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Squeezing Terahertz Light into Nanovolumes: Nanoantenna Enhanced Terahertz Spectroscopy (NETS) of Semiconductor Quantum Dots Andrea Toma, Salvatore Tuccio, Mirko Prato, Francesco De Donato, Andrea Perucchi, Paola Di Pietro, Sergio Marras, Carlo Liberale, Remo Proietti Zaccaria, francesco de angelis, Liberato Manna, Stefano Lupi, Enzo Di Fabrizio, and Luca Razzari Nano Lett., Just Accepted Manuscript • Publication Date (Web): 25 Nov 2014 Downloaded from http://pubs.acs.org on November 25, 2014

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Squeezing Terahertz Light into Nanovolumes: Nanoantenna Enhanced Terahertz Spectroscopy (NETS) of Semiconductor Quantum Dots Andrea Toma1, Salvatore Tuccio1,2, Mirko Prato1, Francesco De Donato1, Andrea Perucchi3, Paola Di Pietro3, Sergio Marras1, Carlo Liberale1,4, Remo Proietti Zaccaria1, Francesco De Angelis1, Liberato Manna1, Stefano Lupi5, Enzo Di Fabrizio4,6, Luca Razzari2,* 1

Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy INRS Énergie, Matériaux et Télécommunications, 1650 Blvd Lionel Boulet, J3X 1S2 Varennes (Québec), Canada 3 INSTM UdR Trieste-ST and Sincrotrone Trieste, Area Science Park, Basovizza, 34012 Trieste, Italy 4 Biological and Environmental Science and Engineering (BESE) division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia 5 CNR-IOM and Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale A. Moro 2, I-00185, Roma, Italy 6 Physical Science and Engineering (PSE) division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia 2

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ABSTRACT: Terahertz spectroscopy has vast potentialities in sensing a broad range of elementary excitations (e.g., collective vibrations of molecules, phonons, excitons, etc.). However, the large wavelength associated with terahertz radiation (about 300 µm at 1 THz) severely hinders its interaction with nano-objects, such as nanoparticles, nanorods, nanotubes and large molecules of biological relevance, practically limiting terahertz studies to macroscopic ensembles of these compounds, in the form of thick pellets of crystallized molecules or highly concentrated solutions of nanomaterials. Here we show that chains of terahertz dipole nanoantennas spaced by nanogaps of 20 nm allow retrieving the spectroscopic signature of a monolayer of cadmium selenide quantum dots, a significant portion of the signal arising from the dots located within the antenna nanocavities. A Fano-like interference between the fundamental antenna mode and the phonon resonance of the quantum dots is observed, accompanied by an absorption enhancement factor greater than one million. NETS can find immediate applications in terahertz spectroscopic studies of nanocrystals and molecules at extremely low concentrations. Furthermore, it shows a practicable route towards the characterization of individual nano-objects at these frequencies. KEYWORDS: terahertz spectroscopy, surface enhancement, nanoantenna, quantum dot, phonon resonance, Fano-like interference


