Angular orientation of nanorods using nanophotonic tweezers.

Near-field optical techniques have enabled the trapping, transport, and handling of nanoscopic materials much smaller than what can be manipulated with traditional optical tweezers. Here we extend the scope of what is possible by demonstrating angular orientation and rotational control of both biological and nonbiological nanoscale rods using photonic crystal nanotweezers. In our experiments, single microtubules (diameter 25 nm, length 8 μm) and multiwalled carbon nanotubes (outer diameter 110-170 nm, length 5 μm) are rotated by the optical torque resulting from their interaction with the evanescent field emanating from these devices. An angular trap stiffness of κ = 92.8 pN·nm/rad(2)·mW is demonstrated for the microtubules, and a torsional spring constant of 22.8 pN·nm/rad(2)·mW is measured for the nanotubes. We expect that this new capability will facilitate the development of high precision nanoassembly schemes and biophysical studies of bending strains of biomolecules.