Elongation of surface plasmon polariton propagation ... - OSA Publishing

Elongation of surface plasmon polariton propagation length without gain G. Zhu, M. Mayy, M. Bahoura, B. A. Ritzo, H. V. Gavrilenko, V. I. Gavrilenko, M. A. Noginov Center for Materials Research, Norfolk State University, 700 Park Ave., Norfolk, VA, USA 23504 * Corresponding author:[email protected]

Abstract: We have demonstrated that an addition of highly concentrated rhodamine 6G chloride dye to the PMMA film adjacent to a silver film can cause 30% elongation of the propagation length of surface plasmon polaritons (SPPs). The possibility to elongate the SPP propagation length without optical gain opens a new technological dimension to low-loss nanoplasmonic and metamaterials. ”2008 Optical Society of America OCIS codes: (250.5403) Plasmonics; (240.5420) Polaritons; (240.6680) Surface plasmons

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17.

M. Moskovits, “Surface-enhanced spectroscopy,” Rev. Mod. Phys. 57, 783-826 (1985). K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, “Single molecule detection using surface-enhanced Raman scattering (SERS),” Phys. Rev. Lett. 78, 1667–1670 (1997). S. Nie and S. R. Emory, “Probing single molecules and single nanoparticles by surface-enhanced Raman scattering,” Science 275, 1102–1104 (1997). S. I. Bozhevolnyi, V. S. Volkov, and K. Leosson, “Localization and Waveguiding of Surface Plasmon Polaritons in Random Nanostructures,” Phys. Rev. Lett. 89, 186801 (2002). A. Boltasseva, S. Bozhevolnyi, T. Søndergaard, T. Nikolajsen, and K. Leosson, “Compact Z-add-drop wavelength filters for long-range surface plasmon polaritons,” Opt. Express 13, 4237-4243 (2005). S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Hare, B. E. Koe and A. Requicha, “Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides,” Nature Materials 2, 229-232 (2003). A. Karalis, E. Lidorikis, M. Ibanescu, J. D. Joannopoulos, and M. Solja, “Surface-plasmon-assisted guiding of broadband slow and subwavelength light in air,” Phys. Rev. Lett. 95, 063901 (2005). M. Stockman, “Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides,” Phys. Rev. Lett. 93, 137404 (2004). C. Sirtori, C. Gmachl, F. Capasso, J. Faist, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, “Long-wavelength (l ~ 8-11.5 µm) semiconductor lasers with waveguides based on surface plasmons,” Opt. Lett. 23, 1366-1368 (1998). Thomas A. Klar; Alexander V. Kildishev; Vladimir P. Drachev; Vladimir M. Shalaev, “Negative-index metamaterials: going optical,” IEEE J. Sel. Top. Quantum Electron. 12, 1106-1115 (2006). R. Wangberg, J. Elser, E. E. Narimanov, and V. A. Podolskiy, “Nonmagnetic nanocomposites for optical and infrared negative-refractive-index media,” J. Opt. Soc. Am. B 23, 498-505 (2006). J. Elser, V. A. Podolskiy, I. Salakhutdinov, I. Avrutsky, “Nonlocal effects in effective-medium response of nanolayered metamaterials,” Appl. Phys. Lett. 90, 191109 (2007) (3 pages). A. N. Sudarkin and P. A. Demkovich, “Excitation of surface electromagnetic waves on the boundary of a metal with an amplifying medium,” Sov. Phys. Tech. Phys. 34, 764-766 (1989). I. Avrutsky, “Surface plasmons at nanoscale relief gratings between a metal and a dielectric medium with optical gain,” Phys. Rev. B 70, 155416 (2004). M. P. Nezhad, K. Tetz and Y. Fainman, “Gain assisted propagation of surface plasmon polaritons on planar metallic waveguides,” Opt. Express 12, 4072-4079 (2004). N. M. Lawandy, “Localized surface plasmon singularities in amplifying media,” Appl. Phys. Lett. 85, 50405042 (2004). M. A. Noginov, G. Zhu, M. Bahoura, J. Adegoke, C. E. Small, B. A. Ritzo, V. P. Drachev, and V. M. Shalaev, “Enhancement of surface plasmons in an Ag aggregate by optical gain in a dielectric medium,” Opt. Lett. 31, 3022-3024 (2006).