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Improving the sensitivity of terahertz (THz) spectroscopy is a long-standing challenge that basically deals with enhancing the interaction of THz radiation with the specimen under investigation. Waveguide-assisted THz sensing1 implements this strategy by increasing the effective interaction length up to several centimeters by means of a guided geometry (typically in a parallel plate configuration). This technique has been successfully employed for a variety of spectroscopic investigations on thin layers of biomolecules2, explosives3 and drugs4, to name but a few examples. However, these layers need to be deposited throughout the whole length of the waveguide to effectively exploit the augmented interaction. In a similar manner, THz sensing using spoof plasmons (i.e. bound electromagnetic modes on corrugated metal surfaces) also necessitates the sensed substance to be deposited along the entire propagation path of the THz surface wave5. Conversely, metamaterial-based THz sensors6,7 have proven to be particularly effective in sensing thin (down to sub-micrometer) films of various materials, exploiting a characteristic narrowband resonance, whose frequency position is particularly sensitive to changes in the dielectric constant of the surrounding environment. This makes metamaterials excellent refractive-index sensors, but not suitable for a complete characterization aiming at identifying the spectroscopic signatures of the investigated analyte. Recently, a new approach based on nanoslits (i.e. nano-apertures on a thin metallic layer8,9) resonating at THz frequencies has been proposed10. By exploiting the field enhancement within the slit, detection of small molecules at the nanogram-level has been demonstrated. The presence of the molecules results in a reduced THz transmission of the slit when its resonance is matched to a particular molecular absorption band. Nonetheless, this technique requires an exact and a priori knowledge of the absorption properties of the investigated specimen and does not retrieve information regarding the main spectroscopic features of the sample (in terms of absorption peak position and bandwidth). Surface enhancement is a powerful concept that has been widely employed to increase the sensitivity of traditional spectroscopies11. First observed and exploited in Raman studies12, it has brought to the development of a groundbreaking technique such as SERS (Surface Enhanced Raman Spectroscopy)13, which allows spectroscopic investigations down to the single-molecule level14-16. Similarly, SEIRA (Surface Enhanced InfraRed


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Absorption)17,18 has shown to have a clear potential for enhancing infrared spectroscopy19-21 and was proven to be effective in sensing molecules with sensitivities down to the sub-attomolar level22. The central idea underlying surface enhancement stems from the fact that both Raman and direct absorption spectroscopies are sensitive to the local electric field, which can be greatly amplified on rough metal surfaces or in close proximity of properly-shaped metallic nanostructures. In this work, we make use of nanoengineered metallic structures to exploit localized surface enhancement in the THz spectral region. In particular, the use of resonant dipole nanoantennas we propose herein allows for the retrieval of the spectroscopic response of the investigated samples, thanks to the broad resonance properties of these nanoplasmonic structures, requiring only a coarse spectral alignment with the sample absorption features. In addition, it offers a unique three-dimensional localization of THz radiation, which enables unprecedented THz studies within nanovolumes.

Nanoantenna arrays for enhanced THz spectroscopy. It is well known that metallic nanoantennas can efficiently convert free-propagating radiation into strongly localized fields23. This effect arises from the resonant coupling between the incoming light and the collective excitation (plasmon) of the conduction electrons of the metal. The simplest nanoantenna design consists of a metallic nanorod with a length approximately equal to half of the effective wavelength of the exciting radiation24. This configuration, which realizes a resonant half-wavelength dipole nanoantenna, localizes the electric field into two nanoscale “hot-spots” placed respectively at the structure surface ends. Resonant nanoantennas have been realized and investigated over a wide region of the electromagnetic spectrum from the ultra-violet down to the THz range25-27. In order to further improve the enhancement of the local electric field, a different geometrical arrangement can be exploited. In fact, by moving from a single nanoantenna to nanostructures coupled end-to-end through a narrow gap, it is possible to generate a higher and strongly localized electric field inside the gap26. Fig. 1a shows an exemplified schematic of the nanoantenna arrangement we have employed in our investigation. Arrays of gold dipole nanoantennas covering an area of 5x5 mm2 were prepared on high-


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resistivity silicon substrates using e-beam lithography28 (for fabrication details, see Supporting Information). The nanoantenna height and width were fixed at h = 60 nm and w = 200 nm respectively. THz “hot-spots” featuring high field enhancement were obtained by coupling the nanoantennas along their long-axis, in chains separated by nanogaps of nominal width of 20 nm (Gx in Fig. 1a; see SEM image in Fig. 1d).

Figure 1. a, Sketch of the THz nanoantenna array and definition of geometrical antenna parameters. b, Transmission Electron Microscope (TEM) image of the synthetized CdSe QDs. c, THz transmittance of a 100 nm thick layer of CdSe QDs. d, Scanning Electron Microscope (SEM) detail of a nanogap region (upper panel); two-dimensional surface plot of the field amplitude enhancement factor F around the gap region at the QD resonance frequency, for L=8um and Gy=14um (lower panel). e, F in the center of the nanogap as a function of frequency, for two arrays with different design characteristics.

