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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 20 — Sep. 26, 2011
  • pp: 19310–19322
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Continuous layer gap plasmon resonators

Michael G. Nielsen, Dmitri K. Gramotnev, Anders Pors, Ole Albrektsen, and Sergey I. Bozhevolnyi  »View Author Affiliations


Optics Express, Vol. 19, Issue 20, pp. 19310-19322 (2011)
http://dx.doi.org/10.1364/OE.19.019310


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Abstract

We demonstrate both theoretically and experimentally that a gold nanostrip supported by a thin dielectric (silicon dioxide) film and a gold underlay forms an efficient (Fabry-Perot) resonator for gap surface plasmons. Periodic nanostrip arrays are shown to exhibit strong and narrow resonances with nearly complete absorption and quality factors of ~15-20 in the near-infrared. Two-photon luminescence microscopy measurements reveal intensity enhancement factors of ~120 in the 400-nm-period array of 85-nm-wide gold strips atop a 23-nm-thick silica film at the resonance wavelength of ~770nm. Excellent resonant characteristics, the simplicity of tuning the resonance wavelength by adjusting the nanostrip width and/or the dielectric film thickness and the ease of fabrication with (only) one lithography step required make the considered plasmonic configuration very attractive for a wide variety of applications, ranging from surface sensing to photovoltaics.

© 2011 OSA

1. Introduction

Subwavelength confinement and enhancement of light in metallic nanostructures due to resonant excitation of surface plasmon polaritons is a rapidly growing research direction in nano-optics and nanophotonics with the major focus onto the development of new efficient approaches for the delivery of light energy to nanoscale objects and single molecules [1

1. L. Novotny and B. Hecht, “Principles of Nano-Optics,” Cambridge University Press, Cambridge, (2006).

]. This is because of the unique opportunities offered by plasmonic subwavelength resonators for the design of plasmonic nanosensors, nanomanipulation and near-field trapping techniques [2

2. W. Zhang, L. Huang, C. Santschi, and O. J. F. Martin, “Trapping and sensing 10 nm metal nanoparticles using plasmonic dipole antennas,” Nano Lett. 10(3), 1006–1011 (2010). [CrossRef] [PubMed]

, 3

3. M. L. Juan, M. Righini, and R. Quidant, “Plasmon nano-optical tweezers,” Nat. Photonics 5(6), 349–356 (2011). [CrossRef]

], high-resolution probes for nanoimaging and new information processing approaches [4

4. A. Weber-Bargioni, A. Schwartzberg, M. Schmidt, B. Harteneck, D. F. Ogletree, P. J. Schuck, and S. Cabrini, “Functional plasmonic antenna scanning probes fabricated by induced-deposition mask lithography,” Nanotechnology 21(6), 065306 (2010). [CrossRef] [PubMed]

, 5

5. J. N. Farahani, D. W. Pohl, H.-J. Eisler, and B. Hecht, “Single quantum dot coupled to a scanning optical antenna: a tunable superemitter,” Phys. Rev. Lett. 95(1), 017402 (2005). [CrossRef] [PubMed]

], improved photovoltaics [6

6. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

], nanoscale photodetectors with significantly enhanced signal-to-noise ratio [7

7. L. Tang, S. Latif, A. K. Okyay, D.-S. Ly-Gagnon, K. C. Saraswat, and D. A. B. Miller, “Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna,” Nat. Photonics 2(4), 226–229 (2008). [CrossRef]

, 8

8. J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater. 9(3), 193–204 (2010). [CrossRef] [PubMed]

], catalysis applications [8

8. J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater. 9(3), 193–204 (2010). [CrossRef] [PubMed]

], efficient coupling of light energy to nanoscale structures, quantum dots and single molecules [9

9. P. Bharadwaj, B. Deutsch, and L. Novotny, “Optical antennas,” Adv. Opt. Photon. 1(3), 438–483 (2009). [CrossRef]

11

11. D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4(2), 83–91 (2010). [CrossRef]

]. Plasmonic resonators are also expected to result in observation and applications of highly localized and enhanced non-linear effects and near-field spectroscopy, including spectroscopic analysis, imaging and identification of nanoscale amounts of substances and single molecules [8

8. J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater. 9(3), 193–204 (2010). [CrossRef] [PubMed]

12

12. A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Mullen, and W. E. Moerner, “Large single-molecule fluorescence enhancements produced by bowtie nanoantenna,” Nat. Photonics 3(11), 654–657 (2009). [CrossRef]

].

In this respect, special interest present the retardation-based resonant nanostructures [13

13. T. Søndergaard and S. I. Bozhevolnyi, “Slow-plasmon resonant nanostructures: Scattering and field enhancements,” Phys. Rev. B 75(7), 073402 (2007). [CrossRef]

], which are realized by truncating the metal-insulator-metal (MIM) or insulator-metal-insulator (IMI) configurations to achieve multiple reflections of (slow) surface plasmon-polariton modes (gap and short-range surface plasmon modes, respectively) from the terminations (edges) of the obtained structures, similar to a conventional Fabry-Perot resonator [13

13. T. Søndergaard and S. I. Bozhevolnyi, “Slow-plasmon resonant nanostructures: Scattering and field enhancements,” Phys. Rev. B 75(7), 073402 (2007). [CrossRef]

19

19. J. Jung, T. Søndergaard, J. Beermann, A. Boltasseva, and S. I. Bozhevolnyi, “Theoretical analysis and experimental demonstration of resonant light scattering from metal nanostrips on quartz,” J. Opt. Soc. Am. B 26(1), 121–124 (2009). [CrossRef]

