Letter pubs.acs.org/NanoLett
All-Semiconductor Plasmonic Nanoantennas for Infrared Sensing Stephanie Law, Lan Yu, Aaron Rosenberg, and Daniel Wasserman* Department of Electrical and Computer Engineering, Micro and Nanotechnology Lab, University of Illinois Urbana−Champaign, 208 N. Wright St., Urbana, Illinois 61801, United States ABSTRACT: Infrared absorption spectroscopy of vibrorotational molecular resonances provides a powerful method for investigation of a wide range of molecules and molecular compounds. However, the wavelength of light required to excite these resonances is often orders of magnitude larger than the absorption cross sections of the molecules under investigation. This mismatch makes infrared detection and identification of nanoscale volumes of material challenging. Here we demonstrate a new type of infrared plasmonic antenna for long-wavelength nanoscale enhanced sensing. The plasmonic materials utilized are epitaxially grown semiconductor engineered metals, which results in high-quality, low-loss infrared plasmonic metals with tunable optical properties. Nanoantennas are fabricated using nanosphere lithography, allowing for cost-effective and large-area fabrication of nanoscale structures. Antenna arrays are optically characterized as a function of both the antenna geometry and the optical properties of the plasmonic semiconductor metals. Thin, weakly absorbing polymer layers are deposited upon the antenna arrays, and we are able to observe very weak molecular absorption signatures when these signatures are in spectral proximity to the antenna resonance. Experimental results are supported with finite element modeling with strong agreement. KEYWORDS: Midinfrared, plasmonics, antenna, sensing
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by adjusting the shape of the nanoparticle away from a perfect sphere, the LSPR can be pushed into the near-infrared portion of the spectrum.8−11 However, extending the LSPR further into the IR becomes challenging, as the permittivity of most traditional plasmonic metals at these wavelengths becomes increasingly large in magnitude, and the metal begins to more closely approximate a perfect electrical conductor (PEC).12 This can be seen in Figure 1b and c, where the response of 26 nm and 300 nm radius gold nanoparticles are modeled using the quasi-static approximation.4,13 At short wavelengths (Figure 1b), when the magnitude of the Au permittivity is close to that of the surrounding dielectric, a strong extinction coefficient as well as field enhancement near the Au nanoparticle can be achieved (though this effect is decreased somewhat by the large losses associated with interband transitions in Au near the plasma frequency). However, at longer wavelengths (Figure 1c), the large and negative value of the Au permittivity results in a weak, nonresonant, scattering from the Au nanoparticle, preventing strong field enhancement or subwavelength localization. However, the importance of the mid-IR for sensing applications has driven continued efforts to enhance the interaction of long wavelength light with molecular species, even if subwavelength plasmonic structures utilizing traditional plasmonic metals cannot be used. One approach has been to use microantennas fabricated from traditional metals to localize
nfrared (IR) spectroscopy offers a powerful tool for analysis and characterization of a wide range of molecular species and compounds via their distinct vibrational and rotational absorption resonances, often referred to as molecular fingerprints.1 However, the wavelength of light at these infrared frequencies is significantly larger than the absorption crosssection of the molecules of interest, making absorption spectroscopy of thin layers or nanoscale volumes challenging. At shorter wavelengths, such as in the near-IR and visible range, a similar but less extreme mismatch between wavelength and sample volume has been tackled by use of plasmonic nanoparticles.2,3 It has been known for some time that ultrasubwavelength metallic particles (usually fabricated from the noble metals Ag or Au) can support localized surface plasmon resonances (LSPR).4 At resonance, these nanoparticles concentrate electric fields in the near-field of the nanoparticle, as shown in Figure 1b. When the molecules of interest coat or chemically attach to the nanoparticles, this field enhancement results in an enhancement of the interaction of the incident light with the molecular species and thus an enhanced absorption signal. It is for this reason that plasmonic nanoparticles are playing an important role in next-generation sensing architectures.5−7 However, the picture changes rather dramatically at longer wavelengths. The expression for the localized surface plasmon resonance of an isolated spherical metallic nanoparticle results in a resonance when the magnitude of the metal permittivity (|εm|) is twice that of the surrounding dielectric matrix (εd), or εm = −2εd, taking into account the negative sign of the metal permittivity. By changing the surrounding dielectric matrix, or © XXXX American Chemical Society
Received: July 25, 2013 Revised: August 26, 2013
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Figure 1. (a) Real (solid) and imaginary (dotted) parts of the permittivity for Au (blue) and heavily doped InAs (red) in the visible and mid-IR wavelength ranges, respectively. Quasi-static calculations of electric field profiles for spherical nanoparticles: (b) Au (r = 26 nm) at resonance (λ = 520 nm), (c) Au (r = 300 nm) at λ = 6 μm, and (d) doped InAs (r = 300 nm) at resonance (λ = 6 μm). (e) Scattering cross sections as a function of wavelength for r = 26 nm Au and r = 300 nm InAs nanoparticles, normalized to the particles’ spherical cross-sectional area. Au permittivity taken from Johnson and Christy,12 InAs permittivity calculated using the Drude formalism, with λp = 5.5 μm and Γ = 2 × 1013 s−1, extracted from experimental measurements of as-grown doped InAs layers.