To prove the potentials of this type of device for ultrasensitive THz spectroscopy, we have selected cadmium selenide (CdSe) quantum dots (QDs) as test-bed nano-objects, since they are endowed with a clear phonon resonance in the THz frequency range29. Furthermore, they represent an excellent model system for our investigation, due to the fact that they can be prepared with extreme precision in size and shape and are known to form a compact and uniform layer with accurate thickness control. CdSe QDs with an average diameter of 5.2 nm were chemically synthetized using a well-established


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protocol30 (for further details, see Supporting Information). Fig. 1b shows a TEM image of the dots, evidencing good size uniformity. The THz response of the QDs was characterized by means of Fourier Transform Spectroscopy31 in a transmission configuration (see Supporting Information). Fig. 1c presents the THz transmittance of a 100 nm thick layer of QDs drop-cast on a bare silicon substrate: a clear absorption resonance centered at around 5.65 THz is evidenced (with a transmission change of about 3.5%), which can be ascribed to the optical phonon resonance (Fröhlich mode) of CdSe QDs29. In order to tune the resonance of our nanoantenna arrays to the QD phonon resonance, we have performed extensive three-dimensional electromagnetic simulations (Supporting Information), employing both the length L of the nanoantennas and their spacing Gy in the y direction as tuning parameters. In fact, on one hand the nanostructure length directly determines the resonance of the single nanoantenna, and can thus be used to spectrally shift the response of the whole array. On the other hand, the array spacing has shown to be a sensitive parameter for resonance engineering22,32. In practice, this spacing modifies the mutual interaction between the nanoantennas promoting, through their in-phase coupling, a collective excitation that can bring to resonance shift and narrowing (Supporting Information), and a higher field enhancement. Fig. 1e shows two representative examples of how the array geometry can be engineered to match the nanoantenna response to the QD resonance. The displayed quantity is the field (amplitude) enhancement factor F in the center of the gap, being F(x0,y0,z0) defined as the ratio of the local electric field at position (x0,y0,z0) in the presence of the nanoantennas to the field in the same position considering a bare substrate with no nanoantennas. The effective squeezing of THz radiation into nanometric volumes is highlighted in Fig. 1d (lower part), which reports the two-dimensional plot of F around the gap region under resonant conditions. In the center of the nanogap, values of F at resonance of about a thousand and higher were found for all the arrays considered in this work. An extremely high value of the near field enhancement is a fundamental requirement for successful applications in enhanced THz spectroscopy of nanomaterials. In fact, in the close proximity of the gap, the usually small effective absorption cross-section of a nano-object at THz frequencies can be greatly amplified (up to more than a million times in our case), since it scales with || 23.


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Figure 2. a, Transmittance of the array with L = 8µm and Gy = 14µm, for THz polarization set along the nanoantenna long axis (i.e. the x axis, being y the perpendicular axis on the sensing plane) and without QDs over the surface (green curve); transmittance of the same array covered with a QD monolayer, for THz polarization set along the y (black curve) and x (red curve) axis. b, Pictorial representation of a monolayer of QDs covering a nanogap region. c, SEM image of a nanogap area covered with QDs (a two-dimensional map of the field enhancement is superimposed to the image, to give an idea of the field distribution over the sensing surface). d, Further magnification of the gap region, highlighting the uniformity of the QD layer.

NETS measurements of a monolayer of CdSe QDs. The green curve in Fig. 2a shows the transmittance of one of the fabricated arrays (L = 8 µm and Gy = 14 µm) as a function of frequency, when the polarization of THz light was set parallel to the long axis of the nanoantennas (illumination geometry exciting the plasmon resonance27,28): a clear dip was revealed as the signature of the array resonance, located in proximity of the QD phonon resonance position (black dotted line in Fig. 2a). A compact monolayer of CdSe QDs was then spin-coated on this array (Fig. 2b-d) and subsequently transmittance measurements were again taken on the sample and are reported in Fig. 2a. When the polarization of THz light was set perpendicular to the long axis of the nanoantennas (nonresonant illumination, black curve) the transmission was found to be the same of a bare silicon substrate and the presence of the QDs could not be detected. This is consistent with the QD response reported in Fig. 1c for a 100nm thick layer, since for such thin layers the relative transmission change (i.e. the difference in transmittance between the reference silicon substrate and an equivalent substrate covered with the tested layer)


