:: Repositorio Institucional


Título:	*Experimental verification of the spectral shift between near- and far-field peak intensities of plasmonic infrared nanoantennas*
Fuente:	Physical Review Letters 110, 203902 (2013)
Autor/es:	Alonso-González, P.; Albella, P.; Neubrech, F.; Huck, C.; Chen, J.; Golmar, F.; Casanova, F.; Hueso, L.E.; Pucci, A.; Aizpurua, J.; Hillenbrand, R.
Materias:	Antenas; Nanotecnología; Nanoestructuras; Microscopía; Rayos infrarrojos; Espectroscopía
Editor/Edición:	American Physical Society; 2013
Licencia:
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PRL 110, 203902 (2013)



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Experimental Veriﬁcation of the Spectral Shift between Near- and Far-Field Peak Intensities of Plasmonic Infrared Nanoantennas



P. Alonso-Gonza´lez,1 P. Albella,1,2 F. Neubrech,3 C. Huck,3 J. Chen (陈佳宁),1,2 F. Golmar,1,4 F. Casanova,1,5 L. E. Hueso,1,5 A. Pucci,3 J. Aizpurua,2 and R. Hillenbrand1,5,* 1CIC nanoGUNE, 20018 Donostia–San Sebastia´n, Spain

2Centro de F´ısica de Materiales (CSIC-UPV/EHU) and Donostia International Physics Center (DIPC), 20018 Donostia–San Sebastia´n, Spain

3Kirchhoff Institute for Physics, University of Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany 4I.N.T.I.-CONICET, Avenida General Paz 5445, Ediﬁcio 42, B1650JKA, San Mart´ın, Bs As, Argentina 5IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain (Received 16 August 2012; published 14 May 2013)

Theory predicts a distinct spectral shift between the near- and far-ﬁeld optical response of plasmonic

antennas. Here we combine near-ﬁeld optical microscopy and far-ﬁeld spectroscopy of individual

infrared-resonant nanoantennas to verify experimentally this spectral shift. Numerical calculations

corroborate our experimental results. We furthermore discuss the implications of this effect in surface-

enhanced infrared spectroscopy.



DOI: 10.1103/PhysRevLett.110.203902



PACS numbers: 42.79.Gn, 42.65.Wi, 73.20.Mf, 78.67.Bf



When a metal nanostructure is illuminated by light, the excitation of surface plasmons yields strongly concentrated optical ﬁelds at the metal surface, often referred to as ‘‘hot spots’’ [1]. Metallic nanostructures can be thus considered as effective optical nanoantennas for converting propagating plane waves into localized ﬁelds [2]. This antenna function enables the control of electromagnetic ﬁelds at the nanometer scale [3] and thus has promoted the development of a vast variety of applications including surface-enhanced Raman scattering spectroscopy (SERS) [4–7], surface-enhanced infrared spectroscopy (SEIRS) [8,9], antenna-enhanced ultrafast nonlinear spectroscopy [10], near-ﬁeld microscopy [11–13], and novel photodetection schemes [14,15].

To explore new antenna functionalities or to optimize the antenna performance, different antenna designs have been developed [16–26]. A major goal is to achieve the highest local ﬁeld enhancement, sensitivity, and tunability of the antennas’ optical response. These properties are essentially determined by the spectral position and width of the antenna resonance, which are typically studied by far-ﬁeld spectroscopy [27]. However, surface-enhanced spectroscopies rely on light-matter interaction in the near ﬁeld of the antenna, where an object is exposed to high ﬁeld intensities in ultrasmall volumes. Early theoretical studies [28] and recent publications [29–34] predict and indicate [35] that the spectral near- and far-ﬁeld response of the antennas are shifted against each other, which might have implications for the application and optimization of optical antennas. Particularly, this spectral shift has been indicated in SERS [35] and antenna-mediated ﬂuorescence [36] studies. However, inelastic scattering experiments present several difﬁculties such as the inherent difference between excitation and scattered frequencies or the chemical bonding and charge transfer between molecules and



metal nanostructures, preventing a rigorous experimental veriﬁcation of the shift. Additionally, SERS experiments usually rely on measurements of samples exhibiting heterogeneously distributed hot spots of random ﬁeld enhancement, which forces a statistical evaluation of the enhancement. In this work we circumvent these difﬁculties by measuring the local near ﬁelds of single antennas with a scattering-type near-ﬁeld microscope, thus avoiding any inherent frequency shift between incident and scattered light or any chemical bonding or charge transfer between the probe and the antenna. Particularly, we study the fundamental dipolar mode of antennas resonant in the midinfrared spectral range and subsequently discuss implications for surface- and antenna-enhanced infrared spectroscopy.

