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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 5 — Mar. 2, 2009
  • pp: 3603–3609
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Tailored second harmonic generation from self-organized metal nano-wires arrays

Alessandro Belardini, Maria Cristina Larciprete, Marco Centini, Eugenio Fazio, Concita Sibilia, Mario Bertolotti, Andrea Toma, Daniele Chiappe, and Francesco Buatier de Mongeot  »View Author Affiliations


Optics Express, Vol. 17, Issue 5, pp. 3603-3609 (2009)
http://dx.doi.org/10.1364/OE.17.003603


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Abstract

Here we report the second harmonic emission properties of self-organized gold nanowires arrays supported on dielectric substrates with a sub-wavelength periodic pattern. The peculiar morphology of the nanowires, which are locally tilted with respect to the average plane of the substrate, allows to generate maximum second harmonic signal at normal incidence with a polarization direction driven by the orientation of the wires (perpendicular to the wires). The generation efficiency was increased by tailoring the growth process in order to tune the metal plasmon resonance close to the pump field frequency and also by increasing the local tilt of the nanowires.

© 2009 Optical Society of America

1. Introduction

Metal nanostructures supported on dielectric substrates have attracted great interest as building blocks of nanoscale optical devices. Large interest was devoted in developing molecular sensors, by using metal enhanced fluorescence [1–2

1. M. H. Chowdhury, K. Ray, K. Aslan, J. R. Lakowicz, and C. D. Geddes, “Metal-Enhanced Fluorescence of Phycobiliproteins from Heterogeneous Plasmonic Nanostructures,” J. Phys. Chem. C 111, 18856 (2007). [CrossRef]

], surface enhanced Raman scattering [3–4

3. P. Measor, L. Seballos, D. Yin, J. Z. Zhang, E. J. Lunt, A. R. Hawkins, and H. Schmidt, “On-chip surface-enhanced Raman scattering detection using integrated liquid-core waveguides,” Appl. Phys. Lett. 90, 211107 (2007). [CrossRef]

] or second harmonic (SH) emission from nanostructures synthesised by e-beam lithography [5–9

5. M. Airola, Y. Liu, and S. Blair, “Second-harmonic generation from an array of sub-wavelength metal apertures,” J. Opt. A: Pure Appl. Opt. 7, S118 (2005). [CrossRef]

]. The interest in the development of metallo-dielectric substrates providing high SH conversion efficiency is due to the possibility of realising new optical (multiphoton) microscopy schemes where vertical illumination and detection are adopted.

Here we studied the SH emission properties of a self-assembled gold nanowire arrays supported on sub-wavelength (160 nm) patterned glass templates produced recurring to a novel self-organised maskless approach. Different morphologies were here investigated in order to highlight and overcome the typical constraints which frustrate SH generation on flat metal surfaces: the emission is forbidden at normal incidence and it is always polarized along the plane of incidence, when the pumping light is set to be either s- or p- polarized; these peculiarities arise because most metals posses cubic crystal structure and the electric dipole term in the Maxwell equations, responsible for the standard SH signal, vanishes when inversion symmetry is present in the lattice structure. In a metal, the SH source terms consist of a magnetic dipole term, originating from the Lorentz force on the conduction electrons, and of an electric quadrupole contribution, through the Coulomb force. The expression of the nonlinear polarization at frequency 2ω which acts as a source term for the SH field was calculated by considering a free electron model for the metal [10

10. J. Rudnick and E. A. Stern, “Second-Harmonic Radiation from Metal Surfaces,” Phys. Rev. B 4, 4274 (1971). [CrossRef]

,12

12. 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, 4389 (1980). [CrossRef]

]. Starting from the equation of motion for the free electrons, within a perturbative approach we can write:

P2ω=(iγ2ω2)[e2mω2(·Jω)Eωeμ0(Jω×Hω)],
(1)

where e is the modulus of the electron charge, m is the electron effective mass, γ is a damping coefficient taking into account ohmic losses. The contribution of the second term on the right hand side of Eq. (1) is limited by the weak penetration of the electric current inside the metal due to the skin effect (≈15 nm at optical frequencies); the first term gives a nonzero contribution only at the surface [12

12. 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, 4389 (1980). [CrossRef]

] (≈ 1 nm, much smaller than the skin depth). We note that both terms in Eq. (1) are always polarized in the incidence plane (p- polarized) when the pump is either s- or p- polarized. Moreover, pumping at normal incidence, the transverse component of the nonlinear polarization (responsible for collinear SH generation) is zero. We experimentally demonstrate that the nonlinear properties of our samples, consisting of a tilted array of self-organized gold nanowires (NW) with sub-wavelength separation, are characterized by maximum SH emission at normal incidence with a polarization direction driven by the orientation of the wires (perpendicular to the wires). On the contrary, measurements of SH generation from reference samples (flat gold layer, corrugated gold layer, untitled wires) are in agreement with the expectations of ordinary centro-symmetric metal films i.e. they exhibit a minimum in the SH generation efficiency at normal incidence.

