Letter pubs.acs.org/NanoLett
Linear and Nonlinear Optical Characterization of Aluminum Nanoantennas Patrick M. Schwab,† Carola Moosmann,† Matthias D. Wissert,† Ekkehart W.-G. Schmidt,‡ Konstantin S. Ilin,‡ Michael Siegel,‡,§ Uli Lemmer,†,§ and Hans-Jürgen Eisler*,† †
Light Technology Institute (LTI), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany Institute for Micro- and Nanoelectronic Systems (IMS), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany § DFG Center for Functional Nanostructures (CFN), 76131 Karlsruhe, Germany ‡
S Supporting Information *
ABSTRACT: We experimentally determine the order of multiphoton induced luminescence of aluminum nanoantennas fabricated on a nonconductive substrate using electron-beam lithography to be 2.11 (±0.10). Furthermore, we optically characterize these nanostructures via linear dark-field microscopy and nonlinear multiphoton laser excitation. We hereby observe different spectral response functions that can be seen as a splitting of peak positions when the antenna arm length is increased to Larm > 150 nm which has not yet been reported for aluminum nanostructures.
KEYWORDS: Plasmon, aluminum nanoantenna, dark-field microscopy, laser induced photoluminescence, nonlinear plasmonics
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Aluminum shows some major peculiarities: First of allas it is not one of the noble metalsthere is a native oxide layer forming at the surface of the metallic structure with a thickness of several nanometers14 which reduces the effective volume and changes the refractive index in the vicinity of the nanostructure from nair ≈ 1 to nAl2O3 ≈ 1.7716 and thus red-shifts the resonance as reported in detail by Langhammer et al.17 Beside this, there is a strong interband transition at about 1.5 eV16 due to almost parallel bands at the W-point for bulk aluminum18 which heavily influences its dielectric function and subsequently its optical response. Calculating the skin-depth of aluminum, one discovers that electromagnetic fields can only weakly penetrate into the metal, and therefore aluminum is almost a perfect conductor in the visible.19 That makes aluminum nanoantennas promising candidates to extend the optical response of gold antennas into the green-blue wavelength regime opening pathways for nanooptics in the UV and applications such as UV Raman sensing. Nonlinear laser excitation has already been applied to optical antennas,4,12,20 and it has been experimentally demonstrated that the scattering spectra of gold nanoantennas are in good agreement with those spectra measured via two-photon laser excitation.10 Here we present the results for aluminum
ver the last few years, metallic nanostructures showing resonances in the optical regime have gained a renaissance due to their unique properties when interacting with light.1 Via the excitation of localized surface plasmons, they offer the possibility of enhancing and concentrating electric fields to a subwavelength volume.2 As they are able to receive and transmit electromagnetic waves, the term optical antenna has been established for these nanoscale objects3 typically fabricated by means of focused-ion-beam (FIB) milling4 or electron-beam lithography.5 Although they cannot be simply scaled down from the radio wavelength regime to operate at optical frequencies,6 it is possible to tailor the optical responseas known from their macroscopic counterpartsby an architecture going beyond what is possible with colloidal chemistry approaches, for example, Yagi-Uda antenna designs.7 With the dielectric function as one of the most important properties of the material itself and therefore its optical behavior, gold (Au) with its plasmonic resonance in the visible range is often the material of choice.8 As a consequence, a lot of attention has been put to the characterization of its optical, that is, both the linear9 and the nonlinear10−12 response. Other metals such as silver (Ag), copper (Cu), and aluminum (Al) show a different plasmonic behavior, and therefore they are also very attractive for nanoengineering structures showing plasmon emission.13 Due to the fact that aluminum nanostructures are difficult to fabricate via chemical bottom-up approaches, only few examples exist so far for this material.14,15 © XXXX American Chemical Society
Received: December 20, 2012 Revised: February 25, 2013
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Figure 1. Scattering spectra for (a) two-arm and (b) single-arm aluminum nanoantennas of 30 nm width, 30 nm height, a 20 nm gap (for two-arm structures), and arm lengths as indicated. The corresponding numerical calculations are shown in (c) and (d) where a 4 nm oxide layer covering the aluminum core is taken into account. All spectra have been normalized to unity, and an offset has been introduced between individual spectra to facilitate discrimination between them.
