Highly efficient broadband ultrafast plasmonics ∗
Brian Ashall,1,2, Jos´e Francisco L´opez-Barber´a,1,3 ´ Eadaoin McClean-Ilten,1 and Dominic Zerulla.1,4,5 1 School
of Physics, University College Dublin, Belfield, Dublin 4, Ireland of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland 3 ICN2 - Institut Catala de Nanociencia i Nanotecnologia, Campus de la UAB, Bellaterra, Barcelona, 08193, Spain 4 Mechanical Engineering, 5130 Etcheverry Hall, University of California Berkeley, CA 94720-1740, USA 5 [email protected] 2 School
∗ [email protected]
Abstract: To date, considerable experimental and theoretical focus has been placed on the spatial control of Surface Plasmon Polaritons (SPPs) using nanostructured surfaces; however, research aimed toward accessing the ultrafast dynamics of SPPs remains vastly unexplored. Despite this, SPPs have the potential to exhibit some of the fastest possible optical processes, while maintaining the advantage of nanoscale spatial manipulation. Here, we present an experimental and computational investigation of a system that provides access to the efficient excitation of broadband, propagating SPP modes. To achieve this, a surface array of tailor designed, reduced symmetry nanostructures has been fabricated to enable the required control of the plasmon dispersion map to match sub 20 fs pulses in the near infra-red. Using a combination of optical spectroscopy and frequency resolved optical gating techniques, complimented by finite element computational analysis, the efficient excitation of propagating broadband plasmonic modes is demonstrated. © 2013 Optical Society of America OCIS codes: (240.6680) Surface plasmons, (240.5420) Polaritons, (320.2250) Femtosecond phenomen)
References and links 1. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer Verlag, 1988). 2. 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. Mat. 9, 193-204 (2010). 3. M. I. Stockman, M. F. Kling, U. Kleineberg, and F. Krausz, “Attosecond nanoplasmonic field microscope,” Nat. Photon. 1, 539-544 (2007). 4. M. I. Stockman, “Ultrafast nanoplasmonics under coherent control,” N. J. Phys. 10, 025031 (2008). 5. C. S¨onnichsen, T. Franzl, T. Wilk, G. von Plessen, J. Feldmann, O. Wilson, and P. Mulvaney, “Drastic reduction of plasmon damping in gold nanorods,” Phys. Rev. Lett. 88, 077402-077405 (2002). 6. B. Lamprecht, J. R. Krenn, A. Leitner, and F. R. Aussenegg, “Resonant and off-resonant light-driven plasmons in metal nanoparticles studied by femtosecond-resolution third-harmonic generation,” Phys. Rev. Lett. 83, 44214424 (1999). 7. A. Wokaun, J. P. Gordon, and P. F. Liao, “Radiation damping in surface-enhanced raman scattering,” Phys. Rev. Lett. 48, 957-960 (1982).
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27383
8. T. Klar, M. Perner, S. Grosse, G. von Plessen, W. Spirkl, and J. Feldman, “Surface-plasmon resonances in single metallic nanoparticles,” Phys. Rev. Lett. 80, 4249-4252 (1998). 9. S. Link and M. A. El-Sayed, “Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods,” J. Phys. Chem. B 103, 8410-8426 (1999). 10. A. Dogariu, T. Thio, L. J. Wang, T. W. Ebbesen, and H. J. Lezec, “Delay in light transmission through small apertures,” Opt. Lett. 26, 450-452 (2001). 11. R. M¨uller, V. Malyarchuk, and C. Lienau, “Three-dimensional theory on light-induced near-field dynamics in a metal film with a periodic array of nanoholes,” Phys. Rev. B 68, 205415-205423 (2003). 12. T. Zentgraf, A. Christ, J. Kuhl, and H. Giessen, “Tailoring the ultrafast dephasing of quasiparticles in metallic photonic crystals,” Phys. Rev. Lett. 93, 243901-243904 (2004). 13. D. S. Kim, S. C. Hohng, V. Malyarchuk, Y. C. Yoon, Y. H. Ahn, K. J. Yee, J.W. Park, J. Kim, Q. H. Park, and C. Lienau, “Microscopic origin of surface-plasmon radiation in plasmonic band-gap nanostructures,” Phys. Rev. Lett. 91, 143901-143904 (2003). 14. C. Ropers, M¨uller, C. Lienau, G. Stibenz, G. Steinmeyer, D-J. Park, Y-C. Yoon, and D-S. Kim, “Ultrafast dynamics of light transmission through plasmonic crystals,” International Conference on Ultrafast Phenomena, Niigata, Japan (2004). 15. R. Rokitski, K. A. Tetz, and Y. Fainman, “Propagation of femtosecond surface plasmon polariton pulses on the surface of a nanostructured metallic film: space-time complex amplitude characterization,” Phys. Rev. Lett. 95, 177401-177404 (2005). 16. W. Zhou, H. Gao, and T. W. Odom, “Toward broadband plasmonics: tuning dispersion in rhombic plasmonic crystals,” ACS Nano 4, 1241-1247 (2010). 17. J.-S Bouillard, S. Vilain, W. Dickson, G. A. Wurtz, and A. V. Zayats, “Broadband and broadangle SPP antennas based on plasmonic crystals with linear chirp,” Scientific Reports 2, 829 (2012). 18. S. Rehwald, M. Berndt, F. Katzenberg, S. Schwieger, E. Runge, K. Schierbaum, and D. Zerulla, “Tunable nanowires: an additional degree of freedom in plasmonics,” Phys. Rev. B 76, 085420-085423 (2007). 19. S. E. Yalcin, Y. Wang, and M. Achermann, “Spectral bandwidth and phase effects of resonantly excited ultrafast surface plasmon pulses,” Appl. Phys. Lett. 93, 101103-101105 (2008). 20. R. Trebino, Frequency-Resolved Optical Gating: the Measurement of Ultrashort Pulses (Kluwer Academic Publishers, 2000). 21. J. Chen, Z. Li, M. Lei, S. Yue. J. Xiao, and Q. Gong, “Broadband unidirectional generation of surface plasmon polaritons with dielectric-film-coated asymmetric singl-slit,” Opt. Express 19, 26463-26469 (2011). 22. B. Ashall, M. Berndt, and D. Zerulla, “Tailoring surface plasmon polariton propagation via specific symmetry properties of nanostructures,” Appl. Phys. Lett. 91, 203109 (2007). 23. B. Ashall, B. Vohnsen, M. Berndt, and D. Zerulla, “Controlling polarization twisting of light resulting from surface plasmon interactions with threefold symmetric nanostructures,” Phys. Rev. B 80, 245413-245417 (2009). 24. A. Marini, D. V. Skryabin, and B. Malomed, “Stable spatial plasmon solitons in a dielectric-metal-dielectric geometry with gain and loss,” Opt. Express 19, 6616-6622 (2011). 25. W. L. Barnes, “Surface plasmon-polariton length scales: a route to sub-wavelength optics,” J. Opt. A 8, S87 (2006). 26. P. Scholz, S. Schwieger, B. Ashall, D. Zerulla, and E. Runge, “The influence of wire shape on surface plasmon mode distribution,” Appl. Phys. B 93, 111-115 (2008).
