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Nanofocusing in laterally tapered plasmonic waveguides

Ewold Verhagen, Albert Polman, and L. (Kobus) Kuipers

Open Access Open Access

Abstract

We investigate the focusing of surface plasmon polaritons (SPPs) excited with 1.5 µm light in a tapered Au waveguide on a planar dielectric substrate by experiments and simulations. We find that nanofocusing can be obtained when the asymmetric bound mode at the substrate side of the metal film is excited. The propagation and concentration of this mode to the tip is demonstrated. No sign of a cutoff waveguide width is observed as the SPPs propagate along the tapered waveguide. Simulations show that such concentrating behavior is not possible for excitation of the mode at the low-index side of the film. The mode that enables the focusing exhibits a strong resemblance to the asymmetric mode responsible for focusing in conical waveguides. This work demonstrates a practical implementation of plasmonic nanofocusing on a planar substrate.

©2008 Optical Society of America

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Figures (10)
Fig. 1.
Fig. 1. (a) Energy level diagram of Er3+ ions. The black arrows depict the cooperative upconversion mechanism which causes the excitation of higher energy levels by energy transfer between two excited ions. (b) SEM image of the end of a fabricated, tapered Au waveguide on an Er-implanted sapphire substrate. The scale bar is 1 µm.

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Fig. 2.
Fig. 2. Schematic of the experimental geometry in the case of upconversion luminescence detection through the substrate (a), or from the air side of the sample (b). In both cases the SPPs are excited with infrared light at the Au/Al2O3 interface in the direction of the arrow. The red line schematically indicates the Er depth profile.

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Fig. 3.
Fig. 3. (a) Upconversion luminescence image from the hole array (left side) and 60 µm long tapered waveguide (right side) sections, taken at 550 nm. The excitation spot is marked by the arrow on the left side of the image. (b) and (c): Detailed maps of the upconversion intensity near the tip of the waveguide, taken at 550 nm and 660 nm, respectively. A 1.48 µm pump at a power of 20 mW was used. The arrows indicate the positions at which power dependency curves were obtained. (d) Optical microscopy image of the same region as (b) and (c), obtained by detecting reflected light from a halogen lamp at a wavelength of 550 nm.

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Fig. 4.
Fig. 4. Excitation laser power dependence (λ=1.48 µm) of the upconversion luminescence at 550 and 660 nm collected at positions A and B (indicated with arrows in Fig. 3(b)). The lines are linear fits to the data for luminescence intensities between 100 and 1000 counts/s, and the slopes of these fits are indicated. The error in the fit is of order 0.01.

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Fig. 5.
Fig. 5. Upconversion luminescence images taken from the air side of the film at (a) 550 nm and (b) 660 nm. The edge of the taper is indicated by the dotted line. Upconversion luminescence excited by SPPs on the substrate side of the film is observed from the edges of the taper, and the maximum intensity is detected at the taper tip.

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Fig. 6.
Fig. 6. (a) Sketch of the electric field of the two SPP modes in an infinitely extended metal film, showing the symmetry of the transverse electric field and the surface charge. The direction of propagation is normal to the image plane. (b) Schematic of the geometry used in the FDTD calculations. The Au taper has a length of 7.8 µm and an apex diameter of 60 nm. The refractive index of the substrate is n 2, and that of the surrounding medium is n 1.

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Fig. 7.
Fig. 7. Mode profile of the asymmetric bound mode used as excitation source at x=0 in the FDTD simulation. Shown are the electric field intensity (a) and the real part of the electric field component in the z direction (b).

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Fig. 8.
Fig. 8. Electric field intensity in the planes z=-35 nm (a), and y=0 (b). The scale is normalized to the average intensity at the start of the tapered waveguide. Both color scales are saturated to improve the visibility away from the tip. The intensity enhancement at z=-10 nm below the tip apex is 100. The inset of (a) shows a detail of the electric field intensity in the plane z=-35 nm at the tip. The intensity distribution at the tip has a full width at half maximum of 92 nm. Normalized vertical cross sections of (b) at the start (x=0.5 µm, black) and the end (x=7.77 µm, red) of the taper are depicted in (c), showing the increase of vertical confinement towards the taper tip.

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Fig. 9.
Fig. 9. Cross section of the electric field intensity near the tip of the tapered waveguide, at x=7.77 µm. The field is predominantly localized at the metal corners. No field nodes are present along the metal surface.

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Fig. 10.
Fig. 10. Electric field intensity in the y=0 plane for the asymmetric bound mode at the high index side of the film (a), and the symmetric leaky mode at the low index side of the film (b). The color scale is saturated in (a), and the same for both figures.

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