exemplary SEM image can be found in the Supporting Information. Results and Discussion. In the following, we present the polarization dependence of optical aluminum two-arm gap antennas to determine the order of multiphoton induced luminescence after plasmon excitation when irradiated with a laser at a fixed wavelength of λlaser = 810 nm. This experiment was motivated in part by the recently stated high capability of aluminum nanoantennas to support strong second harmonic generation (SHG).23 In this context, we point out the influence of ITO as conductive layer on SHG. We first of all present the spectral characterization of coupled and uncoupled aluminum nanorods, subsequently termed two-arm gap antennas and single-arm antennas, respectively, under scattering and multiphoton excitation conditions and compare the results with numerical simulations. Furthermore, we investigate the lengthdependent antenna coupling efficiency to the incident far-field by measuring the emission intensity to demonstrate the antenna character, that is, the geometric resonance tuning of our structures. Dark-Field Microscopy. The linear optical characterization was performed using scattering far-field detection via dark-field microscopy in transmission mode (see the Supporting Information for further details). In this configuration, a polarization filter can be introduced in the detection path for
nanoantennas that go beyond of what is known so far as well as a detailed optical study of polarization dependency for the first time. Experimental Section. The sample preparation used for the experiments presented in this work is based on the fabrication protocol described in previous publications by Wissert et al.5,10,21 In a nutshell, some minor modifications had to be established compared to the previous work, as the conductive indium-tin-oxide (ITO) layer for electron-beam lithography was omitted to increase surface smoothness and thereby reducing sample autofluorescence when illuminated with a highly intense laser. Furthermore, this gives us the opportunity to be as close as possible to the antenna specific plasmonic resonance as any change of refractive indices in the vicinity of the nanoantenna will change both the spectral position and the intensity of the plasmon emission and modify accessible relaxation channels.22 Instead, an additional niobium layer was deposited which was then photolithographically structured resulting in a window pattern. The necessary conductivity for electron-beam focusing and alignment was provided by the metallic layer while exposing the resist directly on glass where the niobium had been removed by etching. The metal also prevents the electron beam from being deflected due to charging effects. In addition, aluminum antennas on ITO have been fabricated for comparison. Details including an B
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polarization sensitive measurements. With the polarization filter set to perpendicular polarization, that is, along the short antenna axis, we were hardly able to measure any signal such that only the longitudinal scattering spectra are presented in the following. The transversal components of the resonance for antennas with a width of 20−40 nm as fabricated for this experiment are supposed to be both very weak and located in the deep UV spectral range where it is very challenging to resolve the signal with the current setup. As a consequence, the unpolarized spectral response is dominated by the longitudinal emission component. Normalized spectra for single-arm and two-arm gap antennas are depicted in Figure 1a and b, summarizing the general trend that both types of antennas yield a red-shifted optical response with an increase in arm length due to the more pronounced enlongated shape. This corresponds to an increased so-called plasmon length24 describing the length-dependent effect on plasmon resonance. For smaller structures having a smaller charge separation along the metal and thus higher restoring forces, spectra can also reach peak wavelengths in the green which was hardly possible for gold nanoantennas of similar geometry. As expected from a symmetric bonding mode of lower energy,25 the resonance of the two-arm gap antennas is at a longer wavelength than the single-arm antenna response for a given arm length. For both types of antenna geometry, we observe the peak scattering resonance energies to converge for arm lengths larger than 150 nm: Although the longitudinal dimension is further increased, there is no additional red-shift to be seen. Coming closer to the interband transition at about 1.5 eV where damping is increased, the scattering resonance peaks stop shifting and stay approximately constant at about 680 nm for single-arm and 700 nm for two-arm gap antennas. As depicted in Figure 1, these results agree very well with the finite difference time domain (FDTD) simulations of ideal aluminum nanoantennas covered by a 4 nm oxide layer. These numerical results go hand in hand with the general trend of our experimental findings. The good accordance leads to the assumption that by measuring the linear darkfield response, we observe the localized plasmon polariton with longitudinal polarization. Coming closer to the interband transition either by increasing the arm length or due to the coupling in the case of a two-arm gap antenna, scattering intensity competes with the Joule heating relaxation channel due to interband absorption. For a discussion concerning the quality factor Q of the optical aluminum antennas, absolute scattering, absorption, and extinction cross sections (FDTD simulations) and a computational investigation of the spectral influence of the oxide thickness please refer to the Supporting Information. Multi-Photon Laser Excitation. To investigate the antenna response under nonlinear laser excitation both qualitatively and quantitatively, we first examined the polarization dependence of both the emission and excitation of aluminum gap antennas to determine the order of the excitation process before measuring the spectral characteristics. Therefore, a pulsed Ti:Sa laser at λlaser = 810 nm in combination with a short-pass filter with cutoff wavelength of 785 nm to remove the exciting laser line was used. See the Supporting Information for a detailed explanation of the setup. The length-dependent emission intensity for two-arm gap antennas for detection polarization along both the long and the short antenna axis is depicted in Figure 2. To circumvent thermal damage to the best matched structures, we applied a
Figure 2. Longitudinal (a) and transversal (b) emission intensity for two-arm gap antennas with excitation polarization along the long antenna axis. The red dashed line is a Gaussian-shaped guide to the eye.