mid-IR light to small volumes.14,15 Such antennas have subsequently been used to couple incident mid-IR radiation to absorbing molecules.16−18 This work has resulted in significant improvements in sensitivity for absorption spectroscopy measurements resulting from the field concentration obtained by the miniature antenna structures, sometimes coupled with grating resonances originating from the periodicity of the antenna array.17 However, though these antennas are sometimes referred to as plasmonic, the resonant wavelength of an isolated antenna at mid-IR wavelengths scales almost linearly with antenna length, a result of the quasi-PEC nature of traditional metals in this wavelength range.19 Thus, the observed resonances are closer to those of a traditional antenna than the localized plasmonic resonances utilized at shorter wavelengths. The wavelength-scale antenna length requirement imposes very real limitations on the antenna array density and the achievable field enhancement of such structures. To replicate at long wavelengths the ultrasubwavelength field localization achievable with traditional plasmonic metals at short wavelengths, it is therefore not simply the structures’ size which must be scaled, but the metals’ permittivity that must also be translated to longer wavelengths. In essence, one must “engineer” mid-IR plasmonic metals to behave at long wavelengths similarly to traditional plasmonic metals at short wavelengths. This can be achieved by utilizing heavily doped semiconductors. The optical response of free carriers in a doped semiconductor can be modeled by the Drude formalism. In such materials, high doping concentrations result in a plasma frequency in the infrared, and this plasma frequency can be tuned by adjusting the doping density. Highly doped semiconductors can therefore mimic, at long wavelengths, the optical properties of traditional plasmonic metals at shorter wavelengths, as seen in Figure 1a. In fact, because doped semiconductors do not have the strong interband absorption of traditional metals near their plasma frequencies, these engineered metals, in many respects, offer a “purer” plasmonic metal with stronger resonant effects than their short wavelength plasmonic counterparts such as Au, as is demonstrated in Figure 1b−e.
Plasmonic behavior in highly doped semiconductors has been observed in n- and p-doped silicon,20,21 as well as in epitaxially grown InAs.22,23 In the latter work, resonant absorption was observed in optically thick microparticles and attributed to the excitation of LSPR modes in the doped semiconductor microparticles. The use of doped semiconductors as plasmonic materials opens the mid-IR to a variety of plasmon-based applications which are not possible using traditional metals. Moreover, the use of epitaxially grown mid-IR plasmonic metals offers a material system with high carrier mobility and single-crystal quality, as well as sharp interfaces and atomic-level control of layer thicknesses for multilayered plasmonic structures. Finally, semiconductorbased plasmonic materials also offer access to the wide range of available semiconductor fabrication and processing techniques and expertise, as well as the potential for integration with mid-IR semiconductor optoelectronic devices.24 In this work we demonstrate that, in addition to supporting LSPR resonances, properly designed semiconductor plasmonic structures can act as subwavelength nanoantennas, collecting incident long-wavelength light and enhancing the light’s interaction with thin-film layers of absorbing molecules. Our antenna arrays are characterized optically as a function of antenna geometry, as well as the optical properties of the constituent plasmonic material. We demonstrate that such structures hold promise as high-density, subwavelength sensing arrays for enhancement of roto-vibrational resonances in nanovolumes of molecular species. Samples were grown by molecular beam epitaxy in an SVT Associates reactor on semi-insulating GaAs substrates. An initial buffer layer (1.3 μm) of undoped InAs is first grown, to isolate the defects induced by the lattice mismatched growth on GaAs far from the doped InAs layer. Following the buffer layer, a 200 nm layer of silicon-doped InAs is grown. Two films (samples A and B) were grown with different doping densities and, consequently, different plasma wavelengths. Undoped epitaxial InAs samples (sample C) were fabricated simultaneously with the doped material, to act as control samples. The films were then patterned using nanosphere lithography (NSL), allowing for large-area and low-cost nanopatterning of our antenna arrays.24 A schematic of the NSL process is shown B