]. The comparative analysis of plasmonic IMI and MIM nanoresonators has revealed significant radiative losses from the former due to large electric dipole moments associated with the fundamental resonance modes [14

14. S. I. Bozhevolnyi and T. Søndergaard, “General properties of slow-plasmon resonant nanostructures: Nano-antennas and resonators,” Opt. Express 15(17), 10869–10877 (2007). [CrossRef] [PubMed]

]. Significant advantages of MIM resonators involving gap surface plasmons (GSPs), as compared to IMI nanostrip resonators, were demonstrated [20

20. H. T. Miyazaki and Y. Kurokawa, “Squeezing visible light waves into a 3-nm-thick and 55-nm-long plasmon cavity,” Phys. Rev. Lett. 96(9), 097401 (2006). [CrossRef] [PubMed]

,21

21. P. Bouchon, F. Pardo, B. Portier, L. Ferlazzo, P. Ghenuche, G. Dagher, C. Dupuis, N. Bardou, R. Haidar, and J.-L. Pelouard, “Total funneling of light in high aspect ratio plasmonic nanoresonators,” Appl. Phys. Lett. 98(19), 191109 (2011). [CrossRef]

] and analyzed theoretically [16

16. T. Søndergaard and S. I. Bozhevolnyi, “Strip and gap plasmon polariton optical resonators,” Phys. Stat. Solidi B 245(1), 9–19 (2008). [CrossRef]

,22

22. T. Søndergaard, J. Jung, S. I. Bozhevolnyi, and G. D. Valle, “Theoretical analysis of gold nanostrip gap plasmon resonators,” N. J. Phys. 10(10), 105008 (2008). [CrossRef]

]. Quality factors up to ~20 were predicted for such GSP-based resonators [23

23. G. Lerosey, D. F. P. Pile, P. Matheu, G. Bartal, and X. Zhang, “Controlling the phase and amplitude of plasmon sources at a subwavelength scale,” Nano Lett. 9(1), 327–331 (2009). [CrossRef] [PubMed]

].

In this paper, we introduce and investigate, both theoretically and experimentally, a simple plasmonic resonant structure termed a continuous layer gap plasmon resonator (CL-GPR), for which nanoscale truncation is carried out only for the top layer in a MIM structure (that can be achieved by only a single lithographic step). Spatial extension of the considered GSPs is defined by the width of a metal strip fabricated on a (thin) continuous dielectric layer supported by an underlying thick metal film (underlay). Arrays of CL-GPRs are fabricated and analyzed for different periods and widths of the metal strips featuring strong and narrow resonances. We demonstrate that the predicted and observed resonances occur due to GSPs experiencing multiple reflections from the edges of each of the metal strips.

2. Structure and numerical methods

The considered CL-GPR structure consists of a continuous SiO2 film (nd = 1.45) of thickness t sandwiched between a 200nm-thick continuous gold underlay and a gold nanostrip of height h and width w (Fig. 1
Fig. 1 The configuration of a CL-GPR unit-cell. A continuous SiO2 film with the thickness t is sandwiched between a continuous 200nm thick gold underlay and a gold strip with the height h and width w; the vertical dashed lines indicate the effective resonator length w' > w, caused by an additional plasmon phase shift acquired by the GSP upon reflection from an edge of the metal strip. The domain above the SiO2 film is assumed to be air. The CL-GPR unit-cell is periodically repeated along the x-axis with period Λ. A plane wave is incident normally onto the structure (along the y-direction) and is polarized along the x-axis (TM polarization).
). The cladding above the structure is assumed to be air (n = 1), and the CL-GPR structure is uniform along the z-direction (Fig. 1). The physical principles for the considered resonator structures are similar to those of a conventional Fabry-Perot resonator. The GSP guided by the gap (filled with SiO2) between the metal strip and the underlay experiences multiple reflections from the terminations (edges) of the strip. The radiation losses at the edges of the strip are due to electric dipole radiation caused by the oscillating opposite charges across the gap. However, because the size of this electric dipole moment is ~t (i.e., in tens of nanometers - much smaller than the wavelength), these radiation losses are significantly weaker than those in the case of the metal strip (IMI) resonator [17

17. T. Søndergaard, J. Beermann, A. Boltasseva, and S. I. Bozhevolnyi, “Slow-plasmon resonant-nanostrip antennas: Analysis and demonstration,” Phys. Rev. B 77(11), 115420 (2008). [CrossRef]

]. As a result, the typical Q-factors of CL-GPR are expected to be significantly higher than for the IMI resonators.

To model the resonator's optical response, we use the finite-element method (FEM) implemented in the commercial software COMSOL MULTIPHYSICS, and consider a monochromatic plane wave with the amplitude E0, polarized along the x-axis (TM-wave) and incident normally onto the CL-GPR structure (Fig. 1). The perfect electric conductor boundary conditions are applied to the unit-cell side walls, thereby mimicking structural periodicity in the x-direction [28

28. S. H. Lim, W. Mar, P. Matheu, D. Derkacs, and E. T. Yu, “Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles,” J. Appl. Phys. 101(10), 104309 (2007). [CrossRef]

]. The corners of the gold nanostrip are assumed to be rounded with the radius 5nm, and the permittivity of gold is described by a complex-valued frequency-dependent dielectric function εm(ω) obtained by the cubic interpolation of the tabular values [29

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

]. The CL-GPR reflectivity is calculated as a function of wavelength by integrating the Poynting vector along a first-order absorption boundary [30

30. J. Jin, “The finite element method in electromagnetics,” Wiley: New York, p 429 (1993).

] also generating the incident wave and positioned at 800nm above the gold underlay. The calculated reflectivity is normalized to the reflectivity from a uniform smooth gold surface with the thin SiO2 layer. The accuracy and validity of the obtained results were also confirmed by using a perfectly matched layer, instead of the first-order absorption boundary, to suppress the artificial reflections.

