Non-spherical gold nanoparticles trapped in optical tweezers: shape matters

Abstract

We present the results of a theoretical analysis focused on three-dimensional optical trapping of non-spherical gold nanoparticles using a tightly focused laser beam (i.e. optical tweezers). We investigate how the wavelength of the trapping beam enhances trapping stiffness and determines the stable orientation of nonspherical nanoparticles in the optical trap which reveals the optimal trapping wavelength. We consider nanoparticles with diameters being between 20 nm and 254 nm illuminated by a highly focused laser beam at wavelength 1064 nm and compare our results based on the coupled-dipole method with published theoretical and experimental data. We demonstrate that by considering the non-spherical morphology of the nanoparticle we can explain the experimentally observed three-dimensional trapping of plasmonic nanoparticles with size higher than 170 nm. These results will contribute to a better understanding of the trapping and alignment of real metal nanoparticles in optical tweezers and their applications as optically controllable nanosources of heat or probes of weak forces and torques.

Invited Article: Optical trapping of ultrasmooth gold nanoparticles in liquid and air

“…This result is in agreement with the previous report that the particle morphology affects the trapping forces. 11 Importantly US NPs exhibit about a two-fold reduction in the measurement error (σ rms ) for stiffness (see Table II) compared to NS counterparts due to the improved circularity of US gold NPs (see Table I). We note that when trapped with a circularly polarized beam, one can expect laserinduced rotation of gold NPs, 16 in which case the stiffness is averaged over azimuth angles.…”

“…2 This experimental result was revisited by a recent study including a theoretical model based on the coupled dipole method. 11 The model suggests that particles larger than 170 nm can be stably trapped only if they deviate from a perfect spherical shape, which is commonly assumed for gold NPs. Indeed, scanning electron microscopy has revealed that commercially available gold NPs are inherently non-spherical and rather appear in the shape of icosahedron, decahedron, triangular, or hexagonal prism structures.…”

“…Indeed, scanning electron microscopy has revealed that commercially available gold NPs are inherently non-spherical and rather appear in the shape of icosahedron, decahedron, triangular, or hexagonal prism structures. 11,12 The morphological features of such particles are responsible for their orientation with respect to the beam polarization, which enhances their trap stiffness. A step forward would be to avoid such non-uniformity in shape to enable more reproducible studies, including consideration of heating with gold NPs for a wide range of trapping experiments.…”

Theory and Practice of Computational Modeling and Simulation of Optical Tweezers

“…Brzobohatý et al (2015b) used simulations to explore the shape dependence of the trapping behaviour of nonspherical gold nanoparticles. Brzobohatý et al (2015a) used simulations to support experiments rotational dynamics of multiple spheroidal particles in a dual beam trap.…”

Quantitative optical trapping and optical manipulation of micro-sized objects

“…Optical tweezers are commonly used for trapping and manipulation with micro-and nanoparticles [1][2][3][4]. Since their invention in 1986, the field of optical trapping has become a hot-spot topic for research, and its applications have extended from microfabrication [5] to drug delivery [6].…”