Applied Physics A
DOI: 10.1007/s00339-006-3635-8
Materials Science & Processing
¨ l. landstrom d. brodoceanu∗ k. piglmayer u ¨ d. bauerle
Extraordinary optical transmission through metal-coated colloidal monolayers Institut für Angewandte Physik, Johannes-Kepler-Universität Linz, 4040 Linz, Austria
Received: 24 January 2006/Accepted: 16 May 2006 Published online: 20 June 2006 • © Springer-Verlag 2006 ABSTRACT Extraordinary optical transmission through metal-coated close-packed monolayers has been observed. The monolayers consist of silica (a-SiO2 ) or polystyrene microspheres that form two-dimensional close-packed lattices by self-assembly. Metal layers of Ag, Au and Ni with different thicknesses (larger than the skin depth) were evaporated onto such lattices by means of standard techniques. The optical transmission spectra investigated between 300 and 2500 nm show pronounced peaks that scale with the diameter and the optical properties of the composite slabs. The enhanced transmission observed is most likely mediated via plasmons. PACS 78.66.-w;
1
81.16.Dn; 82.70.Dd
Introduction
Regular two-dimensional (2D) lattices of microspheres formed by self-assembly from colloidal solutions have recently been applied for different types of laser-induced micro/nanosurface patterning, where the monolayers are utilized as a close-packed lens array [1–4]. Among those are patterns generated from metal-coated monolayers of microspheres by laser-induced forward transfer (LIFT) [5, 6]. In addition to acting like a lens array, the bare close-packed monolayers also behave like a 2D photonic crystal slab (PCS) with photonic modes that the incident light may couple to. Because of the poor refractive-index contrast in aSiO2 PCSs, they do not possess full band gaps. However, the positions of transmission dips and the coupling strength can easily be altered in the PCS by deposition of, for example, amorphous Si onto the microsphere arrays [7, 8]. Anomalous transmission mediated by plasmons and plasmonic nano-optics
have attracted an increasing interest in the last few years [9]. The observed extraordinary transmission in symmetrically perforated thin metal films [10] and the possibility of squeezing the optical near-field zone by plasmon coupling resulting in focusing light into very small volumes [11] are a couple of interesting examples. Possible applications are wavelength-tunable filters, sub-wavelength lithography, near-field microscopy, etc. In the present paper we report the optical properties of metal-covered monolayers of microspheres. The optical transmission properties show similarities with those observed with subwavelength hole arrays in thin planar metal films [10, 12–15]. 2
Experiment
Close-packed monolayers of amorphous silica (a-SiO2 ) or polystyrene (PS) microspheres (diameters d = 0.72, 1.0 and 1.42 µm) were deposited on 1-mm-thick quartz supports from
u Fax: +43-2468-9242, E-mail:
[email protected] ∗ Also at: NILPRP Bucharest.
colloidal suspensions in a similar fashion as described in [16]. This technique allows rapid formation of relatively large areas of close-packed monolayers (∼ cm2 ). However, the deposited monolayers exhibit a polycrystalline structure with a typical domain size ∼ 100 µm. The deposited monolayers were thereafter covered with different metals (Ag, Au, Ni) of different thicknesses (30 – 300 nm) by a standard evaporation technique. Zero-order transmission spectra at different incident light angles (θ = 0–π/6) were recorded in the spectral region 300 – 2500 nm by means of an ultraviolet to near-infrared spectrometer (Cary 500) for both metal-covered and bare microsphere arrays. Both polarized and non-polarized light were used for the transmission measurements. Furthermore, metal-covered spheres were also investigated by means of scanning electron microscopy (SEM), and the metal deposit on the quartz support was probed by atomic force microscopy (AFM). 3
Results and discussion
Figure 1 shows a scanning electron microscope (SEM) picture of a monolayer of a-SiO2 microspheres on a thin quartz platelet as support. The lattice is coated with a thin Ni film which was deposited by standard thermal evaporation in a vacuum chamber. Here, approximately the upper half of single spheres becomes metal coated, while the lower half remains uncoated. At the top of the spheres (center of spheres in Fig. 1) the thickness of the coating is about equal to that measured with a nearby quartz crystal microbalance (QCM). Towards the edge of the
Rapid communication
Appl. Phys. A 84, 373–377 (2006)
374
Applied Physics A – Materials Science & Processing
Furthermore, the plasmons can scatter off these periodic structures, resulting in plasmon modes which may exhibit plasmonic band gaps. The diffraction and coupling follow Bragg diffraction and the coupling to a surface plasmon with wavevector kSP can be written as kSP = k + G hk = k sin θ + m 1 b1 + m 2 b2 . FIGURE 1 Scanning electron microscope (SEM) picture of a Ni-coated monolayer of quartz (aSiO2 ) microspheres of diameter d = 0.72 µm. The support is a 1.0-mm-thick a-SiO2 platelet
