Optical Spectroscopy of Nanometric Holes in Thin Gold Films

ABSTRACT. Elastic scattering measurements show that isolated nanometric holes in optically thin Au films exhibit a localized surface plasmon resonance...
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VOLUME 4, NUMBER 6, JUNE 2004 © Copyright 2004 by the American Chemical Society

Optical Spectroscopy of Nanometric Holes in Thin Gold Films Juris Prikulis,† Per Hanarp, Linda Olofsson, Duncan Sutherland, and Mikael Ka1 ll* Applied Physics, Chalmers UniVersity of Technology, S-41296 Go¨teborg, Sweden Received February 20, 2004; Revised Manuscript Received March 31, 2004

ABSTRACT Elastic scattering measurements show that isolated nanometric holes in optically thin Au films exhibit a localized surface plasmon resonance in the red to near-infrared region. The hole plasmon red shifts with increasing hole diameter or increasing refractive index of the surrounding medium, analogous to a dipolar particle plasmon. A pronounced blue shift is observed when the distance between holes is decreased, indicating an enhanced coupling between holes mediated by surface plasmon polaritons of the intervening flat film surface.

Because of their fascinating optical properties, originating in surface plasmon resonance phenomena,1 sub-wavelength metal structures have attracted great interest in recent years. In the case of the noble metals, in particular gold and silver, the plasmon modes are well defined and usually occur in the visible wavelength range. This has made these materials important for applications, including near-field microscopy,2 surface-enhanced Raman scattering,3 and biochemical sensing.4 Future applications may include plasmonic waveguides and various nanooptical devices.1 For a confined geometry, such as a nanoparticle, the surface plasmon is localized and its energy crucially depends on the size and shape of the object. This dependence has been investigated in numerous experiments on nanoparticles, rods, shells, disks, and rings.5-9 A much less understood type of nanostructure is a subwavelength hole in an extended metal film.10-12 It has been shown that the light transmission through holes in an optically thick gold or silver film can be substantially * Corresponding author. E-mail [email protected]. † Present address: Institute of Chemical Physics, University of Latvia, Raina blvd. 19, Riga LV-1586, Latvia. 10.1021/nl0497171 CCC: $27.50 Published on Web 04/27/2004

© 2004 American Chemical Society

enhanced through resonant excitation of spatially extended surface plasmon polaritons at the metal surface. This is usually achieved through some form of grating coupling of the incident light.10 Although electrodynamics calculations of nanostructured films point toward the importance of localized modes in these phenomena,13,14 experimental evidence for localized hole plasmons is lacking. In this letter, we show that scattering and extinction spectra of optically thin Au films perforated by sub-wavelength holes exhibit a clear optical resonance in the visible to near-infrared spectral range. Single holes and short-range ordered holearrays exhibit qualitatively similar behavior, indicating that the primary excitation is confined to the nanometric hole. Based on the overall spectral similarity between holes and Au disks, we assign the excitation to a dipolar localized surface plasmon (LSP) resonance at the hole circumference. The variation in LSP wavelength with hole size and surrounding refractive index is found to be similar to that of gold nanodisks. However, the scattering cross-section is substantially larger and the coupling between holes is stronger than for particles, indicating a potential for novel

surface-enhanced spectroscopy and nanooptical biosensing applications. The nanohole samples were prepared by colloidal lithography, in a process similar to that used for nanodisks7 and nanorings.8 Polystyrene particles (white-sulfate latex, IDC) were adsorbed on cleaned glass slides at submonolayer coverage. The average distance between particles could be tuned by electrostatic interactions, controlled through the salt concentration in the colloidal solution. An h ) 20 nm Au film was then thermally evaporated on the sample, after which the gold-capped polystyrene particles were removed by tape stripping. This results in a film containing circular holes with diameter controlled by the particle size, which varied between 60 and 500 nm. The holes have small size variation and no long range order. Optical measurements were performed using a dark-field (DF) setup, which records elastic scattering,5,15 and a UVvis-NIR spectrophotometer (Cary 500), which records forward transmission. In the DF setup, white light from a 100 W halogen lamp passes through a DF condenser (Nikon dry, NA 0.95-0.80) on an inverted microscope (Nikon TE300) and illuminates the sample at an angle-of-incidence that is larger than the collection angle of the microscope objective (Nikon 100×/NA 0.5-1.3 or 60×/NA 0.7). Light scattered from the holes is fiber-coupled to a miniature grating spectrometer (Avantes AVS-SD2000 or AvaSpec2048) with spectral resolution better than 5 nm. The raw data was corrected by subtracting a background recorded from a hole-free area of the sample and dividing the result by a bright-field spectrum of the light source. In Figure 1, we show representative scanning electron microscopy (SEM) images and elastic scattering spectra of holes and disks of similar size and arrangement. Both spectra exhibit a pronounced resonance peak, which gives the samples a reddish color when observed under DF illumination. The hole resonance is about twice as broad as the disk resonance. This difference is not due to inhomogeneous broadening effects, as the same kind of polystyrene particles were used as templates for both samples. The peak intensity per scattering object is approximately five times larger for the holes compared to the disks. In the case of the nanodisks, the scattering peak can be identified as a localized surface plasmon resonance and discussed in terms of resonant dipole polarizability R(λ). If the disk is treated as an oblate spheroid of dielectric function (λ) and volume V, electrostatic theory9 yields Rj(λ) )