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goes linearly with the layer thickness d ( 1 , where α is the layer attenuation coefficient), and thus a transmission change of less than 0.2% would be expected for a monolayer, which is below the sensitivity of our experimental setup. When instead the sample was resonantly illuminated, a modified array response arose (red curve), highlighting a particular spectral feature over the array resonance, an apparent local peak in correspondence of the QD phonon resonance. This Fano-like behavior33 is the result of the interference between the nanoantenna (plasmonic) mode and the QD phonon resonance and is similar to the one traditionally observed in SEIRA measurements19-22.

Figure 3. a, Experimental transmittance of nanoantenna arrays covered with a monolayer of CdSe QDs, for different antenna lengths and array spacings (fc is the central frequency of the phonon resonance). b, Corresponding numerical simulations.

To further clarify the nature and behavior of this plasmon-phonon coupling, a similar set of experiments was performed on arrays featuring different plasmon resonance frequencies. Figure 3a shows the results of this investigation: as one can see, the resonance hybridization clearly appears for all the tested samples, thus demonstrating that only a coarse alignment between the interacting phonon and plasmon resonances is required to observe the phenomenon. Moreover, regardless of the mutual position of the two resonant modes, the spectral position of the interference peak always displays in correspondence of the vibrational frequency of CdSe QDs (black dashed line), further hinting at the generation of a Fano-like interference. The observed plasmon-phonon coupling has been additionally investigated by means of numerical simulations. To


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simplify the simulated structure and significantly reduce the required meshing elements and thus the computational time, we have considered a uniform layer with thickness equal to the QD diameter (5.2 nm). The permittivity of the layer was taken from Ref. 34, which reports the experimental characterization of the THz response of CdSe QDs with a diameter (6.3 nm) similar to the one investigated in our work. Regardless of these approximations, the results of the simulations (see Fig. 3b) well reproduce the overall behavior of the experimental measurements and further support our findings. Absorption enhancement. The results presented in Fig. 2a and Fig. 3a shows that our particular implementation of NETS allows sensing a monolayer of CdSe QDs, through the formation of a Fano-like resonance that has a clear visibility and corresponds to a spectral feature in the array transmittance equivalent to a local peak. This is made possible by a significant absorption enhancement induced by the nanoantennas. We can again make use of numerical simulations to quantitatively estimate this effect. The overall array absorption enhancement at the QD phonon resonance frequency can be in fact evaluated by calculating the surface integral of || at resonance and dividing it by the total sensing area A = 25 mm2:

!

∬$%$ ,, " #

.

&

(1)

For the case of the array whose characterization is presented in Fig. 2a (L = 8 µm and Gy = 14 µm), we find: 70, a significant value considering that the nanoantenna covering factor (defined as the ratio of the area covered by the nanoantennas to the total area of the array) is only 1.4%. Following a similar procedure, we can also evaluate in which proportion the QDs located within the nanogaps contribute to the overall absorption. This can be done by comparing the surface integral of || taken over the area covered by the nanogaps with the one taken over the entire array: *+,+-., *$%$

!

∬+,+-., ,, " # !

∬$%$ ,, " #

.