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Received 11 Dec 2007; revised 2 May 2008; accepted 26 May 2008; published 18 Sep 2008

29 September 2008 / Vol. 16, No. 20 / OPTICS EXPRESS 15576

18. 19. 20. 21. 22. 23. 24. 25.

J. Seidel, S. Grafstroem, and L. Eng, “Stimulated emission of surface plasmons at the interface between a silver film and an optically pumped dye solution,” Phys. Rev. Lett. 94, 177401 (2005). M. A. Noginov, V. A. Podolskiy, G. Zhu, M. Mayy, M. Bahoura, J. A. Adegoke, B. A. Ritzo, and K. Reynolds, “Compensation of loss in propagating surface plasmon polariton by gain in adjacent dielectric medium,” Opt. Express 16, 1385-1392 (2008). P. B. Johnson and R. W. Christy, “Optical Constants of the Noble Metals,” Phys. Rev. B 6, 4370-4379 (1972). D. W. Lynch, W. R. Hunter, “Comments on the Optical Constants of Metals and an Introduction to Data for Several Metals”, In Handbook of optical constants of solids, Part II, ed. by E. D. Palik, Academic Press, NY, 1985. M. V. Klein and T. E. Furtak, Optics, 2nd ed. (Wiley, New York, 1986). Calculations are performed with ab initio norm-conserving pseudopotentials within standard Density Functional Theory. Optical functions are calculated in Random Phase Approximation using the lattice constant of 4.023 Å generated within Local Density Approximation. K. Stahrenberg, T. Herrmann, N. Esser, J. Sahm, W. Richter, S. V. Hoffmann, Ph. Hofmann. “Surface state contrubution of the optical anisotropy of Ag(110) surface: a reflectance anisotropy spectroscopy and photoemission study,” Phys. Rev. B 58, R10207 (1998). H. Raether, “Surface plasmons on smooth and rough surfaces and on gratings”, Springer-Verlag, (Berlin, 1988).

1. Introduction and model predictions Localized surface plasmons (SPs), resonant oscillations of electron density in metallic nanostructures, and propagating surface plasmon polaritons (SPPs), electromagnetic waves coupled to electron density oscillations and propagating along the interface between metal and dielectric, have recently become a hot research topic because of their ability to confine and enhance electromagnetic radiation at the nanometer scale. Localized and propagating surface plasmons and relevant phenomena, which are described by a common term nanoplasmonics, have numerous exiting applications in a variety of sensors [1-3], optoelectronic devices [4-9], and photonic metamaterials [10-12]. However, many of these applications are hindered by one common cause – absorption loss in metal. The known solution to the loss problem is optical gain added to a dielectric medium. It has been predicted that gain can compensate for absorption loss in propagating [13-15] and localized [16] surface plasmons. Experimentally, six-fold enhancement of localized SPs by optical gain has been demonstrated in Ref. [17]. The compensation of loss in propagating SPP by optical gain, although very small, was first demonstrated in Ref. [18], and the gain sufficient to fully compensate SPP loss in high-quality silver films was achieved in Ref. [19]. In spite of significant progress made in this direction, maintaining required optical gain (of the order of 103 cm-1) is a technologically difficult task, which often requires a Q-switched laser [17,19]. This makes the gain solution to the loss problem unpractical for many applications. Ideally, one would want to have a low loss in passive systems, without any gain. Two metals commonly used in nanoplasmonic and metamaterials applications are silver and gold. Silver has a smaller absorption loss, and gold is a better technological material. Unfortunately, the propagation length of SPP in these metals (in the visible range) is of the order of ten micrometers or shorter. Can metal-dielectric interfaces be improved to become more suitable for photonic applications? In silver, there is a significant difference between the experimentally measured absorption loss in the visible and ultraviolet ranges of the spectrum [20, 21] and that predicted by the Drude model, which takes into account the contribution of free electrons only [22]. By the first principles calculations [23], we have shown that this difference is due to bound electrons in the layers of silver atoms adjacent to the metal surface. This is evidenced by Fig. 1 comparing the experimental absorption loss with that calculated in bulk silver and silver slabs of different thickness.