3. Reflectivity dependence on structural parameters

Figure 2(a) also demonstrates that decreasing thickness t of the SiO2 layer results in increasing resonant wavelength. This can be regarded as further confirmation that the predicted resonances are caused by GSPs between the gold strips and gold underlay, because decreasing thickness of the SiO2 layer (gap width) results in decreasing GSP wavelength, i.e., increasing vacuum wavelength corresponding to the fundamental resonance at a fixed width of the gold strip. However, this simple trend of increasing resonant wavelength with decreasing t does not seem to hold well at larger thicknesses of the SiO2 layer [solid curves in Fig. 2 (c)]. For example, for the CL-GPR array with the period Λ = 400nm [the lower solid curve in Fig. 2(c)], the resonant wavelength tends to be approximately constant for the SiO2 thicknesses t > 50nm. Furthermore, for the CL-GPR array with the period Λ = 600nm [the upper solid curve in Fig. 2 (c)], the resonant wavelength tends to increase (the resonance is redshifted) for t > 30 nm. This observed behavior of the resonant wavelength can be explained as follows.

The resonance condition for CL-GPR can be written as for any other Fabry-Perot-type resonator:
w2πλngsp=mπφ,
(1)
where λ is the free-space wavelength, ngsp is the effective index for the GSP, m is an integer determining the order of a resonance mode, and φ is the phase acquired by the GSP upon its reflection at the resonator terminations (edges of the strip). Considering only the fundamental mode (m = 1) and defining the phase parameter η=12(1φ/π), Eq. (1) can be rearranged as:

wngsp=λη.
(2)

The obtained results and interpretations can be further illustrated by the distributions of the electric field in CL-GPRs (Fig. 3
Fig. 3 Typical distributions of the resonant electric field magnitude in the (x,y)-plane for the two different SiO2 thicknesses (a) t = 20nm and (b) t = 70nm in a periodic CL-GPR array with Λ = 400nm. (c) The dependences of the normalized (to the amplitude of the incident wave E0) local electric field in the middle of the SiO2 layer for Λ = 400nm; both the dependences were plotted for the resonant wavelengths of 706nm and 616nm for the respective values of t = 20nm and t = 70nm.
and Fig. 4
Fig. 4 (a,b) Typical distributions of the resonant electric field magnitude in the (x,y)-plane for the two different SiO2 layer thicknesses (a) t = 20nm and (b) t = 70nm in a periodic CL-GPR array with Λ = 600nm. (c) The dependences of the normalized (to the amplitude of the incident wave E0) local electric field in the middle of the SiO2 layer for Λ = 600nm; both the dependences were plotted for the resonant wavelengths of 746nm and 762nm for the respective values of t = 20nm and t = 70nm.
). The presented electric field distributions clearly display the standing wave patterns for the GSP with a node under the center of the gold strip, as expected for the fundamental resonance mode. It is also evident that even for very thin SiO2 layers the mode fields significantly extend beyond the edges of the gold strip [Fig. 3(a) and Fig. 4(a)], which is a confirmation of the above conclusion that the effective resonator length w' appears significantly larger than the width of the gold strip w. If the SiO2 thickness is increased [e.g., to ~t = 70nm - Fig. 3(b) and Fig. 4(b)] the fundamental mode becomes practically delocalized and effectively spreads over the whole CL-GPR array. Importantly, this delocalization is stronger for the larger array period of 600 nm [Fig. 4(b)], which is probably the consequence of the discussed coupling of the CL-GPR cavities by means of surface plasmons in the gaps between the gold strips (see above). This delocalization of the fundamental CL-GPR mode in the x-direction at larger values of t is also demonstrated by the dependences of the normalized electric field in the middle of the SiO2 layer as a function of the x-coordinate [Fig. 3(c) and Fig. 4(c)]. Despite the discussed delocalization of the fundamental CL-GPR mode (and thus efficient coupling between the CL-GPRs in the array), the electric field is still significantly localized within or near the thin SiO2 layer [Fig. 3(b) and Fig. 4(b)]. Thus the delocalization caused by the cavity coupling occurs anisotropically and mainly along the dielectric layer, rather than in the direction perpendicular to it. This is a practically important aspect as it allows localization of the plasmonic resonantly enhanced field within a thin dielectric (semiconductor) layer, which could be a significant opportunity for increasing efficiency of photovoltaic devices and photodetectors.

3. Linear reflectivity measurements

The experimental investigation of CL-GPRs was conducted using several 30µm × 30µm periodic arrays - each consisting of 5 rows of 5µm long and 53nm thick gold nanostrips [Fig. 5(a,b)
Fig. 5 (a) Schematic of the 30µm × 30µm CL-GPR array in the form of gold strips on a 23nm thick SiO2 film and 100nm thick gold underlay. (b) A representative scanning-electron microscopy image showing a small section of a CL-GPR array with w = 135nm, h = 53nm, t = 23nm and Λ = 600nm.
]. The separation between the neighboring rows of strips was 1µm [Fig. 5(a)]. The strip width w and the period Λ within each row were fixed for the same array, but different for different arrays. The nanostrip arrays were fabricated in a single-step lithography, followed by lift-off applied to a 3nm thick titanium adhesion layer and 50nm thick gold film, on a silicon wafer which was pre-coated by a 100nm thick gold film (gold underlay) and 23nm thick SiO2 film, deposited by electron-beam deposition and RF-sputtering, respectively.

