Template-Stripping Fabricated Plasmonic Nanogratingsfor Chemical Sensing

Koh Yiin Hong1 & Jacson W. Menezes2 & Alexandre G. Brolo1

Received: 1 September 2016 /Accepted: 2 January 2017# Springer Science+Business Media New York 2017

Abstract Template stripping has been applied to transfer one-dimensional nanograting structures fabricated by interferencelithography (IL) onto planar supports (i.e., glass slides andplane-cut optical fiber tips). A thin adhesive layer of epoxyresin was used to facilitate the transfer. The UV-Vis spectro-scopic response of the nanogratings supported on glass slidesdemonstrated a strong dependency on the polarization of theincident light. The bulk refractive index sensitivities of thenanogratings supported on glass were dependent on the typeof metal (Ag or Au) and the thickness of the metal film. Thedescribed methodology provides an efficient low-cost fabrica-tion alternative to produce metallic nanostructures for plas-monic chemical sensing applications.

Keywords Plasmonic . Nanogratings . Template stripping .

Optical fiber . Interference lithography (IL)

Introduction

The fabrication of nanostructures has been a vibrant researcharea in the last few years. A main drive for the wide interest inthe field is the potential for applications of nanostructures in

chemical sensing and detection [1–3]. In the specific case ofmetallic nanostructures, the chemical sensing mechanism isbased on the excitation of surface plasmon (SP) modes atthe interface between the metal and the medium containingthe analyte of interest. Several types of metallic nanostruc-tures, such as nanoparticles [4, 5], nanoholes [6, 7], nanoslits[8], and nanogratings [9], are able to support SP modes whenthe resonance conditions are satisfied. The developments ofthese Bplasmonic^ sensors have been centered in the desire ofgenerating structures that optimize analytical figure of merits,such as sensitivity, resolution, and limit of detection (LOD).Nonetheless, the possible commercial translation of thosetypes of nanosensors also depends on the complexity of thefabrication process. In this regard, processing time, cost, andease for mass production also become important evaluationparameters.

Plasmonic nanosensors have been fabricated by differenttechniques, ranging from chemical synthesis to nanolithography[10]. Avariety of nanofabricationmethods have been explored inthe recent decades, mainly due to the emergence of advancedlithography facilities in various research centers around theworld. Among them, electron beam lithography (EBL) and fo-cused ion beam (FIB) milling are the two techniques that allowmaximum control over the fabrication precision and resolution.Therefore, these techniques have been widely used to fabricatedifferent types of nanostructures for prototypes and proof-of-concept experiments [11]. The limitations of these techniques,however, are related to their low throughput and their inabilityto quickly pattern larger areas. Interference lithography (IL) of-fers an alternative that provides large area patterning of periodicnanostructures with easy manipulation of the nanostructure geo-metric parameters, which is useful for sensor optimization [12].This maskless lithography technique normally utilizes the opticalinterference between two laser beams to produce a uniform pe-riodic pattern that impress a photoresist [13]. The technology is

Electronic supplementary material The online version of this article(doi:10.1007/s11468-017-0503-7) contains supplementary material,which is available to authorized users.

* Alexandre G. Broloagbrolo@uvic.ca

1 Department of Chemistry, University of Victoria, Victoria, BC V8P5C2, Canada

2 Telecommunication Engineering (GOMNDI), Federal University ofPampa, Alegrete, RS, Brazil

PlasmonicsDOI 10.1007/s11468-017-0503-7

relatively time and cost efficient and compatible with the com-mon approaches currently used in microfabrication [14].

