Towards a tunable nanoscopic light source

Semiconductor nanowires have witnessed an explosion of interest in the last several years due to advances in synthesis and the unique thermal, optoelectronic, chemical, and mechanical properties of these materials. The potential applications of single-crystalline nanowires are truly impressive, including computational technology, communications, spectroscopic sensing, alternative energy, and the biological sciences. While lithographic silicon processes are rapidly approaching their physical size limits, optical information processing promises to be a low-power, high-bandwidth alternative for the continuation of Moorepsilas Law. Semiconductor systems with photon, phonon and/or electron confinement in two dimensions offer a distinct way to study electrical, thermal, mechanical, and optical phenomena as a function of dimensionality and size reduction. These structures have cross-sectional dimensions that can be tuned from 5 to 500 nm, with lengths spanning hundreds of nanometers to millimeters. The vapor-liquid-solid crystal growth mechanism has been utilized for the general synthesis of nanowires of different compositions, sizes, and orientation. Precise size control of the nanowires can be readily achieved using metal nanocrystals as the catalysts. Epitaxial growth plays a significant role in making such nanowire heterostructures and their arrays. Achieving such high level of synthetic control over nanowire growth allows us to explore some of their very unique physical properties. For example, semiconductor nanowires can function as self-contained nanoscale lasers, LEDs, sub-wavelength optical waveguides, photodetector. We have also recently developed 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 colo- - urs 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.