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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 18 — Sep. 9, 2013
  • pp: 21607–21617
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Probe-sample optical interaction: size and wavelength dependence in localized plasmon near-field imaging

Terefe G. Habteyes, Scott Dhuey, Karissa I. Kiesow, and Alexander Vold  »View Author Affiliations


Optics Express, Vol. 21, Issue 18, pp. 21607-21617 (2013)
http://dx.doi.org/10.1364/OE.21.021607


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Abstract

The probe-sample optical interaction in apertureless near-field optical microscopy is studied at 633 nm and 808 nm excitation wavelengths using gold nanodisks as model systems. The near-field distributions of the dipolar and quadrupolar surface plasmon modes have been mapped successfully using metal coated probes with different polarization combinations of excitation and detection except when the incident and the scattered light polarizations are chosen to be parallel to the probe axis. For the parallel polarization of the incident and the scattered light, the pattern of the near-field distribution differs from the inherent plasmon mode structures of the sample, depending sensitively on the sample size and excitation energy. For a given excitation energy, the near-field amplitude shifts from one pole to the other as the sample size increases, having nearly equal amplitude at the two poles when the plasmon resonance peak spectrally overlaps with the excitation energy.

© 2013 OSA

1. Introduction

Optical imaging with spatial resolution beyond the diffraction limit has long been the subject of active scientific research [1

1. D. Courjon, Near Field Microscopy and Near Field Optics (Imperial College Press, 2003).

,2

2. L. Novotny and B. Hecht, Principles of Nano-optics (Cambridge University Press, 2006).

]. Apertureless (scattering type) near-field scanning optical microscope (ANSOM) [3

3. Y. Inouye and S. Kawata, “Near-field scanning optical microscope with a metallic probe tip,” Opt. Lett. 19(3), 159–161 (1994). [CrossRef] [PubMed]

,4

4. F. Zenhausern, M. P. Oboyle, and H. K. Wickramasinghe, “Apertureless near-field optical microscope,” Appl. Phys. Lett. 65(13), 1623–1625 (1994). [CrossRef]

] has been demonstrated to image the near-field characteristics of materials including dielectric contrasts [5

5. B. Knoll and F. Keilmann, “Enhanced dielectric contrast in scattering-type scanning near-field optical microscopy,” Opt. Commun. 182(4-6), 321–328 (2000). [CrossRef]

8

8. Z. H. Kim, S. H. Ahn, B. Liu, and S. R. Leone, “Nanometer-scale dielectric imaging of semiconductor nanoparticles: Size-dependent dipolar coupling and contrast reversal,” Nano Lett. 7(8), 2258–2262 (2007). [CrossRef] [PubMed]

], infrared properties [9

9. R. Hillenbrand, T. Taubner, and F. Keilmann, “Phonon-enhanced light matter interaction at the nanometre scale,” Nature 418(6894), 159–162 (2002). [CrossRef] [PubMed]

11

11. J. M. Stiegler, Y. Abate, A. Cvitkovic, Y. E. Romanyuk, A. J. Huber, S. R. Leone, and R. Hillenbrand, “Nanoscale infrared absorption spectroscopy of individual nanoparticles enabled by scattering-type near-field microscopy,” ACS Nano 5(8), 6494–6499 (2011). [CrossRef] [PubMed]

] and plasmon modes [12

12. R. Hillenbrand, F. Keilmann, P. Hanarp, D. S. Sutherland, and J. Aizpurua, “Coherent imaging of nanoscale plasmon patterns with a carbon nanotube optical probe,” Appl. Phys. Lett. 83(2), 368–370 (2003). [CrossRef]

18

18. S. Mastel, S. E. Grefe, G. B. Cross, A. Taber, S. Dhuey, S. Cabrini, P. J. Schuck, and Y. Abate, “Real-space mapping of nanoplasmonic hotspots via optical antenna-gap loading,” Appl. Phys. Lett. 101(13), 131102 (2012). [CrossRef]

] with spatial resolution independent of the wavelength of light, resolving optical and spectroscopic properties on the order of 10 nm. The basic principle of ANSOM imaging involves focusing external laser light at the tip of atomic force microscope (AFM) probe and measuring the scattered light as the sample is raster scanned in close proximity to the tip. For a given sample and AFM operating parameters, the nature and strength of the probe-sample interaction depends on the radius of curvature of the probe tip [19

19. M. B. Raschke and C. Lienau, “Apertureless near-field optical microscopy: Tip-sample coupling in elastic light scattering,” Appl. Phys. Lett. 83(24), 5089–5091 (2003). [CrossRef]

], the material properties of the probe [16

16. A. García-Etxarri, I. Romero, F. García de Abajo, R. Hillenbrand, and J. Aizpurua, “Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes,” Phys. Rev. B 79(12), 125439 (2009). [CrossRef]

], the polarization of the excitation light with respect to the probe axis [20

20. O. J. F. Martin and C. Girard, “Controlling and tuning strong optical field gradients at a local probe microscope tip apex,” Appl. Phys. Lett. 70(6), 705–707 (1997). [CrossRef]

22

22. L. Aigouy, A. Lahrech, S. Grãsillon, H. Cory, A. C. Boccara, and J. C. Rivoal, “Polarization effects in apertureless scanning near-field optical microscopy: An experimental study,” Opt. Lett. 24(4), 187–189 (1999). [CrossRef] [PubMed]

] and the wavelength of the excitation light [23

23. A. Bouhelier and R. Bachelot, in Scanning Probe Microscopy, edited by S. Kalinin and A. Gruverman (Springer New York, 2007), 254–279.

]. The probe is most sensitive to the electric field of light that is parallel to the probe axis [20

20. O. J. F. Martin and C. Girard, “Controlling and tuning strong optical field gradients at a local probe microscope tip apex,” Appl. Phys. Lett. 70(6), 705–707 (1997). [CrossRef]

22

22. L. Aigouy, A. Lahrech, S. Grãsillon, H. Cory, A. C. Boccara, and J. C. Rivoal, “Polarization effects in apertureless scanning near-field optical microscopy: An experimental study,” Opt. Lett. 24(4), 187–189 (1999). [CrossRef] [PubMed]

], and earlier ANSOM experiments were based on this vertical (P) polarization of the excitation light. While the P-polarization affords detection of scattered light with high sensitivity, the excitation of the probe leads to strong probe-sample optical interactions, resulting in significant distortion of the intrinsic near-field distribution on the sample, particularly when a metallic probe is used [16

16. A. García-Etxarri, I. Romero, F. García de Abajo, R. Hillenbrand, and J. Aizpurua, “Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes,” Phys. Rev. B 79(12), 125439 (2009). [CrossRef]

].