In this work, the IL technique was used to fabricatenanograting templates that were transferred to different glasssurfaces. The template-stripping procedure has been reported[15] to yield metallic nanostructures capable of plasmonic-based chemical sensing. This strategy has not only leveragedthe available nanofabrication methods but also greatly im-proved the fabrication throughput. Here, we demonstratedthe implementation of IL template-stripping procedure totransfer the nanograting pattern to glass slides (1 in.2) andoptical fiber tips (OFTs) with small contact surfaces(∼50 μm of core diameter). Plasmonic nanograting structures(made of either Au or Ag) have been explored for chemicalsensing [8, 16]. Typically, the transmission of white lightthrough those one-dimensional nanostructures (with groovesand ridges) presents clear dips and peaks in the visible NIRrange. Another aspect of the one-dimensional nanostructurespresented here is their dependence on the incident light polar-ization. This property has been explored to eliminate non-polarization-dependent background interferences [17].

Experimental Section

Template Preparation via IL

Microscope glass slides (1 in2) and optical fiber tips (OFT—50-μm core diameter) were used as support for the plasmonicstructures. The glass slides were first sonicated in a water bathand then cleaned with copious amounts of acetone, methanol,ethanol, and isopropanol, respectively. After oven-drying (T∼120 °C) and cooling to room temperature, the glass slides wereready to be spin-coated with 1:1 Microposit SC 1827 positivephotoresist diluted with Microposit thinner type P at 2000 rpmfor 30 s. The substrates were then pre-baked in an oven at 120 °Cfor 10min to remove the solvent from the photoresist layer. Next,the glass substrate coated with the photoresist was then mountedon the interference lithography setup (Lloyd’s interferometer)[18] and exposed to a 458-nm laser (Coherent argon ion laser)once for 45 s (the energy density of the laser was 100 mJ/cm2).After the laser exposure, the photoresist film was developed in1:3 Microposit 351 developer diluted with deionized water (pro-duced through Barnstead Millipore system, resistivity of18.2 MΩ cm) for 45 s under gentle motion parallel to the gratingaxis to generate a uniform one-dimensional photoresist template.The photoresist templates were then evaporated with either goldor silver using electron beam and thermal evaporator (AngstromEngineering Glove-box Evaporator, deposition rate of 1 Å/s).The film thicknesseswere either 100 or 200 nm. Similar cleaningprocedure was applied to a bare OFT (model AFS fromThorlabs, core diameter of 50 μm), which has been previouslyplane cleaved and polished.

Template-Stripping Procedure

The stripping procedure is summarized in Fig. 1. A silver (orgold) evaporated on the photoresist template was pressedagainst a glass slide coated with the epoxy adhesive (step ain Fig. 1). The sandwiched system was then exposed to lightfrom a 100-W incandescent lamp. The sandwiched systemwas left in a benchtop area and illuminated overnight(∼12 h). When the epoxy was cured, the patterned silver (orgold) was stripped from the original template (step b, inFig. 1). The stripped substrate was subject to a final rinsingwith ∼5% NaOH to remove any remaining photoresist fromthe metal surface. The surface morphology of the substrateswas then characterized.

Surface Plasmon Resonance (SPR) Refractive IndexSensing with Template-Stripped (Large Area) Substrates

The SPR measurements for the glass slides coated with me-tallic nanogratings were realized in transmission mode at nor-mal incidence using an UV/Vis/NIR spectrometer (PerkinElmer Lambda 1050). Small modifications were required onthe sample and reference compartments to host a liquid cellwith the patterned samples. The cell was fabricated from trans-parent polydimethylsiloxane (PDMS) which allow liquid tobe flown to a 400-μL sealed pocket in contact to the surface(sensing area 2 × 2 cm2) of the metallic substrate. Glucosesolutions with different concentrations were prepared to testthe SPR sensor response to different bulk refractive indexes.

Optical Fiber Experiments

The surface plasmon resonance-based refractive index mea-surements were also realized using a bifurcated optical fiber(model BIF400-VIS-NIR fromOcean Optics) and bare opticalfiber (OF) adapter (model F-AM-SMA from Newport). Onearm of the fiber was illuminated from a white light source(model LS-1 from Ocean Optics), the second arm was con-nected to a spectrometer (model USB 2000 from OceanOptics), and the sensing tip was a gold nanograting structureprepared using the same IL template-stripping procedure de-scribed above.