The spectral shift between near-ﬁeld intensity INF and far-ﬁeld extinction Iext at the fundamental plasmon resonance is demonstrated in Fig. 1 for linear dipole antennas. We show numerical (ﬁnite difference in time domain, Lumerical Solutions) calculations of the nearand far-ﬁeld optical response of 40 nm high and 140 nm wide Au rods of varying length L on a CaF2 substrate. In Fig. 1(a) we display the spectral positions of the nearﬁeld peak intensity and the far-ﬁeld extinction peak for different antenna lengths L. They were obtained by calculating INF at the antenna extremity [marked by the red cross in the inset of Fig. 1(a)] and Iext ¼ Iin À Itrans, respectively, as a function of the illumination wavelength . Iin is the incident intensity and Itrans is the calculated transmitted intensity in the far ﬁeld of the antennas. The polarization of the incident light was parallel to the antenna axes. A spectral shift between the near-ﬁeld and far-ﬁeld peak intensities is observed throughout the whole spectral range from visible to midinfrared wavelengths. At a ﬁxed antenna length L, the near-ﬁeld peak is



0031-9007= 13=110(20)=203902(6)



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(a)



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Si tip



Iin INF



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Itrans



Iext

x INF



(b)



(c)



(b)



(a)



Iin



FIG. 2 (color online). Illustration of the detection schemes employed for measuring the near- and far-ﬁeld response of individual infrared antennas: (a) s-SNOM, and (b) infrared microspectroscopy.



FIG. 1 (color online). Numerical study of the near-ﬁeld intensity INF and far-ﬁeld extinction Iext of linear dipole Au antennas on a CaF2 substrate. (a) Spectral positions of the near-ﬁeld peak intensity and the far-ﬁeld extinction peak. The lines are guides

to the eye. The inset illustrates where the near-ﬁeld intensity and

the far-ﬁeld extinction were evaluated. (b) Near-ﬁeld intensity INF and far-ﬁeld extinction Iext as a function of wavelength  for an antenna length L ¼ 3:1 m [spectra along the vertical dashed line in (a)]. (c) Near-ﬁeld intensity INF and far-ﬁeld extinction Iext as a function of nanorod length L for a ﬁxed illumination wavelength  ¼ 9:3 m [spectra along the horizontal solid line

in (a)]. The spectra in (b) and (c) were normalized to their

maximum values.



large in the case of strong plasmon damping, for example in plasmonic Ni antennas [33] or in strongly scattering antennas [37].

Here we experimentally verify the shift between nearand far-ﬁeld peak intensities for midinfrared antennas. To that end, we measure at a ﬁxed wavelength both the near-ﬁeld intensity [Fig. 2(a)] and far-ﬁeld extinction [Fig. 2(b)] of individual antennas of varying length L, thus tracing the resonances according to Fig. 1(c). This approach is used because quantitative near-ﬁeld data are readily obtained by recording a single near-ﬁeld image [Fig. 3(a)] of an antenna set where L is systematically varied [38–40]. Furthermore, in applications such as in SEIRS, the antenna length is the essential parameter to



(a)



A



B



Max



shifted to a longer wavelength compared to the far-ﬁeld extinction maximum. This shift is shown in more detail in Fig. 1(b), where we plot Iext and INF along the vertical dashed line in Fig. 1(a) (antenna length L ¼ 3:1 m). Considering a ﬁxed illumination wavelength , the nearﬁeld peak appears at smaller L compared to the far-ﬁeld extinction. We illustrate this effect in Fig. 1(c) by plotting Iext and INF along the horizontal line in Fig. 1(a) ( ¼ 9:3 m).