In order to measure the reflectivity spectra, light from a broad band halogen light source was TM-polarized and subsequently focused by an objective, having 60x magnification and 0.85 numerical aperture, onto the 30µm × 30µm array. The reflected light was collected by the same objective, filtered spatially such that only the reflected light from the array was collected, sent through the analyzer (parallel to the polarizer) and finally collected by an optical fiber connected to a VIS/NIR spectrometer. The reflectivity from the array was normalized to the reflectivity from a similar-sized control patch of the continuous thick (100nm) gold underlay covered in the 23nm SiO2 film with no gold strips forming CL-GPR arrays. The control patch was positioned close to the analyzed CL-GPR arrays to ensure approximately the same structural and material parameters as within the arrays.

Figure 6(a) and Fig. 6(b) also suggest that there are optimal structural parameters such as period Λ and width of the strips w at which the reflectivity of the incident TM radiation from the structure may be close to zero due to nearly 100% coupling of this radiation into the fundamental CL-GPR mode [see the solid curve in Fig. 6(b) and the solid and dashed curves in Fig. 6(a)]. In particular, the obtained dependences [Fig. 6(a) and Fig. 6(b)] suggest that the separation between the neighboring gold strips may be one of the most important optimization parameters. For example, increasing strip width w at a fixed period Λ = 600nm (i.e., decreasing separation between the gold strips) results in increasing strength of the CL-GPR fundamental resonance [Fig. 6(a)]. Similarly, decreasing period Λ at a fixed width w = 85nm (i.e., again decreasing separation between the gold strips) also results in increasing strength of the CL-GPR fundamental resonance [Fig. 6(b)]. Optimal separation between the strips may be expected to ensure the optimal coupling efficiency of the incident radiation into the fundamental mode of CL-GPRs, leading to the nearly 100% absorption of the incident radiation.

The Rayleigh anomaly (the grating resonance) is not clearly seen in the experimental spectra for Λ = 600nm, which could be explained by the fact that we used a high numerical aperture in order to strongly focus radiation from the halogen light source (achieving the spot size of ~80µm) instead of the normally incident plane wave in the simulation (see also [26

26. Y. Chu, M. G. Banaee, and K. B. Crozier, “Double-resonance plasmon substrates for surface-enhanced Raman scattering with enhancement at excitation and stokes frequencies,” ACS Nano 4(5), 2804–2810 (2010). [CrossRef] [PubMed]

]). This might also be one of the reasons for the fact that the experimentally obtained reflectivity minimum for Λ = 600nm is less pronounced than for Λ = 400nm [the solid and dashed curves in Fig. 6 (b)], which is the opposite tendency compared to the theoretical predictions [Fig. 6 (d)].

The theoretically predicted Q-factors for the fundamental CL-GPR resonance are up to ~20 when Λ = 600nm, which is close to the quasistatic limit [31

31. F. Wang and Y. R. Shen, “General properties of local plasmons in metal nanostructures,” Phys. Rev. Lett. 97(20), 206806 (2006). [CrossRef] [PubMed]

]. For Λ = 400nm the theoretically predicted Q-factors are ~8, which is comparable with the experimental value of ~6.

4. Scanning two-photon luminescence microscopy

The predicted enhancement of the electric field was ~22 in the middle of the SiO2 layer for Λ = 600nm [Fig. 4(c)] i.e., larger than for Λ = 400nm - Fig. 3(c). However, the experimentally observed fundamental CL-GPR resonance was stronger at Λ = 400nm and, therefore, the field enhancement is expected to be larger for Λ = 400nm. The local field intensity enhancements (|E|/|E0|)2 in the CL-GPR arrays were evaluated experimentally using the scanning two-photon luminescence (TPL) microscopy whose experimental setup was described for the enhancement measurements for resonances in gold strips [32

32. J. Beermann, S. M. Novikov, T. Søndergaard, A. E. Boltasseva, and S. I. Bozhevolnyi, “Two-photon mapping of localized field enhancements in thin nanostrip antennas,” Opt. Express 16(22), 17302–17309 (2008). [CrossRef] [PubMed]

], split-ring resonators [33

33. M. G. Nielsen, A. Pors, R. B. Nielsen, A. Boltasseva, O. Albrektsen, and S. I. Bozhevolnyi, “Demonstration of scattering suppression in retardation-based plasmonic nanoantennas,” Opt. Express 18(14), 14802–14811 (2010). [CrossRef] [PubMed]

], gold disk fractal patterns [34

34. J. Beermann, I. P. Radko, A. Boltasseva, and S. I. Bozhevolnyi, “Localized field enhancements in fractal shaped periodic metal nanostructures,” Opt. Express 15(23), 15234–15241 (2007). [CrossRef] [PubMed]

,35

35. J. Beermann, A. Evlyukhin, A. Boltasseva, and S. I. Bozhevolnyi, “Nonlinear microscopy of localized field enhancements in fractal shaped periodic metal nanostructures,” J. Opt. Soc. Am. B 25(10), 1585–1592 (2008). [CrossRef]

], and tapered periodic slits in gold [36

36. J. Beermann, T. Søndergaard, S. M. Novikov, S. I. Bozhevolnyi, E. Devaux, and T. W. Ebbesen, “Field enhancement and extraordinary optical transmission by tapered periodic slits in gold films,” N. J. Phys. 13(6), 063029 (2011). [CrossRef]

]. To measure TPL from the CL-GPR arrays, the first harmonic (FH) wavelength of a pulsating mode-locked Ti-Sapphire laser was used with ~200fs pulse, ~80MHz repetition rate, fixed average incident power of ~0.1mW, and tight focusing (spot size ~0.8µm) by means of a Mitutoyo infinity-corrected (x100, numerical aperture 0.70) objective. The reflected FH and TPL were concurrently collected by the same objective and by using a scanning computer-controlled translation stage with the 200nm step size and accuracy of ~4nm a 10µm × 10µm region partly overlapping with a CL-GPR array was scanned. The FH- and TPL signals were separated by the appropriate filters and detected by photomultiplier tubes (PMTs). The PMT for detection of the TPL signal was connected to a photon counter with ~10 dark counts per second (cps) and the integration time of 100ms. As for the linear reflectivity measurements [Fig. 6(a,b)], the polarizer and analyzer were both aligned perpendicular to the nanostrips.

