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Tunable nanowire nonlinear optical probe


One crucial challenge for subwavelength optics has been the development of a tunable source of coherent laser radiation for use in the physical, information and biological sciences that is stable at room temperature and physiological conditions. Current advanced near-field imaging techniques using fibre-optic scattering probes1,2 have already achieved spatial resolution down to the 20-nm range. Recently reported far-field approaches for optical microscopy, including stimulated emission depletion3, structured illumination4, and photoactivated localization microscopy5, have enabled impressive, theoretically unlimited spatial resolution of fluorescent biomolecular complexes. Previous work with laser tweezers6,7,8 has suggested that optical traps could be used to create novel spatial probes and sensors. Inorganic nanowires have diameters substantially below the wavelength of visible light and have electronic and optical properties9,10 that make them ideal for subwavelength laser and imaging technology. Here we report the development of an electrode-free, continuously tunable coherent visible light source compatible with physiological environments, from individual potassium niobate (KNbO3) nanowires. These wires exhibit efficient second harmonic generation, and act as frequency converters, allowing the local synthesis of a wide range of colours via sum and difference frequency generation. We use this tunable nanometric light source to implement a novel form of subwavelength microscopy, in which an infrared laser is used to optically trap and scan a nanowire over a sample, suggesting a wide range of potential applications in physics, chemistry, materials science and biology.

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Figure 1: KNbO3 nanowires and their structural analysis.
Figure 2: Radiation from optically trapped single KNbO3 nanowires.
Figure 3: Transmission line scan of metallic surface pattern with laser trapped KNbO3 nanowire.
Figure 4: POPO-3 bead excitation by waveguided SHG signal from an optically trapped KNbO3 nanowire.


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This work was supported in part by the Dreyfus Foundation and the US Department of Energy (P.Y.), the University of California, Berkeley (J.L.), the Experimental Physical Chemistry Program of the National Science Foundation, and the NASA SRLDA program (R.M.O. and R.J.S.). Y.N. thanks SONY for a research fellowship and P.J.P. thanks the NSF for a graduate research fellowship. Work at the Lawrence Berkeley National Laboratory was supported by the Office of Science, Basic Energy Sciences, Division of Materials Science of the US Department of Energy. We thank T. Kuykendall for transmission electron microscope observations and the National Center for Electron Microscopy for the use of their facilities, L. Sohn for AFM facilities, N. Switz for comments on the manuscript and W. Liang for microfabrication of gold patterns.

Author Contributions Y.N. performed the synthesis and structural characterization of the KNbO3 wires. Y.N. and R.M.O. designed, performed and analysed the wave mixing experiment. P.J.P. and A.R. designed, performed and analysed the laser trapping and nanoprobe imaging experiments.

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Correspondence to Jan Liphardt or Peidong Yang.

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This file contains Supplementary Notes, Supplementary Figures S1-S4 with Legends, Supplementary Table S1 and references. (PDF 314 kb)

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Nakayama, Y., Pauzauskie, P., Radenovic, A. et al. Tunable nanowire nonlinear optical probe. Nature 447, 1098–1101 (2007).

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