2. Sample fabrication

Nanowire arrays have been fabricated by combining ion beam sputtering (IBS) [13

13. U. Valbusa, C. Boragno, and F. Buatier de Mongeot, “Nanostructuring surfaces by ion sputtering,” J. Phys.: Condens. Matter 14, 8153–8175 (2002). [CrossRef]

] to kinetically controlled deposition (KCD) [14–15

14. F. Buatier de Mongeot, W. Zhu, A. Molle, R. Buzio, C. Boragno, U. Valbusa, E. G. Wang, and Z. Zhang, “Nanocrystal Formation and Faceting Instability in Al(110) Homoepitaxy: True Upward Adatom Diffusion at Step Edges and Island Corners,” Phys. Rev. Lett. 91, 016102 (2003). [CrossRef]

]. The dielectric substrates (0.2 mm standard microscope glass slides) were patterned via IBS. The substrates were irradiated by a 800 eV Ar+ defocused ion beam at an incidence angle of 35 deg from the surface normal. Under such conditions we obtained the formation of a nanoscale ripple pattern with a periodicity of 160 +/- 20 nm and an amplitude of 16+/-5 nm (samples A,B,C,D,G). Then the synthesis of the metal nanowires is performed by means of glancing angle deposition of gold (80 deg with respect the surface normal). In this way, due to the shadowing effect imposed by the glass ripples, agglomeration of a disconnected array of metal nanowires with a tilt of 12 deg takes place onto the illuminated ridges of the glass ripple pattern. The derivative of the AFM image in the inset of Fig. 1(a) highlights the contrast between the unexposed glass and the polycrystalline structure of the gold NWs. In Fig. 1(b) a comparison between the derivative of two line profiles (before and after Au deposition) is shown, in order to retrieve information on the nanowire lateral size and disconnection.

Fig. 1. Samples morphology. (a) AFM representing morphology after Au deposition (the same sample in Fig. 2(a). The inset shows a 2.5 X zoom of the derivative signal of the AFM topography. (b) a comparison between the derivative of two line profiles is shown: top trace, before Au deposition, bottom trace, after Au deposition. The lateral width (w) of the Au NWs is identified by the presence of additional dips (indicated by dashed lines).

By modifying the Au dose we fabricated NW test samples (samples A,B,C,D) with different NW cross-sections ranging from 11 nm to 70 nm. The absorption spectrum for each sample (see Figs. 2(a)2(d)) has been measured revealing the direct correlation between the resonance in the transmission spectrum and the absorbance, in correspondence to the excitation of localized plasmons by TM polarized light. An increase in the cross section of the NWs when increasing Au dose results in a red-shift of the plasmon resonance. This fact allows to tune the plasmon resonance closer to the pumping wavelength at λω = 800 nm.

Fig. 2. Test samples. (a)-(e) red and black traces refer to the optical transmission spectra of samples A,B,C,D,E obtained at normal incidence for two perpendicular polarization states of the incoming light (parallel-red and perpendicular-black with respect to the NW axis). The blue stars represent the differential absorption spectra (ΔA = A - A //). The black arrows indicate the estimated plasmon absorption peak position. The Au coverage was varied from h ≅ 11 nm (Sample A) to h ≅ 70 nm (Sample D and E). In panels (f)-(l) the corresponding SH conversion efficiency η I /I ω is plotted. On the left side the samples morphology is sketched.

3. Second harmonic generation measurements

Fig. 3. Reference samples: in panels (a)-(c) the optical transmission spectra of the three reference samples F,G,H and their differential absorbance are plotted. (a - Sample F) is a flat Au film (20 nm thick), (b- Sample G) represents an Au film conformal capping a nanostructured glass template and (c - Sample H) is an array of Au nanowires locally parallel with respect to the glass substrate. In panels (d)-(f) the corresponding SH conversion efficiency η = I /I ω is plotted. On the left side the samples morphology is sketched.