The typical FH and TPL images at the wavelength 760nm are shown in Fig. 7(a)
Fig. 7 Typical FH (a) and TPL (b) images near the corner of a CL-GPR array with w = 85nm and Λ = 400nm; the polarization of the incident beam is in the x-direction. (c) The typical measured x-dependencies of the FH reflectivity and normalized TPL signals along the x-direction across the array edge at x~4μm. (d) The estimated intensity enhancement factor α as a function of the incident wavelength, superimposed with the measured reflectivity spectrum.
and Fig. 7(b), respectively. The corner of a CL-GPR array is seen as the black region [Fig. 7(a)] and the bright (yellow) region [Fig. 7(b)] in the FH and TPL images. The x-dependence of the FH signal [Fig. 7(c)] clearly shows that the FH-reflectivity is low inside (~0.2) and high outside (~0.9) the array. These values are in good agreement with the reflectivities obtained by means of the broadband light source. The TPL dependence in Fig. 7(c) features high contrast with the strong signal with the average of ~350 cps inside the array, and nearly zero outside the array. This confirms that the CL-GPR arrays significantly localize and enhance light intensity compared to the control patch with no gold strips [the black region in Fig. 7(b)].

In order to estimate the intensity enhancement as a function of wavelength, TPL scanning was conducted for the wavelengths between 730nm and 820nm in 10nm wavelength steps. The intensity enhancement factor α can be estimated for each wavelength via the expression [37

37. P. J. Schuck, D. P. Fromm, A. Sundaramurthy, G. S. Kino, and W. E. Moerner, “Improving the mismatch between light and nanoscale objects with gold bowtie nanoantennas,” Phys. Rev. Lett. 94(1), 017402 (2005). [CrossRef] [PubMed]

]:
TPLarrayTPLref=α2Parray2AarrayPref2Aref,
(3)
where TPLarray is the average TPL signal per one pixel (i.e., one scanning step), obtained from the ~6µm × 4µm region inside the CL-GPR array [bright region in Fig. 7(b)], Parray is the average power of ~0.1mW in the focal spot during scanning, Aarray is the area with the locally enhanced fields within a region whose area is equal to Aref that is the area of the focused FHspot (typically, Aarray< Aref), TPLref is the average TPL signal per one pixel, obtained from scanning a 2µm × 2µm reference patch of the smooth continuous gold film of 50nm thickness (deposited onto the same structure and in the same process as the CL-GPR gold strips), Pref2 is the average power between 4mW and 8mW (depending on the wavelength) in the focal spot during scanning of the reference patch of smooth gold film.

As it is difficult to evaluateAarrayand the coupling of TPL from the structure to free space without comprehensive theoretical investigations, the enhancement factor α [Eq. (3)] should be understood as a measure of average enhancement in the structure as compared to smooth gold. In this particular case the enhancement factor α was estimated under the assumption: Aarray = Aref. The resultant experimental dependence of α on wavelength is shown in Fig. 7(d) and demonstrates excellent agreement of the enhancement maximum with the minimum of the measured reflectivity spectrum for the CL-GPR array with Λ = 400nm - both occurring at the same incident wavelength of 760nm [Fig. 7(d)]. Because TPL is a surface-sensitive technique, the reflectivity minimum coinciding with the enhancement maximum is important as this verifies that the estimated enhancement is caused by the local intense fields in CL-GPRs, rather than by surface roughness or other irregularities. It is also important to note that the assumption Aarray = Aref (instead of the actual Aarray<Aref) results in a likely underestimate of the obtained experimental value of α ~126. This is in agreement with the theoretical predictions of larger intensity enhancements of ~225 [Fig. 3(c)].

5. Conclusions

In summary, we have demonstrated numerically and validated experimentally efficient resonant excitation of GSPs in the CL-GPR structures formed by gold nanostrips (fabricated by single-step lithography) on a continuous SiO2 film on a thick gold underlay. The possibility of nearly 100% resonant absorption of the incident TM radiation due to its efficient coupling into the fundamental CL-GPR mode was demonstrated theoretically and confirmed experimentally for thin SiO2 films with the thickness of ~10 - 20nm. Though obtained for the retardation-based resonances involving GSPs, the theoretically predicted Q-factors for the fundamental CL-GPR modes were shown to be close to the quasistatic limit [31

31. F. Wang and Y. R. Shen, “General properties of local plasmons in metal nanostructures,” Phys. Rev. Lett. 97(20), 206806 (2006). [CrossRef] [PubMed]

]. The measurements using the scanning TPL microscopy confirmed major local field enhancement in the considered structures.

The localization of the significant portion of the enhanced local field inside the thin dielectric layer opens excellent opportunities for using the considered CL-GPR arrays to increase efficiency of photovoltaic devices and design photodetectors with enhanced signal-to-noise ratio. The CL-GPR structures with the obtained high Q-factors can also form an efficient and cost-effective basis for other plasmonic applications such as nano-optical sensors and surface-enhanced Raman spectroscopy techniques, including single-molecule detection and identification.

Acknowledgements

We acknowledge financial support for this work from the VELUX Foundation and from the Danish Council for Independent Research (the FTP project ANAP, contract no. 09-072949).