(λ) - m m + Lj[(λ) - m]

j ) 1,2,3

(1)

Here, the geometrical constants L1 ) L2 ) (1 - L3)/2 are determined by the aspect ratio D/h of the spheroid while m denotes the dielectric constant of the embedding medium. For a high aspect ratio Au particle, the optical response is almost completely dominated by the long-axis polarizability R1(λ). The elastic scattering cross section can be then be computed from σs ) k4/(6π)|R(λ)|2, while the extinction cross section is σe ≈ σs + kIm[R(λ)], which includes a contribution from absorption. For a layer of particles, σe can be quantified 1004

Figure 1. Light scattering from holes in a thin Au film compared to scattering from Au nanodisks. (a) Representative SEM images of lithographically prepared holes in a 20 nm thick film and (b) 20 nm high disks. (c) DF scattering spectra of ≈110 nm diameter holes and disks in ambient air normalized by number density N ) 17 and 22 µm-2 for holes and disks, respectively. The measurement area was 5 µm in diameter, i.e., spectra are averaged over ≈400 scattering objects. Note that the recorded signal is free from diffraction effects as the samples have no long-range periodicity.

from the measured forward transmission through T(λ) ) exp[-Nσe(λ)], where N is the number density. Equation 1 predicts that the resonance position λLSP, determined by Re{m + L1[(λ) - m]} ) 0, should red shift approximately linearly with increasing disk diameter D for constant height h. This prediction is fulfilled extraordinarily well for a range of disk samples.7 In the case of large D values, retardation effects can be taken into account through a renormalized dipole polarizability.7 If the hole resonance has a similar origin, i.e., a LSP, we thus expect a pronounced red shift with increasing hole diameter. Unfortunately, for the type of dense array samples shown in Figure 1, a variation in polystyrene particle size also results in a variation in number density N, which would affect the spectrum through a variation in electromagnetic coupling between holes, as detailed below. To circumvent this problem, we modified the preparation protocol and used highly diluted colloidal solutions and short adsorption times. This resulted in an extremely low density of well-separated holes. Figure 2 shows representative scattering spectra, together with DF and SEM images, for single holes of varying size. First, it is clear that also a single hole exhibits a clear resonance, although the peak obviously shifts out of the measurement range for the larger diameters. The peak positions for the 60 and 110 nm single hole spectra indicate that ∆λSPR/∆D ≈ 2.4. This is of the same order as for nanodisks of the same height, for which7 ∆λLSP/∆D ≈ 1.6. Equation 1 also predicts that the Nano Lett., Vol. 4, No. 6, 2004

Figure 2. Characterization of single holes of different diameters using (a) scanning electron microscopy, (b) DF scattering spectra, and (c) DF optical microscopy. The scattering intensity has been normalized by the hole area a.

Figure 3. Extinction spectra of 60 nm holes (40 µm-2) in different refractive index environments. Measurements were performed using a probe size of approximately 0.7 cm2, i.e., data were averaged over ≈3 × 109 holes).