Based on Mie theory for small spheres, the shift between near- and far-ﬁeld peak intensities was already studied by Messinger et al. in the 1980s [28]; however, only recently this phenomenon has been intuitively explained by describing metallic antennas as classically driven harmonic oscillators [32,33]. When the harmonic oscillator is damped (which can be associated with dissipation in the antenna and with scattering losses), the maximum oscillation amplitude (which can be associated with the near-ﬁeld amplitude) appears at lower frequencies than the maximum dissipation (which can be associated with the far-ﬁeld absorption). While the spectral position of the maximum dissipation does not depend on the damping, the maximum oscillation amplitude generally shifts to lower energies with increasing damping. This explains why spectral shifts between near- and far-ﬁeld peak intensities can be quite



C



D



0



(b)



E 5µm



FIG. 3 (color online). Experimental near- and far-ﬁeld study of

linear infrared dipole antennas. (a) s-SNOM image of the nanoantennas at  ¼ 9:3 m. From the top to the bottom and from the left to the right, the antenna length increases from L ¼ 2 m to L ¼ 4:4 m. The cross indicates the position where the near-ﬁeld intensities displayed in Fig. 4 have been measured. (b) Far-ﬁeld extinction spectra of the antennas marked in Fig. 3(a) by the letters A-E. The vertical dashed line indicates the wavelength  ¼ 9:3 m, where the extinction values displayed in Fig. 4 have been extracted.



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be matched to the ﬁxed vibrational resonance of the mole-

cules under study [9,41].

The antennas are Au rods of 40 nm height, 140 nm width, and a length varying from L ¼ 2 m to L ¼ 4:4 m, fabricated by electron beam lithography on a CaF2 substrate. The distance between the different antennas is 10 m, which allows for measuring a far-ﬁeld extinction spectrum of each individual antenna, as well

as for recording a single near-ﬁeld image of the whole

antenna set [Fig. 3(a)]. We thus obtain for each individual

antenna both near-ﬁeld intensity and far-ﬁeld extinction,

allowing for a quantitative comparison of the two quantities as a function of the antenna length L.

Near-ﬁeld imaging [Fig. 2(a)] is performed with a side-

illumination scattering-type scanning near-ﬁeld optical microscope (s-SNOM, from Neaspec GmbH). A Si tip oscillating vertically at frequency  is used for locally scattering the antenna near ﬁelds [21,33,38,39,42–44]. Both tip and antenna are illuminated with s-polarized infrared light from a CO2 laser at an angle of 50 from the surface normal. The light scattered by the tip is

recorded with a pseudoheterodyne interferometer [45].

By locating a polarizer in front of the detector, we select

the horizontally polarized scattered light. Demodulation of the detector signal at a higher-harmonic frequency n yields background free near-ﬁeld signals [21,43]. We note that by (i) illuminating the antennas with s-polarized light and (ii) detecting the s-polarized backscattered light, the demodulated amplitude signal sn yields the square of the local near-ﬁeld amplitude [39], sn ¼ INF.

Far-ﬁeld spectroscopy [Fig. 2(b)] is performed with an

infrared microscope (Bruker Hyperion 1000) coupled to a

Fourier transform spectrometer (Bruker Tensor 27), yield-

ing infrared extinction spectra of the same set of individual

antennas. The antennas are illuminated with thermal radia-

tion (polarization parallel to the long axis of the antenna) of intensity Iin under normal incidence. To address individual antennas, the illumination is through a 10 Â 10 m size aperture. The transmitted light Itrans is recorded with a resolution of 8 cmÀ1, yielding far-ﬁeld extinction spectra Iext ¼ Iin À Itrans. These are normalized to the extinction measured at least 30 m away from the antennas (reference spectrum).

Figure 3(a) shows the near-ﬁeld image of the antenna set,

revealing the typical dipolar mode pattern for each antenna

(two bright spots, indicating the strongly concentrated near

ﬁelds at the rod extremities) [42]. The near-ﬁeld signal

increases with increasing antenna length, until it reaches its maximum at L%3:1 m (marked by position C). With further increasing antenna length, the near-ﬁeld signal

decreases. This observation clearly reveals the resonance

behavior of the antennas [46]. In Fig. 3(b) we show the far-

ﬁeld extinction spectra of the individual antennas marked in

Fig. 3(a). We observe how the far-ﬁeld resonance (extinction

maximum) shifts to longer wavelengths when the antenna

length increases, following the typical behavior of dipole

antennas [9].