References and links

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

2.

W. Zhang, L. Huang, C. Santschi, and O. J. F. Martin, “Trapping and sensing 10 nm metal nanoparticles using plasmonic dipole antennas,” Nano Lett. 10(3), 1006–1011 (2010). [CrossRef] [PubMed]

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M. L. Juan, M. Righini, and R. Quidant, “Plasmon nano-optical tweezers,” Nat. Photonics 5(6), 349–356 (2011). [CrossRef]

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A. Weber-Bargioni, A. Schwartzberg, M. Schmidt, B. Harteneck, D. F. Ogletree, P. J. Schuck, and S. Cabrini, “Functional plasmonic antenna scanning probes fabricated by induced-deposition mask lithography,” Nanotechnology 21(6), 065306 (2010). [CrossRef] [PubMed]

5.

J. N. Farahani, D. W. Pohl, H.-J. Eisler, and B. Hecht, “Single quantum dot coupled to a scanning optical antenna: a tunable superemitter,” Phys. Rev. Lett. 95(1), 017402 (2005). [CrossRef] [PubMed]

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H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

7.

L. Tang, S. Latif, A. K. Okyay, D.-S. Ly-Gagnon, K. C. Saraswat, and D. A. B. Miller, “Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna,” Nat. Photonics 2(4), 226–229 (2008). [CrossRef]

8.

J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater. 9(3), 193–204 (2010). [CrossRef] [PubMed]

9.

P. Bharadwaj, B. Deutsch, and L. Novotny, “Optical antennas,” Adv. Opt. Photon. 1(3), 438–483 (2009). [CrossRef]

10.

L. Novotny and N. Van Hulst, “Antennas for light,” Nat. Photonics 5(2), 83–90 (2011). [CrossRef]

11.

D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4(2), 83–91 (2010). [CrossRef]

12.

A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Mullen, and W. E. Moerner, “Large single-molecule fluorescence enhancements produced by bowtie nanoantenna,” Nat. Photonics 3(11), 654–657 (2009). [CrossRef]

13.

T. Søndergaard and S. I. Bozhevolnyi, “Slow-plasmon resonant nanostructures: Scattering and field enhancements,” Phys. Rev. B 75(7), 073402 (2007). [CrossRef]

14.

S. I. Bozhevolnyi and T. Søndergaard, “General properties of slow-plasmon resonant nanostructures: Nano-antennas and resonators,” Opt. Express 15(17), 10869–10877 (2007). [CrossRef] [PubMed]

15.

T. Søndergaard and S. I. Bozhevolnyi, “Metal nano-strip optical resonators,” Opt. Express 15(7), 4198–4204 (2007). [CrossRef] [PubMed]

16.

T. Søndergaard and S. I. Bozhevolnyi, “Strip and gap plasmon polariton optical resonators,” Phys. Stat. Solidi B 245(1), 9–19 (2008). [CrossRef]

17.

T. Søndergaard, J. Beermann, A. Boltasseva, and S. I. Bozhevolnyi, “Slow-plasmon resonant-nanostrip antennas: Analysis and demonstration,” Phys. Rev. B 77(11), 115420 (2008). [CrossRef]

18.

J. Jung, T. Søndergaard, and S. I. Bozhevolnyi, “Gap plasmon-polariton nanoresonators: Scattering enhancement and launching of surface plasmon polaritons,” Phys. Rev. B 79(3), 035401 (2009). [CrossRef]

19.

J. Jung, T. Søndergaard, J. Beermann, A. Boltasseva, and S. I. Bozhevolnyi, “Theoretical analysis and experimental demonstration of resonant light scattering from metal nanostrips on quartz,” J. Opt. Soc. Am. B 26(1), 121–124 (2009). [CrossRef]

20.

H. T. Miyazaki and Y. Kurokawa, “Squeezing visible light waves into a 3-nm-thick and 55-nm-long plasmon cavity,” Phys. Rev. Lett. 96(9), 097401 (2006). [CrossRef] [PubMed]

21.

P. Bouchon, F. Pardo, B. Portier, L. Ferlazzo, P. Ghenuche, G. Dagher, C. Dupuis, N. Bardou, R. Haidar, and J.-L. Pelouard, “Total funneling of light in high aspect ratio plasmonic nanoresonators,” Appl. Phys. Lett. 98(19), 191109 (2011). [CrossRef]

22.

T. Søndergaard, J. Jung, S. I. Bozhevolnyi, and G. D. Valle, “Theoretical analysis of gold nanostrip gap plasmon resonators,” N. J. Phys. 10(10), 105008 (2008). [CrossRef]

23.

G. Lerosey, D. F. P. Pile, P. Matheu, G. Bartal, and X. Zhang, “Controlling the phase and amplitude of plasmon sources at a subwavelength scale,” Nano Lett. 9(1), 327–331 (2009). [CrossRef] [PubMed]

24.

G. Lévêque and O. J. F. Martin, “Tunable composite nanoparticle for plasmonics,” Opt. Lett. 31(18), 2750–2752 (2006). [CrossRef] [PubMed]

25.

Y. Chu and K. B. Crozier, “Experimental study of the interaction between localized and propagating surface plasmons,” Opt. Lett. 34(3), 244–246 (2009). [CrossRef] [PubMed]

26.

Y. Chu, M. G. Banaee, and K. B. Crozier, “Double-resonance plasmon substrates for surface-enhanced Raman scattering with enhancement at excitation and stokes frequencies,” ACS Nano 4(5), 2804–2810 (2010). [CrossRef] [PubMed]

27.