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in Figure 2a−e. The NSL samples were first coated with 70 nm of SiO2 deposited with plasma enhanced chemical vapor
exact plasma wavelength and scattering time. This is because the penetration depth of mid-infrared light is around 1 μm for the highly doped InAs, and thus the spectral features of the thin (200 nm) n+ InAs layer are not as pronounced as they would be for thicker layers. We therefore used our COMSOL model to fine-tune the plasma wavelength of our n+ InAs layers so that the spectral position of the simulated reflection peaks match those of the experimentally observed peaks. The scattering times used in the simulations were obtained from reflectivity measurements for thicker films grown under similar conditions. The underlying undoped InAs in samples A and B, and the entirety of sample C, was modeled as a lossless dielectric with ϵs = 12.3. As can be seen in Figure 2g, rather than producing straight sidewalls, the ion milling resulted in sloped sidewalls. This effect is accounted for in our simulations and was found to shift the position of the resonance but did not change overall trends in our data. Figure 3a−c shows the
Figure 2. (a−e) Process flow for fabrication of doped semiconductor nanoantennas using nanosphere lithography. (a) Nanosphere deposition over sample coated with SiO2 etch mask, (b) resizing of nanospheres in O2 plasma, (c) CHF3 RIE etch of oxide with nanospheres as the etch mask, (d) ion milling of InAs using a combination nanosphere/SiO2 etch mask, and (e) CF4 RIE removal of oxide etch mask and nanospheres. (f) Top view and (g) 60° scanning electron micrographs of fabricated InAs nanoantennas (r ∼ 217 nm) following step e.
deposition (PECVD). Next, 500 nm polystyrene nanospheres (NS) in methanol were applied to the sample which was then spun at 600 rpm for 10 s, increased to 1500 rpm for 10 s, and finally 2000 rpm for 40 s. The NS were then resized in an O2 plasma, and the SiO2 between the spheres was etched away using a CHF3 reactive ion etch (RIE). The samples were then placed in an ion mill, and approximately 250 nm of InAs was removed using a beam voltage of 250 V and a beam current of 63 mA. The NS and SiO2 masks were then removed in a CF4 RIE, leaving the nanopatterned InAs. Scanning electron microscope (SEM) images of the NSL samples are shown in Figure 2f and g. While the NS arrays do not have long-range order (order exists for areas approximately 10 μm × 10 μm, much smaller than the sampling area), the array periodicity (Λ) is significantly smaller than the wavelengths of interest (λo = 10−11 μm) and should not play a dominant role in the optical properties of the patterned surfaces. Furthermore, scattering from the fabricated nanopillars on the undoped InAs samples (sample C) should be minimal and nonresonant, again due to the large discrepancy between the wavelengths of interest and the array periodicity. The NSL technique allows for low-cost, large-area nanoantenna fabrication, resulting in highly uniform nanoantennas, which gives the fabricated nanoantenna arrays significant potential for use as enhanced sensing substrates. Our structures were modeled numerically in two-dimensions using the COMSOL multiphysics module.27 The doped top layers were modeled as Drude metals with plasma wavelengths (λp,A = 5.5 μm, λp,B = 6.2 μm) and scattering times (ΓA = ΓB = 2 × 1013 s−1) for samples A and B, respectively, as extracted from a combination of reflectivity measurements on the unpatterned, as-grown wafers and finite element modeling of the patterned structures. For the unpatterned films, modeling the reflectivity data with a transfer matrix approach provides a coarse estimate of our doped InAs layer’s optical properties, rather than an
Figure 3. Two-dimensional modeled electric field amplitudes for r = 200 nm and Λ = 550 nm InAs nanoantenna from sample A (λp,A = 5.5 μm, ΓA = 2 × 1013 s−1) at two resonant wavelengths (a) λ = 6 μm and (b) λ = 9.8 μm. (c) Modeled reflection (solid lines) and nanoparticle absorption (dotted lines) for samples A (blue) and B (red) (r = 200 nm, Λ = 550 nm).