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594.1 nm

1 0.9

4

6

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e"

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7

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0.5 0.4 0.3 0.2 0.1 0 7000

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17000

22000

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1 32000

-1

Energy (cm )

Fig. 1. Imaginary part of the dielectric constant of silver e” calculated (1) according to the Drude model with plasma frequency wp =9.1 eV and loss parameter G =0.021 eV; (2) from the first principles for bulk silver; (3-6) for the (1,1,1) surface of silver slabs consisting of 7, 10, 13, and 16 monolayers. Trace 7 - experimental data [20].

Adsorption of molecules, ions or radicals onto the silver surface can modify surface electronic states and reduce the absorption loss of silver. This effect was demonstrated in Ref. [24], where adsorption of oxygen on Ag(110) removed the effect of surface states and substantially reduced optical response around 1.7 eV (730 nm, 13700 cm-1). In this work, we have demonstrated elongation of the SPP propagation length by bringing the silver film in contact with the polymer heavily doped with rhodamine 6G chloride dye. 2. Experimental results and discussion Experimentally, we have studied SPPs in the attenuated total reflection (ATR) setup shown in Fig 2. Silver films were deposited on the 90 degree glass prism. The thickness of the silver film in different samples varied between 35 and 72 nm. Rhodamine 6G chloride dye (R6G) and polymethyl methacrylate (PMMA) were dissolved in dichloromethane. The solutions were applied to the surface of silver and dried in air, providing a PMMA film doped with

Fig. 2. Experimental setup: excitation of SPPs in an attenuated total reflection (ATR) geometry.

R6G. The thickness of the PMMA film was ~10 mm and the concentration of R6G varied between 0 and 100 g/l (2.1x10-1 M). In the measurements of the angular dependence of the reflectivity R( q ), the prism was mounted on a motorized goniometer stage. The reflectivity was probed with p polarized 594.1 nm He-Ne laser light. Reflected light was detected by a photomultiplier tube connected to an integrating sphere, which was moved during the scan to follow the walk of the reflected beam. The reflectivity profile R(q) had a characteristic dip [25] at the angle at which the wave vector #90796 - $15.00 USD

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of the SPP matched the projection of the photon wave vector to the plane of metal-dielectric interface, Fig. 3. It is described by the known formula

R= where rikp =

(kzie k - kzke i ) (kzie k + kzke i )

r01p + r12p exp( 2ikz1d ) 1 + r01p r12p exp( 2ikz1d )

2

,

(1)

is the amplitude reflection coefficient for

p polarized light at the

interface between media†i and k (i, k=0,1,2), ei,k is the dielectric constant, d is the thickness of 2

the metallic film, k zi = e i (w /c ) - k x2 is the wave vector in the direction perpendicular to

Reflectivity

† the surface of the metal, k x = k phot sin q 0 is the wave vector in the direction of the SPP propagation, kphot is the wave vector of a photon in the glass prism, and q is the angle of incidence;†the subscripts 0, 1, and 2 correspond to glass, silver, and PMMA, respectively. Here it is assumed that each of the three media (glass/silver/polymer with dye) is spatially † uniform, the boundaries between media are sharply defined, and effects of hybrid states at the interfaces are neglected. 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

R6G = 30 g/l

W2 W1 no R6G

61

63

65 67 Angle (degree)

69

Fig. 3. Angular dependence of the reflectivity R(q ) recorded in the setup of Fig. 3, with the concentration of the R6G dye equal to 0 g/l (squares) and 30 g/l (circles). With the addition of dye, the width of the reflectivity profile decreased and the position of its minimum shifted.