constant nondestructive laser fluence of 1−2 × 10−3 J/cm2 for two-arm gap and single-arm antennas, respectively. Starting with very short antennas whose resonances are completely mismatched to the incident field, the measured intensity is close to zero for our signal-to-noise detection capabilities. By increasing the arm length, we come closer to the perfectly matched geometry yielding a maximum intensity at Larm ≈ 170 nm. Further increasing the arm length again results in an increased mismatch and a lowered emission intensity. Despite this strong geometrical mismatch, we still observe a residual intensity which can be attributed to the photoluminescence signal from the larger metal volume. By comparing the normalized length-dependent emission intensity for linear polarization along the long and the short antenna axis, no significant difference in length-dependence can be observed. This can be understood in terms of coupling efficiency: Depending on the geometry and the thereby defined plasmon resonance, each antenna couples differently to the incident field. The better the geometry is tuned to be resonant at the wavelength of the incident laser radiation, the better the matching and the electron excitation are. After the excitation of electrons with linearly polarized light and subsequent generation of a plasmonpotentially similar to the model of Dulkeith et al. for gold nanoparticles12it can then decay radiatively with polarization components along both the long C
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for off-resonant antennas to a level which would have been detrimental for resonant antennas to achieve a sufficient signalto-noise level. This procedure is even more critical for the antennas made of aluminum as the laser excited structures tend to undergo shape transformations up to a complete disruption of the material thus leading to nonreproducible photoluminescence spectra. The former may be due to the differences in thermal expansion for aluminum and its oxide or a nonideal thermal coupling of the metal to the underlying substrate. These irreversible structural changes follow the exposure with a too high laser intensity which depends on the specific geometry of the antenna. In the case of a resonant two-arm gap antenna, this critical value is in the order of 4 × 10−3 J/cm2. Exceeding this limit results in a drastically increased emission intensity. In most cases this enhanced light output is followed by a blue-shift due to the shape transformation and the reduction of the aspect-ratio Larm/width which is induced by the melting of the material as, e.g., done in a controlled manner for single gold nanorods by Yorulmaz et al.27 But in most cases, the structure is completely removed, and it is not possible to detect any optical signal afterward or to take topography information thereafter. Measuring the spectrally resolved emission of single-arm and two-arm gap antennas (Figure 4, normalized due to different excitation powers for different types of antennas), a resonance shift toward lower peak energy, i.e., a red-shift while increasing the arm length, can be observed as well as the lower energy binding mode for coupled structures. This red-shift Δλres/ΔLarm seems to be weaker than for other metallic nanostructures in the frequency range under investigation which is in good agreement with Novotny’s simulations.6 Somewhat surprisingly and in contrast to the linear antenna response, the peak energies keep shifting continuously toward lower energy when increasing the arm length. The intrinsic absorption of bulk aluminum is obviously overcome by nonlinearly exciting the nanostructures at a wavelength close to its interband transition. We may call this observation a photoluminescence splitting comparing one-photon (elastic scattering) excitation with twophoton (inelastic) laser excitation. It seems like the plasmonic aluminum structure representing a multielectron system can be compared by analogy to a molecular system having a distinct dipole moment. It is well-known from molecular physics that a discrepancy in spectra can be observed depending on the explicit matrix elements of transition dipole moments addressed by either one- or two-photon excitation. As already shown for gold in the visible,10 and now also for aluminum away from the interband transition, both optical response spectra match well. However, close to the interband transition, the nonlinear excitation unveils another possible effective radiative decay pathway. One may think of a pumping-like process much as in a molecular system where the nonlinear excitation process yields different radiative relaxation channels due to relaxed selection rules by the additional momentum of the second photon involved in the excitation process. As a result, radiative contribution from formerly forbidden transitions can be brought out. Besides this approach of a molecular physical inspired view, the observed photoluminescence splitting might also be interpreted in terms of local field enhancement due to the two-photon excitation and the nearly impedance matched optical antenna structure, effectively accelerating radiative decay channels following the plasmon length scaling of these structures. While the elastic scattering process after linear excitation experiences the influence of competitive nonradiative