Results and Discussion

Substrate Characterizations

The IL template-stripping procedure, illustrated in Fig. 1, yieldedone-dimensional grating nanostructures with a final patternedarea of 1 in.2 Figure 2 shows the SEM image of the patternedsurfaces. The periodicity of the gratings was ∼450 nm and theheight of the ridges was 75 nm. Small morphological defects

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(roughness) can be noticed in Fig. 2, which partly justified thevariation of the optical sensitivity on refractive index (RI) varia-tions (substrate-to-substrate sensitivity variation was less than20%—see below). Additionally, the roughness of the substratewas examined before and after the template-stripping procedurethrough atomic force microscopy (AFM). The AFM result, pre-sented as Supplementary Information (Figure S1), shows that theoverall roughness of the nanostructures was significantly reducedafter the stripping procedure.

Zero-degree transmission spectrum through those large areapatterned substrates led to clear peaks and dips at TM polariza-tion (linear polarization perpendicular to the direction of the grat-ing lines), as shown in Fig. 3a. The plasmonic response ofnanoslit structures has been extensively studied by several re-search groups [8, 16]. Briefly, the transmission bands throughperiodic nanoslit structures have been assigned to contributionsfrom different surface modes on the plasmonic grating [16].These include contributions from (i) localized SPR from thenanostructures and roughness, (ii) cavity modes (Fabry-Perottype of effect), and (iii) coupling from the nearby nanograting

features [16]. The transmission through a 200-nm thickness sil-ver nanogratings presented in Fig. 3a agrees well with the opticaltransmission properties reported on the previous findings [8].The features (peaks and valleys) in the transmission spectrumin Fig. 3b vanishes when the measurement was taken at TE(linear polarization parallel to the direction of the grating lines)polarization. The lack of features for TE excitation can be ex-plained by the homogeneity of the electric field when the inci-dence polarization is parallel to the structures. The strong polar-ization dependence observed in Fig. 3 can potentially provide acontrol channel in chemical sensing experiments, for instance, toeliminate interferences from experimental variables that are po-larization independent [17].

Surface Plasmon Resonance (SPR) Band Assignments

The SPR from an one-dimensional metallic nanogratings canbe approximated through Eq. 1 [8, 16, 18]:

λspr ¼ Pm

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiεdεm

εd þ εm

r;m ¼ 1; ð1Þ

where λspr is the surface plasmon resonance wavelength, P isthe periodicity of the nanogratings, m is the mode of the res-onance, εd is the dielectric constant for the dielectric side ofthe interface, and εm is the dielectric constant for the metal sideof the interface. The transmission spectrum from silvernanogratings in air was found to be significantly different ascompared to that in water (refer to SI, Fig. S3). The calculated(using Eq. 1 for m = 1) SPR mode of a silver nanograting(λspr) is expected to shift from ∼470 nm (dielectric propertiesof Ag obtained from [19]) in air to about 585 nm in water. This

Fig. 2 Scanning electron micrograph of the template-strippednanograting silver substrate. The blue scale bar represents 600 nm

Fig. 3 Transmission spectra for the silver nanogratings under twodifferent polarization states of incidence light. a (red line) lightpolarization perpendicular to the structure orientation (TM polarization).b (blue line) light polarization parallel polarization to the structure (TEpolarization)

Fig. 1 Template-stripping procedure. Step a—silver- or gold-evaporatedphotoresist template was pressed against a glass slide with the epoxyadhesive. Step b—when the epoxy cured, the patterned silver wasstripped off from the original template. The substrate was then rinsed withdilute NaOH to remove the remaining photoresist from the metal

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resonance in water matches reasonably to the observed trans-mission features in Fig. 3a. Another resonant mode in Fig. 3ashould arise from the other interface between epoxy and silver.This resonance is expected to occur at ∼630 nm (consideringnepoxy = 1.46). This is, again, comparable to the observedtransmission features in Fig. 3a.

