Optical near field imaging of localized surface ... - Nanophotonics

We used the technique of near-field scanning optical microscopy in perturbation mode. ... R.S.M.: E-mail: [email protected], S.B.: E-mail: [email protected].
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Optical near field imaging of localized surface plasmons modes in metallic nanostructures integrated on dielectric waveguides Josslyn Beltran Madrigal a , Natalia Dubrovinab , Rafael Salas-Montielc , Heriberto M´arquez Becerraa , Andr´e de Lustracb , Micka¨el Fevrierc , Aniello Apuzzoc , Gilles Lerondelc , Anatole Lupub,d , and Sylvain Blaizec a

Center for Scientific Research and Higher Education at Ensenada, Applied Physics Division, Carretera Ensenada-Tijuana No. 3918, Zona Playitas, C.P. 22860, Ensenada, B.C., Mexico b Univ. Paris-Sud, Institut d’Electronique Fondamentale, UMR 8622, 91405 Orsay, France c Laboratoire de Nanotechnologie et d’Instrumentation Optique, UTT, Troyes 10010, France d CNRS, Orsay, F-91405, France ABSTRACT

The study of surface plasmon-polaritons interactions in metallic nanostructures has been a topic of interest during last years due to their use in various areas such as the photonics, chemistry and biology. Example of use is found in biosensors for the efficient detection of biological analyte and in nanophotonic elements for on-chip photonics. Here, we study the interactions properties of localized surface plasmons in a hybrid waveguiding structure made of bi-dimensional array of gold nanowires vertically integrated on silicon-on-insulator waveguides across the near infrared spectrum. With the use of near-field scanning optical microscopy (NSOM) in perturbation mode, we qualitatively obtained the spectral response of such hybrid structure through intensity near field maps of the light propagation. These experimental results demonstrate that metallic nanostructures integrated on silicon are suitable for the development of localized surface plasmon integrated devices or metallic metamaterials. Keywords: silicon photonics, integrated optics, near-field scanning optical microscopy, metallic metamaterials

1. INTRODUCTION Today scientific research has focused on the study of the microscopic world due to the current needs in medical and technological fields. The study of microscopic organisms and the dispatch of information have enabled optics to be essential part of the latest technological developments as result of its versatility to these scales and its speed of propagation. Particularly, plasmonics offers an analysis of the light interaction with nanometric structures by the appearance of surface plasmon-polaritons.1 These surface waves allow the energy transport and confinement of light at the nanometer scale.2 The hybrid photonic-plasmonic structures combine high optical confinement and low-loss that allows the light propagation for micrometers even for millimeters and bridges the macro and microscopic worlds.3, 4 The object of study in this paper is precisely a hybrid device consisting of a bidimensional array of gold cut nanowires integrated on silicon waveguides. We characterized the hybrid structure with the use of a near-field scanning optical microscope. Intensity maps were obtained and plotted. The experimental results were compared to numerical simulations based on the finite differences time domain method.

2. METHODS AND MATERIALS 2.1 Description of the sample The samples are silicon waveguides with height of 200 nm and variable width along the direction of propagation. A two dimensional array of Nx by Nz gold nanowires is deposited on top of the larger section (10 µm) of the silicon waveguide. The size of each unit nanowires is W = 50 nm, L = 200 nm, and thickness H = 50 nm. The

Plasmonics: Metallic Nanostructures and Their Optical Properties XI, edited by Mark I. Stockman, Proc. of SPIE Vol. 8809, 880932 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2024812

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Figure 1: Scheme of the hybrid photonic-plasmonic structure. array has a period d = W + D with D = 100 nm along the direction of propagation and S = 100 nm along the direction perpendicular to the direction of propagation (Fig. 1). The structure is excited from the left side. The light propagates through the input zone and arrives into narrow section of width 600 nm that filters higher order modes and allows the propagation of the fundamental TE mode. An adiabatic taper increase the size of the fundamental mode. Then, this mode interacts with the LSP structure.

2.2 Near-field scanning optical microscopy in perturbation mode We used the technique of near-field scanning optical microscopy in perturbation mode. This technique allows the measurement of the distribution of the intensity of the electromagnetic field confined over the surface of the metallic nanostructure.5 In this technique, the nanostructure is perturbed periodically with a AFM probe at a frequency ω0 . The evanescent field is thus scattered and transformed in radiative field that can be detected in the far-field with the use of far field detectors. In perturbation mode, the collection is done in transmission through the optical silicon waveguide, that is, with the use of a second lensed optical fiber butt-coupled to the output of the SOI waveguide. The NSOM probes we used have diameters between 2 and 50 nm in its apex. Such small diameters allow the measurement of the optical intensity with high spatial resolution. The AFM enables us to simultaneously measure the topography of the sample. Figure 2 shows the scheme of a Mach-Zehnder interferometer.