1.
Introduction
SPPs are electromagnetic surface waves confined to the interface of two materials with dielectric functions of opposite signs, e.g. conductor and dielectric [1]. They occur as a result of a resonant interaction between an illuminating electromagnetic wave, and localized surface density oscillations of the free electrons of the conductor. The field of plasmon photonics, referred to as plasmonics, offers numerous research possibilities and application prospects. The main driving force behind plasmonics research is the fact that SPs couple to light in a manner that allows for an extremely localized control of the coupled light, thus granting access to sub-diffraction limited optical processes, while maintaining optical frequencies. As a result of this, vast experimental and theoretical research working toward the goal of plasmonic nanoscale confinement and manipulation has been carried out (e.g. [2]), while the potential offered by ultrafast SPP processes remains vastly unexplored. Despite this, broadband SPPs (in the visible and near IR regimes) offer access to temporal processes ranging from evolution times on the hundreds of attoseconds [3], to dephasing times on the 10’s to 100’s of femtoseconds [4]. To date, aspects of the ultrafast kinetics and dephasing times of nanoparticle plasmons have #194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27384
Fig. 1. Illustration of the excitation of propagating plasmonic pulses on the surface of the examined tailor designed periodic nanostructured surface. Sub 20 fs, near IR (centered at 800 nm), 88 MHz repetition rate pulses are incident on the sample (from the left). The propagating SPP pulses along with six elongated exemplary diffracted channels are illustrated.
been investigated [5–9], along with the temporal analysis of extraordinary optical transmission processes [10–15]. However, as a result of the characteristic dispersive nature of propagating SPPs, there is an inherent difficulty in exciting broadband coherently propagating SPPs [16]. Recently, the importance of the efficient excitation of broadband SPPs has been addressed, where a linearly chirped micro-grating was used for the broadband excitation of unidirectional SPPs [17]. In that work, a spatially dependant range of surface momenta provided by the chirped micro-grating permitted the excitation of SPP modes of different frequencies in spatially neighbouring locations. In our presented work, we assume a similar goal of broadband propagating SPP excitation, with the extension to achieve this excitation in a spatially uniform manner; i.e. broadband SPP excitation at all points on the nanostructured surface that is illuminated by the laser pulse. 2.
Ultrashort SPP Pulses on Nanostructured Surfaces
The presented experimental and computational examinations consider the dynamics of broadband propagating SPP pulses excited on square arrays of tailor designed, three fold symmetric, silver coated nanostructures, as illustrated in Fig. 1. Such a surface presents two mechanisms whereby the surface topography can provide the required momentum to overcome the wavevector mismatch between free space light and SPs: grating [18] and nanofeature coupling [1]. Of particular importance here is a modified grating coupling technique. For this excitation scheme, when considering SPP temporal characteristics on the femtosecond (fs) scale, two temporal aspects of propagating SPPs must be distinguished; the SPP propagation lifetime (i.e. the time after which the amplitude of the excited SPP is reduced by a factor of 1/e), and the SPP temporal pulse length. When considering SPP excitation using ultrafast laser pulses (e.g. sub 20 fs), to date it has been the restrictive SPP spectral acceptance bandwidth for a fixed incoming angle that has limited SPP pulses to be temporally elongated in comparison to the driving illumina#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27385
Fig. 2. Characteristics of a sub 20 fs laser pulse used for SPP excitation measured by SHG FROG. (A) Temporal and spectral intensity plot. (B) Spectral intensity profile (red) and spectral phase (green). (C) Temporal pulse intensity (blue) and temporal phase (yellow). Flat spectral and temporal phase profiles across the pulse bandwidth (as in (B) and (C)) indicate a transform limited pulse.