The relative straightforward assignment obtained withEq. 1 can be further confirmed experimentally by Fig. 4.Figure 4 shows the dependence of the transmission featureswith changes in the refractive index of the aqueous dielectricin contact to the metallic grating. It is possible to observe inFig. 4 that the transmission wavelength redshifts in responseto the refractive index changes (refer to Fig. 4) only for thetransmission features ∼550 nm. In contrast, the resonant fea-tures at ∼630 nm were almost unaffected by the changes ofrefractive indexes in the solution, as seen in Fig. 4. This can beexplained by the consideration that the buried metal-epoxyinterface is not significantly altered by the solution refractiveindex. On the other hand, the mode assigned to the water/analyte solution silver nanograting interface is expected todepend strongly on the optical properties of the aqueoussolution.

Sensing Measurements

The polarization dependence observed in Fig. 3 was used tocorrect for polarization-independent background, such as, forinstance, the effect of room temperature variations. The trans-mission spectra were then expressed using the following equa-tion [17] (Eq. 2):

Tcorrected ¼ I⊥−I∥I⊥ þ I∥

ð2Þ

where I⊥ is the transmission intensity at TM polarization andI ‖ is the transmission intensity at TE polarization.

The transmission measurements for the sensing experi-ments were taken using an ordinary UV/Vis/NIR spectrome-ter. A thin PDMS cell was fabricated and used to expose thenanograting substrates to aqueous solutions with different re-fractive indexes. The refractive index of glucose solutions ofdifferent concentrations ranged from 1.3354 to 1.3550.Normalized transmission spectra (according to Eq. 2) fromdifferent glucose solutions are presented in Fig. 4.

The SPR wavelength shifts with refractive index changeswere then quantified using the integrated response (IR) meth-od [20]. In this case, the whole visible spectrum, from 400 to700 nm, were taken into account. Previously, our group andcollaborators have employed this IR method to quantifynanohole SPR sensing and demonstrated an improvement ofup to 90% to the signal-to-noise ratio [24]. The IR equation isgiven below (Eq. 3):

IR ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi∫λ1

λ2

D λð Þ2−D λð Þ2��� ��� dλ

s; ð3Þ

whereD(λ) is the difference in signal between a reference and

a measured spectrum andD λð Þ is the average of the differencein signal over the entire spectrum. In our particular case, thespectrum obtained from the aqueous solution with the lowestrefractive index (n = 1.3354) was used as reference.

The normalized IR response for 200-nm-thick Agnanogratings was plotted against the solution refractive indexin Fig. 5. The calibration sensitivity of the large area SPRsubstrates, calculated from the slope of Fig. 5, were in therange between 50 and 160 IR/RIU, depending on the type ofmetal (Ag or Au) and on the thickness of the metal film. Asummary of the experimental sensitivity values are shown inFig. 6. Typically, in the literature, calibration sensitivity ofplasmonic sensors are given in nanometers/RIU. Therefore,for sake of comparison, the calibration sensitivity was alsorecorded in the nanometer/RIU scale. In this case, the wave-length shifts of the SPR band (in the region between 545 and560 nm, see Fig. 4) were quantified. The shifts were measuredat the full width of half maximum (FWHM) of the SPR bandsof the normalized transmission spectra. The first transmissionspectrum in the series (i.e., in solution medium of the smallestrefractive index) was used as reference, and the wavelengthshifts were read relative to that reference spectrum. The wave-length shifts as a function of refractive index changes are alsoshown in Fig. 5. The values of the calibration sensitivity (slopeof the line in Fig. 5) were between 500 and 600 nm/RIU (againdepending of parameters, such as nature of the metal and filmthickness; see Fig. 6). This calibration sensitivity range iscomparable to values that has been reported from othergrating-based plasmonic substrates [6].