When the electric field of the excitation light is oriented perpendicular to the probe axis [in-plane (S) polarization], the tip is expected to remain essentially unexcited regardless of the materials properties of the probe [20

20. O. J. F. Martin and C. Girard, “Controlling and tuning strong optical field gradients at a local probe microscope tip apex,” Appl. Phys. Lett. 70(6), 705–707 (1997). [CrossRef]

22

22. L. Aigouy, A. Lahrech, S. Grãsillon, H. Cory, A. C. Boccara, and J. C. Rivoal, “Polarization effects in apertureless scanning near-field optical microscopy: An experimental study,” Opt. Lett. 24(4), 187–189 (1999). [CrossRef] [PubMed]

]. The use of silicon probes with in-plane polarized excitation light while detecting the in-plane [24

24. M. Schnell, A. Garcia-Etxarri, J. Alkorta, J. Aizpurua, and R. Hillenbrand, “Phase-resolved mapping of the near-field vector and polarization state in nanoscale antenna gaps,” Nano Lett. 10(9), 3524–3528 (2010). [CrossRef] [PubMed]

26

26. P. Alonso-González, P. Albella, F. Golmar, L. Arzubiaga, F. Casanova, L. E. Hueso, J. Aizpurua, and R. Hillenbrand, “Visualizing the near-field coupling and interference of bonding and anti-bonding modes in infrared dimer nanoantennas,” Opt. Express 21(1), 1270–1280 (2013). [CrossRef] [PubMed]

] or out-of-plane [13

13. R. Esteban, R. Vogelgesang, J. Dorfmüller, A. Dmitriev, C. Rockstuhl, C. Etrich, and K. Kern, “Direct near-field optical imaging of higher order plasmonic resonances,” Nano Lett. 8(10), 3155–3159 (2008). [CrossRef] [PubMed]

,14

14. M. Rang, A. C. Jones, F. Zhou, Z. Y. Li, B. J. Wiley, Y. N. Xia, and M. B. Raschke, “Optical near-field mapping of plasmonic nanoprisms,” Nano Lett. 8(10), 3357–3363 (2008). [CrossRef] [PubMed]

,17

17. P. Alonso-Gonzalez, M. Schnell, P. Sarriugarte, H. Sobhani, C. H. Wu, N. Arju, A. Khanikaev, F. Golmar, P. Albella, L. Arzubiaga, F. Casanova, L. E. Hueso, P. Nordlander, G. Shvets, and R. Hillenbrand, “Real-space mapping of Fano interference in plasmonic metamolecules,” Nano Lett. 11(9), 3922–3926 (2011). [CrossRef] [PubMed]

,24

24. M. Schnell, A. Garcia-Etxarri, J. Alkorta, J. Aizpurua, and R. Hillenbrand, “Phase-resolved mapping of the near-field vector and polarization state in nanoscale antenna gaps,” Nano Lett. 10(9), 3524–3528 (2010). [CrossRef] [PubMed]

26

26. P. Alonso-González, P. Albella, F. Golmar, L. Arzubiaga, F. Casanova, L. E. Hueso, J. Aizpurua, and R. Hillenbrand, “Visualizing the near-field coupling and interference of bonding and anti-bonding modes in infrared dimer nanoantennas,” Opt. Express 21(1), 1270–1280 (2013). [CrossRef] [PubMed]

] vector components of the near-field has been demonstrated to result in near-field images of the plasmon modes with field distribution patterns consistent with theoretical predictions for the samples in the absence of the tip. However, there are only a few reports that suggest metal coated probes can be used to map plasmonic modes [15

15. D. S. Kim, J. Heo, S. H. Ahn, S. W. Han, W. S. Yun, and Z. H. Kim, “Real-space mapping of the strongly coupled plasmons of nanoparticle dimers,” Nano Lett. 9(10), 3619–3625 (2009). [CrossRef] [PubMed]

,27

27. R. L. Olmon, P. M. Krenz, A. C. Jones, G. D. Boreman, and M. B. Raschke, “Near-field imaging of optical antenna modes in the mid-infrared,” Opt. Express 16(25), 20295–20305 (2008). [CrossRef] [PubMed]

] and it still remains unclear to what extent the influence of metallic tips can be decoupled by polarization selection. More importantly, while excitation with in-plane polarized light is suitable for plasmon near-field imaging, systematic investigation of the probe-sample optical interactions, which is significant when the electric field of the excitation light is parallel to the probe axis, is lacking. The parallel (vertical) polarization produces a large enhancement of the field that is localized at the tip of a conical metallic probe, which is a desired characteristic for spectroscopic applications [28

28. E. J. Sànchez, L. Novotny, and X. S. Xie, “Near-field fluorescence microscopy based on two-photon excitation with metal tips,” Phys. Rev. Lett. 82(20), 4014–4017 (1999). [CrossRef]

34

34. N. Jiang, E. T. Foley, J. M. Klingsporn, M. D. Sonntag, N. A. Valley, J. A. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam, and R. P. Van Duyne, “Observation of multiple vibrational modes in ultrahigh vacuum tip-enhanced Raman spectroscopy combined with molecular-resolution scanning tunneling microscopy,” Nano Lett. 12(10), 5061–5067 (2012). [CrossRef] [PubMed]

]. Understanding the probe-sample optical interaction is crucial not only for mapping near-field distribution but also for the advancement of tip-enhanced spectroscopy and nanoscopy.

In this work, using lithographically fabricated gold nanodisks as model systems, it is first demonstrated that metal coated probes can be used to successfully map the dipolar and quadrupolar plasmon near-field distributions when in-plane polarized light is used for excitation and the scattered light that corresponds to both the in-plane and the out-of-plane near-field vector components is detected. Second, considering an incident light with significant projection of its electric field parallel to the probe axis and selectively detecting the near-field vector that is oriented parallel to the probe axis, the probe-sample interaction is investigated at λ = 633 nm and 808 nm as the disk diameter is systematically increased from 64 nm to 210 nm. For a given excitation wavelength, the near-field distribution is observed to evolve as a function of disk diameter showing characteristic trend as the plasmon resonance overlaps with the excitation energy.

