Brownian dynamics of optically trapped semiconductor nanowires

In the follow paper I will discuss the recent progress in investigating the optical trapping dynamics of semiconductor nanowires and other high aspect-ratio nanoscale objects. Owing to their perculiar geometry, these objects present rich and complex behaviour that departs significantly from high-symmetry particles typically employed in tweezers experiments. Early work in the field focused on holographic manipulation of nanowires with a view to achieve post-fabrication nanoscale-assembly of nanowire devices [1]. Whilst the presence of multiple trapping points can be used to configure the nanowire orientation in the imaging plane, it is also know that nanowires will naturally align in a single beam gradient force optical tweezers with the long axis oriented parallel to the laser propagation direction [2]. This behaviour is relatively independent of material properties, but is rather a characteristic of particles and particle-aggregates that present a dimension that exceeds the depth of focus of the trapping laser. To a first approximation, the trapped nanowires experience linear restoring forces and torques that counter the effects of stochastic Brownian motion [3]. However, more complex cross-coupling between translational and rotational degrees of freedom are also possible [4]. The influence of non-conservative effects (i.e. radiation pressure) is considered a perturbative effect in normal trapping configurations, however for strongly scattering objects, such as nanowires, radiation pressure can play a dominating role, leading to observations include cyclical winding and resonant oscillations in the power spectral density of the nanowire motion [5]. The origin of this type of behaviour may also be linked to the hydrodynamic properties of the individual nanowires.