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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 23 — Nov. 8, 2010
  • pp: 23698–23710
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Second harmonic generation from metallo-dielectric multilayered structures in the plasmonic regime

Nadia Mattiucci, Giuseppe D’Aguanno, and Mark J. Bloemer  »View Author Affiliations


Optics Express, Vol. 18, Issue 23, pp. 23698-23710 (2010)
http://dx.doi.org/10.1364/OE.18.023698


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Abstract

We present a theoretical study on second harmonic generation from metallo-dielectric multilayered structures in the plasmonic regime. In particular we analyze the behavior of structures made of Ag (silver) and MgF2 (magnesium-fluoride) due to the straightforward procedure to grow these materials with standard sputtering or thermal evaporation techniques. A systematic study is performed which analyzes four different kinds of elementary cells- namely (Ag/MgF2)N, (MgF2/Ag)N, (Ag/MgF2/Ag)N and (MgF2/Ag/MgF2)N-as function of the number of periods (N) and the thickness of the layers. We predict the conversion efficiency to be up to three orders of magnitude greater than the conversion efficiency found in the non-plasmonic regime and we point out the best geometries to achieve these conversion efficiencies. We also underline the role played by the short-range/long-range plasmons and leaky waves in the generation process. We perform a statistical study to demonstrate the robustness of the SH process in the plasmonic regime against the inevitable variations in the thickness of the layers. Finally, we show that a proper choice of the output medium can further improve the conversion efficiency reaching an enhancement of almost five orders of magnitude with respect to the non plasmonic regime.

© 2010 OSA

1. Introduction

Second harmonic generation (SHG) from metal surfaces in the non-plasmonic regime has been studied since the beginning of nonlinear optics [1

1. F. Brown, R. E. Parks, and A. M. Sleeper, “Nonlinear Optical Reflection from a Metallic Boundary,” Phys. Rev. Lett. 14(25), 1029–1031 (1965). [CrossRef]

,2

2. N. Bloembergen, R. K. Chang, S. S. Jha, and C. H. Lee, “Optical Second-Harmonic Generation in Reflection from Media with Inversion Symmetry,” Phys. Rev. 174(3), 813–822 (1968). [CrossRef]

] in the 60s. It is well known that metals posses both a volume and a surface quadratic nonlinearity [3

3. Y. R. Shen, The Principles of Nonlinear Optics, (Wiley, 1984)

]. The volume nonlinearity originates from the Lorentz force exerted on the free electrons of the metal, while the surface nonlinearity is due to the symmetry breaking at the metal surface [3

3. Y. R. Shen, The Principles of Nonlinear Optics, (Wiley, 1984)

]. The subject of plasmonic SHG was addressed just several years later in 1974 [4

4. H. J. Simon, D. E. Mitchell, and J. G. Watson, “Optical Second-Harmonic Generation with Surface Plasmons in Silver Films,” Phys. Rev. Lett. 33(26), 1531–1534 (1974). [CrossRef]

] when SH from a single layer of silver in the Kretschmann geometry [5

5. E. Kretschmann, “The Determination of the Optical Constants of Metals by Excitation of Surface Plasmons,” Z. Phys. 241(4), 313–324 (1971). [CrossRef]

] was studied both experimentally and theoretically. The work of Ref. [4

4. H. J. Simon, D. E. Mitchell, and J. G. Watson, “Optical Second-Harmonic Generation with Surface Plasmons in Silver Films,” Phys. Rev. Lett. 33(26), 1531–1534 (1974). [CrossRef]

] was then followed by several other works studying SHG in the Kretschmann configuration from smooth surfaces [6

6. J. C. Quail and H. J. Simon, “Second-harmonic generation from silver and aluminum films in total internal reflection,” Phys. Rev. B Condens. Matter 31(8), 4900–4905 (1985). [CrossRef] [PubMed]

8

8. R. Naraoka, H. Okawa, K. Hashimoto, and K. Kajikawa, “Surface plasmon resonance enhanced second-harmonic generation in Kretschmann configuration,” Opt. Commun. 248(1-3), 249–256 (2005). [CrossRef]

] and rough surfaces [9

9. T. A. Leskova, M. Leyva-Lucero, E. R. Mendez, A. A. Maradudin, and I. V. Novikov, “The surface enhanced second harmonic generation of light from a randomly rough metal surface in the Kretschmann geometry,” Opt. Commun. 183(5-6), 529–545 (2000). [CrossRef]

]. A more complex structure was analyzed in Ref. [10

10. Q. Chen, X. Sun, I. R. Coddington, D. A. Goetz, and H. J. Simon, “Reflected second-harmonic generation with coupled surface-plasmon modes in Ag/liquid/Ag layers,” J. Opt. Soc. Am. B 16(6), 971–975 (1999). [CrossRef]

] where a thin layer of metal was grown on the base of a prism, then a second layer of metal was squeezed toward the prism with an index matching fluid in between. Beside the harmonic generation from the metal itself, plasmons have also been used to enhance nonlinear effects in an external medium [11

11. J. G. Rako, J. C. Quail, and H. J. Simon, “Optical second-harmonic generation with surface plasmons in noncentrosymmetric crystals,” Phys. Rev. B 30(10), 5552–5559 (1984). [CrossRef]