and the short axis; that is, the plasmon emission does not conserve the original polarization. The short pass filter which removes the excitation laser also removes parts of the spectral response which is again redshifted with increasing arm length. These longer antennas with emission intensity partly beyond the filter edge are thereby slightly underweighted compared to structures whose response can be completely detected. Therefore, the maximum intensity is supposed to be at a slightly larger arm length than indicated by the results plotted in Figure 2. All results shown here were again achieved with the excitation polarization along the long antenna axis. Single-arm antennas show a similar lengthdependent emission intensity (not shown here), but in contrast to two-arm gap antennas no transversal emission signal could be measured. Especially the reduced volume compared to twoarm gap antennas, i.e., the missing second arm, and the nonexistent field-enhancement inside the feed gap give rise to this intensity decrease in the far-field. Due to the risk of thermal destruction, laser excitation intensity is a limiting factor10 which did not allow for a closer investigation of the order of the process by measuring the emission intensity dependence on the excitation intensity for specific structures. Therefore, we examined the order of the laser excitation process by measuring the emission intensity as a function of excitation polarization as, e.g., earlier shown by Rothenberg et al. for single CdSe/ZnS quantum rods.26 To do so, we installed a linear polarizer in parallel to the laser polarization, a Berek compensator used as quarter-waveplate and a second linear polarizer in the excitation path of our setup to be able to select the effective angle of linear polarization. The measured intensity distribution can be seen in Figure 3. By
Figure 3. Measured emission intensity for varied excitation polarization on two-arm gap aluminum antennas. Error bars take into account the differences for the overall number of nine antennas investigated in this case. The red line indicates a cosine to the power of 4.22 as best fit. A cos4 as well as a cos6 are depicted for comparison.
fitting a cosn function to the measured data we determine the order of excitation to be (n/2) ≈ (4.22/2) ≈ 2.11(±0.1), which is in line with the often stated assumption of a two-photon excitation process. To obtain spectral information with a sufficient SNR via laser induced emission without damaging the antennas or introducing ablation, the excitation power has to be carefully adjusted via optical density filters to the individual arm length to account for the geometrical mismatch to the fixed laser wavelength. While we were able to reduce the laser power for antennas with resonance close to the laser wavelength, we had to increase it D
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Figure 5. Comparison of multiphoton-luminescence (MPL) peak energy and scattering peak energy for single-arm and two-arm gap antennas as a function of antenna arm length.
the laser interaction with the sputtered ITO layer which satisfies the symmetry-breaking condition for a second order nonlinear processwhich in addition might be enhanced due to the interaction with the antenna structure and the intrinsic high second order nonlinear susceptibility χ(2) of metalsbut is not a feature related to the material the antenna is made of. SHG from ITO has been reported in detail by, e.g., Wang et al.28 The corresponding spectra of SHG signal with and without antenna related plasmon emission are depicted in Figure 6.
Figure 4. Laser induced photoluminescence spectra for (a) two-arm and (b) single-arm aluminum nanoantennas of 30 nm width, 30 nm height, a 20 nm gap (for two-arm structures), and arm lengths as indicated. All spectra have been normalized to unity, and an offset has been introduced between individual spectra to facilitate discrimination between them.
decay channels and therefore the corresponding peak positions converge the interband energy, the inelastic scattering induced by the nonlinear excitation unveils the continuous plasmon length dependent shift in peak energy. By comparing the peak energies of single-arm and two-arm gap antennas for both types of excitation, i.e., linear dark-field microscopy and nonlinear laser excitation, this splitting of peak positions emerges (see Figure 5). In contrast to the peak position, the spectral width is very similar to the scattering results with values of about 0.5 eV for all measured structures. In a recent publication, aluminum was claimed to be highly efficient concerning two-photon luminescence compared to gold and silver and to show strong nanoplasmonic second harmonic generation (SHG).23 However, throughout all our experiments with aluminum nanoantennas on a glass-only substrate, we could not measure any SHG signal. Therefore, we investigated the influence on the emission properties of ITO used as conductive material covering the glass substrate. We thereby observed a strong SHG signal at 405 nm just by focusing the laser onto the ITO surface without any aluminum close bywhile there was no SHG signal measurable at the same intensity on a glass-only substrate. We therefore think that the previously reported SHG signal mostly originates from
Figure 6. Pure second harmonic signal at 405 nm in the absence of an antenna nanostructure (black line) compared to the signal with added plasmon contribution at a longer wavelength measured when an optical antenna is present in the laser focus (red line).