spheres the film thickness slightly decreases. In the interstices between the spheres, the coating is placed on the quartz support. Within these areas, the film thickness measured by means of an atomic force microscope (AFM) is equal to that measured by the QCM. Figure 2 shows the optical transmission spectra of monolayers with different metal coatings. In all of these cases, the thickness of the metal layer measured with the QCM significantly exceeds the optical penetration depth lα = α−1 of the corresponding metal (see e.g. [5]). Thus, within the spectral region under consideration, the optical transmission through such metal films is well below 1%. This can also be seen from the figure, in which we have included the spectrum of a 75-nm-thick Ag film on a plane a-SiO2 substrate. The peak at ∼ 325 nm corresponds to the well-known plasma resonance of Ag. In the spectrum for Ag-coated microspheres, this peak is still present, although strongly suppressed. Importantly, the transmission peaks at longer
FIGURE 2 Transmission spectra measured at normal incidence for monolayers of a-SiO2 microspheres (d = 1.42 µm) coated with different metals. The dash-dotted curve shows the spectrum for a 75-nm-thick Ag film on a plane a-SiO2 substrate
FIGURE 3 Normalized transmission spectra for monolayers of a-SiO2 microspheres with diameters d = 1.42 µm (full curve) and d = 0.72 µm (dashed curve) coated with a 75-nm-thick Ni film
wavelengths are only observed when a metal film is evaporated onto the monolayer of microspheres. Figure 3 shows spectra for Ni coatings on monolayers fabricated from silica microspheres with diameters d = 2rsp = 1.42 µm (full curve) and d = 0.72 µm (dashed curve). The figure shows that the peak positions scale with the diameter (periodicity) of the spheres. It is known that the transparent microspheres act as lenses but, since the thicknesses of the metal layers are significantly larger than the skin depth, it seems unlikely that this effect is responsible for the observed transmission features. Furthermore, the maximum transmission peak is at wavelengths slightly longer as compared to the diameter of the spheres, which results in poor focusing of the light. The optical transmission spectra depicted in Fig. 2 show similarities with those observed with sub-wavelength hole arrays (or dielectric sphere inclusions) in thin plane metal films and/or reflectance spectra measured on metal films with ordered corrugations [10, 12–14, 17, 18]. In all these reports, incident light interaction with plasmons is believed to be mainly responsible for the observed phenomena. The similarities in transmission, and the fact that the effect is only observed when a metal is deposited on the ordered monolayers, suggest that the enhanced transmission is also governed by plasmons in our case. Along the boundary of a metal and a dielectric, surface plasmon polaritons can propagate. If the boundary is periodically patterned, for example with holes, an incident optical field can be diffracted and coupled to the plasmons.
(1)
For a close-packed monolayer (and also for the interstices between the spheres), the diffraction vector is given √ by G hk = 4π m 21 + m 22 + m 1 m 2 3d . b are primitive reciprocal lattice √vectors with absolute value b = 4π/ 3d . m 1 and m 2 are integers. From (1) we obtain the resonant wavelength (possible transmission peak in our case) √ 3d εm εd λSP = 2 εm + εd 2 −1/2 × m 1 + m 22 + m 1 m 2 , (2) where εm and εd are the dielectric functions of the metal and dielectric, respectively. The above equations describe so-called Bragg plasmons, where resonance (and possible transmission) occurs when the coupling condition is fulfilled. Furthermore, localized ‘Mie’ plasmons with high-field enhancement have also been observed, where the localized plasmons arise if the field fluctuations become pinned and disconnected from each other. As reported in [18], localized Mie plasmons were observed within metal voids when the voids become deep enough. It was reported that for voids shallower than ≈ 0.2d , the plasmon dispersion follows Bragg plasmons. For the metal-covered spheres in our investigation the metal layer forms a hemisphere, which may introduce localized plasmons. The positions of the transmission peaks are only slightly dependent on the type of metal employed in the investigations; see Fig. 2. This behavior can be expected because |εm | |εd | for the metals in the wavelength region of interest. That is, the value of the square root in (2) (which can be interpreted as an ‘effective’ refractive√index, n eff ) is approximately equal to εd . At shorter wavelengths in the observed spectral region, smaller and less pronounced peaks are observed for the
LANDSTRÖM et al.