To compare the near-ﬁeld and far-ﬁeld optical responses of the antennas, we plot in Fig. 4 both INF and Iext as a function of the antenna length L for the ﬁxed illumination wavelength  ¼ 9:3 m. The near-ﬁeld intensities have been extracted from Fig. 3(a) at the extremities of the antennas where the maximum value is obtained (the cross in Fig. 3(a) marks the typical position). We point out that we study the fundamental antenna mode, which implies that the near-ﬁeld spectrum is the same at every point on the surface of the antenna. The extinction has been extracted from the individual far-ﬁeld extinction spectra of the antennas, some of them shown in Fig. 3(b). We clearly see in Fig. 4 that the near-ﬁeld and far-ﬁeld peak intensities are shifted against each other. The far-ﬁeld extinction maximum occurs at L ¼ 3:30 m (marked by the black dashed line in Fig. 4), while the near-ﬁeld maximum appears at a shorter antenna length of L ¼ 3:10 m (marked by the red dashed line in Fig. 4). These experimental results indeed verify the shift between the calculated near-ﬁeld intensity spectrum INF (red line in Fig. 4, obtained at the antennas’ extremities as in Fig. 1) and the calculated far-ﬁeld extinction spectrum Iext ¼ Iin À Itrans (black curve in Fig. 4, obtained in the far ﬁeld of the antennas as in Fig. 1) of the antennas. We note that no ﬁtting of the numerical results has been applied, just normalization of all the curves to their maximum values. We also note that the use of dielectric tips together with the s-polarized sample illumination has been shown to faithfully measure the spectral near-ﬁeld response of plasmonic antennas without introducing spectral shifts [47–49]. We thus can exclude that the signiﬁcant spectral shift between the near- and far-ﬁeld response of the antennas is introduced by the tip.

FIG. 4 (color online). Experimental and calculated near-ﬁeld intensity INF (red) and far-ﬁeld extinction Iext (black) as a function of the antenna length. The experimental near-ﬁeld intensities were measured at the extremity of the individual nanoantennas, at the position indicated by a cross in Fig. 3(a). The experimental far-ﬁeld extinction values were extracted from the far-ﬁeld spectra at  ¼ 9:3 m, as indicated by a vertical dashed line in Fig. 3(b). The red solid line INF shows the numerically calculated near-ﬁeld intensity at the rod extremity [as indicated by the inset in Fig. 1(a)]. The black solid line Iext shows the numerically calculated total extinction evaluated in the far ﬁeld of the antennas. Both experimental and calculated data were normalized to the corresponding maximum values.



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The shift between near- and far-ﬁeld peak intensities may

have important implications for sensing applications, as the

optical interaction between molecules and antennas is

mediated by the near ﬁeld. The spectroscopic information

about the molecules, however, is measured in the far ﬁeld.

In order to elucidate the inﬂuence of the shift in antenna-

enhanced infrared extinction spectroscopy, we performed

a numerical study of the near- and far-ﬁeld response of

molecules in the vicinity of infrared antennas. We consider

Au antennas covered with a 20-nm-thick layer of polymethyl methacrylate (PMMA) molecules on a CaF2 substrate. PMMA is used as an example because of its

well-deﬁned infrared vibrational resonance. Figure 5(a) shows the calculated extinction IePxMt MA of PMMA-coated antennas of different lengths L (black curves). With increasing L, the antenna resonance shifts to longer wavelengths, whereas the vibrational response of PMMA appears ﬁxed at  ¼ 5:8 m, independently of L. Importantly, the spectral PMMA response is enhanced when it is close to the antenna

resonance. Simultaneously, its line shape is modiﬁed, which

results from the Fano-like interference of both infrared

resonances [9,41,50–52]. To isolate and quantify the

antenna-enhanced spectral response of PMMA, we show in Fig. 5(b) the difference spectra C ¼ IePxMt MA À IeÃxt, where IeÃxt [green curves in Fig. 5(a)] is the resonance of the antenna when covered by a 20-nm-thick reference layer with a dielectric value "Ã ¼2:1 (baseline subtraction). We now deﬁne the ‘‘ﬁngerprint contrast’’ ÁC as the difference



between the maximum and the minimum values of each individual spectrum C [illustrated by the schematics in Fig. 5(b)]. In Fig. 5(c) we show ÁC for the different antenna lengths L. We ﬁnd that the largest ﬁngerprint contrast is obtained for L ¼ 1:7 m. By comparing ÁC with the near-ﬁeld intensity and far-ﬁeld extinction at the vibrational

