Jun 14, 2012
Nanowire makes a sensitive force sensor
Vertical optically trapped dielectric nanowires with high aspect ratios are known to have very low trap stiffnesses parallel to the laser beam. This makes them potentially useful as sensitive force probes, with applications in the sub-piconewton range. Theoretical modelling and computer simulations have been applied to this system, and revealed that the vertical trap stiffness decreases as the inverse cube of the nanowire length, but increases with the refractive index. The work will help in the design of bespoke probes for specific applications.
Researchers from the Nanophysics and Soft Matter research group in the School of Physics at Bristol University, UK, are studying the use of geometrically anisotropic particles as force probes in holographically controlled optical tweezers. An important aspect of this work is to develop an understanding of the origins of the forces involved, which has been achieved through the use of computer simulations, using the discrete dipole approximation, as well as theoretical models.
The team has investigated the stability and thermal motion of a range of optically trapped nanowires having a radius of 50 nm and aspect ratios in the range 10 to 100. As expected, the models predict that the nanowires will align with the laser beam axis. In this configuration the vertical trap stiffness falls as the inverse cube of the nanowire length, in the long nanowire limit. On the other hand, the transverse trap stiffnesses tend to finite limits with nanowire length. Other parameters explored include the trap width and the refractive index of the nanowire. It was observed that all trap stiffnesses increased with the refractive index, while an increase in the beam width caused an increase in the vertical stiffness and a decrease in the lateral stiffnesses.
For short nanowires, around a micron in length, and with relatively high refractive indices, the electromagnetic model suggested that vertical trapping would no longer be stable, and that the nanowire would tend to rotate to the horizontal orientation. However, a full analysis, including the hydrodynamic friction acting on the cylinder, indicates that the presence of a coupling between the rotational and translational motions in the trap actually stabilises the vertical orientation for all aspect ratios considered.
Full details of the work can be found in the journal Nanotechnology.
About the author
Simon Hanna is a Reader in Physics and Stephen Simpson is a post-doctoral researcher in physics at the University of Bristol.