The absorption spectra of PMMA/R6G films featured a characteristic band centered at 530nm, inset of Fig. 4. At high concentration of dye, its long-wavelength wing significantly increased in comparison to that recorded at low concentration of R6G. The imaginary part of the dielectric constant of the PMMA/R6G film e "2 , calculated at l = 594 nm from the absorption data, is plotted as a function of concentration of R6G (in log-log scale) in Fig. 4. At high concentration of dye, the slope of the curve approached 2, which indicated dimerization of dye molecules. Large scatter of the data was due to the known relatively low † reproducibility of PMMA films. We started reflectivity experiments with measuring the angular profile R(q) in pure glass prism, without any deposited films. By fitting the function R(q) with Eq. (1) at d=0 and e2=1, we determined the index of refraction of glass n0 = (e0)1/2 to be equal to 1.7835 at 594.1 nm, in a very good agreement with the data provided by the manufacturer. We then repeated the same measurements with the PMMA/R6G films (with different concentrations of the R6G dye) deposited directly onto the prism. The rear surfaces of the polymeric films were intentionally roughened (with very fine sandpaper) to prevent the reflection of light from the polymer/air interface back to the prism. Each reflectivity profile #90796 - $15.00 USD

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R( q ) was fitted with Eq. (1) at d=0 to determine the dielectric constant of the PMMA/R6G film, Fig. 5.

Abs. Coeff. (cm -1)

4000

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3000 2000

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x20

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0.001

0.0001 h=2 0.00001

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R6G Concentration N (g/L)

100

Fig. 4. Imaginary part e2” of the dielectric constant of the PMMA/R6G film as a function of concentration of R6G, N. Diamonds: data calculated from the absorption measurements. Solid line: data points e2” fitted with a second order polynomial. Dotted line has the slope equal to h=2. Circles: the values e 2” used at the fitting of the R(q) profiles in glass/silver/polymer structures. Inset: Absorption spectrum of PMMA film doped with R6G; trace 1 – N=100 g/l; trace 2 – N=5 g/l.

2.32 2.3

e'2

2.28 2.26 2.24 2.22 2.2 0

25

50

75

100

R6G concentration N (g/l)

Fig. 5. Real part of the dielectric constant e 1’ of PMMA/R6G as a function of R6G concentration N.

The reflectivity studies carried out in three-layer structures (glass/silver/polymer) at different concentrations of dye resulted in profiles R( q ) characteristic of SPPs [25], Fig. 3. The most remarkable result of this measurement is that the width W of the dip of the angular profile R(q ) decreased with an addition of R6G to PMMA, up to the concentration of dye N≈30 g/l, and then increased again when the concentration of dye was increased further (compare widths W1 and W 2 in Fig. 3). According to Ref. [19], W is inversely proportional to the SPP propagation length L. Correspondingly, the obtained experimental result implies that an addition of R6G dye to PMMA helps to reduce the loss and elongate the propagation length of SPPs. #90796 - $15.00 USD

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We then fitted the reflectivity profiles R(q ) using Eq. (1), with the fitting parameters being real and imaginary parts of the dielectric constant of silver, e1’ and e1”. The real and imaginary parts of the dielectric constants of PMMA/R6G, e2’ and e 2”, were chosen in accord with the measurements presented in Figs. 4 and 5. The determined this way values e1’ and e1” (at l=594.1 nm) are plotted against the concentration of R6G in Fig. 6. In the absence of R6G, the value of e1’ coincided within 5% with those published in Refs. [20,21]. The value of e 1” was smaller than that in Ref. [21] and larger than that in Ref. [20]. Both extracted values e1’ and e1” were strongly influenced by the presence of R6G dye in the PMMA film, Fig. 6. A particularly strong effect (40% reduction) has been observed in the dependence e1”(N) with the increase of the concentration of dye from 0 g/l to 30 g/l. -12

a

-13

e1 '

-14 -15 -16 -17 0

20

40

60

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100

R6G Concentration (g/l)

0.8

b

0.7 0.6

e 1"

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60

R6G concentration (g/l)

Fig. 6. Obtained from the fitting of R( q ) real (a) and imaginary (b) parts of the dielectric constant of silver, e1, as a function of the R6G concentration N. Solid lines – interpolations with second order polynomials.