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The NSOM in perturbation mode is inserted in the lower arm of the interferometer. In the upper arm, a reference optical signal propagates with a frequency shifted by the action of the acousto-optic frequency shifters. The optical signal from the two arms are sent to a optical fiber coupler. The detector converts the optical signal into an electrical signal that is sent to a lock-in amplifier locked at ωL = ω0 − ∆ω, where ∆ω = ω1 − ω2 . Note that the optical scattered power is not detected in this configuration. To estimate the light transmission change due to scattered light the transmit power is considered with and without the presence of the probe6 as: ∆T = 1 − TP1sca P1 .

2.3 Finite difference time domain simulations (3D FDTD) The simulations were performed only in the section of the waveguide where the nanostructure is placed. The structure is excited with a temporal pulse containing wavelengths in a range from 900 nm to 1800 nm. We then calculated amplitude and phase distributions over the planes m1 and m2 as shown in Fig. 3.

Figure 3: Simulation scheme for the simple. In this simulation Maxwells equations are solved for light propagated through the sample, which is discretized in a 3D mesh. In this case, the solutions to these equations are given in finite intervals and allow the imaging the amplitude and phase distribution of the electromagnetic field.

3. RESULTS AND DISCUSSION Here we present experimental and numerical results obtained at different wavelengths over the observation planes described in Fig. 3.

3.1 Experimental results With p-SNOM we obtained intensity distribution imaging of the confined field in the sample with D = 100 nm for various excitation wavelengths and its topography images (Fig. 4). In these images we observed exchange of intensity distribution for different wavelength. We observed that some wavelength the confined light is higher in the nanowires.

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3.2 FDTD simulation results To illustrate the results that can be obtained with the simulation, we show in Fig. 5 the results for different incident field of different wavelength for a nanostructure with D = 50 nm. In the observation plane m2 we have a profile that allows us to observe the exchange of energy between the waveguide and the nanostructure. Illustratively, we plotted the amplitude distribution in the middle of the waveguide and in the middle of the metallic nanostructure (Fig. 6).

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(uni)x Figure 6: Amplitude distribution imaging and amplitude distribution plot in the middle of the waveguide (blue) and the middle of the nanostructure (green) for D = 100 nm at a) λ = 1280 nm, b) λ = 1450 nm, c) λ = 1540 nm, and d) λ = 1610 nm. In this figure we observe that some wavelengths are better than others for the exchange of information between waveguide and nanostructure. In a range of 900 nm to 1800 nm we found that this wavelength is 1360 nm. These simulations allow us to observe the phase distributions too. With these amplitude and phase distributions we constructed the complex amplitude of the field and work in the Fourier space to find the propagation constants (k) of propagation modes in the hybrid structure. Fig. 7 shows the collection of these dates for various wavelengths (the dispersion relation) represented by the angular frequency ω and k in terms of nanostructure period in the z direction (d = W + D). In this figure we can observe the propagation constants location of hybrid structure in greater intensity areas for different excitation ω. In Fig. 7 we can see that if d, the nanostructure period, increases its propagation constants has higher values causing that propagation constants of hybrid structure increases, while the dispersion relation of the guide appears to limit to lower values of the propagation constants of the hybrid structure. For D = 100 nm we observed that the dispersion relations values of the guide and the nanostructure intersect about at ω = 1.35×1015 rad/s (λ = 1396 nm). This result indicates that silicon waveguide and nanostructure have similar propagation constants in this ω allowing a good coupling between them. These results can be made for each associated wavelength and we can be compared with the experimental results.

4. CONCLUSION Photonic and plasmonic hybrid structures are able to transport electromagnetic radiation by metal structure for distances of microns due the interaction of propagating modes of a waveguide dielectric and localized surface plasmons in the nanostructure. The FDTD allowed us to observe the behavior of amplitude and phase

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ACKNOWLEDGMENTS This work was supported by the French National Research Agency (ANR Metaphotonique, contract number 7452RA09) and the Champagne-Ardenne region. J.B.M thanks the Mexican National Council of Sciences and Technology (CONACyT, Mexico) and internal project of the CICESE (632110) for the financial support.

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