Conclusion. We have carried out a detailed study of the optical properties of aluminum nanoantennas measuring the spectral response of single-arm and two-arm gap antennas and investigating the emission after both linear and nonlinear excitation. In both cases, the resonance is spectrally red-shifted with increased arm length. It turns out thatin contrast to e.g. gold nanoantennasthe spectra measured via linear and nonlinear excitation do not match for all geometries under investigation. Instead, we observe a splitting of peak positions when increasing the arm length above a critical value of about 150 nm and thereby approaching the interband transition of aluminum while comparing one-photon with two-photon excitation. It is found that the main contribution of second harmonic signal originates from the ITO layer and is not a direct aluminum antenna related feature. By varying the linear laser polarization the nonlinear excitation process is determined E
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(12) Dulkeith, E.; Niedereichholz, T.; Klar, T. A.; Feldmann, J.; von Plessen, G.; Gittins, D. I.; Mayya, K. S.; Caruso, F. Phys. Rev. B 2004, 70, 205424. (13) Mohammadi, A.; Sandoghdar, V.; Agio, M. J. Comput. Theor. Nanosci. 2009, 6, 2024−2030. (14) Chan, G. H.; Zhao, J.; Schatz, G. C.; van Duyne, R. P. J. Phys. Chem. C 2008, 112, 13958−13963. (15) Thyagarajan, K.; Rivier, S.; Lovera, A.; Martin, O. J. F. Opt. Express 2012, 20, 12860−12865. (16) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press: New York, 1984. (17) Langhammer, C.; Schwind, M.; Kasemo, B.; Zorić, I. Nano Lett. 2008, 8, 1461−1471. (18) Singhal, S. P.; Callaway, J. Phys. Rev. B 1977, 16, 1744−1746. (19) Schulz, L. G. J. Opt. Soc. Am. 1954, 44, 357−362. (20) Ghenuche, P.; Cherukulappurath, S.; Taminiau, T. H.; van Hulst, N. F.; Quidant, R. Phys. Rev. Lett. 2008, 101, 116805. (21) Wissert, M. D.; Ilin, K. S.; Siegel, M.; Lemmer, U.; Eisler, H.-J. Nanoscale 2010, 2, 1018−1020. (22) Habteyes, T. G.; Dhuey, S.; Wood, E.; Gargas, D.; Cabrini, S.; Schuck, P. J.; Alivisatos, A. P.; Leone, S. R. ACS Nano 2012, 6, 5702− 5709. (23) Castro-Lopez, M.; Brinks, D.; Sapienza, R.; van Hulst, N. F. Nano Lett. 2011, 10, 4674−4678. (24) Ringe, E.; Langille, M. R.; Sohn, K.; Zhang, J.; Huang, J.; Mirkin, C. A.; van Duyne, R. P.; Marks, L. D. J. Phys. Chem. Lett. 2012, 3, 1479−1483. (25) Nordlander, P.; Oubre, C.; Prodan, E.; Li, K.; Stockman, M. I. Nano Lett. 2004, 4, 899−903. (26) Rothenberg, E.; Ebenstein, Y.; Kazes, M.; Banin, U. J. Phys. Chem. B 2004, 108, 2797−2800. (27) Yorulmaz, M.; Khatua, S.; Zijlstra, P.; Gaiduk, A.; Orrit, M. Nano Lett. 2012, 12, 4385−4391. (28) Wang, W.; Xu, J.; Liu, X.; Jiang, Y.; Wang, G.; Lu, X. Thin Solid Films 2000, 365, 116−118.
to be of second orderas often stated but not yet demonstrated. Due to the higher plasmon frequency of aluminum compared to other plasmonic materials, the resonance is shifted toward shorter wavelengths which makes aluminum a promising candidate for UV plasmonics. Furthermore, the resonance is much broader due to the large imaginary part of the dielectric function ε which can be useful if a broadband antenna operating at optical frequencies is desired. As the excitation laser in our experiments emits close to the interband transition peak of aluminum, one could think of testing a different excitation wavelength spectrally well apart from the interband absorption range to circumvent strong interband absorption and to potentially increase the radiative contribution of plasmon decay with enhanced efficiency.
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ASSOCIATED CONTENT
* Supporting Information S
Detailed fabrication and setup information with corresponding figures and SEM image of an aluminum nanoantenna on ITO. MPL rasterscans of coupled aluminum nanoantennas. Q factor discussion. Details on our numerical calculations and further simulations regarding scattering and absorption cross section. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The first author gratefully acknowledges support by the Karlsruhe School of Optics and Photonics (KSOP) and the Helmholtz International Research School for Teratronics (HIRST). The corresponding author gratefully acknowledges support through Deutsche Forschungsgemeinschaft under DFG EI 442/3-1. U.L. and M.S. are partly supported by the Center for Functional Nanostructures (CFN).
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REFERENCES
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