Fig. 4 Transmission spectra for the silver nanogratings underperpendicular polarization of the incidence light in solutions of differentrefractive indexes. a 1.3354. b 1.3401. c 1.3453. d 1.3523

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The structure reported here was prepared in a large areausing a relative simple and inexpensive procedure. Anotherfigure of merit to compare plasmonic sensor is the sensorresolution, defined as the minimum refractive index changedetectable by the device. The resolution was calculated fromthe calibration sensitivity values and the experimental varia-tion measured by our detection system (either in wavelengthor in IR), obtained frommultiple measurements from the samesample [6]. Figure 6 shows the calibration sensitivity and res-olution of nanogratings of different types of metal (silver orgold) and thickness of the metal film (100 or 200 nm). Thereproducibility of our fabrication procedure is represented inFig. 6 as an error bar in the sensitivity values, which werecalculated from the average of three samples. The 100-nm-thickness silver nanogratings demonstrated the sensor resolu-tion (8.35 × 10−5/RIU) in Fig. 6. The 100 nm-transferred gold

nanogratings exhibited the best sensing reproducibility of∼15%, which might be related to the chemically inert proper-ties of goldmetal. In order to further rationalize our findings inFig. 6, the substrates were examined by AFM (refer to SIFig. S2). It was observed that the 100-nm Ag nanogratingpresented more crack lines on the surface and consistentlyshowed more random roughness, as compared to 200-nmAg nanogratings and 100-nm Au nanogratings (Fig. S2 inSI). This might explain the larger variation shown (substrate-to-substrate reproducibility) observed in Fig. 6 for the 100-nmAg nanograting. On the other hand, the thinner Ag film shouldallow better transmission through the interaction between theplasmonic interfaces [21], which in turn improves the sensi-tivity of the system.

Sensing with Optical Fiber Tips

In order to demonstrate the potential of the stripping procedure totransfer pattern to different types of flat substrates, a plasmonicnanograting was fabricated on an optical fiber tip (OFT). Thereare several reports in the literature focused on the transfer ofnanostructure to optical tips [22]. Yang et al. has reported an insitu IL fabrication on OFTs, but that requires a complexsubstrate-hosting setup to achieve the transfer [23]. Template-stripping procedure [24, 25] offers an easy and efficient solutionto the setup complications of conventional lithography fabrica-tion. In this case, the pattern from the IL template can be trans-ferred onto optical fiber facet with the aid of an optical adhesive[25, 26] or some intermediate medias such as flexible polymericstructures [24]. Similarly, in this work, the IL-generated largearea nanograting structures were template stripped onto the fiberfacet with the aid of epoxy adhesive. Specifically, the IL templatewas first depositedwith 100 nm of gold film using electron beam

Fig. 5 Normalized integratedresponse (y-axis at left) andwavelength shift (y-axis at right)plot as a function of the refractiveindex of the solution for 200-nm-thick Ag nanogratings

0

25

50

75

100

125

150

175

200

225

250

200nm Ag 100nm Ag 100nm Au

R

es

olu

tio

n x

1

0-4

/ R

IU

Ca

lib

ra

tio

n s

en

sitiv

ity

/ IR

.R

IU

-1

0

1

2

3

4

Fig. 6 Comparison of the refractive index calibration sensitivity andresolution. Left, 200-nm Ag nanograting; middle, 100-nm Agnanograting; and right, 100-nm Au nanograting

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evaporator. A drop of epoxy was added to the gold-nanostructured face of the IL substrate. The polished end of theOF, which had been held in a simple plastic holder to fix the fiberend at normal position, was brought to contact with the epoxy-gold nanogratings. The system was then allowed overnight ex-posure to 100-Wwhite light source to ensure that the epoxy wascured completely. The photoresist template was then strippedfrom the fiber end, as demonstrated in Fig. 7. Scanning electronmicrographs of the resulting structures are available assupporting information (Fig. S5 in SI). The plane cut of the fibercore was achieved by connectorizing the OFT to a commerciallyavailable ceramic ferrule and polished extensively. Theconnectorized fiber tip was modified using the template-stripping method. The patterned tip of the optical fiber was thenused as a refractive index-sensing element. In this case, the de-tection was done in reflection (backscattering) mode, using abifurcated OF. As shown in Fig. 7, one arm of the bifurcatedfiber carried the white light while the other arm took the reflectedsignal to a miniaturized spectrometer. Either the patterned tip or abare fiber tip can be connected to the system through an OFadapter purchased commercially. The measurements were doneby immersing the fiber tip (either the patterned or the bare tips)into solutions with different refractive indexes. The solutionswith different refractive indexes were placed in 5-mL disposablevials. A lab-lift was used to control the immersion the fiber end ofthe optical setup into the analyte solutions. The system wasequilibrated for ∼5 min before obtaining backscattered spectrausing the spectrometer. The equilibration time was particularlyimportant during solution changes. It allowed the system to settleand ensured a good degree of stability during the measurements.All measurements were done in a dark room to minimize back-ground noise contributions.

