Plasmonic Cup Resonators for Single-Nanohole-Based Sensing and

Jun 6, 2016 - We fabricate a new type of plasmonic resonator, called a cup resonator, consisting of a single nanohole and a cylindrical surrounding re...
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Plasmonic Cup Resonators for Single-Nanohole-Based Sensing and Spectroscopy Stephen A. O. Olson, Daniel A. Mohr, Jonah Shaver, Timothy W. Johnson, and Sang-Hyun Oh* Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States

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S Supporting Information *

ABSTRACT: We fabricate a new type of plasmonic resonator, called a cup resonator, consisting of a single nanohole and a cylindrical surrounding reflector. This device is made using template stripping, which creates a smooth silver surface for the base along with a sidewall mirror in one step to form a compact plasmonic microcavity. When a cup resonator is illuminated, surface plasmon waves, launched by a nanohole in all directions, are reflected off of the cylindrical sidewall, generating cavity resonances that can be observed as interference patterns in the optical transmission spectra. Since the resonances inside the cup depend on the local refractive index, this device can function as a compact optical sensor. With this sensor we observe a bulk index sensitivity of 390 nm/refractive index unit (RIU). A major advantage of this system over other propagation-based plasmonic sensors is that the energy is confined within a single cup, which is on the order of 1 μm2. This means that large arrays can be fabricated and used for parallel ensemble sensing and imaging applications. KEYWORDS: surface plasmon resonance, nanohole, template stripping, extraordinary optical transmission, plasmonic reflector, nanoplasmonic sensing anometric, subwavelength apertures in metal films can exhibit enhanced optical transmission wherein more light is transmitted through a subwavelength aperture than traditionally predicted by Bethe’s theory.1−3 Enhanced optical transmission enabled by plasmonic resonances in metallic films has been investigated in various geometries such as periodic hole arrays,2,4,5 single holes,6−8 rectangular slits,3,9,10 and coaxial apertures. 11−13 These nanoapertures can be used for applications in plasmonic biosensing,5,14−16 spectroscopy,17,18 optical trapping,19 and near-field optics20 because of their ability to tailor plasmon resonances and create intense subdiffraction-limited electromagnetic fields. Optical transmission through these nanoapertures can be modulated and further enhanced by adding grating couplers6,9,21 or Bragg mirrors.22−24 Various plasmon interferometers25−32 and cavity resonators33−35 have been demonstrated by integrating these building blocks. In this work, we demonstrate a new fabrication process that combines a single nanoaperture milled in a smooth metallic surface with a cylindrical surrounding reflector resembling a “cup”. In our device, a single nanohole acts as a point-like source of surface plasmon waves, and a cylindrical surrounding mirror tightly confines plasmons inside the cup. To fabricate this three-dimensional cup resonator, we employ template stripping36−38 with an integrated single nanohole and broadband sidewall reflector. The unique geometry of our structure provides advantages for compact single-nanohole-based spectroscopy and sensing. The template-stripped resonator surface provides a low-loss interface for plasmon propagation,36 while the vertical sidewalls act as mirrors that can reflect plasmons

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© 2016 American Chemical Society

with higher efficiency than distributed Bragg mirrors or gratings. The use of vertical surrounding mirrors for plasmon confinement also reduces the overall footprint of the device, allowing for large arrays to be used for parallel sensing.

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RESULTS AND DISCUSSION The fabrication process for this device is illustrated in Figure 1a−c. A silicon template is prepared by patterning a hydrogen silsesquioxane (HSQ) etch mask via electron-beam lithography (Vistec EBPG5000+). Next, we use reactive ion etching to create silicon pillars, wherein the HSQ etch mask protects the atomically smooth surface of the polished, (100) crystalline, silicon pillar capitals. The HSQ mask is then removed using a 10:1 buffered oxide etch, and a thin film (200 nm) of silver is conformally sputtered onto the template. Nanoapertures are then milled with a gallium focused ion beam (FIB, FEI Quanta 200 3D), manually aligned to the center of the silver-coated pillars, which will become the final device’s epoxy side (Figure 1a). Milling prior to template stripping ensures that the silicon/ silver interface inside the cup cavity remains uncontaminated by excess gallium. Next, the patterned silver film is templatestripped using UV-cured optical adhesive (NOA 61, Norland Products) onto a glass substrate, yielding the final device (Figure 1b,c). To demonstrate the utility of our fabrication process, we present a 15 × 15 array of unmilled 1000 nm diameter cups interspersed with a 14 × 14 array of unmilled 500 nm diameter cups (Figure 1d). While manual FIB Received: February 10, 2016 Published: June 6, 2016 1202

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Figure 1. (a) A silicon mold is coated with a 200 nm thick silver film, and a nanohole aperture is made through the silver via focused ion beam milling. (b) Epoxy is applied to the silver layer, and the patterned structure is template-stripped from the mold. (c) An SEM of the final structure is shown. (d) Using our method it is possible to produce densely packed arrays. Scale bars: (c) 1 μm, (d) 3 μm.