Extraordinary optical transmission through metal-coated colloidal monolayers
nickel-covered spheres (see Figs. 2 and 3). This effect is most likely because of the much larger imaginary part in εm for Ni, resulting in a lower energy transfer through the metal. That is, nickel is not a so-called ideal metal in the shorterwavelength region. (It is noted that poor transmission properties were also found for symmetrically perforated thin nickel films as compared to silver films [19].) Silver has the lowest ε, which probably results in the largest and most narrow transmission maximum; see Fig. 2. The positions of the peaks are only slightly shifted if the thickness of the metal deposit is changed in the range 30 – 300 nm, and the intensity of the main peak follows an approximately exponential decay as the thickness increases. Furthermore, the shape of the spectra is independent of on which side the light is incident. Because of the polycrystalline nature of the deposited monolayers, the observed transmission spectra are also independent of the polarization of the incident light. Estimation of the peak positions from (2) results in very poor agreement with the experimentally observed peaks. Only the largest peak can with some certainty be assigned to Bragg diffraction. Considering that the system is rather complex, with a metal deposit onto the support and the halfcovered close-packed spheres without a continuous metal deposit (holes in the interstices), the complex behavior of the transmission spectra can be expected. In addition, (1) is an approximation for planar geometry, which is clearly a very crude description of our case. Comparisons with metal-covered, purely stochastically arranged a-SiO2 spheres, deposited by means of pulsed
laser deposition (PLD), were performed to see the influence of single spheres. In Fig. 4, the transmission spectra from metal-covered (75 nm Ag) close-packed and stochastic spheres are shown. The transmission spectrum obtained from the stochastic arrangement still exhibits some small peaks, although with much smaller intensities. Importantly, the ‘main’ transmission peak is absent. This effect again suggests that the main peak stems from the ordering of the lattice. Furthermore, the peak positions observed for the randomly ordered spheres fit well with calculated extinction spectra of a single a-SiO2 sphere by Mie theory [20]. (These oscillations are usually referred to as the ripple structure or whispering gallery modes.) The sphere coverage on the substrate in the stochastic case was approximately one-tenth as compared to a close-packed array, so it seems likely that these features (interpreted as localized Mie modes) will be strong enough to also appear in the close-packed arrangement. Finally, the metal pattern deposited onto the support (within the interstices of the spheres) also alters the transmission properties compared to a nonpatterned quartz support. Measurements on the metal-patterned support only revealed broad and shallow transmission dips, also related to the periodicity (results not shown). In conclusion, the observed spectra are most likely a combination of the above-mentioned contributions, which may also be non-trivially coupled. Thus, trying to de-convolute the observed transmission spectra and assign different parts/features to different origins is not a straightforward task and not within the scope of this report.