,12

12. G. M. Wysin, H. J. Simon, and R. T. Deck, “Optical bistability with surface plasmons,” Opt. Lett. 6(1), 30–32 (1981). [CrossRef] [PubMed]

]. The effect of counter-propagating plasmons has also been studied [13

13. M. Fukui, J. E. Sipe, V. C. Y. So, and G. I. Stegeman, “Nonlinear mixing of opposite traveling surface plasmons,” Solid State Commun. 27(12), 1265–1267 (1978). [CrossRef]

,14

14. C. K. Chen, A. R. B. de Castro, and Y. R. Shen, “Coherent second-harmonic generation by counterpropagating surface plasmons,” Opt. Lett. 4(12), 393–394 (1979). [CrossRef] [PubMed]

].

In this paper we analyze plasmonic SHG from metal/dielectric periodic stratifications. In particular we study SHG from structures made of Ag (silver) and MgF2 (magnesium-fluoride) due to the simple procedure to grow these materials with standard sputtering or thermal evaporation techniques [15

15. H. A. Macleod, Thin film optical filters, (Institute of Physics Publishing, 2001)

]. The paper is organized as follows: In Section 2 we describe in details the geometries studied and the model used in order to describe SHG. In Section 3 we discuss the conversion efficiencies achieved in these kinds of structures and the role played by the short-range/long-range plasmons and leaky waves in the generation process. We also perform a statistical study to investigate the robustness of the SH process in the plasmonic regime against the inevitable variations in the thickness of the layers. Finally in Section 4 we give our conclusions.

2. The model

The simulations are based on the theoretical model that we have developed in Ref. [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

], where the non-plasmonic SHG from metallo-dielectric structures has been investigated both theoretically and experimentally. In Ref. [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

] the quadratic nonlinearity of metals was described through two terms: the Lorentz term (volume term) and the surface term. In our case we can neglect the Lorentz term because in the plasmonic regime the field is mainly localized at the metal-dielectric interfaces and therefore just the surface term will be exploited in an efficient way. Under TM polarization, the Helmholtz equation for the H-field at the SH can be written as follows [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

]:

d2H2ωdz2+4ω2c2(n2ω2(z)nin2sin2ϑ)H2ω=4ω2nincsinϑ(ε0ds(2)kδ(zzk)Ez,ω2)   .
(1)

Equation (1) is valid layer by layer and describes the SH magnetic field generated from a TM polarized pump field, in a Cartesian, right-handed, reference system (x,y,z) where z is the direction of the stratification. Moreover, only the (z,z,z) component of the nonlinearities is considered, i.e. TM→TM SH emission. In Eq. (1), ϑ is the angle of incidence, nin is the refractive index of the incident medium (the hemi-cylindrical prism made of fused silica in our case), n(z) is the step-varying, complex refractive index at the SH along the direction of the stratification, ε0 ≅8.85 × 10−12 F/m is the vacuum permittivity, c is the speed of light in vacuo, E z,ω is the z-component of the fundamental frequency (FF) electric field, H2 ω is the SH magnetic field, δ(z-zk) is the Dirac “delta” function calculated at the kth metal/dielectric interface just inside the metal and, finally, ds (2) is the surface nonlinearity. The value of the surface nonlinearity is estimated to be ds (2) = 10−18m2/V, based on the experimental results reported Ref. [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

]. In the undepleted regime, Eq. (1) can be solved using the Green function approach for multi-layered structures described in Ref. [20

20. N. Mattiucci, G. D’Aguanno, M. J. Bloemer, and M. Scalora, “Second-harmonic generation from a positive-negative index material heterostructure,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72(6), 066612 (2005). [CrossRef]

, 21

21. G. D’Aguanno, N. Mattiucci, M. Scalora, M. J. Bloemer, and A. M. Zheltikov, “Density of modes and tunneling times in finite one-dimensional photonic crystals: a comprehensive analysis,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 70(1), 016612 (2004). [CrossRef] [PubMed]

]. and adapted here to handle TM polarization. The reader interested in the mathematical details of the theoretical approach can consult Ref. [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

]. The SH conversion efficiency in reflection is calculated by the ratio of the z-component of the Poynting vector of the backward generated SH divided by the z-component of the Poynting vector of the FF pump field incident on the sample: η=SzSH/Sz,incFF.