2. Method and layout of the experiment

Polarization selective ANSOM imaging has been performed by configuring the optical components around the Neaspec atomic force microscope customized for pseudoheterodyne interferometric detection of scattered light as shown in Fig. 1
Fig. 1 Schematics of the interferometric apertureless near-field scanning optical microscope. The inset shows the calculated near-field amplitude at the tip of a platinum probe of 25 nm radius of curvature and 1 μm cone length (displayed area = 250 × 250 nm2). The calculation is performed using FDTD method introducing a Gaussian source focused at the tip where the grid size is 2 nm. The bulk dielectric constants of platinum [37] are used to model the structure. Note that there is no noticeable field enhancement for the polarization perpendicular to the probe axis (inset, right panel). The angle of incidence (θ) is 60° and hence for the vertical polarization (inset, left panel), the electric field (E) and propagation (k) vectors have projections parallel both to the sample plane (Ey, ky) and to the tip axis (Ez, kz).
. The details of the experimental procedures are available elsewhere [35

35. N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett. 89(10), 101124 (2006). [CrossRef]

,36

36. Z. H. Kim and S. R. Leone, “Polarization-selective mapping of near-field intensity and phase around gold nanoparticles using apertureless near-field microscopy,” Opt. Express 16(3), 1733–1741 (2008). [CrossRef] [PubMed]

], and briefly described as follows. Linearly polarized laser beam is focused at the probe tip using a parabolic mirror (NA = 0.46). The scattered light is collected through the same parabolic mirror and mixed with a reference beam. Polarization selection of the scattered light is achieved by controlling the polarization of the reference beam using a quarter waveplate. The polarization selection is further refined using a polarizer mounted close to the detector. The AFM is operated in a tapping mode (tapping amplitude ≈50 nm) near the resonance oscillation frequency (Ω ≈240 kHz) of the cantilever. The scattered light + reference beam is detected using a silicon photodiode (New Focus, Model 2051). The output of the detector is demodulated at nΩ, and the optical images obtained at n > 2 are used in the subsequent discussions.

The incidence angle with respect to surface normal is 60°, and for vertically polarized light, the electric field vector has components parallel (Ez ≡ EP) and perpendicular (Ey) to the probe axis [see left panel of the inset in Fig. 1]. For the horizontal polarization, the electric field of the laser is entirely in the sample plane (Ex ≡ ES) as seen in the right panel of the inset in Fig. 1. In the subsequent discussion, P and S refer to the polarization of the incident and scattered light that corresponds to the Ez and Ex, respectively. For example, SP polarization refers to excitation of the sample with an in-plane (S) polarized light and detection of the vertical (P) component of the scattered light.

The near-field optical imaging has been implemented on two sets of gold nanodisks, shown in the schematics in Fig. 2(a)
Fig. 2 (a) Schematics showing arrangement of the gold nanodisks (height = 24 nm) in a unit cell of arrays of small and large diameters. The disks are labeled according to their diameters. (b) Calculated scattering cross-section of representative gold nanodisks as labeled. A total-field/scattered-field source scheme [39] is used to introduce light energy into the simulation region where the grid size is 1 nm in x, y and z directions and the bulk dielectric constants of gold [40] is used to model the structures. The dashed vertical lines indicate the wavelengths of the lasers used in the experiment.
. The gold disks are fabricated using electron beam lithography and liftoff procedures on oxide coated (~300 nm thick) silicon wafers without any metallic adhesion layer, which can result in strong chemical interface plasmon damping [38

38. T. G. Habteyes, S. Dhuey, E. Wood, D. Gargas, S. Cabrini, P. J. Schuck, A. P. Alivisatos, and S. R. Leone, “Metallic adhesion layer induced plasmon damping and molecular linker as a nondamping alternative,” ACS Nano 6(6), 5702–5709 (2012). [CrossRef] [PubMed]

]. The nominal height of the disks is 24 nm as determined from the AFM images. The scattering cross-sections of the gold nanodisks have been calculated using FDTD simulations (implemented using Lumerical software) and representative spectra are plotted in Fig. 2(b). The outputs of a HeNe laser (λ = 633 nm) and a laser diode (λ = 808 nm) are used as sources of excitations. The laser lines are indicated by the dashed vertical lines in Fig. 2(b), and it can be seen that the dipolar resonances of the disks of the smaller diameter set (64 – 104 nm) overlap with the 633 nm laser line, with the extent of the overlap depending on the diameter. Similarly, the dipolar resonances of the larger diameter set (140 – 210 nm) overlap with the 808 nm laser line with different extent depending on the diameter. In addition, the larger diameter disk sets have quadrupolar plasmon resonances that are excited due to the retardation effects [13

13. R. Esteban, R. Vogelgesang, J. Dorfmüller, A. Dmitriev, C. Rockstuhl, C. Etrich, and K. Kern, “Direct near-field optical imaging of higher order plasmonic resonances,” Nano Lett. 8(10), 3155–3159 (2008). [CrossRef] [PubMed]

], which are accessible at the 633 nm laser line.

3. Mapping the dipolar and quadrupolar plasmon modes

Representative near-field optical images of the smaller diameter (64 - 104 nm) nanodisks are presented in Fig. 3(b)
Fig. 3 Results on small diameter (64 – 104 nm) gold nanodisks. (a) The topographic image shows that some of the disks are displaced from the intended position during the liftoff and cleaning processes due to the poor adhesion of gold to the silica surface. (b) Near-field amplitude recorded at 4Ω. The sample is excited with in-plane (S) polarized light as indicated by the E vector on the image and the P-polarized scattered light (corresponding to the vertical component of the near-field, Ez) is selectively detected interferometrically. This polarization scheme is referred as SP on the image and in the text. (c) Calculated near-field amplitude with the same excitation conditions as in the experiment but excluding the probe (the diameter of the disks increases from left to right: 64 nm to 104 nm, 8 nm step).
along with the simultaneously recorded topographic image shown in Fig. 3(a). The optical images are acquired with the SP polarization scheme, and the near-field distribution of the individual disks has two lobes that are localized at diametrically opposite ends of the disks, where the diameter is parallel to the electric field vector (indicated by the arrow on the image) of the excitation light. The double lobe structures of the near-field images agree well with the results of FDTD simulation [Fig. 3(c)] calculated in the absence of the probe. These double lobe structures are due to the dipole plasmon resonance modes excited on the nanodisks. The slight variation in the optical images of the disks of the same nominal diameter can be attributed to structural defects and anisotropy in the lithographically fabricated nanostructures.