Figure 2. Device can be illuminated from either the (a) cup side or (b) epoxy side. (c) Optical transmission spectra from cups with radii of 2100, 1150, and 550 nm under epoxy-side illumination normalized to the same peak intensity. It is clear that as the cup size increases, so does the number of resonances. SEMs of the corresponding structures are also shown in (c). The scale bar for all three micrographs is 1 μm.

alignment was used in this work, utilizing pattern recognition software can improve nanohole centering and reduce overall time. To study how the cup radius modulates the optical transmission through the nanohole, a set of 45 devices was fabricated with varying radii (225 to 2425 nm), 180 nm apertures, and a depth, i.e., sidewall height, of 300 nm. The sidewall mirrors incorporated in each resonator enable plasmon confinement inside the structure, allowing standing wave resonances to build up, which, in turn, have the ability to enhance transmission from the nanohole. Experimentally, a transmission enhancement of 3−4 times in the visible regime and up to 10 times in the near-infrared was observed when compared to a lone nanohole milled on the same sample as the cup resonators (for direct comparison). While the device possesses radial symmetry, it is asymmetric in the z-dimension, and therefore different resonances form on either side of the resonator’s base. Even though the resonances inside the cup itself (air/silver interface) are the most important for sensing, it is also important to understand how they interact with the resonances generated on the opposite side of the silver film (epoxy/silver interface). The structure can be illuminated on either interface, giving measurement flexibility (Figure 2a,b). Transmission spectra from devices with three different cup radii are shown in Figure 2c with corresponding scanning electron micrographs (SEMs). As expected, as the cup radius increases, the number of resonant peaks also increases due to the larger number of resonant modes available. We performed 3D finite-difference time-domain (FDTD) simulations to further study the cup resonator structures. The simulated transmission spectra of a cup (1725 nm radius) under epoxy-side illumination is shown in Figure 3a. Dips and peaks can be seen, which correspond to resonances on the two sides of the cup. Electric field maps from three wavelengths corresponding to the two dips and one peak indicated in Figure 3a are shown in Figure 3b−d. The z-component of the electric field is plotted to view the behavior of the surface plasmons on either side of the cup as the transmission intensity varies. Figure 3b and c indicate strong cup- and epoxy-side resonances at wavelengths of 586 and 617 nm, respectively. At wavelengths of minimum transmission we observe a standing wave symmetric about the hole. This standing wave is formed by the interference of propagating surface plasmons along the surface,

Figure 3. Three-dimensional FDTD simulations for a single cup resonator. The featured cup has a 1725 nm radius, is 300 nm deep, and utilizes 200 nm thick silver films with 50 nm sidewalls. (a) Transmission spectra from the device while illuminating the epoxy side using a total-field scattered-field source. The vertical lines indicate where field maps (|Ez|) are generated, for wavelengths of 586 nm (b), 617 nm (c), and 652 nm (d). A transmission minimum is created when there is destructive interference at the nanohole on either the cup side (b) or epoxy side (c). (d) When there is constructive interference on both sides, a transmission maximum occurs.

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Figure 4. Contour plot of (a) 3D FDTD simulated and (b) measured transmission through cup structures with varying radii under epoxy-side illumination. (c) Simulated contour plot from (a) with analytical phase interference fits where the yellow lines represent cup-side resonances and the magenta lines represent epoxy-side resonances. (d) Same as (c), with ϕGS for the cup-side resonances qualitatively fit to the data. ϕGS = 1.2π is used for the epoxy-side resonances in both cases. The difference observed in the overall intensity between the simulation and experimental spectra in the NIR is likely due to gallium implantation and/or tapering effects from FIB milling of the nanohole.

estimate that destructive interference (θ = (2m − 1)π) at the nanohole occurs when

which destructively interfere at the hole itself. In contrast, the electric fields at transmission maxima (e.g., 652 nm) in Figure 3d constructively interfere at the nanohole. Interestingly, the cup-side portion of field maps is superficially similar to field distributions (see also Figure S4) observed when plasmons scattered from a nanohole in a thin metallic film interfere with light transmitted through the film,39,40 but as the metal deposited in this case was on the order of 200 nm, the pattern observed here is due to the SPP reflection from the surrounding mirror. The cup-side plasmon resonances are relavent for sensing applications since they are contained in the cup’s volume. With this in mind, we begin the following analysis: inside the cup, a first-order approximation assumes that plasmons are generated at the nanohole and propagate to and from it with a single reflection from the sidewall.30,41 Depending on losses accrued during reflection and propagation, there may be multiple reflections, but we find a simpler approximation useful for qualitatively interpreting our results. After a single round trip from the nanohole to the sidewall and back, the phase of the propagated wave with respect to light transmitted directly from the nanohole is estimated by θ=2