4. Numerical simulations

Numerical simulations were performed in order to calculate the linear fields and then to retrieve the origin of the nonlinear response by using a dedicated numerical algorithm based on Eq. (1). In Fig. 4 the calculated radiative nonlinear polarization terms are shown for two configurations of the wires, tilted (Fig. 4(b)) and un-tilted (Fig. 4(a)), respectively corresponding to the tilted nanowire arrays (Figs. 2(a)-2(d)) and to the un-tilted nanowire array (Fig. 3(c)) when the p-polarized pump is set to normal incidence.

Fig. 4. z-component of the nonlinear polarization vector at 2ω (arbitray units) calculated for a p-polarized wave propagating along the x axis in the case: (a) untilted wires; (b) tilted wires.

We remind the reader that the nonlinear polarization vector acts as a source term for the SH field and we note that, in the un-tilted case (i.e. H sample), the polarization contribution from one wire is balanced by a reverse contribution of the adjacent wire (Fig. 4(a)). Thus it is possible to qualitatively predict a vanishing macroscopic SH signal at normal incidence, in agreement with experimental results (Fig. 3(f)). On the contrary, in the tilted case, the net contribution is different from zero due to the asymmetry of the system on a subwavelength scale, thus producing a radiative SH signal (Figs. 2(f)-2(l)). The linear fields have been retrieved by comparing experimental and numerical transmission spectra. The spectral behaviour is strongly dependent on the wire thickness, periodicity and degree of disconnection. For instance, the absorption spectrum for each sample has been performed revealing the direct correlation between the resonance in the transmission spectrum (E field perpendicular to wires) and the absorbance. Numerical results are in agreement with experiments; enhanced absorption (black curves in Fig. (5)) is obtained as long as wires are sufficiently separated from each other (Figs. 5(a)-5(b)). On the contrary, when distance between wires is less than 10 nm, interaction between wires is responsible for increased reflectivity (red curves) and lower absorption (Fig. 5(c)).

Fig. 5. Transmission (blue), reflection (red) and absorption (black) spectra calculated for different gold wires distance as described in the inset.

5. Conclusions

In conclusion our experiments demonstrate that it is possible to fabricate low cost metal nanowire arrays of subwavelength periodicity and size, which are supported on locally tilted transparent glass substrates. Their SHG has maximum emission efficiency at normal incidence with a polarization direction driven by the orientation of the wires (perpendicular to the wires). The efficiency of the SHG can be optimized by tailoring the growth process in order to tune the metal plasmon resonance close to the pump field frequency. The SHG efficiency increases by two orders of magnitude with respect to planar films or to un-tilted NWs due to the strong field localization at the dielectric-metal interfaces, as demonstrated by the theoretical and numerical calculations. A further increase of the SH generation efficiency at normal incidence was obtained by increasing the local tilt of the nanowires, thus demonstrating that adaptable effective medium supported over large areas can be synthesized by the proposed self-organised method. These results appear of clear interest in view of applications in the field of high sensitivity bio-sensing where multiphoton confocal microscopy detection schemes are employed for the study of biological fluorescent samples supported on glass substrates. The amplification of the SH signal, enhanced in the near field of the gold nanoparticles at normal light incidence, provides in fact an efficient way for boosting fluorescence emission from fluorochromes located at the surface of the nanoparticles.

Acknowledgments

This work was supported by the European Community under the Network of Excellence PHOREMOST, by CNISM (fondo Innesco CNR-INFM), and by MAE under program Italia-Polonia. The authors gratefully acknowledge O. Buganov, M. Braccini and G. Leahu for experimental support. Useful discussions with U. Valbusa, A. Diaspro and C. Boragno are acknowledged.

References and links

1.

M. H. Chowdhury, K. Ray, K. Aslan, J. R. Lakowicz, and C. D. Geddes, “Metal-Enhanced Fluorescence of Phycobiliproteins from Heterogeneous Plasmonic Nanostructures,” J. Phys. Chem. C 111, 18856 (2007). [CrossRef]

2.

J. Zhang, Y. Fu, M. H. Chowdhury, and J. R. Lakowicz, “Single-Molecule Studies on Fluorescently Labeled Silver Particles: Effects of Particle Size,” J. Phys. Chem. C 112, 18 (2008). [CrossRef]

3.