electric field amplitude profiles of sample A for two wavelengths of interest in our COMSOL models, as seen in the reflection and absorption spectra shown in Figure 3d. The (weak) short wavelength resonance (λ = 6 μm) corresponds to the LSPR of a doped InAs nanoparticle surrounded by air, with remarkably similar spectral features to the idealized nanoparticle modeled analytically in Figure 1. The strong and broad reflection peak at λ = 9.8 μm corresponds to the doped InAs LSPR near the undoped InAs. Reflection spectra from the fabricated nanoantenna samples were obtained using a Bruker IRII microscope coupled to a Bruker V80v Fourier transform infrared (FTIR) spectrometer. Reflection spectra were normalized to reflection from a gold surface, to remove the spectral dependence of the FTIR’s internal broadband source and detector. Transmission spectra C
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immediately obvious in our experimental data. However, this resonance is predicted to be much weaker than the primary, long wavelength reflection peak which is the focus of this work. At these shorter wavelengths, a greater penetration of the incident light into the sample is expected, resulting in a stronger influence of the highly defective layer at the GaAs/InAs interface (not included in our simulations) on our observed spectra. This, in combination with its spectral proximity to Fabry−Perot features from the undoped InAs layer, may result in the air/InAs LSPR being somewhat masked in our experimental results. For both samples, increasing the nanoantenna radius to 215 nm results in a slight redshift in the observed reflection peak, as can be seen in Figure 4b and c. When the puck radius is decreased to 165 nm, the peaks for samples A and B shift to shorter wavelengths (9.0 and 9.85 μm, respectively). This reflection peak redshift with increasing sample diameter has been observed in noble metal nanoparticles in the visible region8 and is replicated in our simulations. In fact, the reflection spectra for the fabricated doped InAs nanoparticles and their dependence on nanoparticle size very closely resemble those of scaled nanoparticles fabricated from traditional plasmonic materials at short wavelengths, suggesting that these structures are in fact mimicking the LSP resonances long associated with traditional plasmonic particles in the visible and near-IR regions. As can be seen in Figure 4b and c, the intensity of the reflection peak varies among the samples, an effect we attribute partly to fabrication-related variations between samples. However, it is expected that smaller nanoantennas will give weaker reflection signals (as the scattering cross section depends on the nanoantenna area) and will also suffer more from surface damage during fabrication due to their larger surface area/volume ratio, so it is not entirely surprising that the smaller nanoantennas seem to uniformly show weaker resonances. The r = 200 nm nanoantennas from samples A, B, and C were coated in a thin (∼50 nm) layer of polymethylmethacrylate (PMMA) by spinning a heavily diluted solution of 20 mg/ mL PMMA in chloroform on the samples at 2000 rpm for 60 s, followed by baking for 60 s at 125 °C. PMMA is an ideal coating for determining the sensitivity of our nanoantennas, as it can be spun on in very thin layers, and because it has numerous weak absorption lines across the spectral range of our nanoantenna resonances. A cross-sectional SEM image of PMMA-coated nanoantennas is shown in Figure 5b. The thickness of the PMMA on top of the pucks is approximately 50 nm, and the PMMA appears to have at least partially filled in the trenches between the pucks. Reflection spectra for all three samples were obtained as described previously. As shown in Figure 5a, coating the samples with PMMA causes the scattering peak to redshift, a result of the change in the local index of refraction surrounding the nanoantenna. For sample A, the shift is 71 cm−1, while for sample B, it is 44 cm−1. The spectral shift in a plasmonic resonance resulting from an index change in the surrounding dielectric matrix is a highly sensitive and well-established technique for sensing applications.5−7,28 A typical plasmonic sensing system will prepare the plasmonic surface to bind to the analyte under investigation. Upon binding, the local refractive index will change in a very small volume surrounding the particle, with the magnitude of the shift dependent on the analyte concentration in the nearfield of the plasmonic surface. Such a sensing system relies on both chemical (analyte binding) and optical (refractive index
were collected as well, though the magnitude of the transmitted signal is small with weak signal-to-noise, due to large losses from the defects at the GaAs/InAs interface.22 A broad, strong peak in reflection is seen near 9.8 μm for sample A with r = 200 nm and Λ = 550 nm, while sample B, patterned with the same geometry, shows a peak near 11 μm, as shown in Figure 4a.