The near-field imaging with the SP polarization scheme is extended on the larger diameter (140 - 210 nm) disk sets, and the topographic and optical images are presented in Figs. 4(a)
Fig. 4 Results on large diameter (140 – 210 nm) gold nanodisks. (a) Topography. (b) Near-field amplitude recorded at 4Ω. The excitation and detection polarizations are the same as in Fig. 2, which is referred as SP polarization. (c) Calculated near-field amplitude with the same excitation conditions as in the experiment but excluding the probe (the diameter of the disks increases from left right: 154 nm to 210 nm, 14 nm step).
and 4(b), respectively. The near-field distribution on the nanodisks has four lobes with more pronounced amplitude at the far-edge of the disk with respect to the laser propagation direction. The observed field distribution pattern is in agreement with the results of FDTD simulation [Fig. 4(c)] and is characteristic of the quadrupolar plasmon mode structures of the nanodisks. Referring to the calculated scattering cross-sections plotted in Fig. 2(b), it can be seen that the quadrupolar mode structures become more apparent as the spectral overlap of the resonance peak with the laser line (633 nm) becomes significant.

4. Size and wavelength dependent probe-sample interaction

When the incident light is P-polarized and the P-polarization component of the scattered light is detected, the observed near-field distribution differs significantly from what is expected for the isolated nanodisks, depending sensitively on the sample size as seen in Figs. 5(d). For the smallest disk diameter (64 nm), the near-field amplitude obtained at λ = 633 nm is larger on the near-edge (NE) of the gold disk than on the far-edge (FE). For the context of NE and FE see label in Fig. 5(d) and the discussion in [16

16. A. García-Etxarri, I. Romero, F. García de Abajo, R. Hillenbrand, and J. Aizpurua, “Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes,” Phys. Rev. B 79(12), 125439 (2009). [CrossRef]

]. As the diameter of the disks increases, the NE to FE amplitude ratio decreases, attaining larger amplitude at the far-edge for diameter greater than 88 nm as seen in the line profile plotted in Fig. 6(a)
Fig. 6 Analysis of the relative near-field amplitude at the near-edge (NE) and far-edge (FE) of the gold nanodisks. The line profile in (a) is a plot of the near-field amplitude across the dashed line (as shown on the inset image), which is parallel to the ky component of the laser propagation direction. (b) Calculated near-field amplitude where the tip is located 3 nm above the top surface of the disk at near-edge (left panel) and far-edge (right panel). (c) Plot of ρ = |E|NE/|E|FE as a function of disk diameter.
. In contrast, as can be seen in the corresponding simulation results in Fig. 5(e), the calculated near-field amplitude for the isolated 64 nm diameter disk is greater at the FE than at the NE, and the NE to FE ratio increases as the diameter of the disks increases from 64 nm to 104 nm. Clearly, the probe-sample interaction has altered not only the pattern of the inherent near-field distribution of the sample when a single disk is considered but also the trend as the sample size changes. Repeating the experiment at λ = 808 nm reveals that the reversal of the relative amplitude as a function of the sample size depends on the wavelength of the excitation energy. While at 633 nm excitation the maximum amplitude is shifted from NE to FE at diameter 96 nm [see the inset and the corresponding line profile in Fig. 6(a)], at 808 nm, the FE amplitude becomes dominant for diameters greater than 154 nm.

In order to gain insight into the probe-sample optical interaction, the field distribution on the gold nanodisks has been calculated in the presence of a platinum probe with 1 μm cone length and 25 nm radius of curvature of the tip. Representative results for the 633 nm excitation are presented in Fig. 6(b) showing the near-field distribution when the probe is located at the near-edge (left panel) and far-edge (right panel) 3 nm above the top surface of a 64 nm diameter disk. The difference in the near-field distributions for the NE and FE probe location can be explained in terms of the probe-sample configuration with respect to the laser field, indicated on the right panel in Fig. 6(b). For the NE arrangement, the polarizations of the tip and the sample are nearly anti-parallel, which is not favorable for strong dipole-dipole interaction as described by Aizpurua and associates [16

16. A. García-Etxarri, I. Romero, F. García de Abajo, R. Hillenbrand, and J. Aizpurua, “Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes,” Phys. Rev. B 79(12), 125439 (2009). [CrossRef]

].

On the other hand, the far-edge probe-sample arrangement results in parallel orientation of the tip and sample polarizations, leading to a strong dipole-dipole interaction that localizes the field in the tip-sample gap as shown in Fig. 6(b) right panel. In this case the inherent optical response of the sample is influenced considerably due to the near-field interaction with the tip of the probe, as it is evident in the calculated near-field distribution. Approximating a platinum probe as a point dipole, a spectral red-shift of the dipole plasmon resonance energy of a gold nanodisk has been predicted for P-polarized excitation light [16

16. A. García-Etxarri, I. Romero, F. García de Abajo, R. Hillenbrand, and J. Aizpurua, “Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes,” Phys. Rev. B 79(12), 125439 (2009). [CrossRef]

]. However, it should be noted that the spectral red-shift for the small nanodisks (diameter less than 88 nm) is expected to increase the plasmon resonance overlap with the 633 nm laser line [see Fig. 2(b)], and does not explain the smaller near-field amplitude at the FE than at the NE. Despite this apparent contradiction between the results of the measurement and the simulation when a fixed sample size is considered, interesting trend can be extracted by analyzing the relative near-field amplitude ratio as a function of the diameter of the gold nanodisks as described next.

We first introduce a parameter ρ that is defined as NE to FE electric field amplitude ratio, ρ = |E|NE/|E|FE (the deviation of ρ from unity indicates the extent of asymmetry in the dipolar near-field distribution). When ρ extracted from the 633 nm excitation experiment is plotted as a function of the diameter of the nanodisks, it decreases as shown in Fig. 6(c) [blue curve]. We note that for the 633 nm excitation ρ is plotted up to 104 nm diameter because for larger disks the quadrupolar field distribution affects the simple dipolar field localization at the NE and FE poles. Similar to the experimental result, the maximum calculated ρ value is obtained for 64 nm diameter disk and it decreases with increasing diameter. The calculated absolute ρ value is arbitrary as it depends on the tip-sample gap. Therefore, the theoretical plot is displaced along the y-axis such that the theoretical maximum is equal to the experimental maximum as plotted in Fig. 6(c). Each experimental data point is an average of up to 15 nominally the same but different disks and also of measurements with different tips. The error bar is mainly due to variation from particle to particle rather than from measurement to measurement, and therefore can be attributed to the sensitivity of the FE probe-sample coupling to slight structural variations/defects. It is interesting to note that when silicon tip is used, ρ < 1 for all the disks with slight increase with decreasing disk diameter [black curve in Fig. 6(c)], indicating insignificant influence of the tip on the optical characteristics of the sample.