LSP should red shift with increasing m. We tested this using dense array hole samples immersed in media with different refractive index n ) xm. Figure 3, which shows extinction ln(1/T) for D ) 60 nm holes in dry N2 gas, water, and p-xylene, clearly demonstrates a substantial red shift with increasing n, analogous to what has been observed for the corresponding nano disk samples. The data yield ∆λLSP/∆n ≈ 100 nm, which can be compared to ∆λLSP/∆n ≈ 60, 100, and 200 nm for Au disks of aspect ratio 1, 3.5, and 7, Nano Lett., Vol. 4, No. 6, 2004

respectively.7 Note that the elastic scattering from holes yields a peak in the extinction superimposed on a background from hole-free regions. The wavelength-dependent skin depth of the Au film determines the spectral shape of this background. The overall similarity in resonance behavior between holes and disks, together with the observation of well-defined excitations for isolated holes, clearly support the hypothesis that the hole resonance is a dipolar localized surface plasmon. We note, however, that the quasi-static polarizability of an oblate spheroid cannot, in principle, be used to describe a hole. By simply exchanging m and (λ) in eq 1, one obtains the polarizability of a spheroidal void, with a dipolar LSP that is strongly blue shifted compared to the corresponding particle plasmon. This reflects a general sum rule for 2 2 + ωvoid-LSP ) ω2P, complimentary structures, ωparticle-LSP 16 where ωP is the bulk plasmon frequency. Although no simple electrostatic polarizability model exists for a hole in a thin film, it is likely that the dominant resonance is similar to the dipolar mode of a cylinder. In the limit of a very thin cylinder, the optical cross sections for polarization perpendicular to the cylinder axis can be obtained from a dipole polarizability analogous to eq 1, but with the resonance denominator replaced by9 m + (λ). In this case, an exchange of m with (λ) has no effect, i.e., the complimentary structures have the same resonance frequency ωLSP ) ωP/x2, which corresponds to ωLSP ≈ 500 nm for Au. This means that the two modes also exhibit the same degree of red shift with increasing m. For a disk or a hole, the restoring force acting on the dipolar charge displacement will be lower than for an infinite cylindrical object. This will tend to red shift the LSP in a fashion similar to that for an oblate spheroid as a function of increasing aspect ratio. Although there are no detailed electrodynamics calculations available for holes in thin gold films, recent data on silver films support the interpretation above. Using the multiple multipole method applied to nanometric holes (D ) 50-100 nm) in thin Ag films, Wannemacher14 found that the angle-integrated transmission (proportional to the scattering cross section for small size parameters) was resonantly enhanced in the blue-green spectral range due to plasmon excitation. The resonance wavelength was found to red shift with increasing diameter in a manner similar to the results in Figure 2. We now turn to the question of hole-hole interactions. When comparing spectra for single holes and short-range ordered arrays, we found that the respective resonance positions do not generally coincide. This is apparent from a comparison of the D ) 100 nm hole spectra in Figure 1 (λLSP ≈ 700 nm) and Figure 2 (λLSP ≈ 860 nm). To study the variation in LSP with interhole distance further, we prepared a set of D ) 60 nm samples with varying hole densities. Figure 4a,b shows scattering and extinction spectra, respectively, for three hole samples with N ) 70, 40, and 20 µm-2. The corresponding average center-to-center distances and peak-positions in nm are (d, λLSP) ≈ (110, 550), (140, 575), and (230, 625), respectively. The LSP thus exhibit an approximately linear red shift with increasing interhole distance, gradually approaching the single hole resonance 1005

Figure 4. Optical characterization of dense hole arrays. (a) Scattering and (b) extinction spectra of 60 nm diameter holes with different number densities (A) 70 µm-2, (B) 40 µm-2, and (C) 20 µm-2 in ambient air. Note the good agreement between scattering and extinction data in terms of the LSP peak positions and relative intensities. The dashed line in (b) shows the extinction spectrum for a film free of holes. (c) DF images of the samples used for scattering measurements. The image size is ≈10 × 10 µm2. (d) Simultaneously recorded NSOM and (e) AFM images of 110 nm holes at 17 µm-2 density. The NSOM was operated in illumination mode using a 100 nm aperture probe at λ ) 633 nm, as described in ref 18.

position at λLSP ) 730 nm. This behavior is similar in magnitude to that which has been observed for ordered particle arrays, where the LSP shift can be explained by inphase coupling of radiative dipole fields.17 However, radiative interparticle coupling is greatly diminished for samples that lack long range order. In the case of disordered nanodisk samples, a change in particle density similar to that in Figure 4 (a,b) has a negligible effect on the resonance position.7 It is likely that the critical difference between a nanodisk LSP and a hole LSP in this respect is that the latter has an additional possibility to interact through emission of surface plasmon polaritons in the intervening Au film. For a plane SPP wave, the decay-length exceeds 10 µm at λ ) 600 nm and grows rapidly at longer wavelengths. In thin films, however, the SPP loses energy to optical waves in the glass substrate at angles corresponding to total internal reflection. This radiation cannot be detected using an objective lens with NA < 1. In addition, the SPP launched by individual holes have a spherical wavefront and therefore suffer from a 1/r2 decay. Nevertheless, at submicron separations the hole-hole coupling mediated by SPP excitation can be expected to be far more efficient than radiative dipole coupling, resulting in a pronounced LSP renormalization even for disordered samples. Because of this extra coupling possibility, the hole 1006