Figure 4. (a) Experimental (solid) and modeled (dotted) reflection spectra for r = 200 nm nanoantennas fabricated from sample A (blue, λp,A = 5.5 μm, ΓA = 2 × 1013 s−1) and sample B (red, λp,B = 6.2 μm, ΓB = 2 × 1013 s−1). Note that the modeled spectra are scaled to demonstrate the strong spectral agreement of our experimental and modeled data. The modeled reflection peaks, however, are somewhat stronger than those observed experimentally. Experimental reflection spectra for (b) sample A and (c) sample B for varying nanoantenna radii. The reflection of the nanopatterned (r = 200 nm) undoped InAs (sample C) is shown for comparison in both b and c.
Experimental data is compared to the 2D modeling results with strong agreement in the spectral location of the reflection resonance. It should be noted, however, that the magnitude of the experimentally observed resonance is somewhat weaker than the simulated structures. The weaker experimental signal may be a result of fabrication-related damage to the InAs material, which may induce additional losses in the doped InAs, or at the etch floor of the sample, which are not accounted for in the simulations. Our model also assumes 100% areal coverage of our surface with nanoantennas. In fact, over the measurement area (∼1 mm × 1 mm), nanoantenna coverage is less than 100%, due to the imperfect nature of the nanosphere lithography process, an effect which would presumably weaken the experimental reflection peak observed from our samples. Finally, the use of a 2D, as opposed to a full three-dimensional model, might account for the slight discrepancy between our modeled and experimentally observed data. The short wavelength air/InAs LSPR identified in our simulations is not D
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nanoantennas fabricated from samples A, B, and C. All spectra were taken with a resolution of 8 cm−1, over 4000 scans, and corrected to the same gold reflection data. The scattering resonance of sample A occurs near 950 cm−1 and does not overlap strongly with the PMMA lines under investigation, so a significant sensing enhancement is not expected. Indeed, though we are able to observe the two medium strength resonances (752 and 842 cm−1) fairly easily, the very weak resonances (810 and 827 cm−1) show up only as slight inflections on the falling edge of the sample A reflection peak and are difficult to discern. Similarly, we observe peaks from the two medium strength PMMA absorption lines in our reflection from sample C but see no features larger than the spectral noise at the energies corresponding to the very weak PMMA lines. However, for sample B, whose scattering resonance aligns nicely with the very weak PMMA lines at 810 cm−1 and 827 cm−1, we are able to observe the features from both the medium strength as well as the very weak lines. This is due to the electric field enhancement between the nanoantennas at the scattering resonance, which allows more of the incident light to interact with the nanovolume of PMMA and demonstrates the utility of this material system for sensing small volumes of materials with even weak roto-vibrational absorption features. Far from any scattering resonance, the PMMA lines show up well for all three samples, as evidenced by Figure 5d. This indicates that all three samples have similar PMMA coatings and suggests that the scattering resonance and concurrent electric field enhancement is indeed playing a large part in the observation of the very weak PMMA lines. We have demonstrated the ability to observe very weak molecular absorption lines using heavily doped semiconductor nanoantennas at their scattering resonance and have been able to resolve the presence of nanoscale volumes of material via ultraweak absorption resonances in the material. Nonetheless, the observed effect of the absorbing material is weak. This could be due to many factors, including etch-induced material damage, nanoantenna size, and overall geometry. Though the unpatterned material is of high quality and has a low scattering rate, the nanoantennas are created by ion milling, which could damage the sidewalls of the nanoantennas