To confirm that the trend observed in the 633 nm experiment is not simply due to tip-sample size mismatch, similar plots have been reproduced for the experimental and simulation results obtained at 808 nm excitation. As seen in Fig. 6(c), while at λ = 633 nm, ρ < 1 for diameter greater than 88 nm, at λ = 808 nm, ρ > 1 even for diameter 154 nm. Then similar to the trend observed at λ = 633 nm, as the plasmon resonance peak shifts towards the laser line with increasing diameter, ρ decreases, crosses unity and becomes nearly constant after apparently a complete spectral overlap of the plasmon resonance with the laser line [Fig. 6(c), red curve]. In fact, in agreement to the results of the calculation, ρ appears to increase after attaining a minimum at the plasmon resonance. The faster increase of the theoretical ρ values beyond the minimum than the experimental values can be attributed to the reduced signal-to-noise ratio as the spectral overlap with the laser line is decreasing with further increase of diameter. In general the observation at the two excitation wavelengths suggest that, close to resonance, the plasmon near-field amplitude becomes large dominating the tip influence and resulting in the near-field images characteristics of dipolar mode structure. The asymmetry of the field distribution (ρ < 1) can partly be attributed to the oblique incidence of the laser, which favors larger field amplitude at the far-edge as can be seen in Fig. 5(e) last three panels. Off-resonance, the field distribution is mainly characteristic of the tip-sample interaction, resulting in harmonic oscillator type of picture that is more evident in the simulation result, pink curve in Fig. 6(c).

Comparing the near-field images obtained at the two excitation wavelengths with PP polarization, it can be seen that the resolution of the dipolar mode structures is better at λ = 633 nm than at λ = 808 nm. At the 633 nm excitation, the dipolar plasmon mode structure becomes evident with increasing overlap of the resonance peak with the laser line as seen in Figs. 5(d) and 6(a). On the other hand, at 808 nm, the corresponding dipolar mode on the larger disks has not been clearly resolved for any of the disks; instead as the diameter increases, the near-field amplitude shifts from NE to FE before the two poles are discernible as seen in Fig. 5(k). The increased obscurity of the dipolar signature at longer wavelength can be attributed to weaker near-field amplitude on the larger disks and/or greater field enhancement at the tip. Results of FDTD simulation indicate that the near-field amplitude on the small disks at 633 nm is comparable to the near-field amplitude on the large disks at 808 nm. For the platinum probe, the calculated field enhancement at 808 nm is about twice as that calculated at 633 nm, which may result in stronger probe-sample interaction, resulting in greater influence on the inherent near-field distribution of the sample. However, as seen in Fig. 5(l), even for the SS polarization more pronounced asymmetry is observed in the dipolar modes on some of the disks than in the corresponding images obtained at 633 nm. This observation suggests that the poorer resolution at 808 nm can result from the poorer beam quality of the laser diode output and polarization mixing by the parabolic mirror [41

41. B. Deutsch, R. Hillenbrand, and L. Novotny, “Visualizing the optical interaction tensor of a gold nanoparticle pair,” Nano Lett. 10(2), 652–656 (2010). [CrossRef] [PubMed]

] and other optical components on the beam path. In fact, without using polarizers 1 and 2 in Fig. 1, the dipolar modes have not been clearly resolved at 808 nm for any of the nanodisks. On the other hand, at 633 nm excitation, the near-field images obtained in the absence and presence of the polarizers are practically the same.

5. Conclusion

In summary, systematic analyses of the probe-sample optical interactions in apertureless near-field scanning optical microscopy are presented for 633 nm and 808 nm excitation wavelengths using lithographically fabricated gold nanodisks as model systems. The near-field images of the dipolar and quadrupolar plasmon modes of gold nanodisks (diameter 64 – 210 nm) have been successfully mapped using metal coated probes. When the excitation light is polarized perpendicular to the probe axis, the observed near-field distributions agree very well with the results of electromagnetic simulations (performed on the isolated gold nanodisks) for both vertical (out-of-plane) and in-plane detection polarizations. For the incident light polarized parallel to the probe axis, it is shown that the strong background associated to the strong excitation of the probe can be removed through orthogonal detection polarization scheme. For the parallel excitation and detection polarization combination (PP), the probe-sample optical interaction depends on the sample size and excitation wavelength, and the observed near-field distributions differ from the inherent near-field distribution of the sample. For a given excitation wavelength, the near-field distribution is observed to evolve as a function of disk diameter showing characteristic trend as the plasmon resonance overlaps with the excitation energy.

Acknowledgments

The nanofabrication of the gold nanodisks was performed as a user project at the Molecular Foundry, Lawrence Berkeley National Laboratory, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

References and links

1.

D. Courjon, Near Field Microscopy and Near Field Optics (Imperial College Press, 2003).

2.

L. Novotny and B. Hecht, Principles of Nano-optics (Cambridge University Press, 2006).

3.

Y. Inouye and S. Kawata, “Near-field scanning optical microscope with a metallic probe tip,” Opt. Lett. 19(3), 159–161 (1994). [CrossRef] [PubMed]

4.

F. Zenhausern, M. P. Oboyle, and H. K. Wickramasinghe, “Apertureless near-field optical microscope,” Appl. Phys. Lett. 65(13), 1623–1625 (1994). [CrossRef]

5.

B. Knoll and F. Keilmann, “Enhanced dielectric contrast in scattering-type scanning near-field optical microscopy,” Opt. Commun. 182(4-6), 321–328 (2000). [CrossRef]

6.

R. Hillenbrand, B. Knoll, and F. Keilmann, “Pure optical contrast in scattering-type scanning near-field microscopy, ” J. Microsc. 202, 77–83 (2001).

7.

Z. H. Kim, B. Liu, and S. R. Leone, “Nanometer-scale optical imaging of epitaxially grown GaN and InN islands using apertureless near-field microscopy,” J. Phys. Chem. B 109(17), 8503–8508 (2005). [CrossRef] [PubMed]

8.

Z. H. Kim, S. H. Ahn, B. Liu, and S. R. Leone, “Nanometer-scale dielectric imaging of semiconductor nanoparticles: Size-dependent dipolar coupling and contrast reversal,” Nano Lett. 7(8), 2258–2262 (2007). [CrossRef] [PubMed]

9.

R. Hillenbrand, T. Taubner, and F. Keilmann, “Phonon-enhanced light matter interaction at the nanometre scale,” Nature 418(6894), 159–162 (2002). [CrossRef] [PubMed]

10.

M. Brehm, T. Taubner, R. Hillenbrand, and F. Keilmann, “Infrared spectroscopic mapping of single nanoparticles and viruses at nanoscale resolution,” Nano Lett. 6(7), 1307–1310 (2006). [CrossRef] [PubMed]

11.

