arrays. Different wavelength light coupled in each fibre channel will excite different SPR modes. The plasmonic fibre probe is uniquely suited for remote, in vivo and in situ applications. Many techniques have been specially developed for direct fabrication of regular nanostructures on the tip of

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optical fibres,14 such as electron-beam lithography (EBL), focus ion beam (FIB) milling, nanoimprinting, twophoton direct laser writing and interference lithography. For example, a hybrid metallo-dielectric nanohole arrays was fabricated on optical fibres with EBL for refractive index sensing.14 Surface-enhanced Raman scattering (SERS) sensors were demonstrated using FIB to mill gold nanohole arrays on single mode optical fibres.14 These methods either require special and complex apparatus to grip and align the optical fibre or suffer from low yield. By contrast, transfer techniques avoid these shortcomings through fabrication of nanostructures on a traditional planar substrate,14 and then transfer of the nanostructures to fibre tips. This strategy takes advantage of the nanofabrication techniques mentioned above while bypassing the difficulty of directly patterning structures on the fibre tip. However, most of the transfer methods require production of a new pattern each time before transfer. This situation is especially time-consuming in the case of quasiperiodic structures, because none of parallel methods are capable of creating large-area general aperiodic arrays. Here we propose to utilize quasiperiodic nanohole arrays as plasmonic sensors and integrate them with optical fibres by template transfer15 combined with EBL for refractive index sensing. This combined procedure enables efficient fabrication of non-periodic nanostructure arrays with high quality on optical fibre tips. The quasiperiodic structures show more SPR modes with the potential for multiplexing compared to periodic nanohole arrays. These characteristic modes are narrow and highly sensitive to dielectric changes on the probe surface. Previously, we have developed a universal theory to predict the refractive index sensitivity of propagating SPR in nanostructure arrays that covers both periodic and quasiperiodic arrays.16 This theory has been validated using the published results of periodic nanostructure arrays. Here the refractive index sensitivities of quasiperiodic nanohole arrays are measured to be as high as that of periodic arrays and approach the predicted theoretical limit. This result complies with our analysis and validates our prediction for a wider range of nanostructure arrays. These qualities make the quasiperiodic nanohole array based optical fibre sensor a promising candidate in plasmonic sensing.

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EXPERIMENTAL AND RESULTS Fabrication of the quasiperiodic nanohole arrays. In this work, we examine two common quasiperiodic patterns: 5-fold symmetric Penrose tiling17 and 12-fold symmetric square-triangle tiling18 with nanoholes at the vertices of the tiles. Interference lithography19 and recent Moiré lithography20,21 have been demonstrated to be capable of fabricating subwavelength patterns with highrotational symmetries over a large area. However, these patterns do not meet all the requirements for quasiperiodicity as they show incomplete lack of translational symmetry. Therefore, general quasiperiodic structures such as the two patterns used here cannot be made by these methods. Instead, we choose EBL to produce our quasiperiodic nanohole arrays because it is capable of creating arbitrary two-dimensional patterns with high resolution. Rather than directly pattern structures on the fibre tip, we transfer the as-patterned quasiperiodic nanohole arrays from the template to the fibre tip with the template transfer technique.15 In contrast to many other transfer methods that require fabrication of a new pattern each time before transfer, this technique creates a one-off nanohole array pattern on the template, which can be reused to replicate and transfer the same nanohole array onto the fibre tip. Specifically, the pattern of quasiperiodic nanohole arrays is first generated on a silicon wafer using EBL and deep reactive ion etching (DRIE) (Fig. 1, Pattern generation). 100 nm gold film is then deposited onto the template, leading to the formation of corresponding holes during deposition. Next, the gold film with quasiperiodic nanohole arrays is transferred to the fibre tip using an epoxy as the adhesive layer, and the template can be reused after cleaning without damage (Fig. 1, Template transfer). The transfer process is highly robust and can generate exactly the same pattern each time due to the reusability of the template. The combination of EBL with template transfer enables fabrication of arbitrary patterns and overcomes the drawback of the serial and time-consuming operation of EBL.

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Figure 1. Procedure for fabrication of quasiperiodic nanohole arrays on the tip of optical fibres. The entire process is divided into two sections: quasiperiodic pattern generation and template transfer of nanohole arrays.

Characterization. The 5-fold and 12-fold quasiperiodic nanohole arrays are designed with generalized dual multigrid method22 and Stampfli inflation method23 (detailed in Supplement), respectively. The hole diameter is 200 nm and the tile edge length is 600 nm in each pattern. Figure 2 shows the generated patterns on Si wafers. The 5fold Penrose tiling consists of two kinds of rhombuses

with equal edge length and acute angles of ߨ/5 ⁄and 2ߨ/5, respectively. The 12-fold square-triangle tiling is only made of squares and equilateral triangles, in which the distances between nearest holes are the same. The array area is larger than the core of the optical fibre to ensure entire coverage of the core region and eliminate edge effect during characterization of the array.