and lead to a smaller scattering response and a decreased interaction of the nanoantennas with the absorbing medium. This could be overcome by using an anisotropic wet chemical etch (such as a metal-assisted chemical etch31,32) which would minimize etch damage or, perhaps, by annealing the sample post ion mill. By creating the nanoantennas using NSL, we were able to pattern a large area quickly and at low cost. However, in doing so, we sacrificed some control over both size and shape of the nanoantennas. Their diameter can be no larger than the diameter of the available NS (in this case, 500 nm), and their shape is, necessarily, circular. As sharp edges and corners concentrate electric fields, it is likely that moving from circular antennas to nanorods or nanotriangles would further concentrate the electric field in the antennas’ near-field and, therefore, increase our sensing capabilities. In addition, it is expected that interantenna separation will play a role in the potential enhancement of the sensing capabilities of nanoantenna arrays, where “hot-spots” resulting from strong field confinement between antennas provide enhanced interaction with molecular resonances. Again, nanosphere lithography is not the ideal fabrication technique for such an investigation, due to the limited control over array and individual nanoantenna geometries. Future work will investigate the
Figure 5. (a) Normalized reflection spectra for sample A (blue) and sample B (red) before (dotted) and after (solid) coating in a thin PMMA layer. (b) Cross-sectional scanning electron micrograph of r = 200 nm nanopillars coated in PMMA, showing a thin (50 nm) layer above the nanoantennas. Reflection spectra for r = 200 nm samples A (blue), B (red), and C (gray) coated in PMMA (c) in the range of the sample B antenna resonance and (d) far from either sample’s antenna resonance. The spectrum of sample C in c and d has been shifted vertically (but not scaled) to aid in viewing.
shift) processes. However, strong and broadband absorption of almost all fluids in the mid-IR, as well as the technologically mature nature of the above-described plasmonic sensors at shorter wavelengths, gives little incentive for translating this already established technology to longer wavelengths. However, the mid-IR does offer the ability to detect the chemical composition of materials by direct measurement of absorption from roto-vibrational molecular resonances. Such a detection scheme would be all-optical, label-free, and have the potential to identify molecules via their distinct absorption spectra. We investigate two sets of PMMA absorption lines: the first cover the reflection resonance of the r = 200 nm sample B nanoantennas and are expected at 752 cm−1 (C−C stretch, medium strength), 810 cm−1 (very weak), 827 cm−1 (C−16O− C stretch, very weak), and 842 cm−1 (CH2 rocking, medium strength).29 The second group of absorption lines lie far from the nanoantenna resonances and are expected at 1150 cm−1 and 1194 cm−1 (both very strong and associated with PMMA ester groups). The coated sample C is expected to behave similarly to a thin layer of absorbing material (PMMA) deposited upon a dielectric surface. If each PMMA absorption feature is modeled as a harmonic oscillator, the imaginary permittivity of the absorbing layer takes on a Lorentzian line shape. The resulting reflection spectra from the two-layer system will have peaks at the PMMA resonant frequencies. For the nanoantenna structures, the situation is slightly different. Here, one can model the nanoantenna and the absorbing layer as coupled harmonic oscillators, which will result in a dip in the reflection spectra when the nanoantenna and absorbing materials’ resonances overlap.30 Figure 5c shows reflection spectra across the first set of absorption lines for the r = 200 nm coated E