J. M. Stiegler, Y. Abate, A. Cvitkovic, Y. E. Romanyuk, A. J. Huber, S. R. Leone, and R. Hillenbrand, “Nanoscale infrared absorption spectroscopy of individual nanoparticles enabled by scattering-type near-field microscopy,” ACS Nano 5(8), 6494–6499 (2011). [CrossRef] [PubMed]

12.

R. Hillenbrand, F. Keilmann, P. Hanarp, D. S. Sutherland, and J. Aizpurua, “Coherent imaging of nanoscale plasmon patterns with a carbon nanotube optical probe,” Appl. Phys. Lett. 83(2), 368–370 (2003). [CrossRef]

13.

R. Esteban, R. Vogelgesang, J. Dorfmüller, A. Dmitriev, C. Rockstuhl, C. Etrich, and K. Kern, “Direct near-field optical imaging of higher order plasmonic resonances,” Nano Lett. 8(10), 3155–3159 (2008). [CrossRef] [PubMed]

14.

M. Rang, A. C. Jones, F. Zhou, Z. Y. Li, B. J. Wiley, Y. N. Xia, and M. B. Raschke, “Optical near-field mapping of plasmonic nanoprisms,” Nano Lett. 8(10), 3357–3363 (2008). [CrossRef] [PubMed]

15.

D. S. Kim, J. Heo, S. H. Ahn, S. W. Han, W. S. Yun, and Z. H. Kim, “Real-space mapping of the strongly coupled plasmons of nanoparticle dimers,” Nano Lett. 9(10), 3619–3625 (2009). [CrossRef] [PubMed]

16.

A. García-Etxarri, I. Romero, F. García de Abajo, R. Hillenbrand, and J. Aizpurua, “Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes,” Phys. Rev. B 79(12), 125439 (2009). [CrossRef]

17.

P. Alonso-Gonzalez, M. Schnell, P. Sarriugarte, H. Sobhani, C. H. Wu, N. Arju, A. Khanikaev, F. Golmar, P. Albella, L. Arzubiaga, F. Casanova, L. E. Hueso, P. Nordlander, G. Shvets, and R. Hillenbrand, “Real-space mapping of Fano interference in plasmonic metamolecules,” Nano Lett. 11(9), 3922–3926 (2011). [CrossRef] [PubMed]

18.

S. Mastel, S. E. Grefe, G. B. Cross, A. Taber, S. Dhuey, S. Cabrini, P. J. Schuck, and Y. Abate, “Real-space mapping of nanoplasmonic hotspots via optical antenna-gap loading,” Appl. Phys. Lett. 101(13), 131102 (2012). [CrossRef]

19.

M. B. Raschke and C. Lienau, “Apertureless near-field optical microscopy: Tip-sample coupling in elastic light scattering,” Appl. Phys. Lett. 83(24), 5089–5091 (2003). [CrossRef]

20.

O. J. F. Martin and C. Girard, “Controlling and tuning strong optical field gradients at a local probe microscope tip apex,” Appl. Phys. Lett. 70(6), 705–707 (1997). [CrossRef]

21.

L. Novotny, R. X. Bian, and X. S. Xie, “Theory of nanometric optical tweezers,” Phys. Rev. Lett. 79(4), 645–648 (1997). [CrossRef]

22.

L. Aigouy, A. Lahrech, S. Grãsillon, H. Cory, A. C. Boccara, and J. C. Rivoal, “Polarization effects in apertureless scanning near-field optical microscopy: An experimental study,” Opt. Lett. 24(4), 187–189 (1999). [CrossRef] [PubMed]

23.

A. Bouhelier and R. Bachelot, in Scanning Probe Microscopy, edited by S. Kalinin and A. Gruverman (Springer New York, 2007), 254–279.

24.

M. Schnell, A. Garcia-Etxarri, J. Alkorta, J. Aizpurua, and R. Hillenbrand, “Phase-resolved mapping of the near-field vector and polarization state in nanoscale antenna gaps,” Nano Lett. 10(9), 3524–3528 (2010). [CrossRef] [PubMed]

25.

D. S. Kim and Z. H. Kim, “Role of in-plane polarizability of the tip in scattering near-field microscopy of a plasmonic nanoparticle,” Opt. Express 20(8), 8689–8699 (2012). [CrossRef] [PubMed]

26.

P. Alonso-González, P. Albella, F. Golmar, L. Arzubiaga, F. Casanova, L. E. Hueso, J. Aizpurua, and R. Hillenbrand, “Visualizing the near-field coupling and interference of bonding and anti-bonding modes in infrared dimer nanoantennas,” Opt. Express 21(1), 1270–1280 (2013). [CrossRef] [PubMed]

27.

R. L. Olmon, P. M. Krenz, A. C. Jones, G. D. Boreman, and M. B. Raschke, “Near-field imaging of optical antenna modes in the mid-infrared,” Opt. Express 16(25), 20295–20305 (2008). [CrossRef] [PubMed]

28.

E. J. Sànchez, L. Novotny, and X. S. Xie, “Near-field fluorescence microscopy based on two-photon excitation with metal tips,” Phys. Rev. Lett. 82(20), 4014–4017 (1999). [CrossRef]

29.

R. M. Stöckle, Y. D. Suh, V. Deckert, and R. Zenobi, “Nanoscale chemical analysis by tip-enhanced raman spectroscopy,” Chem. Phys. Lett. 318(1-3), 131–136 (2000). [CrossRef]

30.

A. Hartschuh, E. J. Sánchez, X. S. Xie, and L. Novotny, “High-resolution near-field Raman microscopy of single-walled carbon nanotubes,” Phys. Rev. Lett. 90(9), 095503 (2003). [CrossRef] [PubMed]

31.

W. H. Zhang, B. S. Yeo, T. Schmid, and R. Zenobi, “Single molecule tip-enhanced Raman spectroscopy with silver tips,” J. Phys. Chem. C 111(4), 1733–1738 (2007). [CrossRef]

32.

J. Steidtner and B. Pettinger, “Tip-enhanced Raman spectroscopy and microscopy on single dye molecules with 15 nm resolution,” Phys. Rev. Lett. 100(23), 236101 (2008). [CrossRef] [PubMed]

33.