tion field [19]. However, if this restriction could be lifted, applications in nonlinear plasmonics and ultrafast plasmonic data processing can be envisaged; where, for example, a single channel of on chip plasmonic pulse code modulation would result in potential data transmission rates of up to 1014 bits per second. For the Ti:Sa oscillator used in our experiments (Griffin C, Kapteyn-Murnane Laboratories), following group delay dispersion compensation, near transform limited (sub 20 fs) illumination pulses were generated. An essential prerequisite for this work is that the SPP mode must accept the full spectral content of such illumination pulses. Using the pulse characterization technique of Second Harmonic Generation (SHG) Frequency Resolved Optical Gating (FROG) [20], experimental measurements (Fig. 2) show that in order to envelope more than 95% of our neartransform limited sub 20 fs pulse spectrum (with a FWHM bandwidth of approximately 28 nm), an SPP coupling acceptance bandwidth exceeding 80 nm is in practice required. Furthermore, as the illumination pulse must remain spatially and temporally optimized prior to interaction with the surface, the required broadband excitation mechanism must couple this complete illumination bandwidth at a single angle of incidence, and the excitation must occur in a spatially uniform manner. However, for high efficiency SPP excitation architectures (i.e. typical grating or attenuated total reflection - ATR - coupling) the SPP dispersion relation varies rapidly with illumination frequency and angle [1]. For example, in an ATR configuration, the full bandwidth of a spatially and temporally unchirped ultrashort pulse cannot be coupled to an SPP simultaneously [19], thus prohibiting the generation of SPP pulses of comparable duration to the driving ultrashort laser pulse. For some nanofeature based excitation mechanisms (e.g. nanoparticle, rough surface, slit [21], etc.) a suitable broad range of momenta can be inherently provided [1], allowing for broadband plasmon excitation. However, such excitation schemes are limited to comparably weak SPP excitation, with only a very small percentage of the illumination light coupled to the desired SPP mode. Furthermore, such coupling techniques are not suitable for plasmon pulse generation, as the excited plasmons will have a range of group and phase velocities, will not co-propagate, and so will be strongly spatially and temporally chirped.
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27386
Fig. 3. (A) Illustration of setup. Broadband laser light from a post compensated Ti:Sa laser incident on a goniometer housing the sample. The detector (interchangeable between a photodiode, spectrometer, or SHG FROG) can scan any diffraction order with change in θ , φ , or α. Note: In reality, the center of the sample lies at the fulcrum of the θ , φ and α rotations. (B) SEM image of an 8 by 4 µm section of the examined periodic nanostructured array.
3.
Broadband SPP Excitation
In our presented work, an elegant solution for the efficient excitation of broadband propagating SPPs is found whereby a highly ordered periodic array of tailor designed, reduced symmetry nanostructures (Fig. 3(B)) is used for SPP excitation (for fabrication details see [22, 23]). Evidentally, such structures can be used to control and design the plasmon dispersion map. The initial design for the structures was conceived and optimized using reciprocal space analysis followed by a finite element analysis (Comsol Multiphysics). The geometry was chosen to present a particular distribution of inter-structure distances to allow for the required control over the dispersion map, and the physical dimensions of the surface features were optimized for SPP excitation in the near IR (pitch of approximately 1 µm, structure footprint of 740 nm by 750 nm). A further advantage is that the structures can be described by an analytical function; namely an epitrochoidal geometry. For this sample, as a result of the well ordered array basis, a high SPP coupling efficiency is achieved, and as a result of the reduced symmetry nanostructures, the range of momenta provided by the grating is increased, customizing the dispersion map to present the possibility of generating efficient broadband SPPs. A detailed far-field experimental examination of the excitation of SPPs on the nanostructured array was performed using a goniometric set up (Fig. 3(A)) allowing for combined spectral and angular intensity characterization along with Autocorrelation and FROG analysis. By spectrally and angularly mapping individual diffraction orders, the optical dispersion map is obtained, where SPP modes are identifiable as dark bands in the far-field light. Using near-transform limited pulses, three plasmonic modes were observed in the presented (−1, 0) diffraction order (Fig. 4). It is clear that the three SPP modes have different dispersion properties, characterized by their varying angular (θ ) and spectral dependencies. While the mode B plasmon, ranging from 35◦ to 50◦ (Fig. 4), appears to be the strongest, it is important to note that this mode has a strong angular dependence over the spectrum of the laser. This is significant here, as it means
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27387
Fig. 4. Experimentally measured spectral and angular (θ in Fig. 3(A)) reflectivity map (normalized intensity according to color scale) for the (-1, 0) diffraction order, using sub 20 fs illumination pulses as in Fig. 2. Three plasmonic modes (A, B and C) of varying spectral and angular (θ ) dependencies are identifiable as dark bands.
that for a coherent collimated beam of light, while some illumination wavelengths may be strongly in SPP resonance, other wavelengths will be strongly out of resonance. As a result, this SPP mode will always have a reduced bandwidth in comparison to the sub 20 fs illumination pulse, and thus, according to the Fourier limit, a significantly prolonged temporal pulse length. A similar strong angular dependence is present for the mode C plasmon. However, for the mode A plasmon there is virtually no angular variation in the excitation over the entire illumination spectral range; therefore, SPPs of very high bandwidth are accessible. Indeed, for this particular SPP mode, the complete illumination bandwidth (approximately 90 nm) is coupled with a very even efficiency (70% reflectivity dip), presenting the possibility for the excited SPP to have a very comparable spectral and temporal distribution to the sub 20 fs illumination pulses. As a result of the fact that the current experimental investigation uses ultrashort pulses, it is important to consider nonlinear contributions to the observations [24]. To do this we combine experimental and computational methods. From an experimental point of view, we record and compare the optical dispersion maps for two spectrally identical, but temporally significantly different illumination pulses, interacting with our nanostructured surface. To contrast with the sub 20 fs pulse (Fig. 5(i)) the second pulse is chosen as a chirped 785 fs pulse (Fig. 5(ii)). This pulse therefore has a temporal length significantly longer than the SPP decay lifetime [25]. Any dominant nonlinear process contributing to the SPP will be identifiable as variations between the two dispersion maps. It is evident that the general trends of the two experimental dispersion maps are very similar and that there are no significant intensity variations between the SPP bands; however, the two dispersion maps are not identical. For example, at the base of the mode A plasmon (8 eV, 1.5 eV) there is a subtle difference in the width of the SPP resonance, where the chirped pulse resonance is slightly broader than that of the near-transform limited pulse. This type of subtle difference could originate from non-linear contributions which may enhance the coupling between adjacent plasmonic modes. In order to investigate further, we perform a computational analysis, as presented in the next section.