3. Results and discussion

We have performed a systematic study of the SH generated at λ = 400nm as a function of: a) the angle of incidence (ϑ) of the FF pump field at λ = 800nm; b) the thickness of the metal (dAg), and the dielectric (dMgF2) layers; c) the number of elementary cells (N). The variation of the angle of incidence has been limited to the total internal reflection regime, i.e. 90°>ϑ> asin(nout/nin) which in our case corresponds to 90°>ϑ>40.8°. To compare more easily the behavior of the different elementary cells, dAg and dMgF2 refer, respectively, to the total amount of silver and of magnesium-fluoride inside the elementary cell. Moreover, the thickness of the magnesium-fluoride layers has been limited to value below 600nm and above 10nm, while the thickness of the silver layers has been limited to values not exceeding 180nm and not less than 9nm. Note that a 9-10nm layer is currently at the boundary of sputtering or evaporation growing techniques. Moreover, and more importantly, for silver layers much thinner than 10nm, the classical description of the metals might be hampered by the insurgency of quantum-size effects [24

24. S. Ciraci and I. P. Batra, “Theory of the quantum size effect in simple metals,” Phys. Rev. B Condens. Matter 33(6), 4294–4297 (1986). [CrossRef] [PubMed]

]. As we will see later, for some configurations a maximum of the conversion efficiency has been predicted when the thickness of the constituent materials reaches their extreme values. These cases will be discussed later on in the text. All the conversion efficiencies refer to an intensity of the incident FF pump field of 6 GW/cm2 which can be readily achieved by focusing on the sample the light coming from a Ti-sapphire laser [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

], for example. In the calculations we have not included the transmission coefficient at the air/prism interface.

In order to better understand the main mechanisms that bring to an efficient SH generation, and to clarify how the different terminations of the elementary cell affect the nonlinear process, we will start our analysis by studying SH generation as a function of dAg and ϑ, when the thickness of the dielectric material is kept fixed to its maximum value of 600nm and the structures consist of only one period. The results are reported in Fig. 2
Fig. 2 Left side: Log(η) vs. Ag thickness and incident angle for the 1-period structures considered, η is the conversion efficiency. Right side: Schematic representation of the structures considered: Symmetric-1 (2a), Asymmetric-1 (2b), Asymmetric-2 (2c), Symmetric-2 (2d). In Fig. 2(a) the black dashed line superimposed represents the dispersion of the short-range/long-range plasmons for the structure MgF2/Ag/MgF2. For each figure, see also the multimedia material (Media 1, Media 2, Media 3, and Media 4) where the thickness of the MgF2 is varied.
for the four kinds of elementary cells. While here we will show the figures only for the case with the total thickness of the dielectric material kept fixed at 600nm, we have also uploaded multimedia material with animations that show how the conversion efficiency changes by changing the thickness of the dielectric material for the four elementary cells under consideration (see multimedia Media 1, Media 2, Media 3, Media 4).

We now go to analyze the asymmetric-1 elementary cell reported in Fig. 2(b). For large values of dAg and dMgF2 it can be assimilated to a MgF2/Ag interface, with plasmon excitation at 67.7°; and in fact that resonance is clearly visible in the figure. On the other hand, for thin silver thicknesses the field can reach also the interface with air so that a surface plasmon at the Ag/Air interface can be excited at an angle of 41.45° in fused silica for the FF and at an angle of 48.84° at the SH. Both resonances are visible in the figure, even though the 48.84° resonance is very week.

The Asymmetric-2 elementary cell is specular to the Asymmetric-1, although the 1 period Asymmetric-2 structure supports only the Ag/MgF2 interface plasmon. From Fig. 2(c) we can see two distinctive resonances: one resonance is, of course, the single interface plasmon, the second corresponds to the excitation of a leaky mode in MgF2.. In fact, if we change the thickness of MgF2 [see Media 3] from 50nm to 600nm in steps of 50nm, we see that for dMgF2 = 50nm there are two resonances: one weak at 67.7° and one strong at 50°. As dMgF2 grows, the strong resonance move towards the weak until they merge, meanwhile another resonance appears and starts to moves toward the 67.7°. If we let the thickness of MgF2 keep growing (not shown in the multimedia), this new resonance reaches a saturation value below 67.7° and other resonances start to appear. The appearance of new resonances as the thickness of the dielectric material increases should not be too surprising after all: this is what normally happens also in conventional dielectric waveguides that admit more and more guided modes for increasing values of the core thickness.

Lastly we consider the 1period Symmetric-2 structure studied in Fig. 2(d). In this case there are three interfaces that can support plasmonic modes: two MgF2/Ag interfaces, and one Ag/air interface. The Ag/Air plasmon resonance at 41.45° is clearly visible in the figure. The two Ag/MgF2 plasmons couple as in a metal/dielectric/metal waveguide and generate the two resonances around 67.7°. This behavior is confirmed by Media 4, in which it is possible to see how the two resonances tend to collapse as the thickness of the dielectric increases degenerating into the single surface plasmon resonance at 67.7°. A t this regard we note that in both Symmetric-1 and Asymmetric-1 configuration it is the dielectric to be attached to the prism. This allows both structures to efficiently generate SH also for thick layers of metal. On the other hand, in the Asymmetric-2 and in the Symmetric-2 it is the metal in direct contact with the prism. In this second case, the field must reach the second Ag interface in order to produce a relevant conversion efficiency meaning that only structures with thin layers of silver will be efficient.

Moreover, we note that only the asymmetric-1 and the symmetric-2 structures exhibit a silver/air interface. Therefore, only those structures can support Ag/air interface plasmons.