P. Measor, L. Seballos, D. Yin, J. Z. Zhang, E. J. Lunt, A. R. Hawkins, and H. Schmidt, “On-chip surface-enhanced Raman scattering detection using integrated liquid-core waveguides,” Appl. Phys. Lett. 90, 211107 (2007). [CrossRef]

4.

X. Qian, X.-H. Peng, D. O. Ansari, Q. Yin-Goen, G. Z. Chen, D. M. Shin, L. Yang, A. N. Young, M. D. Wang, and S. Nie, “In vivo: tumor targeting and spectroscopic detection with surface-enhanced Raman nanoparticle tags,” Nat. Biotechnol. 26, 83 (2008). [CrossRef]

5.

M. Airola, Y. Liu, and S. Blair, “Second-harmonic generation from an array of sub-wavelength metal apertures,” J. Opt. A: Pure Appl. Opt. 7, S118 (2005). [CrossRef]

6.

J. A. H. van Nieuwstadt, M. Sandtke, R. H. Harmsen, F. B. Segerink, J. C. Prangsma, S. Enoch, and L. Kuipers, “Strong Modification of the Nonlinear Optical Response of Metallic Subwavelength Hole Arrays,” Phys. Rev. Lett. 97, 146102 (2006). [CrossRef] [PubMed]

7.

C. Hubert, L. Billot, P.-M. Adam, R. Bachelot, P. Royer, J. Grand, D. Gindre, K. D. Dorkenoo, and A. Fort, “Role of surface plasmon in second harmonic generation from gold nanorods,” Appl. Phys. Lett. 90, 181105 (2007). [CrossRef]

8.

M. W. Klein, C. Enkrich, M. Wegener, and S. Linden,“Second-Harmonic Generation from Magnetic Metamaterials,” Science 313, 502 (2006). [CrossRef] [PubMed]

9.

M. W. Klein, M. Wegener, N. Feth, and S. Linden, “Experiments on second- and third-harmonic generation from magnetic metamaterials: erratum,” Opt. Express 16, 8055 (2008). [CrossRef]

10.

J. Rudnick and E. A. Stern, “Second-Harmonic Radiation from Metal Surfaces,” Phys. Rev. B 4, 4274 (1971). [CrossRef]

11.

C. H. Lee, “Optical Second-Harmonic Generation in Reflection from Media with Inversion Symmetry,” Phys. Rev. 174, 813 (1968). [CrossRef]

12.

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, 4389 (1980). [CrossRef]

13.

U. Valbusa, C. Boragno, and F. Buatier de Mongeot, “Nanostructuring surfaces by ion sputtering,” J. Phys.: Condens. Matter 14, 8153–8175 (2002). [CrossRef]

14.

F. Buatier de Mongeot, W. Zhu, A. Molle, R. Buzio, C. Boragno, U. Valbusa, E. G. Wang, and Z. Zhang, “Nanocrystal Formation and Faceting Instability in Al(110) Homoepitaxy: True Upward Adatom Diffusion at Step Edges and Island Corners,” Phys. Rev. Lett. 91, 016102 (2003). [CrossRef]

15.

A. Toma, D. Chiappe, D. Massabò, C. Boragno, and F. Buatier de Mongeot, “Self-organized metal nanowire arrays with tunable optical anisotropy,” Appl. Phys. Lett. 93, 163104 (2008). [CrossRef]

16.

S. Rusponi, G. Costantini, F. Buatier-de-Mongeot, C. Boragno, and U. Valbusa, “Patterning a surface on the nanometric scale by ion sputtering,” Appl. Phys. Lett. 75, 3318–3320 (1999). [CrossRef]

17.

F. Buatier de Mongeot et al. Patent pending.

OCIS Codes
(190.2620) Nonlinear optics : Harmonic generation and mixing
(190.4400) Nonlinear optics : Nonlinear optics, materials
(240.4350) Optics at surfaces : Nonlinear optics at surfaces
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Nonlinear Optics

History
Original Manuscript: December 19, 2008
Revised Manuscript: January 29, 2009
Manuscript Accepted: January 29, 2009
Published: February 23, 2009

Citation
Alessandro Belardini, Maria C. Larciprete, Marco Centini, Eugenio Fazio, Concita Sibilia, Mario Bertolotti, Andrea Toma, Daniele Chiappe, and Francesco Buatier de Mongeot, "Tailored second harmonic generation from self-organized metal nano-wires arrays," Opt. Express 17, 3603-3609 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-5-3603