FIGURE 4 Measured transmission spectra for stochastic (full line, ×10) and close-packed monolayers (dashed line) from d = 1.42-µm silica spheres covered with 75 nm of Ag
Measured transmission spectra (normal incidence) for a d = 1.0-µm PS monolayer (ML), a bare 2D PCS (dashed line, right y-scale) and this covered with 75-nm Ag (full line, left yscale) FIGURE 5
375
By using PS monolayers, the effective refractive index of the PCS is increased, resulting in a further red shift of the main transmission peak as compared to the periodicity d (see (2)). Figure 5 shows the transmission properties of the bare d = 1.0-µm polystyrene 2D PCS (dashed line, right y-scale), and the same monolayer covered with 75-nm Ag (full line, left y-scale). The main dip at around 1250 nm observed for the non-covered slab corresponds to strong coupling to photonic mode(s). As mentioned earlier, a close-packed monolayer of transparent spheres (see Fig. 5, dashed line) acts only as a 2D photonic crystal slab with transmission dips related to resonances with photonic modes. Hence, a similar expression as given in (2) holds, with the difference that the effective refractive index, n eff , is related to the periodic refractive-index variation in the slab. A good approximation of n eff can be obtained from filling factors. Because n eff (plasmon) ≈ n eff (PCS) for good metals, one could expect similar positions and angular behavior (dispersion) for the transmission dips in the 2D PCS and the metal-covered monolayer if the plasmons follow Bragg diffraction (1). Since only the main transmission peak
FIGURE 6 Changes in zero-order transmission with the angle of incidence θ. (a) a d = 1.0-µm polystyrene (PS) 2D photonic crystal slab and (b) 75-nm Ag on a PS sphere monolayer. Transmission curves are shifted in y-scale for convenience
376
Applied Physics A – Materials Science & Processing FIGURE 7 Dispersion maps obtained from the transmission spectra in Fig. 6. Brighter colors indicate higher transmission. Also included are the estimated dispersions from (3) for three different azimuthal angles where strong coupling to photonic modes is expected
0.6 and 0.8 for the PCS and metalcovered slab, respectively.) Reasonable agreement between the estimated and observed dispersions can be seen, again suggesting that the main peak arises from Bragg-like plasmons. 4
could be assigned to Bragg plasmons with some certainty, the following discussion will focus on the behavior of this feature. As can be seen in Fig. 5, the main transmission peak can be found slightly red shifted as compared to the main dip. (Similar behavior was also observed for Au-covered spheres.) But, the red shift is larger than expected from (2) if εd (≈ n 2eff ) is calculated from the dip position observed from the 2D PCS. Considering that after metal deposition, the filling factor of the dielectric is increased, thus increasing the effective refractive index and the red shift of the transmission peak, a good agreement of the main peak position is obtained. Recently, the asymmetric shape and additional red shift of transmission peaks in similar systems (sub-wavelength holes/slits in thin metal films) could be explained by a Fano-type interference [21–23]. That is, the interference between resonant and nonresonant (direct) transmission results in Fano profiles. Indeed, the observed main transmission peaks (see e.g. Figs. 4 and 5) do seem to correlate well with such profiles. However, such an analysis is not within the scope of the present report. The angular dependence of the transmission for bare and metal-covered monolayers of d = 1.0-µm PS spheres is depicted in Figs. 6 and 7. The main difference in Fig. 6a and b is the red shift of
the transmission peak compared to the dip in the bare monolayer. Furthermore, the transmission peak is broader. Similarities are the splitting and red shift of the main dip/transmission as the angle of incidence is increased. To further illustrate the dispersion of the resonant modes, dispersion maps for the bare monolayer and a metal-coated one are depicted in Fig. 7. Here, it is even more evident that the main mode is red shifted for the metal-covered composite in comparison to the 2D PCS, but the overall behavior of the main features are similar. By using FDTD (finite difference time domain) calculations, it was shown that strong coupling was observed to diffraction vectors at azimuthal angles (relative to the plane of incidence) ϕ = π/2 for p-polarized light and ϕ = 5π/6 and π for s-polarized light, respectively [8]. An expression for the mode dispersion for the first-order diffraction for arbitrary ϕ is given by √ d 3 λ10 = n 2eff − sin2 θ sin2 ϕ 2 − sin θ cos ϕ . (3) In Fig. 7, the dispersions are plotted for the three azimuthal angles where strong coupling to the PCS is expected. n(PS) = 1.57 (see [24]) and εm = −102 +2.6i were used and filling factors of
Conclusions
Extraordinary transmission through metal-covered microsphere arrays was observed and the observed transmission is most likely mediated by plasmons. The positions of the peaks scale with the periodicity of the lattice and are also related to the effective refractive index of the 2D photonic crystal slab. The influence on the shape of the spectra is slightly dependent on the type of metal deposited and differences are observed because of the different optical properties of the metals. Only the maximum transmission peak could with some certainty be related to Bragg-like plasmons and, most likely, localized Mie plasmons are also present in the measured spectra. The rather complex system investigated, with a metal deposit onto the quartz support and hemispherically covered microspheres resulting in a noncontinuous metal film does not allow straightforward interpretation of the observed spectra. Fabrication and measurements on simpler systems, still based on microsphere arrays, such as corrugated continuous metal films and periodically arranged voids, are in progress and those results will be presented elsewhere. ACKNOWLEDGEMENTS We thank the Austrian Research Fund FWF (Fonds zur F¨orderung der wissenschaftlichen Forschung) under Contract No. P16133-N08 for financial support. One of us (L.L.) would like to thank the Knut and Alice Wallenberg Foundation for a postdoctoral fellowship.