In Fig. 3
Fig. 3 Maximum conversion efficiency accessible as function of the thickness of the layers for a given number of periods and a given type of elementary cell. Superimposed an acronym that indicates the type of resonant mechanism: LR indicates that the enhanced SH generation is due to the excitation of long range plasmons; SR indicates that the enhanced SH generation is due to the excitation of short range plasmons; SPair indicates that the enhanced SH generation is due to the excitation of a plasmon at the Ag/Air interface; SPMgF2 indicates that the enhanced SH generation is due to the excitation of a plasmon at the Ag/MgF2 interface; LW indicates that the enhanced SH generation is due to the excitation of a leaky waveguide mode.
we show topographic views arranged in a table environment of the maximum conversion efficiency as function of the thickness of the layers; then, in Fig. 4
Fig. 4 Angle at which the conversion efficiency reported in Fig. 3 is reached as a function of the thickness of the layers for a given number of periods and a given type of elementary cell.
we show, in a similar way, the angle at which that conversion efficiency is reached. Basically each plot corresponds to the maximum conversion efficiency for a given number N of elementary cell and a given type of elementary cell. Each point of the plot corresponds to a different value of the angle of incidence. Once the type of elementary cell and the number of periods are fixed, the conversion efficiency becomes a function of the angle of incidence and the thickness of the layers. In Fig. 3, dAg and dMgF2 are the variables; this means that the conversion efficiency is still dependent upon a parameter: the angle of incidence. This parameter is chosen point by point in order to maximize the conversion efficiency and its value is reported in Fig. 4. The intensity scale of each conversion efficiency plot (Fig. 4) is logarithmic and runs from 10−10 (dark blue, which means value of the conversion efficiency ≤ 10−10) up to maximum value of 3*10−7 (dark red). The intensity scale of each angle (Fig. 4) is linear and runs from 40.8° (total internal reflection at SiO2/Air interface) up to 90°.

First of all we note that the conversion efficiency can be up to three orders of magnitude greater than the conversion efficiency experimentally found and theoretically predicted in Ref. [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

] in the non-plasmonic regime. Moreover, with our choice of parameters, a classical Kretschmann geometry having air as output medium (like the one analyzed in Ref. [4

4. H. J. Simon, D. E. Mitchell, and J. G. Watson, “Optical Second-Harmonic Generation with Surface Plasmons in Silver Films,” Phys. Rev. Lett. 33(26), 1531–1534 (1974). [CrossRef]

].) will generate SH with a peak efficiency of 5.56*10−8, which is one order of magnitude lower than the one that is possible to achieve with a periodic structure. Second, for a large number of periods all the structures seem to converge to approximately the same conversion efficiency, while major differences can be found for few repetitions of the elementary cell. We must emphasize that for N>3 the structures are almost identical and differ by their end faces only. This is why the SH generation for the various structures tends to converge for increasing number of periods.

We can see from Fig. 3 that the SH resonance associated with short range plasmons requires thinner thicknesses of the constituent materials as the number of periods increases. A similar behavior can be also noted for the long range plasmons: the peak area shrinks on the dAg scale as the number of periods increases. This phenomenon can be explained in the framework of plasmons tunneling: for structures with a large number of periods shorter thicknesses are required in order to allow an efficient coupling of the plasmons excited at each metal/dielectric interface.

From Fig. 3 we can see that the maximum of the conversion efficiency for the 1 period Asymmetric-2 structure is out of the range considered for the MgF2 layers thickness. Therefore, these maxima of the conversion efficiency appear in a geometry in which MgF2 can be considered as a semi-infinite medium. This result indicates that high conversion efficiencies can probably be obtained in a standard Kretschmann geometry by simply changing the output medium. To the authors’ knowledge, up to now, in Kretschmann geometry, the output medium has always been assumed to be air. We will return later on this interesting aspect. Lastly, we note that when the ratio of the dielectric layer to the metallic layer is about 15 (dMgF2/dAg~15) all the structures show an anti-resonant behavior and it is not possible to efficiently generate SH.

Many factors determine the performance of SH generation in multilayered structures, among them the most important are the field localization at the FF and at the SH and the phase matching conditions [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

, 28

28. G. D’Aguanno, M. Centini, M. Scalora, C. Sibilia, M. Bertolotti, M. J. Bloemer, and C. M. Bowden, “Generalized coupled-mode theory for χ(2) interactions in finite multilayered structures,” J. Opt. Soc. Am. B 19(9), 2111 (2002). [CrossRef]

]. In our case the field localization is associated with the plasmon excitation. High SHG generation is reached when there is high field localization both at the FF and at the SH (i.e. the respective plasmon dispersion curves must someway overlap) and when there is the right interference between these fields (phase matching condition).

In order to check the robustness of SH generation against possible errors in the fabrication of an actual sample, in Figs. 5(a)
Fig. 5 (a) Histogram of the conversion efficiency and (b)angle at which it is obtained when each layer is randomly varied around its central value according to a Gaussian distribution having a standard deviation from the central value equal to 5%. The structure considered is N=5-periods Symmetric-1 for which the maximum conversion efficiency of η=4.9*10-8 is reached when dAg=15nm and dMgF2=101nm which represent our central values in the statistical study. The histograms have been obtained after 6318856 simulations.
and 5(b) we present a statistical analysis of the N = 5-periods Symmetric-1 type structure. In particular, in Figs. 5(a) and 5(b) are reported two histograms: Fig. 5(a) is relative to the maximum conversion efficiency while Fig. 5(b) is relative to the angle at which the maximum conversion efficiency is obtained. Each layer is randomly varied according to a Gaussian distribution centered on the short range resonance as indicated in Fig. 3, and having a standard deviation equal to 5% of the central value. The histograms haves been obtained after 6318856 simulations. We note that the conversion efficiency of the ideal structure is in the middle of the histogram. This means that on one hand the structure is stable and, on the other hand, the periodic structure it is not the most efficient structure and therefore an optimized device for efficient plasmonic SHG might eventually pass through the study of more sophisticated, non-periodic geometries.

