S emiconductor nanowires have received much attentionowing to their potential use as building blocks of miniaturized electrical 1 , nanofluidic 2 and optical devices 3 . Although chemical nanowire synthesis procedures have matured and now yield nanowires with specific compositions 4 and growth directions 5 , the use of these materials in scientific, biomedical and microelectronic applications is greatly restricted owing to a lack of methods to assemble nanowires into complex heterostructures with high spatial and angular precision. Here we show that an infrared single-beam optical trap can be used to individually trap, transfer and assemble high-aspect-ratio semiconductor nanowires into arbitrary structures in a fluid environment. Nanowires with diameters as small as 20 nm and aspect ratios of more than 100 can be trapped and transported in three dimensions, enabling the construction of nanowire architectures that may function as active photonic devices. Moreover, nanowire structures can now be assembled in physiological environments, offering new forms of chemical, mechanical and optical stimulation of living cells.At present, several nanowire assembly techniques are being investigated, including electric 6 and magnetic 7 fields, laminar flow in microfluidic channels 8 , and Langmuir-Blodgett compression 9 . Although these techniques can align groups of nanowires, they lack the ability to control and assemble individual wires into two-or three-dimensional heterostructures 10 . Optical traps are an appealing tool for semiconductor nanowire integration owing to their ability to act in situ in closed aqueous chambers, their potential applicability to a broad range of dielectric materials, their spatial positioning accuracy 11 (<1 nm), and the degree to which their intensity, wavelength and polarization can be controlled using tuneable lasers, acousto-optic modulators and holographic optical elements 12 . Single-beam optical traps have been used for almost two decades 13 to manipulate and interrogate objects of micrometre and nanometre size 14 . In 1994, optical confinement of metal particles in two dimensions was achieved 15 and it was shown 16 that a 36-nm-diameter gold particle could be optically trapped in three dimensions. More recently, birefringent crystals were rotated in an optical trap by angular momentum transfer 17 , and CuO nanorods were manipulated in two dimensions with a line optical trap 18 .In the Mie limit, applicable to objects much larger than the wavelength of light, ray-tracing can be used to calculate the forces exerted on objects by an optical potential 19,20 . Optical trapping of objects much smaller than the wavelength of light, such as certain glass nanorods, is theoretically tractable using the standard framework of the Rayleigh limit 21,22 . However, nanowires targeted for use in integrated optical and electrical circuits usually have aspect ratios larger than 100 and diameters smaller than 80 nm (two dimensions in the Rayleigh limit, the third in the Mie limit), may have asymmetrical ...

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