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Figure 2. Nanohole quasiperiodic arrays on the templates. Scanning electron microscope (SEM) images of entire patterns for (a) 5-fold Penrose tiling and (b) 12-fold square-triangle tiling. The insets are their FFT images. Magnified images (c) and (d) show the long range order and rotation symmetry for (a) and (b), respectively. Scale bars, 10 µm in (a) and (b), 1 µm in (c) and (d), and 500 nm in the insets of (c) and (d).

Fast Fourier transform (FFT) is applied to the individual nanohole quasiperiodic arrays to determine their geometrical structure factor (insets in Fig. 2(a) and 2(b)). Whereas the lack of periodicity is clearly reflected in the FFT images, they both have a long range order and rotation symmetry. In contrast to the case of periodic arrays with two primitive vectors in the form of a Bravais lattice, the Fourier transforms of the quasiperiodic arrays are characterized by their reciprocal vectors. These vectors, for example, in the FFT image of 5-fold Penrose tiling (inset in Fig. 1 (a)), correspond to various series of discrete set of bright points with rotation symmetry. Different point sets can be indexed according to their distance from the centre. These distances have been confirmed to directly relate to certain features of the quasicrystals.11 Thus the quasicrystal reciprocal vectors can be represented by the spatial character lengths. The radii of the five rings, from inside to outside, have a reversely proportional relationship with five feature lengths: long diagonals for thin rhombus, long diagonals for thick rhombus, short diagonals for thick rhombus, rhombus edges, and short diagonals for thin rhombus, respectively. Based on the fact that the wave vectors at resonance are equal to the reciprocal vectors at normal incidence, the relationship between rings and feature lengths implies the pattern of a quasicrystal determines the momentum matching condition for the SPR excitation. These resonances are expected to be highly sensitive to dielectric change in the proximity of the structure. Optical performance. To determine the sensitivity, quasiperiodic nanohole array fibres are equipped with a fluidic channel (Fig. 3). The light transmitted through the nanohole array is directly collected by another face-toface configured fibre coupled to a spectrometer. The nanohole array on the fibre tip maintains the same quasiperiodic pattern as the corresponding template because the integrity of the nanohole array is perfectly preserved. The fine grain boundary on the gold film visually implies a minimal surface roughness due to our template transfer method.15 The high surface quality of these structures would facilitate the surface plasmon propagation and en-

able the quasiperiodic nanohole arrays to achieve high optical performance. The merit of a plasmonic nanostructure in sensing is determined by its sensitivity, which indicates the signal variation responding to a refractive index change in the vicinity of the senor surface and provides an upper bound to the biosensing. Among the most common performance indicators is the wavelength sensitivity, i.e. wavelength shift (nm) per refractive index unit (RIU), which has been measured by numerous experiments for periodic nanohole arrays. We use refractive index calibration solutions to determine the sensitivity of our two 5-fold and 12-fold quasiperiodic nanohole arrays. When a nanohole array on a fibre tip is exposed to the solutions, distinct spectral features are observed between 700 nm and 900 nm in transmission due to excitation of multiple SPR modes (Fig. 4). These peaks and troughs proportionally shift to longer wavelengths as the refractive index increases due to the nature of SPR.

Figure 3. Optical setup for performance test of quasiperiodic nanohole arrays attached on the tip of optical fibres. The left inset is a SEM image of the 12-fold gold nanohole quasiperiodic array; the left inset is a photo of the optical fibre device mounted in a flow cell.

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Figure 4. Transmission spectra measured in water and NaCl solutions for nanohole quasiperiodic arrays of (a) 5-fold Penrose tiling and (c) 12-fold square-triangle tiling. Refractive index sensitivities obtained by linear fitting of (b) three peak (P1, P2, P3) shifts and two trough (T1, T2) shifts in (a), as well as (d) two peak (P1, P2) shifts and two trough (T1, T2) shifts in (c). The corresponding peaks and troughs are labelled in (a) and (c).

To determine the performance of the fabricated quasiperiodic nanohole arrays, we selected the peaks/troughs that show the largest wavelength shift of all the modes. The refractive index sensitivity of these peaks or troughs (Fig. 4(b) and 4(d)) is obtained by linearly fitting three peak shifts and two trough shifts for the 5-fold quasiperiodic array, as well as two peak shifts and two trough shifts for the 12-fold quasiperiodic array. The highest sensitivities of the lowest order SPR modes for the Penrose and square-triangle tilings are 607 nm/RIU and 557 nm/RIU, respectively. The overall performance of a SPR sensor is dominated by both the sensitivity of the resonance and its spectral linewidth.24 The full width at half maximum (FWHM) of the narrowest resonance is down to 8 nm for the Penrose tiling and 5 nm for the square-triangle tiling, which is comparable to the narrowest resonance linewidth of periodic arrays reported to date.4 Such narrow linewidth indicates the long lifetime of these SPR modes and high structure quality of our quasiperiodic nanohole arrays. Accordingly, the figure of merit (FOM),24 defined as the refractive index sensitivity divided by the corresponding FWHM, achieves 51 for the peak with 408 nm/RIU sensitivity. This FOM is three times higher than the previously reported value for periodic nanoholes on planar substrates.25