A. Weber-Bargioni, A. Schwartzberg, M. Cornaglia, A. Ismach, J. J. Urban, Y. J. Pang, R. Gordon, J. Bokor, M. B. Salmeron, D. F. Ogletree, P. Ashby, S. Cabrini, and P. J. Schuck, “Hyperspectral nanoscale imaging on dielectric substrates with coaxial optical antenna scan probes,” Nano Lett. 11(3), 1201–1207 (2011). [CrossRef] [PubMed]

34.

N. Jiang, E. T. Foley, J. M. Klingsporn, M. D. Sonntag, N. A. Valley, J. A. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam, and R. P. Van Duyne, “Observation of multiple vibrational modes in ultrahigh vacuum tip-enhanced Raman spectroscopy combined with molecular-resolution scanning tunneling microscopy,” Nano Lett. 12(10), 5061–5067 (2012). [CrossRef] [PubMed]

35.

N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett. 89(10), 101124 (2006). [CrossRef]

36.

Z. H. Kim and S. R. Leone, “Polarization-selective mapping of near-field intensity and phase around gold nanoparticles using apertureless near-field microscopy,” Opt. Express 16(3), 1733–1741 (2008). [CrossRef] [PubMed]

37.

E. D. Palik, Handbook of Optical Constants of Solids (Elsevier, 1998).

38.

T. G. Habteyes, S. Dhuey, E. Wood, D. Gargas, S. Cabrini, P. J. Schuck, A. P. Alivisatos, and S. R. Leone, “Metallic adhesion layer induced plasmon damping and molecular linker as a nondamping alternative,” ACS Nano 6(6), 5702–5709 (2012). [CrossRef] [PubMed]

39.

A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method (Artech House, 2005).

40.

P. B. Johnson and R. W. Christy, “Optical-constants of noble-metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

41.

B. Deutsch, R. Hillenbrand, and L. Novotny, “Visualizing the optical interaction tensor of a gold nanoparticle pair,” Nano Lett. 10(2), 652–656 (2010). [CrossRef] [PubMed]

42.

M. Esslinger and R. Vogelgesang, “Reciprocity theory of apertureless scanning near-field optical microscopy with point-dipole probes,” ACS Nano 6(9), 8173–8182 (2012). [CrossRef] [PubMed]

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(180.4243) Microscopy : Near-field microscopy

ToC Category:
Microscopy

History
Original Manuscript: June 17, 2013
Revised Manuscript: August 23, 2013
Manuscript Accepted: September 2, 2013
Published: September 6, 2013

Citation
Terefe G. Habteyes, Scott Dhuey, Karissa I. Kiesow, and Alexander Vold, "Probe-sample optical interaction: size and wavelength dependence in localized plasmon near-field imaging," Opt. Express 21, 21607-21617 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-18-21607