R. Ameling, D. Dregely, and H. Giessen, “Strong coupling of localized and surface plasmons to microcavity modes,” Opt. Lett. 36(12), 2218–2220 (2011). [CrossRef] [PubMed]

28.

S. H. Lim, W. Mar, P. Matheu, D. Derkacs, and E. T. Yu, “Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles,” J. Appl. Phys. 101(10), 104309 (2007). [CrossRef]

29.

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

30.

J. Jin, “The finite element method in electromagnetics,” Wiley: New York, p 429 (1993).

31.

F. Wang and Y. R. Shen, “General properties of local plasmons in metal nanostructures,” Phys. Rev. Lett. 97(20), 206806 (2006). [CrossRef] [PubMed]

32.

J. Beermann, S. M. Novikov, T. Søndergaard, A. E. Boltasseva, and S. I. Bozhevolnyi, “Two-photon mapping of localized field enhancements in thin nanostrip antennas,” Opt. Express 16(22), 17302–17309 (2008). [CrossRef] [PubMed]

33.

M. G. Nielsen, A. Pors, R. B. Nielsen, A. Boltasseva, O. Albrektsen, and S. I. Bozhevolnyi, “Demonstration of scattering suppression in retardation-based plasmonic nanoantennas,” Opt. Express 18(14), 14802–14811 (2010). [CrossRef] [PubMed]

34.

J. Beermann, I. P. Radko, A. Boltasseva, and S. I. Bozhevolnyi, “Localized field enhancements in fractal shaped periodic metal nanostructures,” Opt. Express 15(23), 15234–15241 (2007). [CrossRef] [PubMed]

35.

J. Beermann, A. Evlyukhin, A. Boltasseva, and S. I. Bozhevolnyi, “Nonlinear microscopy of localized field enhancements in fractal shaped periodic metal nanostructures,” J. Opt. Soc. Am. B 25(10), 1585–1592 (2008). [CrossRef]

36.

J. Beermann, T. Søndergaard, S. M. Novikov, S. I. Bozhevolnyi, E. Devaux, and T. W. Ebbesen, “Field enhancement and extraordinary optical transmission by tapered periodic slits in gold films,” N. J. Phys. 13(6), 063029 (2011). [CrossRef]

37.

P. J. Schuck, D. P. Fromm, A. Sundaramurthy, G. S. Kino, and W. E. Moerner, “Improving the mismatch between light and nanoscale objects with gold bowtie nanoantennas,” Phys. Rev. Lett. 94(1), 017402 (2005). [CrossRef] [PubMed]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(190.0190) Nonlinear optics : Nonlinear optics
(240.6680) Optics at surfaces : Surface plasmons
(260.3910) Physical optics : Metal optics
(300.1030) Spectroscopy : Absorption
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Optics at Surfaces

History
Original Manuscript: July 26, 2011
Revised Manuscript: August 25, 2011
Manuscript Accepted: September 7, 2011
Published: September 20, 2011

Citation
Michael G. Nielsen, Dmitri K. Gramotnev, Anders Pors, Ole Albrektsen, and Sergey I. Bozhevolnyi, "Continuous layer gap plasmon resonators," Opt. Express 19, 19310-19322 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-20-19310