REFERENCES 1 D. Bäuerle, T. Gumpenberger, D. Brodoceanu, G. Langer, J. Kofler, J. Heitz, K. Piglmayer, in Laser Cleaning II, ed. by D.M. Kane (World Scientific, Singapore, 2005) 2 D. Bäuerle, G. Wysocki, L. Landström, J. Klimstein, K. Piglmayer, J. Heitz, Proc. SPIE 5063, 8 (2003) 3 K. Piglmayer, R. Denk, D. Bäuerle, Appl. Phys. Lett. 80, 4693 (2002) 4 G. Wysocki, R. Denk, K. Piglmayer, N. Arnold, D. Bäuerle, Appl. Phys. Lett. 82, 692 (2003)
LANDSTRÖM et al.
Extraordinary optical transmission through metal-coated colloidal monolayers
5 D. Bäuerle, Laser Processing and Chemistry (Springer, Berlin, 2000) 6 L. Landström, J. Klimstein, G. Schrems, K. Piglmayer, D. Bäuerle, Appl. Phys. A 78, 537 (2004) 7 L. Landström, D. Brodoceanu, N. Arnold, K. Piglmayer, D. Bäuerle, Appl. Phys. A 81, 911 (2005) 8 L. Landström, N. Arnold, D. Brodoceanu, K. Piglmayer, D. Bäuerle, Appl. Phys. A 83, 271 (2006) 9 W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424, 824 (2003) 10 T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391, 667 (1998) 11 J.R. Krenn, A. Dereux, J.C. Weeber, E. Bourillot, Y. Lacroute, J.P. Goudonnet, G. Schider, W. Gotschy, A. Leitner, F.R. Aussenegg,
12
13 14 15 16 17
C. Girard, Phys. Rev. Lett. 82, 2590 (1999) L. Mart´ın-Moreno, F.J. Garc´ıa-Vidal, H.J. Lezec, K.M. Pellerin, T. Thio, J.B. Pendry, T.W. Ebbesen, Phys. Rev. Lett. 86, 1114 (2001) W.L. Barnes, W.A. Murray, J. Dintinger, E. Devaux, T.W. Ebbesen, Phys. Rev. Lett. 92, 107 401 (2004) K.J. Klein Koerkamp, S. Enoch, F.B. Segerink, N.F. van Hulst, L. Kuipers, Phys. Rev. Lett. 92, 183 901 (2004) Q. Wang, J. Li, C. Huang, C. Zhang, Y. Zhu, Appl. Phys. Lett. 87, 091 105 (2005) R. Micheletto, H. Fukuda, M. Ohtsu, Langmuir 11, 3333 (1995) F.J. Garcia de Abajo, G. Gomez-Santos, L.A. Blanco, A.G. Borisov, S.V. Shabanov, Phys. Rev. Lett. 95, 067 403 (2005)
377
18 T.A. Kelf, Y. Sugawara, J.J. Baumberg, M. Abdelsalam, P.N. Bartlett, Phys. Rev. Lett. 95, 116 802 (2005) 19 D.E. Grupp, H.J. Lezec, T.W. Ebbesen, K.M. Pellerin, T. Thio, Appl. Phys. Lett. 77, 1569 (2000) 20 C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley, New York, 1983) 21 C. Genet, M.P. van Exter, J.P. Woerdman, Opt. Commun. 225, 331 (2003) 22 M. Sarrazin, J.-P. Vigneron, J.-M. Vigoureux, Phys. Rev. B 67, 085 415 (2003) 23 C. Ropers, D.J. Park, G. Stibenz, G. Steinmeyer, J. Kim, D.S. Kim, C. Lienau, Phys. Rev. Lett. 94, 113 901 (2005) 24 X. Ma, J.Q. Lu, R.S. Brock, K.M. Jacobs, P. Yang, X.-H. Hu, Phys. Med. Biol. 48, 4165 (2003)