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References

  1. D. Courjon, Near Field Microscopy and Near Field Optics (Imperial College Press, 2003).
  2. L. Novotny and B. Hecht, Principles of Nano-optics (Cambridge University Press, 2006).
  3. Y. Inouye and S. Kawata, “Near-field scanning optical microscope with a metallic probe tip,” Opt. Lett.19(3), 159–161 (1994). [CrossRef] [PubMed]
  4. F. Zenhausern, M. P. Oboyle, and H. K. Wickramasinghe, “Apertureless near-field optical microscope,” Appl. Phys. Lett.65(13), 1623–1625 (1994). [CrossRef]
  5. B. Knoll and F. Keilmann, “Enhanced dielectric contrast in scattering-type scanning near-field optical microscopy,” Opt. Commun.182(4-6), 321–328 (2000). [CrossRef]
  6. R. Hillenbrand, B. Knoll, and F. Keilmann, “Pure optical contrast in scattering-type scanning near-field microscopy, ” J. Microsc.202, 77–83 (2001).
  7. Z. H. Kim, B. Liu, and S. R. Leone, “Nanometer-scale optical imaging of epitaxially grown GaN and InN islands using apertureless near-field microscopy,” J. Phys. Chem. B109(17), 8503–8508 (2005). [CrossRef] [PubMed]
  8. Z. H. Kim, S. H. Ahn, B. Liu, and S. R. Leone, “Nanometer-scale dielectric imaging of semiconductor nanoparticles: Size-dependent dipolar coupling and contrast reversal,” Nano Lett.7(8), 2258–2262 (2007). [CrossRef] [PubMed]
  9. R. Hillenbrand, T. Taubner, and F. Keilmann, “Phonon-enhanced light matter interaction at the nanometre scale,” Nature418(6894), 159–162 (2002). [CrossRef] [PubMed]
  10. M. Brehm, T. Taubner, R. Hillenbrand, and F. Keilmann, “Infrared spectroscopic mapping of single nanoparticles and viruses at nanoscale resolution,” Nano Lett.6(7), 1307–1310 (2006). [CrossRef] [PubMed]
  11. J. M. Stiegler, Y. Abate, A. Cvitkovic, Y. E. Romanyuk, A. J. Huber, S. R. Leone, and R. Hillenbrand, “Nanoscale infrared absorption spectroscopy of individual nanoparticles enabled by scattering-type near-field microscopy,” ACS Nano5(8), 6494–6499 (2011). [CrossRef] [PubMed]
  12. R. Hillenbrand, F. Keilmann, P. Hanarp, D. S. Sutherland, and J. Aizpurua, “Coherent imaging of nanoscale plasmon patterns with a carbon nanotube optical probe,” Appl. Phys. Lett.83(2), 368–370 (2003). [CrossRef]
  13. R. Esteban, R. Vogelgesang, J. Dorfmüller, A. Dmitriev, C. Rockstuhl, C. Etrich, and K. Kern, “Direct near-field optical imaging of higher order plasmonic resonances,” Nano Lett.8(10), 3155–3159 (2008). [CrossRef] [PubMed]
  14. M. Rang, A. C. Jones, F. Zhou, Z. Y. Li, B. J. Wiley, Y. N. Xia, and M. B. Raschke, “Optical near-field mapping of plasmonic nanoprisms,” Nano Lett.8(10), 3357–3363 (2008). [CrossRef] [PubMed]
  15. D. S. Kim, J. Heo, S. H. Ahn, S. W. Han, W. S. Yun, and Z. H. Kim, “Real-space mapping of the strongly coupled plasmons of nanoparticle dimers,” Nano Lett.9(10), 3619–3625 (2009). [CrossRef] [PubMed]
  16. A. García-Etxarri, I. Romero, F. García de Abajo, R. Hillenbrand, and J. Aizpurua, “Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes,” Phys. Rev. B79(12), 125439 (2009). [CrossRef]
  17. P. Alonso-Gonzalez, M. Schnell, P. Sarriugarte, H. Sobhani, C. H. Wu, N. Arju, A. Khanikaev, F. Golmar, P. Albella, L. Arzubiaga, F. Casanova, L. E. Hueso, P. Nordlander, G. Shvets, and R. Hillenbrand, “Real-space mapping of Fano interference in plasmonic metamolecules,” Nano Lett.11(9), 3922–3926 (2011). [CrossRef] [PubMed]
  18. S. Mastel, S. E. Grefe, G. B. Cross, A. Taber, S. Dhuey, S. Cabrini, P. J. Schuck, and Y. Abate, “Real-space mapping of nanoplasmonic hotspots via optical antenna-gap loading,” Appl. Phys. Lett.101(13), 131102 (2012). [CrossRef]
  19. M. B. Raschke and C. Lienau, “Apertureless near-field optical microscopy: Tip-sample coupling in elastic light scattering,” Appl. Phys. Lett.83(24), 5089–5091 (2003). [CrossRef]
  20. O. J. F. Martin and C. Girard, “Controlling and tuning strong optical field gradients at a local probe microscope tip apex,” Appl. Phys. Lett.70(6), 705–707 (1997). [CrossRef]
  21. L. Novotny, R. X. Bian, and X. S. Xie, “Theory of nanometric optical tweezers,” Phys. Rev. Lett.79(4), 645–648 (1997). [CrossRef]
  22. L. Aigouy, A. Lahrech, S. Grãsillon, H. Cory, A. C. Boccara, and J. C. Rivoal, “Polarization effects in apertureless scanning near-field optical microscopy: An experimental study,” Opt. Lett.24(4), 187–189 (1999). [CrossRef] [PubMed]
  23. A. Bouhelier and R. Bachelot, in Scanning Probe Microscopy, edited by S. Kalinin and A. Gruverman (Springer New York, 2007), 254–279.
  24. M. Schnell, A. Garcia-Etxarri, J. Alkorta, J. Aizpurua, and R. Hillenbrand, “Phase-resolved mapping of the near-field vector and polarization state in nanoscale antenna gaps,” Nano Lett.10(9), 3524–3528 (2010). [CrossRef] [PubMed]
  25. D. S. Kim and Z. H. Kim, “Role of in-plane polarizability of the tip in scattering near-field microscopy of a plasmonic nanoparticle,” Opt. Express20(8), 8689–8699 (2012). [CrossRef] [PubMed]
  26. P. Alonso-González, P. Albella, F. Golmar, L. Arzubiaga, F. Casanova, L. E. Hueso, J. Aizpurua, and R. Hillenbrand, “Visualizing the near-field coupling and interference of bonding and anti-bonding modes in infrared dimer nanoantennas,” Opt. Express21(1), 1270–1280 (2013). [CrossRef] [PubMed]
  27. R. L. Olmon, P. M. Krenz, A. C. Jones, G. D. Boreman, and M. B. Raschke, “Near-field imaging of optical antenna modes in the mid-infrared,” Opt. Express16(25), 20295–20305 (2008). [CrossRef] [PubMed]
  28. E. J. Sànchez, L. Novotny, and X. S. Xie, “Near-field fluorescence microscopy based on two-photon excitation with metal tips,” Phys. Rev. Lett.82(20), 4014–4017 (1999). [CrossRef]
  29. R. M. Stöckle, Y. D. Suh, V. Deckert, and R. Zenobi, “Nanoscale chemical analysis by tip-enhanced raman spectroscopy,” Chem. Phys. Lett.318(1-3), 131–136 (2000). [CrossRef]
  30. A. Hartschuh, E. J. Sánchez, X. S. Xie, and L. Novotny, “High-resolution near-field Raman microscopy of single-walled carbon nanotubes,” Phys. Rev. Lett.90(9), 095503 (2003). [CrossRef] [PubMed]
  31. W. H. Zhang, B. S. Yeo, T. Schmid, and R. Zenobi, “Single molecule tip-enhanced Raman spectroscopy with silver tips,” J. Phys. Chem. C111(4), 1733–1738 (2007). [CrossRef]
  32. J. Steidtner and B. Pettinger, “Tip-enhanced Raman spectroscopy and microscopy on single dye molecules with 15 nm resolution,” Phys. Rev. Lett.100(23), 236101 (2008). [CrossRef] [PubMed]
  33. A. Weber-Bargioni, A. Schwartzberg, M. Cornaglia, A. Ismach, J. J. Urban, Y. J. Pang, R. Gordon, J. Bokor, M. B. Salmeron, D. F. Ogletree, P. Ashby, S. Cabrini, and P. J. Schuck, “Hyperspectral nanoscale imaging on dielectric substrates with coaxial optical antenna scan probes,” Nano Lett.11(3), 1201–1207 (2011). [CrossRef] [PubMed]
  34. N. Jiang, E. T. Foley, J. M. Klingsporn, M. D. Sonntag, N. A. Valley, J. A. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam, and R. P. Van Duyne, “Observation of multiple vibrational modes in ultrahigh vacuum tip-enhanced Raman spectroscopy combined with molecular-resolution scanning tunneling microscopy,” Nano Lett.12(10), 5061–5067 (2012). [CrossRef] [PubMed]
  35. N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett.89(10), 101124 (2006). [CrossRef]
  36. Z. H. Kim and S. R. Leone, “Polarization-selective mapping of near-field intensity and phase around gold nanoparticles using apertureless near-field microscopy,” Opt. Express16(3), 1733–1741 (2008). [CrossRef] [PubMed]
  37. E. D. Palik, Handbook of Optical Constants of Solids (Elsevier, 1998).
  38. T. G. Habteyes, S. Dhuey, E. Wood, D. Gargas, S. Cabrini, P. J. Schuck, A. P. Alivisatos, and S. R. Leone, “Metallic adhesion layer induced plasmon damping and molecular linker as a nondamping alternative,” ACS Nano6(6), 5702–5709 (2012). [CrossRef] [PubMed]
  39. A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method (Artech House, 2005).
  40. P. B. Johnson and R. W. Christy, “Optical-constants of noble-metals,” Phys. Rev. B6(12), 4370–4379 (1972). [CrossRef]
  41. B. Deutsch, R. Hillenbrand, and L. Novotny, “Visualizing the optical interaction tensor of a gold nanoparticle pair,” Nano Lett.10(2), 652–656 (2010). [CrossRef] [PubMed]
  42. M. Esslinger and R. Vogelgesang, “Reciprocity theory of apertureless scanning near-field optical microscopy with point-dipole probes,” ACS Nano6(9), 8173–8182 (2012). [CrossRef] [PubMed]

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