The backscattered measurements carried information re-garding the SPR mode excited on the metal nanograting tip.The SPR waves on the metal-solution interface were verysensitive to the refractive index changes (i.e., intensity chang-es and wavelength shifts). The recorded spectra in the regionfrom 400 to 850 nmwere then smoothened, polynomial fitted,and subjected to the IR calculation, as shown in Eq. 2. Anexample of the wavelength shifts observed in response to thechanges of refractive index from a nanograting-patterned tip is

shown in the SI file (Fig. S4(a)). The bare fiber ’sbackscattered spectra remained unchanged for all the refrac-tive indexes tested (Fig. S4(b)). Figure 8 shows a comparisonof the sensing performance of a Au nanograting-patternedoptical fiber tip to a bare optical fiber. The non-patterned(bare) fiber showed almost no response as compared to thepatterned tip. In addition, since the measurements were takenwith a bifurcated fiber setup, the results from Fig. 8 illustratethe potential for application of this setup in an optrode geom-etry [27], which could be useful for in-line monitoring ofchemical process, for instance. This relative simple and inex-pensive procedure to modify the OFT with plasmonic nano-structures has also the potential to be utilized in other plas-monic sensing applications; i.e., surface-enhanced Ramanspectroscopy.

It is worth to mention that the polarization-dependent charac-teristic of the nanograting structures has not been utilized in thefiber setup reported here. The manipulation of the polarizationeffect, using a polarization maintaining fiber, for instance, is apotential future improvement that can be implemented for theapplication of this type of OFT-modified plasmonic device.

Light source

SpectrometerBifurcated

optical fiber

Sensing

area

fe

rru

le

cla

dd

in

g

co

re

Fig. 7 Portable assembly for the refractive index calibrationmeasurement using bifurcated optical fiber; one arm of the fiber isilluminated with white light source, the second arm is connected to

ocean optic spectrometer (USB 2000), and the last arm is the sensingarea with gold nanogratings where a similar template-strippingprocedure is applied on plane-cleaved optical fiber tips

Fig. 8 Integrated response measured using the portable fiber assembly asa function of increasing refractive index of the solution (square dots) tip-patterned optical fiber and (triangle dots) bare optical fiber

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Conclusions

Summarizing, we successfully utilized the IL template strip-ping to prepare SPR substrates on both large area (1 in.2) andon small surfaces (50-μm core OPTs). These plasmonic sub-strates were used for bulk refractive index sensing. The suc-cessful integrations of the IL procedure have clearly demon-strated flexibility for the fabrication of good quality plasmonicsubstrates with different dimensions. This allows tailoring thefabrication for different applications. For instance, the realiza-tion of refractive index sensing using the optical fiber setupillustrated the potential of the procedure to prepare miniatur-ized (small area) plasmonic structures, suitable to accommo-date remote sensing applications. Lastly, the substrates pat-terned with nanogratings demonstrated strong polarization de-pendency. This property has the potential to be used as anadditional control in refractive index-sensing applications.For instance, polarization controlled self-referencing can beimplemented to eliminate non-polarization-dependent inter-ferences and backgrounds, such as stray illuminations andtemperature variations.

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