Before going to the concluding remarks, we would like finally to discuss the fact that in this paper we have restricted our study to plasmonic structures in which the output medium is air. We have found that a properly designed structure can have a conversion efficiency η~3*10−7 for ~6GW/cm2 pump intensity. This value of the conversion efficiency was obtained by using just the metal surface nonlinearities that are naturally included in any metallo-dielectric filter. Now, if we had carefully chosen the material of the output medium there would have been even further room for strong improvements. To this end, in Fig. 6(b)
Fig. 6 (a) Classical Kretschmann geometry where the input medium is a fused silica prism, the metal is silver, and the output medium is a generic non absorptive non dispersive dielectric of index of refraction nout. (b) Maximum conversion efficiency achievable vs. the Ag layer thickness and the index of refraction of the output medium (nout). (c) Transmission as a function of the angle of incidence and of nout when dAg = 10nm. The transmission maxima correspond to the dispersion of the leaky wave in the fused silica. The white dashed line is the dispersion of the surface plasmon at the Ag/nout interface calculated according to the standard dispersion law: kSP=k0εAgεout/(εAg+εout). The black dashed line represents instead the loci of the maxima of the SHG. Note how the loci of the maxima of the SHG follow exactly the dispersion of the leaky wave. Note also as the dispersion of the Ag/nout surface plasmon is similar to the dispersion of the leaky wave. (c) Angle at which the maximum conversion efficiency is obtained vs. dAg and nout.
we show SH generation in a classical Kretschmann configuration, see the scheme reported in Fig. 6(a), as a function of the Ag thickness and of the refractive index of the output medium. The figure shows that by considering an output medium of refractive index n~1.4 and a single layer of Ag approximately ~40nm, conversion efficiency of η~2*10−7 (i.e. of the same order of that previously found in more complicate structures) is readily achievable. By further increasing the refractive index of the output medium to arrive around n~1.5 we obtain a conversion efficiency η~4*10−7. However, note from Fig. 6(d) that this high conversion efficiency is reached when the angle of incidence is grazing.

As we will see in a moment, the peak of the conversion efficiency in this geometry is linked to the excitation of leaky surface waves that mimic the surface plasmon at the Ag/nout interface. First we note that in this case the silver layer is placed in an asymmetric environment (nin≠nout). We know from the work of Ref. [29

29. J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B Condens. Matter 33(8), 5186–5201 (1986). [CrossRef] [PubMed]

]. that in general a thin metal layer in an asymmetric environment may admit four modes, two bound modes and two leaky modes. In particular, the two leaky modes show localization at the metal dielectric interface Ag/nin (Ag/nout) very similar to a standard surface plasmon, while they couple to radiation fields, i.e. they leak energy, in the medium nout (nin) [29

29. J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B Condens. Matter 33(8), 5186–5201 (1986). [CrossRef] [PubMed]

]. In the case of our configuration, simple considerations about transverse momentum conservation can show that the only mode compatible with the transverse momentum conservation of the incident field is the one that leaks in the input medium (fused silica) and it is localized like a surface plasmon at the Ag/nout interface. This leaky wave is the one responsible for the enhancement of the SH as we show in Fig. 6(c) where we have reported the dispersion of the leaky wave in the fused silica and superimposed the loci of the maximum SHG (black-dashed line) in the case of a 10nm Ag layer. Once more, we point out that the dispersion of the leaky wave has been calculated by using the transmission function of the structure according to the recipe laid out in Ref. [27

27. N. Mattiucci, G. D’Aguanno, M. Scalora, M. J. Bloemer, and C. Sibilia, “Transmission function properties for multi-layered structures: application to super-resolution,” Opt. Express 17(20), 17517–17529 (2009). [CrossRef] [PubMed]

]. As the reader can easily ascertain, the loci of the maxima of the SHG follow exactly the dispersion of the leaky wave in the fused silica. For the benefit of the reader, we have also reported in Fig. 6(c) the dispersion of the surface plasmon at the Ag/nout (white dashed line) to confirm its close similarity with the dispersion of the leaky wave in agreement with the results reported in Ref. [29

29. J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B Condens. Matter 33(8), 5186–5201 (1986). [CrossRef] [PubMed]

].