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27388
Fig. 5. Experimentally measured dispersion maps for the (-1, 0) diffraction order for (i) 17.5 fs and (ii) 785 fs illumination pulses. Both pulse trains are delivered at a 95 MHz repetition rate with an average power of 250 mW. Dotted lines highlight the three plasmonic modes (A, B and C). Note: Fine detail differences are due to data acquisition resolution being lower for the longer pulse. ω is the illumination frequency, and kx is the x component of the illumination wavevector: kx = ωc sin(θ ).
4.
Simulations
In support of the experimental far-field optical characterisation, a computational analysis (COMSOL multiphysics) of the same system is performed where the near- and far-field optical responses were calculated. In order to better understand the plasmonic and optical processes involved, the spectral range examined is extended beyond that of the experimental analysis, and a flat spectral profile is considered rather than the near-gaussian profile of the laser. As mentioned above, in order to investigate the contribution of non-linear processes, it is most valuable to deliberately contrast the ultrashort experimental pulsed illumination with a continuous wave simulated illumination, as it will not yield non-linear effects. Presented in Fig. 6 is the simulated dispersion map for a nanostructured surface matching the experimental system. As is clear, over the laser spectral range (indicated by the horizontal white dashed lines in Fig. 6) there is a strong correlation between the simulated and experimental (Fig. 5) dispersion maps; however important appreciable differences are present. It is anticipated that some of the differences between the experimental and simulated dispersion maps originate from the manner in which the finite element method simulation considers the broadband illumination. Specifically, the simulation uses a combination of multiple single frequency elements to represent a broadband illumination, and as a result, SPP mode mixing effects (discussed below) are underrepresented. In addition, further experimental / simulation differences originate in the fact that surface imperfections present in the experimental system are not considered in the simulation (e.g. surface roughness and structure edge rounding [26]). However, the simulated dispersion map remains particularly useful in justifying the origin of the broadband mode. In Fig. 6, it is clear that the mode A SPP dispersion is significantly influenced by the presence of other SPP modes; in particular mode B and mode D. Where the modes approach, and even meet (i.e. two propagating modes are excited for the same frequency and energy conditions), the modes mix and there is an apparent modification in the dispersion trend of both effected modes. As discussed above, the finite element simulation method inherently underestimates this mixing; however, as is clear in the experimental data (Fig. 5), for the mode A SPP this mode mixing alters the dispersion map such that it reveals a broadband mode. Furthermore, the comparison of the experimental and simulated dispersion maps indicate that nonlinear effects do not play a
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27389
Fig. 6. Simulated dispersion map. The plasmonic modes A, B and C match those found experimentally, with extra mode detail visible here due to an extended spectral range (e.g. modes D and E). The spectral limits of the experimental laser pulses are indicated by the horizontal dashed lines.
dominant role, but do play a supportive role in terms of enhanced mode coupling resulting in the straightening of the mode A SPP observed in the experimental dispersion map. In addition to the dispersion map, near-field characteristics of the excited broadband plasmonic mode are also investigated. In contrast to the simulated dispersion map presented in Fig. 6, for this investigation the spectrum of the simulated illumination source matches that of the experimental laser, and an angle of incidence of 32 degrees is considered for investigation of the broadband mode A plasmon. As can be seen in Fig. 7, a snap-shot in time of the optical
q Fig. 7. Simulated electric field strength Ex2 + Ey2 + Ez2 at an instant in time during the optical cycle for the mode A plasmon (color scale) including Poynting vectors (red arrows) indicating the direction of energy flow (i.e. SPP propagating direction). For this simulation, the spectral profile of the illumination matches that of the laser (as in Fig. 2(B)), and illumination of the structures is from the left at an angle of 32 degrees out of the plane of the page. Media included in the supplementary material illustrates the evolution of the electric field strength over a complete optical cycle (Media 1).
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27390
near-field of the broadband excited SPP mode A shows both the direction of SPP propagation (Poynting vectors indicated by red arrows) and the total absolute combined electric field strength (according to the color scale). Regarding the electric field strength associated with the mode A SPP, the location of maximum enhancement varies over the optical cycle (see supplemental media), but as is expected, the maximum enhancement is predominantly found at the structure boundaries. Of particular interest in this near-field analysis is verification that the excited broadband SPP is a propagating mode. As is evident at the presented instant in time, the energy flux is predominantly to the left (i.e. a counter-propagating SPP). This negative propagation direction is maintained over the complete optical cycle (see supplemental media), confirming that this broadband mode is a propagating mode, and as such could potentially support SPP pulses of similar temporal duration to the sub 20 fs illumination pulses. 5.
Conclusion
A demonstration of the efficient excitation of broadband plasmonic modes in the near-IR regime is presented. This has been achieved using a tailor designed periodic surface that grants access to an SPP mode which can be simultaneously efficiently excited over the entire bandwidth of Ti:Sa oscillator ultrashort pulses. For the nanostructured surface presented, propagating SPP modes that couple the complete illumination spectrum of a sub 20 fs illumination pulse were demonstrated experimentally, and further examined computationally. This ability to generate propagating broadband SPP modes that exhibit no spatial or temporal chirping in their excitation process is an important step toward accessing the previously predicted ultraquick optical processes associated with SPPs. Acknowledgments The authors thank Dr. F. O’Reilly, Dr. S. Crosbie, and Dr. M. Berndt for their valuable discussions and contributions; and for financial support the authors acknowledge Science Foundation Ireland (Project Codes: 07/RFP/ENEF347, 11/RFP/MTR/3113, 08/RFP/ENEF1198), Enterprise Ireland, and the Irish Research Council.