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References

  1. M. H. Chowdhury, K. Ray, K. Aslan, J. R. Lakowicz, and C. D. Geddes, "Metal-Enhanced Fluorescence of Phycobiliproteins from Heterogeneous Plasmonic Nanostructures," J. Phys. Chem. C 111, 18856 (2007). [CrossRef]
  2. J. Zhang, Y. Fu, M. H. Chowdhury, and J. R. Lakowicz, "Single-Molecule Studies on Fluorescently Labeled Silver Particles: Effects of Particle Size," J. Phys. Chem. C 112, 18 (2008). [CrossRef]
  3. P. Measor, L. Seballos, D. Yin, J. Z. Zhang, E. J. Lunt, A. R. Hawkins, H. Schmidt, "On-chip surface-enhanced Raman scattering detection using integrated liquid-core waveguides," Appl. Phys. Lett. 90, 211107 (2007). [CrossRef]
  4. X. Qian, X.-H. Peng, D. O. Ansari, Q. Yin-Goen, G. Z. Chen, D. M. Shin, L. Yang, A. N. Young, M. D. Wang, and S. Nie, "In vivo: tumor targeting and spectroscopic detection with surface-enhanced Raman nanoparticle tags," Nat. Biotechnol. 26, 83 (2008). [CrossRef]
  5. M. Airola, Y. Liu, and S. Blair, "Second-harmonic generation from an array of sub-wavelength metal apertures," J. Opt. A: Pure Appl. Opt. 7, S118 (2005). [CrossRef]
  6. J. A. H. van Nieuwstadt, M. Sandtke, R. H. Harmsen, F. B. Segerink, J. C. Prangsma, S. Enoch, and L. Kuipers, "Strong Modification of the Nonlinear Optical Response of Metallic Subwavelength Hole Arrays," Phys. Rev. Lett. 97, 146102 (2006). [CrossRef] [PubMed]
  7. C. Hubert, L. Billot, P.-M. Adam, R. Bachelot, P. Royer, J. Grand, D. Gindre, K. D. Dorkenoo, and A. Fort, "Role of surface plasmon in second harmonic generation from gold nanorods," Appl. Phys. Lett. 90, 181105 (2007). [CrossRef]
  8. M. W. Klein, C. Enkrich, M. Wegener, S. Linden," Second-Harmonic Generation from Magnetic Metamaterials," Science 313, 502 (2006). [CrossRef] [PubMed]
  9. M. W. Klein, M. Wegener, N. Feth, S. Linden, "Experiments on second- and third-harmonic generation from magnetic metamaterials: erratum," Opt. Express 16, 8055 (2008). [CrossRef]
  10. J. Rudnick and E. A. Stern, "Second-Harmonic Radiation from Metal Surfaces," Phys. Rev. B 4, 4274 (1971). [CrossRef]
  11. C. H. Lee, "Optical Second-Harmonic Generation in Reflection from Media with Inversion Symmetry," Phys. Rev. 174, 813 (1968). [CrossRef]
  12. 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, 4389 (1980). [CrossRef]
  13. U. Valbusa, C. Boragno and F. Buatier de Mongeot, "Nanostructuring surfaces by ion sputtering," J. Phys.: Condens. Matter 14, 8153-8175 (2002). [CrossRef]
  14. F. Buatier de Mongeot, W. Zhu, A. Molle, R. Buzio, C. Boragno, U. Valbusa, E. G. Wang, Z. Zhang, "Nanocrystal Formation and Faceting Instability in Al(110) Homoepitaxy: True Upward Adatom Diffusion at Step Edges and Island Corners," Phys. Rev. Lett. 91, 016102 (2003). [CrossRef]
  15. A. Toma, D. Chiappe, D. Massabò, C. Boragno, and F. Buatier de Mongeot, "Self-organized metal nanowire arrays with tunable optical anisotropy," Appl. Phys. Lett. 93, 163104 (2008). [CrossRef]
  16. S. Rusponi, G. Costantini, F. Buatier-de-Mongeot, C. Boragno, and U. Valbusa, "Patterning a surface on the nanometric scale by ion sputtering," Appl. Phys. Lett. 75, 3318-3320 (1999). [CrossRef]
  17. F. Buatier de Mongeot et al. Patent pending.

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