1. Department of Chemistry,
   
2. Department of Physics, and,
   
3. Biophysics Graduate Group, University of California
   
4. Physical Biosciences Division and,
   
5. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
   
6. Materials Laboratories, Sony Corporation, 4-16-1 Okata, Atsugi-shi, Kanagawa 243-0021, Japan
   
7. These authors contributed equally to this work.

Correspondence to: Jan Liphardt^2,3,4Peidong Yang^1,5 Correspondence and requests for materials should be addressed to J.L. (Email: liphardt@physics.berkeley.edu) or P.Y. (Email: p_yang@berkeley.edu).

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 probes^{1, 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 tweezers^{6, 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 properties^{9, 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.