This implies that in a framework of a simple model assuming that each of the three media is spatially uniform, the boundaries between media are sharply defined, and the effects of hybrid states at the interfaces are neglected, this reduction of e1” would be required to account for the change in the SPP propagation length observed experimentally. The propagation length L of SPPs can be calculated as [19] -1

L = [ 2(g i + g r )] , 3/2 w Ê e 'e ' ˆ Ê e " e " where gi is the internal SPP loss, g i = Á ' 1 2 ' ˜ Á 12 + 22 2c Ë e1 + e 2 ¯ ÁË e1' † e '2

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(2) ˆ ˜ , and gr is the radiation loss ˜ ¯


†


0 -3 / 2 ¸ Ï Ô 2wr01e i2kz d1 Ê e 2 + e1 ˆ Ô ˝. caused by the decoupling of SPPs back to the prism g r = Ì Im Á ˜ c e 2 - e1 Ë e 2e1 ¯ ÔÓ Ô˛ 0 Here w is the oscillation frequency, d1 is the thickness of the metallic film, kz is the value of kz at the resonance angle, and c is the speed of light. In order to calculate the dependence L(N) for two different thicknesses of the metallic † film, d1=40 nm and d1=80 nm (Fig. 7), we substituted to Eq. (2) functions e 1(N) and e2(N) obtained by interpolation of the experimental data in Figs. 4-6 with the second order polynomials. Because the values of the radiative loss gr and, correspondingly, the SPP propagation lengths L are different at d1=80 nm and d1=40 nm, the data sets L(N) (squares and triangles in Fig. 7) were normalized to unity at N=0 for convenience of presentation.

(

)

1.5

L (rel. units)

1.3 1.1 0.9 0.7 0.5 0.3 0

20

40

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100

R6G Concentration (g/l) Fig. 7. SPP propagation length L as a function of dye concentration N. Solid squares (triangles) – calculations done for real experimental parameters at d 1=80 nm (40 nm); Open squares (triangles) – calculations done for the hypothetic case of the absence of dye absorption, e2”=0, at d 1=80 nm (40 nm). Solid circles – inverse width of the reflectivity profile R(q). All data sets are normalized to unity at N=0.

The fact that the two normalized curves L(N) calculated at d 1=80 nm and d1=40 nm (solid squares and triangles) are close to each other suggests that the relative change of the SPP propagation length with the concentration of dye is not strongly dependent on the thickness of the silver film. This justifies the comparison of the calculated values L(N) with the inverse widths W-1 of the reflectivity profiles R(q ) (circles in Fig. 7) measured in different samples with the thickness of the silver film varying between 35 and 91 nm and the average thickness being equal to 70 nm, Fig. 7. Not surprisingly (since W-1 is proportional to L [19]), these two types of curves closely resemble each other, confirming ~30% elongation of the SPP propagation length with the increase of the dye concentration to N=30 g/l (6.3x10-2 M). (The slight difference between the experimental and the calculated curves in Fig. 7 is not clearly understood. However, the former one appears to be more accurate since it is based on direct measurements rather than recalculated data.) The reduction of L with the increase of N above 30 g/l is due to (i) the increase of the absorption losses e1” and e 2” and (ii) the increase of e1’, which becomes less negative, Figs. 4,6. Figure 7 demonstrates the strong difference between the values of L calculated under the assumption of e2”=0 (no loss in dielectric, open squares and triangles) and at the experimental values of e2” (solid squares and triangles). We tentatively explain the strong change in e1” by the modification of electronic states in silver layer adjacent to the metal-dielectric interface, including possible formation of the AgCl phase. The preliminary results of ab initio modeling show that the formation of AgCl film

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can, indeed, affect the absorption loss at the surface. The results of these studies will be reported elsewhere. 3. Summary To summarize, we have demonstrated that an addition of highly concentrated rhodamine 6G chloride dye to the PMMA film adjacent to the silver film causes 30% elongation of the propagation length of SPP without gain. This opens a new technological dimension to lowloss nanoplasmonics and metamaterials, in which surface phenomena and hybrid electronic states play an enormously important role in determining physical properties of nanocomposites and nanostructures. Acknowledgements The work was supported by the NSF PREM grant # DMR 0611430, NSF CREST grant # HRD 0317722, NSF NCN grant # EEC-0228390, NASA URC grant # NCC3-1035. The authors thank Vladimir Shalaev, Vladimir Drachev, Evgenii Narimanov and Igor Smolyaninov for useful discussions.

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