2π R + ϕR + ϕGS λspp

R=

R=

(2)

λspp ⎡ (2m − 1) ϕ ⎤ − GS ⎥ ⎢ 2 ⎣ 2 2π ⎦

m = 1, 2, 3, ... (3)

These analytical equations describe the features observed in both the simulation and experimental data sets. As expected, when there is constructive interference between plasmons propagating back to the nanohole and light directly transmitted by the nanohole, a transmission maximum is observed, while destructive interference leads to a transmission minimum. From our simulations, we also observe that during transmission maxima the standing wave plasmon resonances inside the cup appear much weaker than during their minimum transmission counterparts. This may be because during peak transmission propagating plasmon waves form an antinode at the nanohole and are more efficiently coupled both into the nanohole33 and back to free space, thus reducing the intensity of the cavity standing wave in the resonator. Additionally, as the cup side of the resonator has highly reflective mirrors surrounding the aperture, plasmons experience much lower scattering losses as compared to the epoxy side of the structure containing the nonreflecting, rounded bottom corner. This is why cup-side resonances appear sharper. FDTD and experimental contour transmission plots of epoxy-side illuminated cup resonators are displayed in Figure 4a and b, respectively. In Figure 4c and d, analytical fits of destructive interference resonances are plotted in yellow (cupside resonances) and magenta (epoxy-side resonances). The

(1)

εm + εd εmεd

m = 1, 2, 3, ...

and constructive interference (θ = (2m)π) occurs when

where θ is the relative phase accumulation, ϕR is an effective phase shift from plasmon reflection, ϕGS is the cumulative phase shift due to plasmon generation and scattering, R is the radius of the cup structure, and λspp = λ 0

λspp ⎡ ϕGS ⎤ ⎢(m − 1) − ⎥ 2 ⎣ 2π ⎦

is the surface

plasmon wavelength with εm and εd being the permittivities of the metal and dielectric, respectively, and λ0 the free space wavelength of the illuminating light. Assuming an effective phase shift of π upon reflection from the sidewall mirror, we 1204

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analysis of the cup-side resonances can be extended to the epoxy-side resonances and is available in the Supporting Information. The resonances from both the cup and epoxy side fit well with the dips in the transmission spectra from the device, but we will still note that the two interfaces are coupled through the nanohole and both will contribute to the total transmission. There are several important factors affecting the transmission spectra and relative contributions of each of the resonances. Cup sidewall height, sidewall thickness, and the base thickness can all influence the quality of the resonances, while a change in the nanohole size, profile, or whether it is filled by epoxy can affect the overall transmission intensity. Finally, the illumination scheme also has a large impact, as cupside illumination largely eliminates the epoxy-side resonances, since plasmons are no longer generated at the bottom corner of the device. Additionally, weaker resonances may also be present in the structure, but the two most relevant in the visible/NIR are captured with this model. Since each nanohole functions as an independent sensor, the reduced footprint of the cup resonator makes it an ideal structure for high-density sensor arrays. Since the sidewalls confine propagating plasmon waves, interference between adjacent sensing elements can be minimized. We fabricated 1 μm diameter unmilled cup resonator arrays with areal sensor densities 4 times higher than previously reported using Bragg mirrors.24 Sensor density can be increased further, to 8 times, by interspersing small cups in the array with the larger ones as in Figure 1d. To demonstrate the feasibility of using our device as a standalone sensor, refractive index sensing was performed (Figure 5a) using a 900 nm radius cup with two different liquids (water and glycerin). A bulk sensitivity of 390 nm/RIU was found, which is comparable to similar nanoplasmonic devices. Based on this bulk sensitivity and the spectral noise measured from our current instrument configuration (∼4 × 10−3 nm),42 spectral resolution is approximately 10−5 RIU. Additionally, it is possible to fabricate devices with submicrometer diameters that exhibit resonant transmission at only one or two wavelengths. This leads to the transmission of characteristic colors when illuminated with a white light source. Figure 5b contains transmission spectra from an array of small cups alongside bright-field images of the corresponding structure under illumination recorded using a color CCD camera (Thorlabs DCC1645C). Red shifts in the transmission spectra correlate with visible red shifts in the bright-field images. This shows that cup resonators could be used in color filter applications. For cups above 1 μm in diameter, multiple resonant transmission peaks cause the cups to emit more neutral white light colors (data not shown). In conclusion, we present a new architecture for a plasmonic cup resonator, which has the ability to be scaled into highdensity arrays. The structure uses template stripping to create metal films with microscale cups forming vertical sidewall mirrors. While a FIB was used to mill holes in this work, it is possible to modify the process to incorporate deep, circular holes in the silicon mold, as we have previously shown.43 Using this technique would both increase process throughput and remove the possibility of damaging the silicon mold by FIB overexposure. The cup confines plasmons and generates resonances, and these experimentally observed resonances match well with both analytical models and FDTD simulations. This has the potential to create a new class of plasmonic biosensors that produce structures that can be packed in very dense arrays and used in imaging and parallel sensing