By considering again the array with L = 8 µm and Gy = 14 µm, we find:

(2) *+,+-., *$%$

0.52,

meaning that about half of the THz absorption in the monolayer takes place in the


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nanogaps of the array. This result is of particular significance, since the gaps cover an area that is about 30,000 times smaller than the overall sensing surface and each gap contains roughly 130 QDs (visual estimate from SEM images; 170 assuming a flat monolayer and the densest – hexagonal – possible packing). Modeling the observed Fano-like interference with a mechanical analog. In addition to numerical simulations, a direct and straightforward analytical model can be applied to describe the observed resonance interference with a deeper physical insight. In fact, a Fano-like interference can be modeled classically by considering a system composed of two coupled harmonic oscillators35,36. In our particular case, the first oscillator, with resonance frequency 1 223 and damping 4 22Δ3 (Δ3 being the full width at half maximum of the resonance), represents the nanoantenna (plasmonic) mode, while the second oscillator, with resonance frequency 1 223 and damping 4 22Δ3 represents the phonon mode. The two systems are coupled through a term proportional to a coupling constant g. Since the phonon mode of the QD monolayer is weakly excited by the far-field radiation, we consider only the first (plasmonic) oscillator to be excited directly by an external driving force (6 789 ). Under this approximation, the equations of motion are: ": + 4 "< + 1 " + =" 6 789 ,

(3)

" + =" 0. ": + 4 "< + 1

(4)

The displacements are harmonic (", >, 789 ) and thus the amplitude of the plasmonic oscillator can be written as: >

! 8 ?7@ 8A8!

! ?7@ 8A8! DB8! ?7@ 8A8! DAE! B8C C

6.

(5)

Figure 4 shows how this simple model can properly reproduce the main characteristics of our experimental results, presenting in the response of the plasmonic oscillator a clear antiresonant feature in correspondence of the QD phonon resonance. More generally, we highlight the fact that a direct comparison between this model and NETS experimental results can be used to extract the main spectroscopic properties of the specimen under


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investigation, in terms of absorption peak position and bandwidth. This has been done in Fig. 4 for the case of the array with L = 8 µm and Gy = 10 µm. As summarized in Table 1, the values employed in the fitting procedure for the phonon resonance frequency (νph = 5.72 THz) and bandwidth (∆νph = 1.12 THz) are in good agreement with the experimental ones that can be directly extracted from Fig. 1c.

Figure 4. Absolute value squared of the plasmonic oscillator amplitude > as a function of frequency (blue curve) and normalized extinction efficiency σext (extracted from the experimental transmittance T as F9 ∝ 1 ) of a nanoantenna array covered with QDs, with L = 8µm and Gy = 10µm (full red circles).

NETS

CdSe QD

Nanoantenna

measurement

absorption peak

array resonance

νph

5.72

5.64

/

∆νph

1.12

1.15

/

νpl

6.08

/

6.03

∆νpl

2.4

/

2.39

TABLE 1. Fitting NETS measurements with the coupled harmonic oscillator model: The second column reports the best-fit parameters used in Fig. 4 (g = 121 ps-2). The corresponding values that can be obtained from a Lorentzian fit of the QD absorption peak (Fig. 1c) and of the extinction resonance of the nanoantenna array with L = 8µm and Gy = 10µm (Fig. 1Sb in Supporting Information) are reported for comparison in the third and fourth column respectively. All values in the table are expressed in THz.


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In conclusion, we have shown that nano-engineered arrays of THz nanoantennas coupled through narrow gaps allow performing THz spectroscopy of a monolayer of CdSe QDs. This is made possible by the direct coupling of the nanoantenna (plasmonic) mode and the phonon resonance of the QDs, which results in the formation of an evident Fano-like interference centered at the phonon resonance frequency. The overall surface enhancement effect is the result of the strong near fields that are generated around the nanoantennas at resonance, with an absorption enhancement that can reach values greater than one million in the center of the nanogaps. Furthermore, we have verified that a simple mechanical model of the Fano-like interference can be used to extract the main spectroscopic characteristics (absorption peak frequency and bandwidth) of the investigated sample. Beside straightforward applications in ultra-sensitive THz spectroscopy of tiny quantities of molecules and nano-compounds, we envision the possibility of utilizing this technique to characterize individual nano-objects. This can be achieved by employing a diffraction-limited illumination of a single resonant nanostructure (such as a dimer with a single nanogap) and by further engineering the near field response, for example using a smaller gap and thinner dipolar nanoantennas. Such an optimized geometry could not only allow the linear characterization of individual nano-objects avoiding population averaging and inhomogeneous broadening, but could also enable unprecedented nonlinear THz studies (like the generation of THz harmonics37 and other high-field investigations) within a nanovolume. ASSOCIATED CONTENT Supporting Information available: Details of numerical simulations, resonance engineering of terahertz nanoantenna arrays, fabrication of the arrays, CdSe quantum dots synthesis, monolayer preparation, and terahertz extinction measurements. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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ACKNOWLEDGEMENT L. R. is grateful for financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Fonds de Recherche du Québec – Nature et technologies (FRQNT).