In Fig. 6(d) we finally show the angle at which the maximum SH conversion efficiency is achieved as function of the thickness of the Ag layer and the refractive index of the output medium. The figure shows that the maximum generation is independent from the Ag thickness, a fact that reinforces the notion that the enhanced SH is due to a surface-plasmon-like mode. In view of the above considerations we may say that a careful optimization of a metallo-dielectric multi-layered structure which includes the optimization of the output medium might reasonably bring an additional enhancement in the conversion efficiency by two or three orders of magnitude. As an example, we consider an 8-periods asymmetric-1 structure. For dAg = 9nm, dMgF2 = 52nm and at an angle of incidence of 83.2°, a SH resonance η~1.2*10−7 associated with short range plasmon excitation is found. We now calculate the conversion efficiency for the optimized structure, which is schematically represented in Fig. 7(a)
Fig. 7 (a) Scheme of the configuration studied. The 8-periods asymmetric-1 structure with dAg=9nm and dMgF2=52nm is grown on the base of a fused silica hemi-cylindrical prism, while the output medium is a generic non absorptive non dispersive dielectric of index of refraction nout. (b) Conversion efficiency vs. the index of refraction of the output medium (nout) and the angle of incidence ϑ.
, as a function of the angle of incidence and of the refractive index of the output medium. The results are reported in Fig. 7(b). According to our calculations the conversion efficiency improves by an additional factor of 20 when the refractive index of the output medium is around 1.4.

4. Conclusions

In this paper we have studied SH generation from periodic metallo/dielectric stratifications in the plasmonic regime. The conversion efficiency is estimated to be some four to five orders of magnitude higher than the conversion efficiency experimentally found and theoretically predicted in the non-plasmonic regime [19

19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

]. We have clarified the role played in the SHG process by the single interface surface plasmons, by the short-range/long-range plasmons at the double interface and by the leaky waves. We have also pointed out that there are still vast margins of improvement by considering more sophisticated, non-periodic structures and by carefully choosing the output medium. The subject of SHG in the plasmonic regime remains still today a vital field of research which we believe has many pleasant surprises yet to reveal.

Acknowledgments

We thank M. Scalora for helpful discussions. This work is supported by Defense Advanced Research Projects Agency (DARPA) project W31P4Q-09-C-0347 “Nonlinear Plasmonic Devices”.

References and links

1.

F. Brown, R. E. Parks, and A. M. Sleeper, “Nonlinear Optical Reflection from a Metallic Boundary,” Phys. Rev. Lett. 14(25), 1029–1031 (1965). [CrossRef]

2.

N. Bloembergen, R. K. Chang, S. S. Jha, and C. H. Lee, “Optical Second-Harmonic Generation in Reflection from Media with Inversion Symmetry,” Phys. Rev. 174(3), 813–822 (1968). [CrossRef]

3.

Y. R. Shen, The Principles of Nonlinear Optics, (Wiley, 1984)

4.

H. J. Simon, D. E. Mitchell, and J. G. Watson, “Optical Second-Harmonic Generation with Surface Plasmons in Silver Films,” Phys. Rev. Lett. 33(26), 1531–1534 (1974). [CrossRef]

5.

E. Kretschmann, “The Determination of the Optical Constants of Metals by Excitation of Surface Plasmons,” Z. Phys. 241(4), 313–324 (1971). [CrossRef]

6.

J. C. Quail and H. J. Simon, “Second-harmonic generation from silver and aluminum films in total internal reflection,” Phys. Rev. B Condens. Matter 31(8), 4900–4905 (1985). [CrossRef] [PubMed]

7.

T. Y. F. Tsang, “Surface-plasmon-enhanced third-harmonic generation in thin silver films,” Opt. Lett. 21(4), 245–247 (1996). [CrossRef] [PubMed]

8.

R. Naraoka, H. Okawa, K. Hashimoto, and K. Kajikawa, “Surface plasmon resonance enhanced second-harmonic generation in Kretschmann configuration,” Opt. Commun. 248(1-3), 249–256 (2005). [CrossRef]

9.

T. A. Leskova, M. Leyva-Lucero, E. R. Mendez, A. A. Maradudin, and I. V. Novikov, “The surface enhanced second harmonic generation of light from a randomly rough metal surface in the Kretschmann geometry,” Opt. Commun. 183(5-6), 529–545 (2000). [CrossRef]

10.

Q. Chen, X. Sun, I. R. Coddington, D. A. Goetz, and H. J. Simon, “Reflected second-harmonic generation with coupled surface-plasmon modes in Ag/liquid/Ag layers,” J. Opt. Soc. Am. B 16(6), 971–975 (1999). [CrossRef]

11.

J. G. Rako, J. C. Quail, and H. J. Simon, “Optical second-harmonic generation with surface plasmons in noncentrosymmetric crystals,” Phys. Rev. B 30(10), 5552–5559 (1984). [CrossRef]

12.

G. M. Wysin, H. J. Simon, and R. T. Deck, “Optical bistability with surface plasmons,” Opt. Lett. 6(1), 30–32 (1981). [CrossRef] [PubMed]

13.

M. Fukui, J. E. Sipe, V. C. Y. So, and G. I. Stegeman, “Nonlinear mixing of opposite traveling surface plasmons,” Solid State Commun. 27(12), 1265–1267 (1978). [CrossRef]

14.

C. K. Chen, A. R. B. de Castro, and Y. R. Shen, “Coherent second-harmonic generation by counterpropagating surface plasmons,” Opt. Lett. 4(12), 393–394 (1979). [CrossRef] [PubMed]

15.

