The synergistic role of light and heat in liquid-based nanoparticle manipulation

Abstract

Light and heat are synergistic tools used in the manipulation of nanoparticles and biomolecules. When optical effects dominate over thermal effects, the motion of nanoparticles can be controlled by optical forces. Here, we study the motion of 100 nm gold particles within a 1D optical potential, created by interfering counterpropagating beams. Tracking of particle trajectories revealed a large and asymmetric reduction in the nanoparticle diffusion constant in the presence of the traps, in agreement with theoretical predictions. When thermal effects dominate, laser light can induce local temperature gradients. Here, this was achieved by absorption of near-infrared (NIR) laser light in a Chromium microdisc. This resulted in thermophoretic separation of sodium azide ions, causing a local electric field that was used to manipulate 26 nm polystyrene beads. The nanoparticles were observed to follow the NIR heating spot, enabling light-controlled nanoparticle swarming. The induced 3D temperature profiles were characterised by time-correlated single-photon counting microscopy, with a temperature-sensitive dye. Through analysis of the particle velocities, the thermoelectric field strength, as well as the previously unknown Soret coefficients of azide ions were quantified. Transmission of laser beams through nanoparticle suspensions can lead to strong nonlinear lensing and soliton-like propagation effects. Literature has attributed these to redistribution of particles by optical gradient forces, and the effect is commonly described as an effective Kerr nonlinearity. To test this hypothesis, beam propagation experiments through a suspension of 40 nm plasmonic gold nanoparticles were carried out, and were found to be in agreement with previously reported results. To verify the nature of the effect, a new time-resolved z-scan technique was developed to measure the timescale and magnitude of the refractive index change. Surprisingly, the data demonstrates that the timescales can only be explained by thermal-absorption, -diffusion, and thermo-optic effects. As a result, the nonlinear effects are non-local and z-scan measurements will underestimate their magnitude.