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Compressing surface plasmons for nano-scale optical focusing

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Abstract

A major challenge in optics is how to deliver and concentrate light from the micron-scale into the nano-scale. Light can not be guided, by conventional mechanisms, with optical beam sizes significantly smaller than its wavelength due to the diffraction limit. On the other hand, focusing of light into very small volumes beyond the diffraction limit can be achieved by exploiting the wavelength scalability of surface plasmon polaritons. By slowing down an optical wave and shrinking its wavelength during its propagation, optical energy can be compressed and concentrated down to nanometer scale, namely, nanofocusing. Here, we experimentally demonstrate and quantitatively measure the nanofocusing of surface plasmon polaritons in tapered metallic V-grooves down to the deep sub-wavelength scale - ~λ/40 at wavelength of 1.5 micron - with almost 50% power efficiency.

©2009 Optical Society of America

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Figures (5)

Fig. 1.
Fig. 1. Illustration of nanofocusing of light. (a) An example of electric field intensity, ∣E∣2, enhancement during nanofocusing in a tapered gap plasmon waveguide illustrates this effect. The electric field is determined by numerical solution of the Maxwell equations for the V-groove output width = 50nm (all other parameters are same as detailed below). (b) Scanning electron microscope image of a typical V-groove’s cross-section showing wide upper region for access of the incoming beam and narrow lower region where nanofocusing takes place.
Fig. 2.
Fig. 2. V-groove fabrication process and optical schematic. (a) Fabrication process for nano scale V-groove. (b) Optical measurement setup for the developed rapid and reliable far-field investigation of nanofocusing.
Fig. 3.
Fig. 3. Illumination by different polarizations of light – plasmon excitation. (a) SEM image of two sub-wavelength width (~100nm) V-groove outputs and a large reference hole as seen from the backside (i.e. looking in the positive x direction (Fig. 1a)) together with a magnified view of the V-groove outputs. (b) TM (E-field in the y-axis) excitation by the incident laser. (c) Illumination by TE (E-field in the z-axis) polarization. Note that the reference hole on the left is sufficiently large to allow both TE and TM wave pass. Scale bars are 10μm.
Fig. 4.
Fig. 4. (a). CCD image of optical output from 17 V-grooves of different output widths. (b) SEM image of the V-groove outputs corresponding to the encircled optical spots in Fig. 4a. (c) Close up view of the CCD image of the four optical spots encircled in Fig. 4a outputted from the structure in Fig. 4b. (d) Calculated square of the electric field amplitude, ∣E∣2 at the approximate far-field (red-line in Fig. 4f). (e) Calculated square of the electric field amplitude, ∣E∣2 at the narrowest part of the V-groove output (green line in Fig. 4f). (f) Schematic for field distributions in Fig. 4d and 4e.
Fig. 5.
Fig. 5. Intensity and electric field dependence on gap width. (a) In the experiments the guided mode’s beam width was scaled down to ~λ/40 (~40nm for λ = 1.53μm). Power (per gap width) dependence on V-groove output width for experiment (crosses) and FDTD simulation (squares). (b) The maximum of ∣E2 dependence on V-groove output width from the experimental power (crosses) and FDTD (squares) revealing a measured enhancement of ~10.
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