DISCUSSION Theoretical analysis shows the sensitivity of the lowest order SPR modes is proportional to the period of nanohole arrays.26 However, it is impossible to extend this analysis to the quasiperiodic arrays as they lack the short range periodicity. Because different geometry parameters always couple together to affect the spectral features, it is challenging to establish an analytical sensitivity model applicable to general two dimensional plasmonic structures. To address this dilemma, we have established a coherent framework to enable sensitivity evaluation of various structures from a general SPR point of view.16 A universal, geometry-independent sensitivity expression is derived by using a black box model of surface plasmon excitation for two-dimensional nanostructures. For SPR of the gold structure arrays in the visible and near infrared range, the sensitivity is

S=

2λε m2 n ( 2ε m2 + ε m n 2 + 34n 2 )

where λ is the resonance wavelength, n is the refractive index of the sensing medium and εm is the real dielectric constant of gold. This expression reveals that the sensitivity is dominated by the SPR wavelength and the dielectric

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property of materials involved in the interaction, whereas metal nanostructures mainly act as a coupling media to generate SPR. Previously, we have validated this theory using experimental and simulated results of periodic nanostructure arrays published by other groups.16 According to our theory, quasiperiodic nanohole arrays are supposed to have similar sensitivity at the same wavelength.16 To verify this prediction, the highest sensitivity of the lowest order SPR is plotted in Fig. 5, along with theoretical values and a series of experimental and simulated data for periodic nanohole arrays. Compared to periodic nanohole arrays, quasiperiodic arrays show equally high sensitivity, approaching the theoretical limit. This result in turn validates our universal sensitivity analysis for a wider range of plasmonic nanostructures.

Figure 5. Sensitivity of quasiperiodic nanohole arrays compared to theoretical values and that of periodic arrays (listed in Supplement).

CONCLUSIONS We have created quasiperiodic nanohole arrays on optical fibre tips and investigated their refractive index sensitivity to show their potential as plasmonic sensors. The combination of template transfer and EBL leads to an efficient procedure for high quality fabrication of arbitrary nanostructure arrays on optical fibres. The obtained quasiperiodic arrays show as high sensitivity as periodic arrays, approaching the theoretical limit. This result complies with our previous analysis and further validates our theoretical model and prediction about propagating SPR on nanostructure arrays. High sensitivity combined with narrow linewidth and multiple modes makes quasiperiodic nanohole arrays a viable alternative to periodic arrays. In addition, their integration with optical fibres would facilitate their wide utilization in real world plasmonic sensing.

METHOD Pattern generation. The Si templates of the quasiperiodic nanohole arrays were patterned using EBL followed by deep ion etching. A 100 nm thick poly(methyl methacrylate) (PMMA) resist was spin-coated on a Si wafer. The

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designed 5-fold Penrose tiling and 12-fold square-triangle tiling of circular holes with 200 nm diameter and 600 nm edge length were first patterned on the PMMA layer using an EBL system (LEO 1530 equipped with a nanopattern generation system). After development, the features of the PMMA mask were then etched into the Si substrate using a deep reactive ion etching machine (Alcatel 601E). After removing the PMMA mask in piranha solution (98% H2SO4 : 30% H2O2 = 3:1, v/v), the template is ready for gold deposition and transfer. Template transfer. A custom electron-beam evaporator was used to deposit 100 nm thick gold onto the Si template without the adhesion layer. The deposition rate of 1 Å/s was maintained at ~ 5 × 10-6 Torr. 1 µL thermalcuring optical epoxy (301, Epoxy Technology Inc.) was applied on one facet of the optical fibre and pre-heated for 10 min under a halogen lamp to be sticky. After attaching the template to the fibre endface, the gold structures on the reliefs of the template adhered to the fibre tip via 3 h heating at 60 ⁰C. The gold quasiperiodic nanohole arrays were then transferred by detaching the template from the fibre facet. After cleaning with gold etchant followed by chlorinated solvents, the template can be reused for a new cycle of transfer without damage. Refractive index sensing. Refractive index solutions were prepared by adding NaCl into deionized water to obtain various concentrations (5%, 10%, 15% and 20%) with different refractive index (1.3418, 1.3505, 1.3594, and 1.3684, respectively). The solutions were injected through the flow cell sequentially using a syringe pump. All transmission spectra were measured after 5 mins flowing and recorded by averaging acquisitions with a spectrometer (USB4000, Ocean Optics Inc.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: The following file is available free of charge. Supplementary Information. The design of quasiperiodic nanohole arrays and the sensitivity list of periodic nanohole arrays.

AUTHOR INFORMATION Corresponding Author * Email: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research is supported by Natural Science and Engineering Research Council of Canada (NSERC) and Canada Foundation for Innovation (CFI). Parts of this work were conducted at western nanofabrication facility. Peipei Jia and Heike Ebendorff-Heidepriem acknowledge the support of the ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP).

REFERENCES (1) Maier, S. A. Plasmonics: Fundamentals and Applications; Springer: Berlin, 2007.

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