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27391
Brian Ashall,1,2, Jos´e Francisco L´opez-Barber´a,1,3 ´ Eadaoin McClean-Ilten,1 and Dominic Zerulla.1,4,5 1 School
of Physics, University College Dublin, Belfield, Dublin 4, Ireland of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland 3 ICN2 - Institut Catala de Nanociencia i Nanotecnologia, Campus de la UAB, Bellaterra, Barcelona, 08193, Spain 4 Mechanical Engineering, 5130 Etcheverry Hall, University of California Berkeley, CA 94720-1740, USA 5 [email protected] 2 School
∗ [email protected]
Abstract: To date, considerable experimental and theoretical focus has been placed on the spatial control of Surface Plasmon Polaritons (SPPs) using nanostructured surfaces; however, research aimed toward accessing the ultrafast dynamics of SPPs remains vastly unexplored. Despite this, SPPs have the potential to exhibit some of the fastest possible optical processes, while maintaining the advantage of nanoscale spatial manipulation. Here, we present an experimental and computational investigation of a system that provides access to the efficient excitation of broadband, propagating SPP modes. To achieve this, a surface array of tailor designed, reduced symmetry nanostructures has been fabricated to enable the required control of the plasmon dispersion map to match sub 20 fs pulses in the near infra-red. Using a combination of optical spectroscopy and frequency resolved optical gating techniques, complimented by finite element computational analysis, the efficient excitation of propagating broadband plasmonic modes is demonstrated. © 2013 Optical Society of America OCIS codes: (240.6680) Surface plasmons, (240.5420) Polaritons, (320.2250) Femtosecond phenomen)
References and links 1. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer Verlag, 1988). 2. 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. Mat. 9, 193-204 (2010). 3. M. I. Stockman, M. F. Kling, U. Kleineberg, and F. Krausz, “Attosecond nanoplasmonic field microscope,” Nat. Photon. 1, 539-544 (2007). 4. M. I. Stockman, “Ultrafast nanoplasmonics under coherent control,” N. J. Phys. 10, 025031 (2008). 5. C. S¨onnichsen, T. Franzl, T. Wilk, G. von Plessen, J. Feldmann, O. Wilson, and P. Mulvaney, “Drastic reduction of plasmon damping in gold nanorods,” Phys. Rev. Lett. 88, 077402-077405 (2002). 6. B. Lamprecht, J. R. Krenn, A. Leitner, and F. R. Aussenegg, “Resonant and off-resonant light-driven plasmons in metal nanoparticles studied by femtosecond-resolution third-harmonic generation,” Phys. Rev. Lett. 83, 44214424 (1999). 7. A. Wokaun, J. P. Gordon, and P. F. Liao, “Radiation damping in surface-enhanced raman scattering,” Phys. Rev. Lett. 48, 957-960 (1982).
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27383
8. T. Klar, M. Perner, S. Grosse, G. von Plessen, W. Spirkl, and J. Feldman, “Surface-plasmon resonances in single metallic nanoparticles,” Phys. Rev. Lett. 80, 4249-4252 (1998). 9. S. Link and M. A. El-Sayed, “Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods,” J. Phys. Chem. B 103, 8410-8426 (1999). 10. A. Dogariu, T. Thio, L. J. Wang, T. W. Ebbesen, and H. J. Lezec, “Delay in light transmission through small apertures,” Opt. Lett. 26, 450-452 (2001). 11. R. M¨uller, V. Malyarchuk, and C. Lienau, “Three-dimensional theory on light-induced near-field dynamics in a metal film with a periodic array of nanoholes,” Phys. Rev. B 68, 205415-205423 (2003). 12. T. Zentgraf, A. Christ, J. Kuhl, and H. Giessen, “Tailoring the ultrafast dephasing of quasiparticles in metallic photonic crystals,” Phys. Rev. Lett. 93, 243901-243904 (2004). 13. D. S. Kim, S. C. Hohng, V. Malyarchuk, Y. C. Yoon, Y. H. Ahn, K. J. Yee, J.W. Park, J. Kim, Q. H. Park, and C. Lienau, “Microscopic origin of surface-plasmon radiation in plasmonic band-gap nanostructures,” Phys. Rev. Lett. 91, 143901-143904 (2003). 14. C. Ropers, M¨uller, C. Lienau, G. Stibenz, G. Steinmeyer, D-J. Park, Y-C. Yoon, and D-S. Kim, “Ultrafast dynamics of light transmission through plasmonic crystals,” International Conference on Ultrafast Phenomena, Niigata, Japan (2004). 15. R. Rokitski, K. A. Tetz, and Y. Fainman, “Propagation of femtosecond surface plasmon polariton pulses on the surface of a nanostructured metallic film: space-time complex amplitude characterization,” Phys. Rev. Lett. 95, 177401-177404 (2005). 16. W. Zhou, H. Gao, and T. W. Odom, “Toward broadband plasmonics: tuning dispersion in rhombic plasmonic crystals,” ACS Nano 4, 1241-1247 (2010). 17. J.-S Bouillard, S. Vilain, W. Dickson, G. A. Wurtz, and A. V. Zayats, “Broadband and broadangle SPP antennas based on plasmonic crystals with linear chirp,” Scientific Reports 2, 829 (2012). 18. S. Rehwald, M. Berndt, F. Katzenberg, S. Schwieger, E. Runge, K. Schierbaum, and D. Zerulla, “Tunable nanowires: an additional degree of freedom in plasmonics,” Phys. Rev. B 76, 085420-085423 (2007). 19. S. E. Yalcin, Y. Wang, and M. Achermann, “Spectral bandwidth and phase effects of resonantly excited ultrafast surface plasmon pulses,” Appl. Phys. Lett. 93, 101103-101105 (2008). 20. R. Trebino, Frequency-Resolved Optical Gating: the Measurement of Ultrashort Pulses (Kluwer Academic Publishers, 2000). 21. J. Chen, Z. Li, M. Lei, S. Yue. J. Xiao, and Q. Gong, “Broadband unidirectional generation of surface plasmon polaritons with dielectric-film-coated asymmetric singl-slit,” Opt. Express 19, 26463-26469 (2011). 22. B. Ashall, M. Berndt, and D. Zerulla, “Tailoring surface plasmon polariton propagation via specific symmetry properties of nanostructures,” Appl. Phys. Lett. 91, 203109 (2007). 23. B. Ashall, B. Vohnsen, M. Berndt, and D. Zerulla, “Controlling polarization twisting of light resulting from surface plasmon interactions with threefold symmetric nanostructures,” Phys. Rev. B 80, 245413-245417 (2009). 24. A. Marini, D. V. Skryabin, and B. Malomed, “Stable spatial plasmon solitons in a dielectric-metal-dielectric geometry with gain and loss,” Opt. Express 19, 6616-6622 (2011). 25. W. L. Barnes, “Surface plasmon-polariton length scales: a route to sub-wavelength optics,” J. Opt. A 8, S87 (2006). 26. P. Scholz, S. Schwieger, B. Ashall, D. Zerulla, and E. Runge, “The influence of wire shape on surface plasmon mode distribution,” Appl. Phys. B 93, 111-115 (2008).