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References

  1. L. Novotny and B. Hecht, “Principles of Nano-Optics,” Cambridge University Press, Cambridge, (2006).
  2. W. Zhang, L. Huang, C. Santschi, and O. J. F. Martin, “Trapping and sensing 10 nm metal nanoparticles using plasmonic dipole antennas,” Nano Lett.10(3), 1006–1011 (2010). [CrossRef] [PubMed]
  3. M. L. Juan, M. Righini, and R. Quidant, “Plasmon nano-optical tweezers,” Nat. Photonics5(6), 349–356 (2011). [CrossRef]
  4. A. Weber-Bargioni, A. Schwartzberg, M. Schmidt, B. Harteneck, D. F. Ogletree, P. J. Schuck, and S. Cabrini, “Functional plasmonic antenna scanning probes fabricated by induced-deposition mask lithography,” Nanotechnology21(6), 065306 (2010). [CrossRef] [PubMed]
  5. J. N. Farahani, D. W. Pohl, H.-J. Eisler, and B. Hecht, “Single quantum dot coupled to a scanning optical antenna: a tunable superemitter,” Phys. Rev. Lett.95(1), 017402 (2005). [CrossRef] [PubMed]
  6. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater.9(3), 205–213 (2010). [CrossRef] [PubMed]
  7. L. Tang, S. Latif, A. K. Okyay, D.-S. Ly-Gagnon, K. C. Saraswat, and D. A. B. Miller, “Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna,” Nat. Photonics2(4), 226–229 (2008). [CrossRef]
  8. J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater.9(3), 193–204 (2010). [CrossRef] [PubMed]
  9. P. Bharadwaj, B. Deutsch, and L. Novotny, “Optical antennas,” Adv. Opt. Photon.1(3), 438–483 (2009). [CrossRef]
  10. L. Novotny and N. Van Hulst, “Antennas for light,” Nat. Photonics5(2), 83–90 (2011). [CrossRef]
  11. D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics4(2), 83–91 (2010). [CrossRef]
  12. A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Mullen, and W. E. Moerner, “Large single-molecule fluorescence enhancements produced by bowtie nanoantenna,” Nat. Photonics3(11), 654–657 (2009). [CrossRef]
  13. T. Søndergaard and S. I. Bozhevolnyi, “Slow-plasmon resonant nanostructures: Scattering and field enhancements,” Phys. Rev. B75(7), 073402 (2007). [CrossRef]
  14. S. I. Bozhevolnyi and T. Søndergaard, “General properties of slow-plasmon resonant nanostructures: Nano-antennas and resonators,” Opt. Express15(17), 10869–10877 (2007). [CrossRef] [PubMed]
  15. T. Søndergaard and S. I. Bozhevolnyi, “Metal nano-strip optical resonators,” Opt. Express15(7), 4198–4204 (2007). [CrossRef] [PubMed]
  16. T. Søndergaard and S. I. Bozhevolnyi, “Strip and gap plasmon polariton optical resonators,” Phys. Stat. Solidi B245(1), 9–19 (2008). [CrossRef]
  17. T. Søndergaard, J. Beermann, A. Boltasseva, and S. I. Bozhevolnyi, “Slow-plasmon resonant-nanostrip antennas: Analysis and demonstration,” Phys. Rev. B77(11), 115420 (2008). [CrossRef]
  18. J. Jung, T. Søndergaard, and S. I. Bozhevolnyi, “Gap plasmon-polariton nanoresonators: Scattering enhancement and launching of surface plasmon polaritons,” Phys. Rev. B79(3), 035401 (2009). [CrossRef]
  19. J. Jung, T. Søndergaard, J. Beermann, A. Boltasseva, and S. I. Bozhevolnyi, “Theoretical analysis and experimental demonstration of resonant light scattering from metal nanostrips on quartz,” J. Opt. Soc. Am. B26(1), 121–124 (2009). [CrossRef]
  20. H. T. Miyazaki and Y. Kurokawa, “Squeezing visible light waves into a 3-nm-thick and 55-nm-long plasmon cavity,” Phys. Rev. Lett.96(9), 097401 (2006). [CrossRef] [PubMed]
  21. P. Bouchon, F. Pardo, B. Portier, L. Ferlazzo, P. Ghenuche, G. Dagher, C. Dupuis, N. Bardou, R. Haidar, and J.-L. Pelouard, “Total funneling of light in high aspect ratio plasmonic nanoresonators,” Appl. Phys. Lett.98(19), 191109 (2011). [CrossRef]
  22. T. Søndergaard, J. Jung, S. I. Bozhevolnyi, and G. D. Valle, “Theoretical analysis of gold nanostrip gap plasmon resonators,” N. J. Phys.10(10), 105008 (2008). [CrossRef]
  23. G. Lerosey, D. F. P. Pile, P. Matheu, G. Bartal, and X. Zhang, “Controlling the phase and amplitude of plasmon sources at a subwavelength scale,” Nano Lett.9(1), 327–331 (2009). [CrossRef] [PubMed]
  24. G. Lévêque and O. J. F. Martin, “Tunable composite nanoparticle for plasmonics,” Opt. Lett.31(18), 2750–2752 (2006). [CrossRef] [PubMed]
  25. Y. Chu and K. B. Crozier, “Experimental study of the interaction between localized and propagating surface plasmons,” Opt. Lett.34(3), 244–246 (2009). [CrossRef] [PubMed]
  26. Y. Chu, M. G. Banaee, and K. B. Crozier, “Double-resonance plasmon substrates for surface-enhanced Raman scattering with enhancement at excitation and stokes frequencies,” ACS Nano4(5), 2804–2810 (2010). [CrossRef] [PubMed]
  27. R. Ameling, D. Dregely, and H. Giessen, “Strong coupling of localized and surface plasmons to microcavity modes,” Opt. Lett.36(12), 2218–2220 (2011). [CrossRef] [PubMed]
  28. S. H. Lim, W. Mar, P. Matheu, D. Derkacs, and E. T. Yu, “Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles,” J. Appl. Phys.101(10), 104309 (2007). [CrossRef]
  29. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B6(12), 4370–4379 (1972). [CrossRef]
  30. J. Jin, “The finite element method in electromagnetics,” Wiley: New York, p 429 (1993).
  31. F. Wang and Y. R. Shen, “General properties of local plasmons in metal nanostructures,” Phys. Rev. Lett.97(20), 206806 (2006). [CrossRef] [PubMed]
  32. J. Beermann, S. M. Novikov, T. Søndergaard, A. E. Boltasseva, and S. I. Bozhevolnyi, “Two-photon mapping of localized field enhancements in thin nanostrip antennas,” Opt. Express16(22), 17302–17309 (2008). [CrossRef] [PubMed]
  33. M. G. Nielsen, A. Pors, R. B. Nielsen, A. Boltasseva, O. Albrektsen, and S. I. Bozhevolnyi, “Demonstration of scattering suppression in retardation-based plasmonic nanoantennas,” Opt. Express18(14), 14802–14811 (2010). [CrossRef] [PubMed]
  34. J. Beermann, I. P. Radko, A. Boltasseva, and S. I. Bozhevolnyi, “Localized field enhancements in fractal shaped periodic metal nanostructures,” Opt. Express15(23), 15234–15241 (2007). [CrossRef] [PubMed]
  35. J. Beermann, A. Evlyukhin, A. Boltasseva, and S. I. Bozhevolnyi, “Nonlinear microscopy of localized field enhancements in fractal shaped periodic metal nanostructures,” J. Opt. Soc. Am. B25(10), 1585–1592 (2008). [CrossRef]
  36. J. Beermann, T. Søndergaard, S. M. Novikov, S. I. Bozhevolnyi, E. Devaux, and T. W. Ebbesen, “Field enhancement and extraordinary optical transmission by tapered periodic slits in gold films,” N. J. Phys.13(6), 063029 (2011). [CrossRef]
  37. P. J. Schuck, D. P. Fromm, A. Sundaramurthy, G. S. Kino, and W. E. Moerner, “Improving the mismatch between light and nanoscale objects with gold bowtie nanoantennas,” Phys. Rev. Lett.94(1), 017402 (2005). [CrossRef] [PubMed]

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