H. A. Macleod, Thin film optical filters, (Institute of Physics Publishing, 2001)

16.

N. Mattiucci, G. D’Aguanno, N. Akozbek, M. Scalora, and M. J. Bloemer, “Homogenization procedure for a metamaterial and local violation of the second principle of thermodynamics,” Opt. Commun. 283(8), 1613–1620 (2010). [CrossRef]

17.

T. Decoopman, G. Tayeb, S. Enoch, D. Maystre, and B. Gralak, “Photonic crystal lens: from negative refraction and negative index to negative permittivity and permeability,” Phys. Rev. Lett. 97(7), 073905 (2006). [CrossRef] [PubMed]

18.

Handbook of Optical constants of solids, E. D. Palik ed., (Academic Press Inc., 1991).

19.

G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]

20.

N. Mattiucci, G. D’Aguanno, M. J. Bloemer, and M. Scalora, “Second-harmonic generation from a positive-negative index material heterostructure,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72(6), 066612 (2005). [CrossRef]

21.

G. D’Aguanno, N. Mattiucci, M. Scalora, M. J. Bloemer, and A. M. Zheltikov, “Density of modes and tunneling times in finite one-dimensional photonic crystals: a comprehensive analysis,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 70(1), 016612 (2004). [CrossRef] [PubMed]

22.

J. E. Sipe, V. C. Y. So, M. Fukui, and G. I. Stegeman, “Analysis of second-harmonic generation at metal surfaces,” Phys. Rev. B 21(10), 4389–4402 (1980). [CrossRef]

23.

M. Scalora, M. A. Vincenti, D. de Ceglia, V. Roppo, M. Centini, N. Akozbek, and M. J. Bloemer, “Second and Third Harmonic Generation in Metal-Based Nanostructures,” at http://arxiv.org/abs/1006.3841

24.

S. Ciraci and I. P. Batra, “Theory of the quantum size effect in simple metals,” Phys. Rev. B Condens. Matter 33(6), 4294–4297 (1986). [CrossRef] [PubMed]

25.

H. Raether, “Surface Plasmons,” Springer Tracts in Modern Physics, (Berlin, 1988)

26.

D. Sarid, “Long-range surface-plasma waves on very thin metal films,” Phys. Rev. Lett. 47(26), 1927–1930 (1981). [CrossRef]

27.

N. Mattiucci, G. D’Aguanno, M. Scalora, M. J. Bloemer, and C. Sibilia, “Transmission function properties for multi-layered structures: application to super-resolution,” Opt. Express 17(20), 17517–17529 (2009). [CrossRef] [PubMed]

28.

G. D’Aguanno, M. Centini, M. Scalora, C. Sibilia, M. Bertolotti, M. J. Bloemer, and C. M. Bowden, “Generalized coupled-mode theory for χ(2) interactions in finite multilayered structures,” J. Opt. Soc. Am. B 19(9), 2111 (2002). [CrossRef]

29.

J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B Condens. Matter 33(8), 5186–5201 (1986). [CrossRef] [PubMed]

OCIS Codes
(190.0190) Nonlinear optics : Nonlinear optics
(240.6680) Optics at surfaces : Surface plasmons

ToC Category:
Optics at Surfaces

History
Original Manuscript: July 30, 2010
Revised Manuscript: September 30, 2010
Manuscript Accepted: October 1, 2010
Published: October 27, 2010

Citation
Nadia Mattiucci, Giuseppe D’Aguanno, and Mark J. Bloemer, "Second harmonic generation from metallo-dielectric multilayered structures in the plasmonic regime," Opt. Express 18, 23698-23710 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-23-23698