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REFERENCES (1)

Gallot, G.; Jamison, S.P.; McGowan, R.W.; Grischkowsky, D. J. Opt. Soc. Am. B

2000, 17, 851-863. (2)

Laman, N.; Harsha, S.S.; Grischkowsky, D.; Melinger, J.S. Biophys. J. 2008, 94,

1010-1020. (3)

Melinger, J.S.; Laman, N.; Grischkowsky, D. Appl. Phys. Lett. 2008, 93, 011102.

(4)

Laman, N.; Harsha, S.S.; Grischkowsky, D. Appl. Spectrosc. 2008, 62, 319-326.

(5)

Ng, B.; Hanham, S.M.; Wu, J.; Fernández-Domínguez, A.I.; Klein, N.; Liew,

Y.F.; Breese, M.B.H.; Hong, M.; Maier, S.A. ACS Photonics 2014, 1, 1059-1067. (6)

O’Hara, J.F.; Singh, R.; Brener, I.; Smirnova, E.; Han, J.; Taylor, A.J.; Zhang, W.

Opt. Express 2008, 16, 1786-1795. (7)

D’Apuzzo, F.; Candeloro, P.; Domenici, F.; Autore, M.; Di Pietro, P.; Perucchi,

A.; Roy, P.; Sennato, S.; Bordi, F.; Di Fabrizio, E.M.; Lupi, S. Plasmonics, 2014, DOI 10.1007/s11468-014-9775-3. (8)

Seo, M.A.; Park, H.R.; Koo, S.M.; Park, D.J.; Kang, J.H.; Suwal, O.K.; Choi,

S.S.; Planken, P.C.M.; Park, G.S.; Park, N.K.; Park, Q.H.; Kim, D.S. Nature Photon. 2009, 3, 152-156. (9)

Chen, X.; Park, H.-R.; Pelton, M.; Piao, X.; Lindquist, N.C.; Im, H.; Kim, Y.J.;

Ahn, J.S.; Ahn, K.J.; Park, N.; Kim, D.-S.; Oh, S.-H. Nature Comm. 2013, 4, 2361. (10)

Park, H.-R.; Ahn, K.J.; Han, S.; Bahk, Y.-M.; Park, N.; Kim, D.-S. Nano Lett.

2013, 13, 1782-1786. (11)

Aroca, R. Surface-Enhanced Vibrational Spectroscopy; John Wiley & Sons: The

Atrium, Southern Gate, Chichester, West Sussex, 2006. (12)

Fleischmann, M.; Hendra, P.J.; McQuillan, A.J. Chem. Phys. Lett. 1974, 26, 163-

166. (13)

Kneipp, K.; Moskovits, M.; Kneipp, H. Surface-Enhanced Raman Scattering:

Physics and Applications; Springer-Verlag: Berlin, Heidelberg, 2006.


Page 14 of 17

Page 15 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(14)

Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L.T.; Itzkan, I.; Dasari, R.R.; Feld,

M.S. Phys. Rev. Lett. 1997, 78, 1667-1670. (15)

Nie, S.; Emory S.R. Science 1997, 275, 1102-1106.

(16)

Chirumamilla, M.; Toma, A.; Gopalakrishnan, A.; Das, G.; Proietti Zaccaria, R.;

Krahne, R.; Rondanina, E.; Leoncini, M.; Liberale, C.; De Angelis, F.; Di Fabrizio, E. Adv. Mater. 2014, 26, 2353-2358. (17)

Hartstein, A.; Kirtley, J.R.; Tsang, J.C. Phys. Rev. Lett. 1980, 45, 201-204.