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(13) Jackson, J. D. Classical Electrodynamics, 3rd ed.; Wiley: New York, 1976. (14) Olmon, R. L.; Krenz, P. M.; Jones, A. C.; Boreman, G. D.; Raschke, M. B. Opt. Express 2008, 16, 20295. (15) Yi, F.; Zhu, H.; Reed, J. C.; Cucukcu, E. Nano Lett. 2013, 13, 1638. (16) Neubrech, F.; Pucci, A.; Cornelius, T. W.; Karim, S. Phys. Rev. Lett. 2008, 101, 157403. (17) Adato, R.; Yanik, A. A.; Amsden, J. J.; Kaplan, D. L.; Omenetto, F. G.; Hong, M. K.; Erramilli, S.; Altug, H. Proc. Natl. Acad. Sci. 2009, 106, 19227. (18) Aksu, S.; Yanik, A. A.; Adato, R.; Artar, A.; Huang, M.; Altug, H. Nano Lett. 2010, 10, 2511. (19) Novotny, L. Phys. Rev. Lett. 2005, 95, 266802. (20) Ginn, J. C.; Jarecki, R. L.; Shaner, E. A.; Davids, P. S. J. Appl. Phys. 2011, 110, 043110. (21) Shahzad, M.; Medhi, G.; Peale, R. E.; Buchwald, W. R.; Cleary, J. W.; Soref, R.; Boreman, G. D.; Edwards, O. J. Appl. Phys. 2011, 110, 123105. (22) Law, S.; Adams, D. C.; Taylor, A. M.; Wasserman, D. Opt. Express 2012, 20, 12155. (23) Law, S.; Yu, L.; Wasserman, D. J. Vac. Sci. Technol., B 2013, 31, 03C121. (24) Li, D.; Ning, C. Z. Opt. Express 2011, 19, 14594. (25) Yu, L.; Law, S.; Wasserman, D. Appl. Phys. Lett. 2012, 101, 103105. (26) Deckman, H. W.; Dunsmuir, J. H. Appl. Phys. Lett. 1982, 41, 377−379. (27) http://www.comsol.com (accessed July 24, 2013). (28) Homola, J. Chem. Rev. 2008, 108, 462. (29) Dirlikov, S.; Koenig, J. L. Appl. Spectrosc. 1979, 33, 551. (30) Mason, J. A.; Allen, G.; Podolskiy, V. A.; Wasserman, D. IEEE Photon. Technol. Lett. 2012, 24, 31. (31) Li, X.; Bohn, P. W. Appl. Phys. Lett. 2000, 77, 2572. (32) DeJarld, M. T.; Shin, J. C.; Chern, W.; Chanda, D.; Balasundaram, K.; Rogers, J. A.; Li, X. Nano Lett. 2011, 11, 5259. (33) Ellis, D. I.; Broadhurst, D.; Goodacre, R. Anal. Chim. Acta 2004, 514, 193. (34) Chan, K. L.; Kazarian, S. G. J. Combin. Chem. 2005, 7, 185−189. (35) Rustichelli, C.; Gamberini, G.; Ferioli, V.; Gamberini, M. C.; Ficarra, R.; Tommansini, S. J. Pharm. Biol. Anal. 2000, 23, 41−45. (36) Walsh, M. J.; Reddy, R. K.; Bhargava, R. IEEE Sel. Top. Quant. Electron. 2012, 18, 1502. (37) Bauer, C.; Sharma, A. K.; Willer, U.; Burgmeier, J.; Braunschweig, B.; Schade, W.; Blaser, S.; Hvozdara, L.; Muller, A.; Holl, G. Appl. Phys. B: Laser Opt. 2008, 92, 327−333. (38) Willer, U.; Saraji, M.; Khorsandi, A.; Geiser, P.; Schade, W. Opt. Lasers Eng. 2006, 44, 699−710.
dependence of the scattering resonance strength and sensing enhancement on the size, shape, periodicity, and separation of the nanoantennas. The ability to detect distinct roto-vibration resonances is a fundamental requirement for IR sensing applications in industries including, but not limited to, food safety,33 pharmaceutical development and monitoring,34,35 health and biosensing,36 security and defense,37 and industrial process control.38 As technologies continue to shrink in size, it is increasingly important to be able to perform such sensing with nanovolumes of materials. However, the plasmonic sensors which have found purchase at shorter wavelengths are not easily translated to longer wavelengths, due to the optical properties of their constituent plasmonic metals. Here we demonstrate that patterned doped semiconductors can act as plasmonic nanoantennas with strongly localized optical modes in the near-field of the nanoantennas. This material system is particularly attractive as it allows for single crystal, low-loss, and perhaps most importantly, engineered plasmonic metals, all of which are difficult, if not impossible, to achieve with traditional plasmonic metals at short wavelengths. Furthermore, we demonstrate that our nanoantennas, patterned to a length scale smaller than λo/20, can detect very weak absorption resonances in nanoscale volumes of absorbing material deposited over the nanoantennas. This work provides evidence for the potential of these novel IR antennas for long wavelength sensing applications and offers a potential route toward highly sensitive nanosensing with micrometer-scale light.
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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 authors gratefully acknowledge support from NSF DMR1210398 (D.W. and A.R.), the AFOSR Young Investigator Program FA9550-10-1-0226 (D.W.), and ECCS-CAREER 1157933 (L.Y.).
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REFERENCES
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