LSP should have shorter lifetime (broader spectrum) than the corresponding particle excitation. This is also the case, as seen from Figure 1. We should note that the dipolar LSP of the cylindrical void discussed above has the same frequency as the SPP in the large k limit, ωLSP ) ωP/x2. For the experimental configuration used here, however, momentum conservation excludes that SPPs are the primary excitations responsible for the single hole resonances shown in Figure 2. The importance of SPP effects can also be seen from optical images of the dense hole samples. In Figure 4c, which shows DF micrographs of the three D ) 60 nm samples discussed above, one notes a characteristic graininess at the µm length scale. We investigated this phenomenon using a near-field scanning optical microscope (NSOM, Nanonics NSOM-100) equipped with 100 nm aperture fiber-probe and operated in illumination mode at λ ) 633 nm.18 Figure 4d,e shows corresponding optical and topographic images for the same D ) 110 nm sample, as shown in Figure 1. It is clear that the grain structure seen in the optical far field persists under near-field illumination conditions and that it is impossible to relate the optical intensity variation to individual holes seen in the topography image. We interpret this effect as the result of a random interference and multiple scattering of SPP scattered to the far-field by holes.11,19 It is of obvious interest to try to relate the optical properties reported here to the question of enhanced transmission.10-12 From Figure 1c, one can conclude that the scattering efficiency, Cs ) σs/a, for a hole is of the order five times larger than for a disk. For Au disks in the size range discussed here, eq 1 yields that Cs ≈ 2, which indicates that the holes scatter of the order 10 times as much light as expected from their geometrical cross-section a. This would then imply that the angle-integrated hole transmission is enhanced approximately five times. However, there is no coherent scattering in the forward direction because of the lack of long-range order in the disordered hole arrays. The result of the resonantly enhanced scattering efficiency is then an enhanced extinction rather than an enhanced forward transmission. Furthermore, from Figure 3b we see that the strength of the extinction and scattering peaks does not simply scale with number density N, as expected for a system of noninteracting sub-wavelength objects. Instead, the peak intensity is maximized at N ≈ 40 µm-2, where the scattering efficiency is ≈ 50% higher than for N ≈ 70 µm-2. This is a further indication for the importance of LSP-SPP coupling, which needs to be taken into account in any model of optical properties of the type of hole arrays discussed here. In summary, we have shown that isolated nanometric holes in optically thin Au films exhibit an optical resonance that behaves in a manner that is qualitatively similar to the dipolar particle plasmon of Au nanodisks. Based on this similarity, we assign the resonance to a localized surface plasmon excitation originating in the dipolar resonance of a small cylindrical void. For arrays of holes, the localized resonance is strongly renormalized by strong interactions with spatially extended surface plasmon polaritons. This interaction and its effect on the transmission, extinction, and imaging of Nano Lett., Vol. 4, No. 6, 2004

dense but short range ordered hole arrays requires further study. However, we note that the existence of localized plasmons in holes points toward a range of applications, including surface-enhanced spectroscopy and nanooptical biosensors, which have previously been associated mainly with particle plasmons. The possibility to tune the properties of hole plasmons through extended SPP modes may then offer a significant advantage to the comparatively weak dipole coupling of metal particles. Acknowledgment. We thank J. Aizpurua, J. Garcı´a de Abajo, P. Apell, and P. Johansson for numerous stimulating discussions and suggestions. Financial support from the Swedish Research Council is gratefully acknowledged. References (1) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824830. (2) Paesler, M. A.; Moyer, P. J. Near-Field Optics: Theory, Instrumentation, and Applications. Wiley-Interscience: 1996. (3) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783-826. (4) Homola, J.; Koudela, I.; Yee, S. S. Sens. Actuator B-Chem. 1999, 54, 16-24. (5) Sonnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P. Phys. ReV. Lett. 2002, 88, 077402.

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