Figure 5. (a) Refractive index sensing data from a 900 nm radius cup comparing water (n = 1.33) and a glycerin solution (n = 1.351). Data were collected using epoxy-side illumination and normalized to 1. Shifts are caused by changes in the surface plasmon resonances and show a bulk index shift of 390 nm/RIU. In general, only the cup-side resonances are expected to shift (Figure S2), but the shifts in the epoxy side seen here could be due to non-plane-wave illumination (i.e., a microscope condenser) or analyte between the silver and epoxy for this particular cup. (b) By altering the radius of the cup structure it is possible to tune the emitted wavelengths, resulting in the transmission of characteristic colors. Small cups have distinct colors due to fewer resonant wavelengths being transmitted. The blue color of the 400 nm cup is due to a transmission resonance outside the limit of our spectral data.

applications. Arrays of these sensors can be used to make many unique measurements across the width of a single microfluidic channel in real time, useful for scenarios with different flow streamlines across channel width or in situations where spatially selective mixing occurs in the channel. Furthermore, the unique geometry of our device can be useful for local dielectrophoretic concentration,16,44 where each micrometer-scale cup can naturally act as a miniaturized well for sample confinement and on-chip sensing.

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METHODS Optical measurements were performed using a broadband, fiber-coupled, laser-driven light source (Energetiq EQ-99FC) to illuminate cup resonators through a 60 cm long fiber, reflective collimator, and condenser on top of an inverted microscope (Nikon Ti-S). Transmitted light was collected with a 50×, 0.9 NA objective and imaged onto the entrance slit of a 300 mm 1205

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receives support from the NSF through the National Nanotechnology Coordinated Infrastructure (NNCI).

focal length imaging spectrometer (Acton SP2300i) equipped with a backside-illuminated, deep-depletion thermoelectrically cooled CCD (Princeton Instruments PIXIS 400B). Multiple cups were imaged onto the entrance slit simultaneously, multiplexing the data collection. Spectra of cup resonators and lone nanoholes were background subtracted and normalized to the spectral transmission through the glass substrate. The transmission spectra from multiple nanoholes (of the same diameter as the cup nanoholes) were averaged together; this was used as the basis for the transmission comparison between cup resonators and lone nanoholes. The spectra presented in Figures 2, 4, and 5 were normalized to spectral transmission through the glass substrate. Closely matching our physical geometry, FDTD simulations of the structures were performed using a 300 nm cup depth, 180 nm aperture, 200 nm thick silver film coverage, 50 nm thick sidewalls, and a 100 nm radius of curvature around the base of the cup. Modeling was performed using Lumerical FDTD Solutions in three dimensions. The dielectric function of silver was incorporated using experimentally measured values.45 A total-field scattered-field plane wave light source was used to illuminate the structures over an area of 42.25 μm2, and a 4 nm staircase mesh was used inside the nanohole and around the base of the cup. Simulations were allowed to run until 0.001% of the initial excitation energy remained in the modeling area. The modeling volume was kept constant for all simulated cups. Perfectly matched layer boundary conditions were used in all directions, while also taking advantage of the 2-fold symmetry available in the structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.6b00091. Description of the calculations for the epoxy-side resonances along with further simulations of the resonances (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail (S.-H. Oh): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Henri Lezec, Amit Agrawal, Wenqi Zhu, and Ting Xu at the National Institutes of Standards and Technology for suggesting the idea of a subwavelength-aperture decorated cup resonator for SPPs operating in transmission. The authors thank Yong-Sang Ryu for assisting with figure preparation. This work was supported by grants from the National Science Foundation (NSF CAREER Award 1054191 for S.A.O.O. and S.-H.O. and CMMI 1363334 for J.S. and S.-H.O.), Seagate Technology through the University of Minnesota MINT (D.A.M., T.W.J., and S.-H.O.), MnDrive Initiative from the State of Minnesota (T.W.J. and S.-H.O.), and the Minnesota Partnership for Biotechnology (S.-H.O.). D.A.M. also acknowledges support from the NIH Biotechnology Training Grant (T32 GM008347). Device fabrication was performed at the Minnesota Nano Center at the University of Minnesota, which 1206

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