resonance of PMMA [both shown in Fig. 5(d)], we interest-

ingly observe that the largest ﬁngerprint contrast is obtained when the near-ﬁeld intensity INPMF MA (rather than the far-ﬁeld extinction IePxMt MA) reaches its maximum. This can be understood when considering (i) that the molecular layer absorbs

energy from the antenna’s near ﬁeld rather from its far

ﬁeld and (ii) that the absorption of the molecules is much

stronger than their scattering. The spectral shift between

near- and far-ﬁeld peak intensities thus has to be considered

when optimizing antennas for spectroscopy applications. In Fig. 5(e) we compare the near-ﬁeld spectrum (INF, red

dashed line) and the far-ﬁeld spectrum (IePxMt MA, black solid line) of the 1.7-mm-long PMMA-coated antennas, where

the ﬁngerprint contrast in the far-ﬁeld extinction spectrum

is largest. We observe that the near-ﬁeld intensity peak of

the antenna matches the molecular vibration, rather than

the far-ﬁeld extinction maximum. To see this more clearly, we show the near-ﬁeld intensity INÃ F (red solid line) and far-ﬁeld extinction IeÃxt (green solid line). In addition to the spectral shift, we observe that the spectral shape of the

molecular ﬁngerprint also differs in the near-ﬁeld and far-

ﬁeld signals. We note that further extended experimental



(a)



I PMMA



ext



PMMA Au



L



CaF2



I* ext

Au



* CaF2



(b)



(c)



(d)



(e)



FIG. 5 (color online). Numerical study of antenna-enhanced extinction spectroscopy of polymethyl methacrylate molecules. (a) Left: Illustration of the spectroscopy conﬁguration and dielectric functions of the PMMA and reference layers, "PMMA and "Ã. Right: Extinction spectra of PMMA-coated Au antennas on a CaF2 substrate for different lengths L (IePxMt MA, black curves). The green curves IeÃxt show the extinction spectra of the antennas covered by a 20-nm-thick reference layer with a dielectric value "Ã. (b) Difference spectra C ¼ IePxMt MA À IeÃxt. (c) ‘‘Fingerprint contrast’’ ÁC as deﬁned in the scheme of (b). (d) Near-ﬁeld intensity (INPMF MA, red curve) and far-ﬁeld extinction (IePxMt MA, black curve) of PMMA-coated Au antennas as a function of antenna length L. The illumination wavelength is  ¼ 5:8 m, matching the vibrational resonance of the PMMA molecules. (e) Near-ﬁeld intensity (INPMF MA, red dashed curve) and far-ﬁeld extinction (IePxMt MA, black solid curve) of PMMA-coated Au antennas as a function of illumination wavelength . For comparison, we also show the near-ﬁeld intensity INÃ F (red solid line) and far-ﬁeld extinction IeÃxt (green solid line). The antenna length is L ¼ 1:7 m, corresponding to the maximum of ÁC in (c).

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and theoretical studies are needed for more detailed insights into this phenomenon, as well as to draw general conclusions regarding the optimal spectral contrast, since a variety of antenna designs, molecule vibrations, and physical spectroscopy processes (extinction, Raman scattering, ﬂuorescence) exists. Nevertheless, the canonical case of a dipolar antenna for SEIRS, as studied in this work, already stresses the importance and necessity of considering spectral shifts between near- and far-ﬁeld peaks, in order to optimize surface- and antenna-enhanced spectroscopies.

In conclusion, having performed experimental studies of individual infrared-resonant antennas by near-ﬁeld microscopy and far-ﬁeld extinction spectroscopy, we conﬁrm experimentally the spectral shift between the near- and far-ﬁeld peak intensities. Furthermore, we have studied numerically the implications of this spectral shift in SEIRS, showing that it has to be considered in order to optimize the molecular spectral absorption contrast in plasmonic (bio)sensing devices.

This work was supported by the European FP7 Project ‘‘Nanoantenna’’ (No. FP7-HEALTH-F5-2009-241818NANOANTENNA), the Spanish National Projects No. MAT2009-08398 and No. FIS2010-19609-C02-01, and the Basque Government Projects ETORTEK (nanoIKER) and excellence program IT756-13.

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