(18)

Osawa, M.; Ataka, K.-I.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47,

1497-1502. (19)

Neubrech, F.; Pucci, A.; Cornelius, T.W.; Karim, S.; García-Etxarri, A.; Aizpurua,

J. Phys. Rev. Lett. 2008, 101, 157403. (20)

D'Andrea, C.; Bochterle, J.; Toma, A.; Huck, C.; Neubrech, F.; Messina, E.;

Fazio, B.; Maragò, O.M.; Di Fabrizio, E.; Lamy de La Chapelle, M.; Gucciardi, P.G.; Pucci, A. ACS Nano 2013, 7, 3522-3531. (21)

Huck, C.; Neubrech, F.; Vogt, J.; Toma, A.; Gerbert, D.; Katzmann, J.; Härtling,

T.; Pucci, A. ACS Nano 2014, 8, 4908-4914. (22)

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, 19227-19232. (23)

Novotny L.; van Hulst, N. Nature Photon. 2011, 5, 83-90.

(24)

Cubukcu, E.; Capasso, F. Appl. Phys. Lett. 2009, 95, 201101.

(25) Knight, M.W.; Liu, L.; Wang,Y.; Brown, L.; Mukherjee, S.; King, N.S.; Everitt, H.O.; Nordlander, P.; Halas, N.J. Nano Lett. 2012, 12, 6000-6004. (26)

Biagioni, P.; Huang, J.-S.; Hecht, B. Rep. Prog. Phys. 2012, 75, 024402.

(27) Razzari, L.; Toma, A.; Shalaby, M.; Clerici, M.; Proietti Zaccaria, R.; Liberale, C.; Marras, S.; Al-Naib, I.A.I.; Das, G.; De Angelis, F.; Peccianti, M.; Falqui, A.; Ozaki, T.; Morandotti, R.; Di Fabrizio, E. Opt. Express 2011, 19, 26088-26094.


Nano Letters

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(28) Razzari, L.; Toma, A.; Clerici, M.; Shalaby, M.; Das, G.; Liberale, C.; Chirumamilla, M.; Proietti Zaccaria, R.; De Angelis, F.; Peccianti, M.; Morandotti, R.; Di Fabrizio, E. Plasmonics 2013, 8, 133-138. (29) Vasilevskiy, M.I.; Rolo, A.G.; Artemyev, M.V.; Filonovich, S.A.; Gomes, M.J.M.; Rakovich, Y.P. Phys. Stat. Sol. (b) 2001, 224, 599-604. (30) Carbone, L.; Nobile, C.; De Giorgi, M.; Della Sala, F.; Morello, G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I.R.; Nadasan, M.; Silvestre, A.F.; Chiodo, L.; Kudera, S.; Cingolani, R.; Krahne, R.; Manna, L. Nano Lett. 2007, 7, 2942-2950. (31) Griffiths, P. R.; de Haseth, J.A. Fourier Transform Infrared Spectrometry; John Wiley & Sons: Hoboken, NJ, 2007. (32)

Zhou, W.; Odom, T.W. Nature Nanotech. 2011, 6, 423-427.

(33)

Fano, U. Phys. Rev. 1961, 124, 1866-1878.

(34)

Mandal, P.K.; Chikan, V. Nano Lett. 2007, 7, 2521-2528.

(35)

Joe, Y.S., Satanin, A.M.; Kim, C.S. Phys. Scr. 2006, 74, 259-266.

(36)

Gallinet, B.; Martin, O.J.F. Phys. Rev. B 2011, 83, 235427.

(37) Shubert, O.; Hohenleutner, M.; Langer, F.; Urbanek, B.; Lange, C.; Huttner, U.; Golde, D.; Meier, T.; Kira, M.; Koch, S.W.; Huber, R. Nature Photon. 2014, 8, 119-123.


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