1.
Introduction
SPPs are electromagnetic surface waves confined to the interface of two materials with dielectric functions of opposite signs, e.g. conductor and dielectric [1]. They occur as a result of a resonant interaction between an illuminating electromagnetic wave, and localized surface density oscillations of the free electrons of the conductor. The field of plasmon photonics, referred to as plasmonics, offers numerous research possibilities and application prospects. The main driving force behind plasmonics research is the fact that SPs couple to light in a manner that allows for an extremely localized control of the coupled light, thus granting access to sub-diffraction limited optical processes, while maintaining optical frequencies. As a result of this, vast experimental and theoretical research working toward the goal of plasmonic nanoscale confinement and manipulation has been carried out (e.g. [2]), while the potential offered by ultrafast SPP processes remains vastly unexplored. Despite this, broadband SPPs (in the visible and near IR regimes) offer access to temporal processes ranging from evolution times on the hundreds of attoseconds [3], to dephasing times on the 10’s to 100’s of femtoseconds [4]. To date, aspects of the ultrafast kinetics and dephasing times of nanoparticle plasmons have #194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27384
Fig. 1. Illustration of the excitation of propagating plasmonic pulses on the surface of the examined tailor designed periodic nanostructured surface. Sub 20 fs, near IR (centered at 800 nm), 88 MHz repetition rate pulses are incident on the sample (from the left). The propagating SPP pulses along with six elongated exemplary diffracted channels are illustrated.
been investigated [5–9], along with the temporal analysis of extraordinary optical transmission processes [10–15]. However, as a result of the characteristic dispersive nature of propagating SPPs, there is an inherent difficulty in exciting broadband coherently propagating SPPs [16]. Recently, the importance of the efficient excitation of broadband SPPs has been addressed, where a linearly chirped micro-grating was used for the broadband excitation of unidirectional SPPs [17]. In that work, a spatially dependant range of surface momenta provided by the chirped micro-grating permitted the excitation of SPP modes of different frequencies in spatially neighbouring locations. In our presented work, we assume a similar goal of broadband propagating SPP excitation, with the extension to achieve this excitation in a spatially uniform manner; i.e. broadband SPP excitation at all points on the nanostructured surface that is illuminated by the laser pulse. 2.
Ultrashort SPP Pulses on Nanostructured Surfaces
The presented experimental and computational examinations consider the dynamics of broadband propagating SPP pulses excited on square arrays of tailor designed, three fold symmetric, silver coated nanostructures, as illustrated in Fig. 1. Such a surface presents two mechanisms whereby the surface topography can provide the required momentum to overcome the wavevector mismatch between free space light and SPs: grating [18] and nanofeature coupling [1]. Of particular importance here is a modified grating coupling technique. For this excitation scheme, when considering SPP temporal characteristics on the femtosecond (fs) scale, two temporal aspects of propagating SPPs must be distinguished; the SPP propagation lifetime (i.e. the time after which the amplitude of the excited SPP is reduced by a factor of 1/e), and the SPP temporal pulse length. When considering SPP excitation using ultrafast laser pulses (e.g. sub 20 fs), to date it has been the restrictive SPP spectral acceptance bandwidth for a fixed incoming angle that has limited SPP pulses to be temporally elongated in comparison to the driving illumina#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27385
Fig. 2. Characteristics of a sub 20 fs laser pulse used for SPP excitation measured by SHG FROG. (A) Temporal and spectral intensity plot. (B) Spectral intensity profile (red) and spectral phase (green). (C) Temporal pulse intensity (blue) and temporal phase (yellow). Flat spectral and temporal phase profiles across the pulse bandwidth (as in (B) and (C)) indicate a transform limited pulse.
