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 fiber-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 depletion (STED)3, structured illumination4, and photoactivated localization microscopy (PALM)5, have also enabled impressive, theoretically-unlimited spatial resolution of fluorescent biomolecular complexes. Previous work with laser tweezers6-8 has suggested the promise of using optical traps to create novel spatial probes and sensors. Inorganic nanowires have diameters substantially below the wavelength of visible light and have unique electronic and optical properties9,10 that make them prime candidates 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 (SHG), and act as frequency converters, allowing the local synthesis of a wide range of colors via sum and difference frequency generation (SFG, DFG). We use this tunable nanometric light source to implement a novel form of subwavelength microscopy, in which an infrared (IR) 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.