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References

  1. F. Brown, R. E. Parks, and A. M. Sleeper, “Nonlinear Optical Reflection from a Metallic Boundary,” Phys. Rev. Lett. 14(25), 1029–1031 (1965). [CrossRef]
  2. N. Bloembergen, R. K. Chang, S. S. Jha, and C. H. Lee, “Optical Second-Harmonic Generation in Reflection from Media with Inversion Symmetry,” Phys. Rev. 174(3), 813–822 (1968). [CrossRef]
  3. Y. R. Shen, The Principles of Nonlinear Optics, (Wiley, 1984)
  4. H. J. Simon, D. E. Mitchell, and J. G. Watson, “Optical Second-Harmonic Generation with Surface Plasmons in Silver Films,” Phys. Rev. Lett. 33(26), 1531–1534 (1974). [CrossRef]
  5. E. Kretschmann, “The Determination of the Optical Constants of Metals by Excitation of Surface Plasmons,” Z. Phys. 241(4), 313–324 (1971). [CrossRef]
  6. J. C. Quail and H. J. Simon, “Second-harmonic generation from silver and aluminum films in total internal reflection,” Phys. Rev. B Condens. Matter 31(8), 4900–4905 (1985). [CrossRef] [PubMed]
  7. T. Y. F. Tsang, “Surface-plasmon-enhanced third-harmonic generation in thin silver films,” Opt. Lett. 21(4), 245–247 (1996). [CrossRef] [PubMed]
  8. R. Naraoka, H. Okawa, K. Hashimoto, and K. Kajikawa, “Surface plasmon resonance enhanced second-harmonic generation in Kretschmann configuration,” Opt. Commun. 248(1-3), 249–256 (2005). [CrossRef]
  9. T. A. Leskova, M. Leyva-Lucero, E. R. Mendez, A. A. Maradudin, and I. V. Novikov, “The surface enhanced second harmonic generation of light from a randomly rough metal surface in the Kretschmann geometry,” Opt. Commun. 183(5-6), 529–545 (2000). [CrossRef]
  10. Q. Chen, X. Sun, I. R. Coddington, D. A. Goetz, and H. J. Simon, “Reflected second-harmonic generation with coupled surface-plasmon modes in Ag/liquid/Ag layers,” J. Opt. Soc. Am. B 16(6), 971–975 (1999). [CrossRef]
  11. J. G. Rako, J. C. Quail, and H. J. Simon, “Optical second-harmonic generation with surface plasmons in noncentrosymmetric crystals,” Phys. Rev. B 30(10), 5552–5559 (1984). [CrossRef]
  12. G. M. Wysin, H. J. Simon, and R. T. Deck, “Optical bistability with surface plasmons,” Opt. Lett. 6(1), 30–32 (1981). [CrossRef] [PubMed]
  13. M. Fukui, J. E. Sipe, V. C. Y. So, and G. I. Stegeman, “Nonlinear mixing of opposite traveling surface plasmons,” Solid State Commun. 27(12), 1265–1267 (1978). [CrossRef]
  14. C. K. Chen, A. R. B. de Castro, and Y. R. Shen, “Coherent second-harmonic generation by counterpropagating surface plasmons,” Opt. Lett. 4(12), 393–394 (1979). [CrossRef] [PubMed]
  15. H. A. Macleod, Thin film optical filters, (Institute of Physics Publishing, 2001)
  16. N. Mattiucci, G. D’Aguanno, N. Akozbek, M. Scalora, and M. J. Bloemer, “Homogenization procedure for a metamaterial and local violation of the second principle of thermodynamics,” Opt. Commun. 283(8), 1613–1620 (2010). [CrossRef]
  17. T. Decoopman, G. Tayeb, S. Enoch, D. Maystre, and B. Gralak, “Photonic crystal lens: from negative refraction and negative index to negative permittivity and permeability,” Phys. Rev. Lett. 97(7), 073905 (2006). [CrossRef] [PubMed]
  18. Handbook of Optical constants of solids, E. D. Palik ed., (Academic Press Inc., 1991).
  19. G. D’Aguanno, M. C. Larciprete, N. Mattiucci, A. Belardini, M. J. Bloemer, E. Fazio, O. Buganov, M. Centini, and C. Sibilia, “Experimental study of Bloch vector analysis in nonlinear, finite, dissipative systems,” Phys. Rev. A 81(1), 013834 (2010). [CrossRef]
  20. N. Mattiucci, G. D’Aguanno, M. J. Bloemer, and M. Scalora, “Second-harmonic generation from a positive-negative index material heterostructure,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72(6), 066612 (2005). [CrossRef]
  21. G. D’Aguanno, N. Mattiucci, M. Scalora, M. J. Bloemer, and A. M. Zheltikov, “Density of modes and tunneling times in finite one-dimensional photonic crystals: a comprehensive analysis,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 70(1), 016612 (2004). [CrossRef] [PubMed]
  22. J. E. Sipe, V. C. Y. So, M. Fukui, and G. I. Stegeman, “Analysis of second-harmonic generation at metal surfaces,” Phys. Rev. B 21(10), 4389–4402 (1980). [CrossRef]
  23. M. Scalora, M. A. Vincenti, D. de Ceglia, V. Roppo, M. Centini, N. Akozbek, and M. J. Bloemer, “Second and Third Harmonic Generation in Metal-Based Nanostructures,” at http://arxiv.org/abs/1006.3841
  24. S. Ciraci and I. P. Batra, “Theory of the quantum size effect in simple metals,” Phys. Rev. B Condens. Matter 33(6), 4294–4297 (1986). [CrossRef] [PubMed]
  25. H. Raether, “Surface Plasmons,” Springer Tracts in Modern Physics, (Berlin, 1988)
  26. D. Sarid, “Long-range surface-plasma waves on very thin metal films,” Phys. Rev. Lett. 47(26), 1927–1930 (1981). [CrossRef]
  27. N. Mattiucci, G. D’Aguanno, M. Scalora, M. J. Bloemer, and C. Sibilia, “Transmission function properties for multi-layered structures: application to super-resolution,” Opt. Express 17(20), 17517–17529 (2009). [CrossRef] [PubMed]
  28. G. D’Aguanno, M. Centini, M. Scalora, C. Sibilia, M. Bertolotti, M. J. Bloemer, and C. M. Bowden, “Generalized coupled-mode theory for χ(2) interactions in finite multilayered structures,” J. Opt. Soc. Am. B 19(9), 2111 (2002). [CrossRef]
  29. J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B Condens. Matter 33(8), 5186–5201 (1986). [CrossRef] [PubMed]

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