tion field [19]. However, if this restriction could be lifted, applications in nonlinear plasmonics and ultrafast plasmonic data processing can be envisaged; where, for example, a single channel of on chip plasmonic pulse code modulation would result in potential data transmission rates of up to 1014 bits per second. For the Ti:Sa oscillator used in our experiments (Griffin C, Kapteyn-Murnane Laboratories), following group delay dispersion compensation, near transform limited (sub 20 fs) illumination pulses were generated. An essential prerequisite for this work is that the SPP mode must accept the full spectral content of such illumination pulses. Using the pulse characterization technique of Second Harmonic Generation (SHG) Frequency Resolved Optical Gating (FROG) [20], experimental measurements (Fig. 2) show that in order to envelope more than 95% of our neartransform limited sub 20 fs pulse spectrum (with a FWHM bandwidth of approximately 28 nm), an SPP coupling acceptance bandwidth exceeding 80 nm is in practice required. Furthermore, as the illumination pulse must remain spatially and temporally optimized prior to interaction with the surface, the required broadband excitation mechanism must couple this complete illumination bandwidth at a single angle of incidence, and the excitation must occur in a spatially uniform manner. However, for high efficiency SPP excitation architectures (i.e. typical grating or attenuated total reflection - ATR - coupling) the SPP dispersion relation varies rapidly with illumination frequency and angle [1]. For example, in an ATR configuration, the full bandwidth of a spatially and temporally unchirped ultrashort pulse cannot be coupled to an SPP simultaneously [19], thus prohibiting the generation of SPP pulses of comparable duration to the driving ultrashort laser pulse. For some nanofeature based excitation mechanisms (e.g. nanoparticle, rough surface, slit [21], etc.) a suitable broad range of momenta can be inherently provided [1], allowing for broadband plasmon excitation. However, such excitation schemes are limited to comparably weak SPP excitation, with only a very small percentage of the illumination light coupled to the desired SPP mode. Furthermore, such coupling techniques are not suitable for plasmon pulse generation, as the excited plasmons will have a range of group and phase velocities, will not co-propagate, and so will be strongly spatially and temporally chirped.
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27386
Fig. 3. (A) Illustration of setup. Broadband laser light from a post compensated Ti:Sa laser incident on a goniometer housing the sample. The detector (interchangeable between a photodiode, spectrometer, or SHG FROG) can scan any diffraction order with change in θ , φ , or α. Note: In reality, the center of the sample lies at the fulcrum of the θ , φ and α rotations. (B) SEM image of an 8 by 4 µm section of the examined periodic nanostructured array.
3.
Broadband SPP Excitation
In our presented work, an elegant solution for the efficient excitation of broadband propagating SPPs is found whereby a highly ordered periodic array of tailor designed, reduced symmetry nanostructures (Fig. 3(B)) is used for SPP excitation (for fabrication details see [22, 23]). Evidentally, such structures can be used to control and design the plasmon dispersion map. The initial design for the structures was conceived and optimized using reciprocal space analysis followed by a finite element analysis (Comsol Multiphysics). The geometry was chosen to present a particular distribution of inter-structure distances to allow for the required control over the dispersion map, and the physical dimensions of the surface features were optimized for SPP excitation in the near IR (pitch of approximately 1 µm, structure footprint of 740 nm by 750 nm). A further advantage is that the structures can be described by an analytical function; namely an epitrochoidal geometry. For this sample, as a result of the well ordered array basis, a high SPP coupling efficiency is achieved, and as a result of the reduced symmetry nanostructures, the range of momenta provided by the grating is increased, customizing the dispersion map to present the possibility of generating efficient broadband SPPs. A detailed far-field experimental examination of the excitation of SPPs on the nanostructured array was performed using a goniometric set up (Fig. 3(A)) allowing for combined spectral and angular intensity characterization along with Autocorrelation and FROG analysis. By spectrally and angularly mapping individual diffraction orders, the optical dispersion map is obtained, where SPP modes are identifiable as dark bands in the far-field light. Using near-transform limited pulses, three plasmonic modes were observed in the presented (−1, 0) diffraction order (Fig. 4). It is clear that the three SPP modes have different dispersion properties, characterized by their varying angular (θ ) and spectral dependencies. While the mode B plasmon, ranging from 35◦ to 50◦ (Fig. 4), appears to be the strongest, it is important to note that this mode has a strong angular dependence over the spectrum of the laser. This is significant here, as it means
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27387
Fig. 4. Experimentally measured spectral and angular (θ in Fig. 3(A)) reflectivity map (normalized intensity according to color scale) for the (-1, 0) diffraction order, using sub 20 fs illumination pulses as in Fig. 2. Three plasmonic modes (A, B and C) of varying spectral and angular (θ ) dependencies are identifiable as dark bands.
that for a coherent collimated beam of light, while some illumination wavelengths may be strongly in SPP resonance, other wavelengths will be strongly out of resonance. As a result, this SPP mode will always have a reduced bandwidth in comparison to the sub 20 fs illumination pulse, and thus, according to the Fourier limit, a significantly prolonged temporal pulse length. A similar strong angular dependence is present for the mode C plasmon. However, for the mode A plasmon there is virtually no angular variation in the excitation over the entire illumination spectral range; therefore, SPPs of very high bandwidth are accessible. Indeed, for this particular SPP mode, the complete illumination bandwidth (approximately 90 nm) is coupled with a very even efficiency (70% reflectivity dip), presenting the possibility for the excited SPP to have a very comparable spectral and temporal distribution to the sub 20 fs illumination pulses. As a result of the fact that the current experimental investigation uses ultrashort pulses, it is important to consider nonlinear contributions to the observations [24]. To do this we combine experimental and computational methods. From an experimental point of view, we record and compare the optical dispersion maps for two spectrally identical, but temporally significantly different illumination pulses, interacting with our nanostructured surface. To contrast with the sub 20 fs pulse (Fig. 5(i)) the second pulse is chosen as a chirped 785 fs pulse (Fig. 5(ii)). This pulse therefore has a temporal length significantly longer than the SPP decay lifetime [25]. Any dominant nonlinear process contributing to the SPP will be identifiable as variations between the two dispersion maps. It is evident that the general trends of the two experimental dispersion maps are very similar and that there are no significant intensity variations between the SPP bands; however, the two dispersion maps are not identical. For example, at the base of the mode A plasmon (8 eV, 1.5 eV) there is a subtle difference in the width of the SPP resonance, where the chirped pulse resonance is slightly broader than that of the near-transform limited pulse. This type of subtle difference could originate from non-linear contributions which may enhance the coupling between adjacent plasmonic modes. In order to investigate further, we perform a computational analysis, as presented in the next section.
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27388
Fig. 5. Experimentally measured dispersion maps for the (-1, 0) diffraction order for (i) 17.5 fs and (ii) 785 fs illumination pulses. Both pulse trains are delivered at a 95 MHz repetition rate with an average power of 250 mW. Dotted lines highlight the three plasmonic modes (A, B and C). Note: Fine detail differences are due to data acquisition resolution being lower for the longer pulse. ω is the illumination frequency, and kx is the x component of the illumination wavevector: kx = ωc sin(θ ).
4.
Simulations
In support of the experimental far-field optical characterisation, a computational analysis (COMSOL multiphysics) of the same system is performed where the near- and far-field optical responses were calculated. In order to better understand the plasmonic and optical processes involved, the spectral range examined is extended beyond that of the experimental analysis, and a flat spectral profile is considered rather than the near-gaussian profile of the laser. As mentioned above, in order to investigate the contribution of non-linear processes, it is most valuable to deliberately contrast the ultrashort experimental pulsed illumination with a continuous wave simulated illumination, as it will not yield non-linear effects. Presented in Fig. 6 is the simulated dispersion map for a nanostructured surface matching the experimental system. As is clear, over the laser spectral range (indicated by the horizontal white dashed lines in Fig. 6) there is a strong correlation between the simulated and experimental (Fig. 5) dispersion maps; however important appreciable differences are present. It is anticipated that some of the differences between the experimental and simulated dispersion maps originate from the manner in which the finite element method simulation considers the broadband illumination. Specifically, the simulation uses a combination of multiple single frequency elements to represent a broadband illumination, and as a result, SPP mode mixing effects (discussed below) are underrepresented. In addition, further experimental / simulation differences originate in the fact that surface imperfections present in the experimental system are not considered in the simulation (e.g. surface roughness and structure edge rounding [26]). However, the simulated dispersion map remains particularly useful in justifying the origin of the broadband mode. In Fig. 6, it is clear that the mode A SPP dispersion is significantly influenced by the presence of other SPP modes; in particular mode B and mode D. Where the modes approach, and even meet (i.e. two propagating modes are excited for the same frequency and energy conditions), the modes mix and there is an apparent modification in the dispersion trend of both effected modes. As discussed above, the finite element simulation method inherently underestimates this mixing; however, as is clear in the experimental data (Fig. 5), for the mode A SPP this mode mixing alters the dispersion map such that it reveals a broadband mode. Furthermore, the comparison of the experimental and simulated dispersion maps indicate that nonlinear effects do not play a
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27389
Fig. 6. Simulated dispersion map. The plasmonic modes A, B and C match those found experimentally, with extra mode detail visible here due to an extended spectral range (e.g. modes D and E). The spectral limits of the experimental laser pulses are indicated by the horizontal dashed lines.
dominant role, but do play a supportive role in terms of enhanced mode coupling resulting in the straightening of the mode A SPP observed in the experimental dispersion map. In addition to the dispersion map, near-field characteristics of the excited broadband plasmonic mode are also investigated. In contrast to the simulated dispersion map presented in Fig. 6, for this investigation the spectrum of the simulated illumination source matches that of the experimental laser, and an angle of incidence of 32 degrees is considered for investigation of the broadband mode A plasmon. As can be seen in Fig. 7, a snap-shot in time of the optical
q Fig. 7. Simulated electric field strength Ex2 + Ey2 + Ez2 at an instant in time during the optical cycle for the mode A plasmon (color scale) including Poynting vectors (red arrows) indicating the direction of energy flow (i.e. SPP propagating direction). For this simulation, the spectral profile of the illumination matches that of the laser (as in Fig. 2(B)), and illumination of the structures is from the left at an angle of 32 degrees out of the plane of the page. Media included in the supplementary material illustrates the evolution of the electric field strength over a complete optical cycle (Media 1).
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27390
near-field of the broadband excited SPP mode A shows both the direction of SPP propagation (Poynting vectors indicated by red arrows) and the total absolute combined electric field strength (according to the color scale). Regarding the electric field strength associated with the mode A SPP, the location of maximum enhancement varies over the optical cycle (see supplemental media), but as is expected, the maximum enhancement is predominantly found at the structure boundaries. Of particular interest in this near-field analysis is verification that the excited broadband SPP is a propagating mode. As is evident at the presented instant in time, the energy flux is predominantly to the left (i.e. a counter-propagating SPP). This negative propagation direction is maintained over the complete optical cycle (see supplemental media), confirming that this broadband mode is a propagating mode, and as such could potentially support SPP pulses of similar temporal duration to the sub 20 fs illumination pulses. 5.
Conclusion
A demonstration of the efficient excitation of broadband plasmonic modes in the near-IR regime is presented. This has been achieved using a tailor designed periodic surface that grants access to an SPP mode which can be simultaneously efficiently excited over the entire bandwidth of Ti:Sa oscillator ultrashort pulses. For the nanostructured surface presented, propagating SPP modes that couple the complete illumination spectrum of a sub 20 fs illumination pulse were demonstrated experimentally, and further examined computationally. This ability to generate propagating broadband SPP modes that exhibit no spatial or temporal chirping in their excitation process is an important step toward accessing the previously predicted ultraquick optical processes associated with SPPs. Acknowledgments The authors thank Dr. F. O’Reilly, Dr. S. Crosbie, and Dr. M. Berndt for their valuable discussions and contributions; and for financial support the authors acknowledge Science Foundation Ireland (Project Codes: 07/RFP/ENEF347, 11/RFP/MTR/3113, 08/RFP/ENEF1198), Enterprise Ireland, and the Irish Research Council.
#194870 - $15.00 USD Received 2 Aug 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 4 Nov 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.027383 | OPTICS EXPRESS 27391
